Arturo Consoli

M.Sc Marco Pileggi

A T M Hasibul Hasan

M.D Alice Venier

M.D Alessandro Sgreccia

M.D Silvia Pizzuto

M.D Oguzhan Coskun

M.D Federico Di Maria

M.D Bertrand Lapergue

PhD Georges Rodesch

PhD Serge Bracard

M.D Bailiang Chen

LIST OF TABLES AND FIGURES vi RESUME DE LA THESE EN FRANÇAIS vii DEFINITION AND PRESENTATION OF THE THESIS xvi

Keywords:

Background: Patients with acute ischemic stroke secondary to large vessel occlusions and good collaterals are frequently associated with favorable outcomes after mechanical thrombectomy (MT), although poor outcomes are observed also in this subgroup. We aimed to investigate the factors associated with unfavorable outcomes (mRS3-6) in this specific subgroup of patients.

Methods: 219 patients (117 females, mean age: 70.6±16 y.o.) with anterior circulation stroke and good collaterals (ASITN/SIR grades 3-4), treated by MT between 2016-2021 at our institution were included in this study. Clinical files and neuroimaging were retrospectively reviewed. Univariate and multivariate analyses were performed to identify the predictors of unfavorable outcomes in the overall population (primary endpoint). Secondary endpoints focused on the analysis of the predic-tors of unfavorable outcomes in the subgroup of successfully recanalized patients, predictors of mortality and symptomatic intracerebral hemorrhages (sICH) in the overall population. Results: Poor outcome was observed in 47% of the patients despite the presence of good collaterals. Older age (p<0.001), baseline mRS2 (p<0.001), higher baseline NIHSS (p<0.001), longer onset-to-recanalization (p=0.03) and "groin-to-recanalization" times (p=0.004), no intravenous thrombolysis administration (no-IVT, p=0.004), >3 passes (p=0.01), partial recanalizations (mTICI0-2a; p=0.002), higher 24h-NIHSS (p<0.001) and sICH (p<0.001) were associated with the primary endpoint. The multivariate analysis showed an independent correlation between unfavorable outcomes and older age, higher 24h-NIHSS, 24h-ASPECTS, no-IVT and with secondary transfers. Conclusions: Despite good collaterals poor outcomes occurred in 47% of the patients. The main factors associated with unfavorable outcomes are comparable to those observed in patients with poor collaterals. Patients with good collaterals not receiving IVT were significantly associated with unfavorable outcomes, whereas FPE was not significantly correlated with clinical outcome in this specific cohort of patients.

LIST OF TABLES AND FIGURES

Introduction

Les Accidents Vasculaires Cérébraux ischémiques (AVCi) représentent une pathologie grave, potentiellement mortelle causée par l'occlusion d'une artère cérébrale. La lésion ischémique déterminée par l'occlusion artérielle peut évoluer vers la nécrose du tissu cérébral si le flux sanguin n'est pas rapidement rétabli. Les AVCi se caractérisent par des taux élevés de mortalité et de morbidité et l'objectif de la prise en charge de cette maladie dans un contexte aigu est basé sur un diagnostic rapide et un choix approprié de traitement.

Les nouvelles preuves apportées par les récents essais contrôlés randomisés et publiés en 2015 ont permis le développement et la diffusion généralisée de la thrombectomie mécanique (TM), une procédure Malgré les excellents résultats obtenus depuis l'introduction de la MT en termes d'augmentation constante du taux de recanalisation des artères obstruées, le taux de bons résultats cliniques n'a pas suivi la même évolution.

Tous les patients traités par MT ne tirent pas bénéfice de ce type de procédure, et on a évoqué pour eux le concept de « reperfusions futiles ». L'hypothèse physiopathologique sous-jacente à ce phénomène semble s'appuyer sur la circulation collatérale (CC), un système d'anastomoses entre les artères léptoméningées corticales du cerveau qui pourrait alimenter le territoire d'une artère occluse par un flux rétrograde au travers de ces anastomoses.

Une attention croissante est accordée à la CC dans la littérature, bien que les informations sur son hémodynamique et la tolérance à l'évolution de l'ischémie restent encore très peu connues.

Objectifs de la thèse

L'objectif de cette thèse de sciences est d'analyser le rôle de la CC selon trois paramètres:

1. L'analyse de l'implication clinique de la CC en tant que facteur pronostique pour les patients présentant un AVCi; Le chapitre 4 est axé sur l'analyse critique de la CC selon les caractéristiques artériographiques décrites dans l'article: Angiographic collateral venous phase: a novel landmark for leptomeningeal collaterals evaluation in acute ischemic stroke. Consoli A, Pizzuto S, Sgreccia A, Di Maria F, Coskun O, Rodesch G, Lapergue B, Felblinger J, Chen B, Bracard S. L'article a été publié dans le

Journal of NeuroInterventional Surgery (JNIS).

Le dernier chapitre est consacré au développement d'un algorithme de post-traitement spécifique pour l'évaluation de la CC directement sur des artériographies 2D. Le flux de travail technique et l'ébauche de la demande de brevet seront présentés dans ce chapitre.

viii Chapitre 1 : INTRODUCTION Les AVCi représentent environ 80 % des AVC à la phase aiguë, tandis que les AVC hémorragiques représentent environ 20 % des cas. En France, les AVCi sont considérés comme la première cause de handicap dans la population adulte, la deuxième cause de démence et troisième cause de mortalité (Haute Autorité de Santé, 2009). Chaque année, environ 150 000 nouveaux AVCi sont signalés avec un taux de mortalité d'environ 15 à 20% au cours du premier mois après l'événement ischémique et environ 50% au cours de la première année [START_REF] Fery-Lemonnier | La prévention et la prise en charge des accidents vasculaires cérébraux en France[END_REF][START_REF] Chevreul | Cost of stroke in France[END_REF]. Le coût annuel de la gestion des AVCi avait été estimé à 8,9 milliards € en 2007, ce qui correspondait à aux 3 % du budget annuel du système de santé [START_REF] Chevreul | Cost of stroke in France[END_REF].

Actuellement, deux stratégies thérapeutiques principales sont disponibles pour le traitement de l'AIS: la thrombolyse intraveineuse (IVT) et la thrombectomie mécanique (MT). L'objectif de la MT est d'obtenir la recanalisation de l'artère obstruée de manière rapide, sûre et complète. Le nombre de procédures de MT suit une croissance constante dans le monde entier et en France environ 7500 procédures/année sont effectuées.

En revanche, certains sous-groupes de patients ne semblent pas bénéficier de la MT, ce qui détermine la condition d'un succès technique (la recanalisation de l'artère occluse) sans atteindre un résultat clinique favorable. Cette condition a été décrite dans la littérature comme une « recanalisation futile ». À l'heure actuelle, le taux de recanalisation futile se situerait entre 45 et 55 % (Goyal M, 2016;van Horn N et coll., 2021). Une possible explication physiopathologique de ce phénomène pourrait être recherchée dans le rôle de la CC.

La CC pourrait être définie comme un apport vasculaire rétrograde de la circulation cérébrale dans un territoire qui ne peut pas être vascularisé en raison de l'occlusion de l'artère qui perfuse normalement des zones cérébrales définies. La CC se développe grâce à des anastomoses préexistantes entre les artères corticales des territoires voisins.

Il est possible de soutenir que la CC intervient entre une macro-circulation, c'est-à-dire la circulation corticale cérébrale (les artères corticales et piales et les artérioles pénétrantes) et une microcirculation, conçue comme le réseau cortical des capillaires qui est responsable de l'autorégulation.

En effet, certains auteurs ont interprété la CC comme un « modulateur » pour la réponse vasculaire d'autorégulation et pour certains phénomènes hémodynamiques spécifiques tels que le couplage neurovasculaire [START_REF] Vasquez | Intracranial collateral circulation and its role in neurovascular pathology[END_REF].

La compréhension de l'hémodynamique des artères corticales du cerveau est encore limitée. Les valeurs de pression et la mesure du flux sanguin dans cette région découlent toujours des mesures ix effectuées au cours des interventions microchirurgicales après craniotomie et qui ont été réalisées à l'aide de microsondes dans patients sans occlusion vasculaires (Carter PL et coll., 1978;Bederson et coll., 1995). Ces travaux préliminaires ont permis de créer le substrat pour le développement technique de l'algorithme de caractérisation.

Chapitre 3: ANALYSE DE L'IMPACT CLINIQUE DE LA CIRCULATION COLLATE-

RALE

Le rôle de la CC dans la détermination des résultats cliniques favorables a été largement décrit dans la littérature. Plusieurs articles ont montré comment la présence des vaisseaux collatéraux est assox ciée à des résultats cliniques favorables après traitement endovasculaire [START_REF] Bang | Impact of collateral flow on tissue fate in acute ischaemic stroke[END_REF][START_REF] Bang | Collateral flow predicts response to endovascular therapy for acute ischemic stroke[END_REF][START_REF] Ribo | Extending the time window for endovascular procedures according to collateral pial circulation[END_REF], Liebeskind et al., 2014, Consoli et al., 2016[START_REF] Mangiafico | Effect of the Interaction between Recanalization and Collateral Circulation on Functional Outcome in Acute Ischaemic Stroke[END_REF][START_REF] Liggins | Interhospital variation in reperfusion rates following endovascular treatment for acute ischemic stroke[END_REF], Seners P et al., 2019, Tong et al., 2018[START_REF] Leng | Impact of Collateral Status on Successful Revascularization in Endovascular Treatment: A Systematic Review and Meta-Analysis[END_REF].

Compte tenu de l'ensemble de la population présentant un AVCi, il a été démontré qu'environ 45 à 55% des patients traité avec succès par TM n'atteindra pas une indépendance fonctionnelle (Goyal et al., 2016, van Horn et al., 2021).

Les patients ayant un bon profil collatéral sont généralement considérés comme les candidats les plus favorables pour la TM, car ceux-ci sont associés à des conditions cliniques moins graves et avec lésions ischémiques plus limitées (Consoli et al., 2016). Cependant, aussi les patients avec de bons collatéraux peuvent ne pas bénéficier de la TM, malgré un tableau de perfusion cérébrale et clinique favorable, bien qu'une analyse des facteurs qui déterminent un mauvais résultat clinique dans ce sous-groupe spécifique de patients n'ait pas déjà été effectuée.

Par conséquent, ce chapitre était axé sur l'analyse de l'impact clinique de la CC en tant que facteur Les résultats de l'étude UNCLOSE ont montré que même les patients ayant de bons collatéraux peuvent ne pas atteindre un résultat clinique favorable en termes, indépendamment de la technique endovasculaire ou du type de anesthésie utilisée. Les résultats globaux étaient partiellement similaires à ceux rapportés dans la littérature et pour les populations de patients avec une CC insuffisante. En effet, il s'agissait de l'un des premiers articles portant sur cette cohorte spécifique de patients avec une bonne CC.

Néanmoins, nous avons pu observer de mauvais résultats cliniques (mRS3-6) chez 47% des patients présentant une bonne CC évaluée à l'aide du score ASITN/SIR. Le taux de mortalité était de 13,6 %. Le rôle de la thrombolyse IV en tant que facteur « protecteur » est conforme aux résultats actuels rapportés par l'étude SWIFT DIRECT, récemment publiée dans le Lancet, qui a montré que l'association IVT + MT était associée à un meilleur résultat clinique que la TM seule (Fischer U et al., 2022).

Ces données sont particulièrement intéressantes si l'on considère que cette cohorte est considérée Hassen W et al., 2019).

Afin d'évaluer l'efficacité de la CC, nous avons proposé l'analyse de la CC sur la base du concept de la perméabilité du côté veineux des vaisseaux collatéraux. Selon cette théorie, l'efficacité de la CC serait également liée à l'efficacité des veinules de la garantie circulation effectuant plusieurs tâches, telles que le dégagement des emboles en aval, le maintien du flux sanguin et effet préventif sur l'adhésion plaquettaire (Tong et al., 2018).

Les résultats de l'étude UNCLOSE avaient montré comme la modalité d'évaluation actuelle de la CC par artériographie, en utilisant les outils actuels, pourrait ne pas être adaptée compte tenu de ce nouveau paradigme qui met en évidence le rôle des veines et des venules de la CC dans la physiopathologie des AVCi.

L'analyse du sous-groupe de patients avec de bonnes collatérales et des résultats défavorables et la subjectivité de la l'évaluation de la CC soulèvent certains points de débat. Cette étude rétrospective réalisée sur une population de 200 patients a permis d'analyser un paramètre pas encore évalué dans la littérature : la phase veineuse de la CC évaluée sur les images artériographiques. L'hypothèse de l'étude était centrée sur l'impact clinique de la visualisation des structures veineuses sous-corticales dans la région de la circulation collatérale. La présence de cette phase veineuse représenterait le signe indirect de la fonctionnalité de la CC, ce qui pourrait permettre de différencier la CC non seulement par rapport à son extension, mais surtout par rapport à la capacité de maintenir un flux fonctionnel dans les vaisseaux collatéraux.

L'étude a montré que l'analyse de la phase veineuse des collatéraux semble affiner l'évaluation de la CC. Bien que l'analyse basée sur l'échelle ASITN/SIR et l'analyse basée sur CVP aient montré des résultats globalement similaires et en ligne avec la littérature médicale (Anadani et al. 2022[START_REF] Mangiafico | Effect of the Interaction between Recanalization and Collateral Circulation on Functional Outcome in Acute Ischaemic Stroke[END_REF], Liebeskind et coll., 2022), il a été possible d'observer une association bien plus significative entre la présence de la phase veineuse collatérale (CVP+) et le résultat clinique favorable (mRS à 3 mois) ainsi qu'en termes de taux de transformation hémorragique et de mortalité en comparaison avec l'évaluation effectuée par l'échelle ASITN/SIR. De plus, l'analyse composite qui correspondait à l'évaluation combinée de la CC par l'échelle ASITN/SIR et la présence/absence de la phase veineuse collatérale ont fourni des résultats solides en termes d'association avec des résultats cliniques favorables (OR: 6.56, CI95% [START_REF] Qian | A meta! analysis of collateral status and outcomes of mechanical thrombectomy[END_REF]39]) et un risque plus faible de transformation hémorragique.

Une validation externe de ces résultats sera obligatoire afin de confirmer ces résultats, qui restent pour l'instant un outil générateur d'hypothèse.

Chapitre 5 : UN APPROCHE TECHNIQUE BASE SUR LA PHYSIOPATHOLOGIE : DÉ-VELOPPEMENT D'UN ALGORITHME POUR L'ÉVALUATION ANGIOGRAPHIQUE DE

LA CIRCULATION COLLATÉRALE

Les résultats de l'analyse clinique et critique de la circulation collatérale, qui ont été discutés dans les chapitres précédents ont soulevé plusieurs questions sur l'évaluation de la circulation de la CC. Afin de caractériser les angiographies, en particulier l'opacification liée au produit de contraste iodé, nous avons analysé de manière approfondie le comportement de la série artériographique, non seulement par régions (cérébrale, région collatérale, région du bassin versant, etc.) mais aussi par pixel afin de comprendre le comportement du contraste du tissu cérébral.

Analyse spatiale des régions observées

Les caractéristiques des images artériographiques ont été analysées sur la base d'une répartition dans 4 types de régions: la région cérébrale, région collatérale étendue, la région « critique » de territoire de dernier pré et la région collatérale pure, afin d'analyser les changements de trajectoire temporelle de l'opacification liée au produit de contraste des vaisseaux collatéraux dans le territoire cortical de l'artère cérébrale moyenne. Un premier processus de segmentation a été effectué manuellement à l'aide du logiciel ITK-SNAP.

Analyse semi-quantitative de la courbe de densité temporelle L'évaluation de l'analyse de la courbe temps-densité basée sur la ROI a été effectuée en calculant les éléments des mappes paramétriques : time-to-peak (TTP), mean transit time (MTT), maximum enhancement projection (MEP). Alors que les interprétations étaient difficiles à réaliser sur les données TTP et MTT, les mappes MEP ont abouti à des résultats plus indicatifs pour donner la première xiv différentiation de la CC en « bonne » et « mauvaise » (nommées respectivement Group-gc « Good collaterals » et Group-pc « Poor collaterals »).

Complexité vasculaire dans la région collatérale

Afin de décrire quantitativement l'observation précédente et aussi prouver l'hypothèse qu'une bonne/mauvaise CC peut être différencié par le niveau de complexité vasculaire de la CC, nous avons utilisé la métrique appelée « dimension fractale », Ds, une méthode mathématique récente qui a été largement utilisée pour quantifier les vaisseaux rétiniens et les profils osseux trabéculaires en radiographie ou tomodensitométrie.

Concepts physiopathologiques intégrés

Pendant les travaux de la thèse, deux concepts physiopathologiques ont été développées et intégrés dans le développement de l'algorithme : la désynchronisation et les « turning points ». formed though an endovascular approach to remove the clot occluding a cerebral artery.

Désynchronisation

Although the excellent results obtained with the introduction of MT in terms of a constant increase of the recanalization rate of the occluded arteries, the rate of clinical outcomes did not follow the same behavior. Therefore, it was clear that not all the patients treated by MT could benefit from this type of procedure, configuring the so-called "futile reperfusions".

The physiopathological hypothesis underlying this phenomenon seems to rely on the collateral circulation (CC), a system of anastomoses between the cortical leptomeningeal arteries of the brain that could supply the territory of an occluded artery through a retrograde flow sustained by these anastomoses. Furthermore, an increasingly growing attention is being paid to CC in the literature, although very little is known about its hemodynamics.

The interest about CC raised up during the period of my Residency program in Radiology at the Careggi University Hospital in Florence (2010)(2011)(2012)(2013)(2014)(2015), where I have started to build my scientific activity under the supervision of my first mentor, Dr Salvatore Mangiafico, the first interventional neuroradiologist in the world to perform a procedure of MT. During those years the knowledge about CC was very limited as well as it was the therapeutic management of AIS.

I have started to deeply study this subject, since I could clearly observe that this type of circulation played a major role in the prognosis and evolution of AIS. I have participated to the publication of two first papers about collaterals in 2013 [START_REF] Mangiafico | Semi-quantitative and qualitative evaluation of pial leptomeningeal collateral circulation in acute ischemic stroke of the anterior circulation: the Careggi Collateral Score[END_REF]) and 2014 [START_REF] Mangiafico | Effect of the Interaction between Recanalization and Collateral Circulation on Functional Outcome in Acute Ischaemic Stroke[END_REF] xvii

The final declared objective of this algorithm will be the assessment of the flow velocity within the CC.

Aim of the PhD thesis

The aim of my PhD Thesis was to investigate the role of the CC through:

1.The analysis of the clinical implication of the CC as a prognostic factor for patients with AIS 2.A critical analysis of the angiographic images in order to evaluate a possible correlation with the physiopathologic events of AIS 3.The development of a dedicated post-processing algorithm for 2D angiograms and the comparison with a pre-existing algorithm.

The PhD thesis will be constituted of three different sections:

-Section 1: INTRODUCTION An introductive chapter (Chapter 1) that will provide an overview about AIS, the diagnosis, the possible treatment and the clinical challenges. This introductive part will serve also to acquaint the reader with the concept and the definition of the CC and the role of CC in AIS. All these notions will be further developed in the following sections;

-Section 2: PREFATORY RESULTS

A second chapter (Chapter 2) that resumes the Prefatory results obtained during my previous scientific experiences in the field of CC during my Residency Program and during the Master 2.

The following paper will be presented:

Consoli A, Andersson T, Holmberg A, Verganti L, Saletti A, Vallone S, Zini A, Cerase A, Romano D, Bracco S, Lorenzano S, Fainardi E, Mangiafico S; CAPRI Collaborative Group. CT perfusion and angiographic assessment of pial collateral reperfusion in acute ischemic stroke: the CAPRI study.

The paper was published in the Journal of Neurointerventional Surgery (JNIS) in 2016.

These preliminary results provided the basic concepts for the present work.

-Section 3: PhD ENDPOINTS xviii Three separate chapters (Chapters 3-5) will focus on the three aforementioned endpoints of the PhD thesis.

Chapter 3 will be centered on the clinical relevance of the CC as a prognostic factor in AIS.

This aspect was analyzed in the paper:

UNfavorable CLinical Outcome in patients with good collateral Scores: the UNCLOSE study. cardioembolic (due to valvular or non-valvular cardiac diseases, atrial fibrillation, intra-cardiac tumors such as mixomas, valvular prosthetic implantation),

atheromatous (secondary to the rupture of an instable atheromatous plaque located in the arte- rial wall of the carotid bifurcation/in the aortic arch or to the presence of an atheromatous plaque in a cerebral artery with an in-situ thrombosis),

infectious (in case of bacterial/mycotic/viral endocarditis)

tumoral (secondary to the hyper-coagulation induced by the biochemical reactions caused by the cancerous lesion)

Hemorrhagic strokes are mainly linked to the rupture of cerebral vascular structures (arteries, capillaries, veins) secondary to the aging or to the development of malformative diseases (aneurysms, ArterioVenous Malformations -AVMs, ArterioVenous cortical or dural fistulae -DAVF). According to the type of bleeding we can distinguish:

typical intra-parenchymal hematomas, which are frequently observed in the elderly population, with associated hypertension and ongoing vascular aging phenomena (amyloid deposits, arteriosclerosis) in the so-called "typical" localizations such as the putamen, the lenticular nucleus, deep cerebral territories;

atypical intra-parechnymal hematomas, which are often observed in younger patients without known vascular risk factors and which are located in atypical sites, such as (cerebral lobes, superficial territories, posterior cranial fossa, except for the pons). This subtype of hemorrhage stroke is frequently linked to vascular malformations (AVMs/DAFVs, dural or cortical fistulas);

sub-arachnoid hemorrhages (SAH), which are mainly caused by the rupture of a cerebral aneurysm, whose fissuration determines a blood collection in the sub-arachnoid spaces, surrounding the brain. Also other diseases can cause SAH, such as DAVFs and cortical AVMs, although this presentation is quite rare.

Epidemiology of Acute Ischemic stroke (AIS)

AIS represent about 80% of acute strokes, while hemorrhagic strokes account for about 20% of the cases. In France, AIS is considered the first cause of handicap in the adult population, the second cause of dementia and the third cause of mortality (Haute Autorité de Santé, 2009).

Every year about 150.000 new AIS are reported with a mortality rate of about 15-20% during the first month after the ischemic event and about 50% during the first year. This rate varies according to the type, localization and extension of the ischemic lesion. Among the survivors, the morbidity rate is about 50-75% because of the persistence of a neurological deficit (Fery-Lemonnier E., 2009; [START_REF] Chevreul | Cost of stroke in France[END_REF]).

Socio-financial burden of AIS

The annual cost of IAS management had been estimated in 8.9 billion € in 2007, which corresponded to the 3% of the annual budget of the Healthcare system [START_REF] Chevreul | Cost of stroke in France[END_REF]. The median cost of direct expenses every ischemic event is estimated at 16.686 €/patient during the first year [START_REF] Berkhemer | A Randomized Trial of Intraarterial Treatment for Acute Ischemic Stroke[END_REF].

Furthermore, one should consider the indirect costs, such as those linked to the rehabilitation programs and the loss of productivity, which had been calculated respectively at 2.4 billions € and 255.9 millions € [START_REF] Berkhemer | A Randomized Trial of Intraarterial Treatment for Acute Ischemic Stroke[END_REF].

Treatment of AIS patients

As aforementioned, AIS are secondary to the occlusion of a cerebral artery. However, the localization of the occlusion site allows to differentiate the lacunar AIS, which are caused by the occlusion of small arterial branches (perforating or isolated cortical branches) due to small clots, and the major strokes, which are due to the occlusion of large arteries of the Willis's polygon (the so-called "large vessel occlusions" -LVO). In general, lacunar strokes are associated with a more favorable prognosis and less neurological sequelae, while LVOs are mostly correlated with worse clinical outcomes and higher mortality rates (Goyal et al., 2016).

Most of the AIS is located in the carotid territory (anterior circulation, about 80-85% of the cases)

and less than 20% are located in the vertebro-basilar territory (posterior circulation).

Clinical severity of AIS depends on the localization of the occluded artery and on the extension of the ischemic lesion in the involved territory. Indeed, the occlusions of an artery of the vertebro-basilar system are associated with higher mortality rates, although these are less frequent. AS far as the anterior circulation is concerned, the occlusions of the carotid siphon of the internal carotid artery (ICA) are associated with more severe clinical conditions as compared with the isolated occlusions of the middle cerebral artery (MCA).

Currently, two main therapeutic strategies are available for the treatment of AIS: the intravenous thrombolysis (IVT) and the mechanical thrombectomy (MT)

-IVT is an inhibitor of the plasminogen activator; therefore, this drug allows the clot lysis. The ECASS-III [START_REF] Hacke | Thrombolysis with alteplase 3 to 4.5 hours after acute ischemic stroke[END_REF] had shown that patients could benefit of IVT administration (Alteplase) during the first 4.5 hours from the onset of the neurological symptoms, concluding that there was a significant improvement of clinical outcome without increasing the risk of a hemorrhagic transformation of the ischemic lesion. More recent trials, such as the DEFUSE-3 (Albers et al., 2017), showed the same clinical benefit without a temporal window limitation, but according to the perfusional status of the brain at the moment of the administration of the IVT.

The effectiveness of IVT seems to be related to the clot length, since it has been shown that clots longer than 8 mm are significantly less responsive to IVT with Alteplase [START_REF] Campbell | Clot Length Assessment in Stroke Therapy Decisions[END_REF]. However, new lytic molecules, such as the Tenecteplase showed even better results in terms of effectiveness and safety profile, slightly increasing the recanalization rates (Seners P et al., 2019). Current guidelines of the AHA and the ESO [START_REF] Powers | Guidelines for the Early Management of Patients With Acute Ischemic Stroke: 2019 Update to the 2018 Guidelines for the Early Management of Acute Ischemic Stroke: A Guideline for Healthcare Professionals From the American Heart Association/American Stroke Association[END_REF]Berge E et al., 2019) consider IVT as a first-line treatment for AIS in patients without contra-indications for IVT), in association with MT in case of LVOs.

-Mechanical Thrombectomy (MT) is a minimally invasive, endovascular procedure performed by Interventional neuroradiologists that allows the removal of the clot which occludes the cerebral artery. Different endovascular techniques are used to perform MT and in particular: the stent-like retriever (SR), the Contact Aspiration (CA) and the Combined Technique (CoT). All these techniques are performed through a femoral/radial/carotid arterial access (through the navigation of the aorta, the aortic arch and of vertebral or carotid arteries, always under angiographic control)

and these allow to reach to the occlusion site and to catch/aspire (or both simultaneously) the clot that obstructs the cerebral artery.

SR technique is based on the catch of the clot though dedicated devices (stent-like retrievers) which can be deployed and re-sheathed after having trapped the clot through their struts. CA is based on the navigation of large bore aspiration catheters until the proximal surface of the clot and a continuous aspiration, performed either manually of through a mechanical pump, and allows to engage the clot inside of the aspiration catheter and to remove it by simple aspiration. Recently, their combined use (SR+CA) which is resumed in the CoT, showed similar results in terms of effectiveness, without however showing a superiority compared to SR alone [START_REF] Seners | Recanalization before Thrombectomy in Tenecteplase vs. Alteplase-Treated Drip-and-Ship Patients[END_REF].

Recent Randomized Controlled Trials (RCTs) showed the significant benefit of MT for patients with LVOs of the anterior circulation [START_REF] Jovin | Thrombectomy within 8 hours after symptom onset in ischemic stroke[END_REF][START_REF] Saver | Stent-retriever thrombectomy after intravenous t-PA vs. t-PA alone in stroke[END_REF][START_REF] Goyal | Randomized assessment of rapid endovascular treatment of ischemic stroke[END_REF][START_REF] Campbell | Endovascular Therapy for Ischemic Stroke with Perfusion-Imaging Selection[END_REF]. Furthermore, also patients with posterior circulation AIS seem to benefit from MT [START_REF] Tao | Endovascular treatment for acute basilar artery occlusion: A multicenter randomized controlled trial (ATTENTION)[END_REF][START_REF] Li | Basilar Artery Occlusion Chinese Endovascular Trial: Protocol for a prospective randomized controlled study[END_REF], although previous trials weren't able to show its superiority compared to the best medical management.

The aim of MT is to achieve the recanalization of the occluded artery rapidly, safely and completely. The number of MT procedures is constantly growing throughout the world and in France about 7500 procedures/year are performed.

Patients selection

As aforementioned, RCTs showed the evidence of the benefit of MT in AIS treatment as well as the need to properly select patients to treat.

Indeed, some subgroups of patients seem not to get a benefit from MT, which determines the condition of a technical success (the recanalization of the occluded artery) without achieving a favorable clinical result. This condition has been described in the literature as "futile recanalization". Currently, the rate of futile recanalization is reported to be between 45 and 55% (Goyal M, 2016;van Horn N et al., 2021), meaning that almost half of the patients treated by MT will not have a favorable clinical outcome. A possible explanation of this phenomenon could be provided by an inappropriate patients' selection. During the last years we observed a progressive widening of the indications to MT in terms of temporal window, age limit and clot location. On the other side, a larger number of patients have had access to MT compared to the last decade.

Patients' selection is currently performed through MRI (in particular in France and Switzerland) and more widely through CT scan (in most European and US countries). These imaging methods allow to assess the extension of the ischemic lesion, through the ASPECT score (Fig. 1), the localization and the length of the occlusion as well as the presence of a collateral circulation at the moment of the treatment. A new paradigm of AIS management is currently being proposed by some recent on- In this model, which includes patients admitted in the early phases of AIS (before 4.5 hours) and within severe neurological deficits, the Digital Subtraction Angiography (DSA) would acquire a central role as a selection tool for patients to treat, since it provides real-time information about the extension of the collateral circulation.

Moreover, it has been clearly showed that patients with LVOs and limited ischemic lesions represent the best target for MT and IVT in order to achieve a favorable clinical outcome [START_REF] Nogueira | Thrombectomy 6 to 24 Hours after Stroke with a Mismatch between Deficit and Infarct[END_REF]Consoli A et al., 2016), while patients with larger ischemic lesions and treated in very late windows represent those subgroup for which a clear response has not been provided yet and an ongoing clinical trial, IN EXTREMIS (https://www.inextremis-study.com/), will hopefully provide solid data about this issue.

The assessment of the collateral circulation

As previously mentioned, collateral circulation assumes a central role in patients' selection.

Collateral circulation (CC) has been widely described in the literature [START_REF] Mangiafico | Effect of the Interaction between Recanalization and Collateral Circulation on Functional Outcome in Acute Ischaemic Stroke[END_REF]Consoli et al., 2016;Rocha M et al., 2017;Qian J et al., 2020) although its hemodynamic characteristics remain still largely unknown.

CC could be defined as a retrograde vascular supply of the cerebral circulation in a territory which cannot be fed because of the occlusion of the artery that normally vascularize defined cerebral areas. CC develops thanks to pre-existing anastomoses between the cortical arteries of neighbor territories (i.e. in case of occlusion of the MCA, the CC develops through anastomoses between the cortical branches of the MCA and the arteries of the ipsilateral Anterior Cerebral Artery -ACA. From a physiopathologic point of view, CC reflects the expression of what we could name the "hemodynamic reserve" of the brain during ischemia. Previously reported studies showed how the development of CC is definitely individual and subjective [START_REF] Khatri | Leptomeningeal collateral response and computed tomographic perfusion mismatch in acute middle cerebral artery occlusion[END_REF][START_REF] Seyman | The collateral circulation determines cortical infarct volume in anterior circulation ischemic stroke[END_REF].

7 A B ACA ACM ACA ACM
Currently, both CT-Angiography with a multiphase acquisition protocol [START_REF] Menon | Multiphase CT Angiography: A New Tool for the Imaging Triage of Patients with Acute Ischemic Stroke[END_REF] and MR-Angiography [START_REF] Hartkamp | Circle of Willis collateral flow investigated by magnetic resonance angiography[END_REF][START_REF] Kim | Multiphase MR Angiography Collateral Map: Functional Outcome after Acute Anterior Circulation Ischemic Stroke[END_REF] could provide useful information about the extension of CC, however without providing further information about the qualitative measurement of the collateral vessels.

On the other side, the DSA, which is performed through the injection of iodine contrast mean directly into the artery, allows to provide a more sensitive assessment of CC in terms of extension and effectiveness, basing on its dynamic profile (Consoli A et al., 2016;[START_REF] Mangiafico | Semi-quantitative and qualitative evaluation of pial leptomeningeal collateral circulation in acute ischemic stroke of the anterior circulation: the Careggi Collateral Score[END_REF].

The role of the collateral circulation in AIS

The endovascular treatment of AIS is continuously evolving. The new devices, stent-like retrievers, net aspiration catheter, new angiographic machines, are considered among the most important technological novelties in the neuroradiological field, both diagnostic and interventional. However, such technical developments need to be associated to the evolution of the comprehension and understanding of the disease.

When we consider the anatomy of the cortical and subcortical arteries, we can observe how the cortical arteries which run on the pial surface of the brain give rise to the penetrating arterioles. These arterioles feed the microcirculation which is mainly constituted by the capillaries located in the cerebral cortex and in the sub-cortex (Fig. 3).

8 10, 1369-1376 (2007). https://doi.org/10.1038/nn2003).

The microcirculation is responsible for the cerebral autoregulation and, therefore, for the maintenance of the perfusion of the cortex and of the subcortical area. In case of AIS with a LVO, the perfusion of the microcirculation can be maintained through some anastomoses that occur between the cortical arteries of two neighbor vascular territories [START_REF] Iadecola | Glial regulation of the cerebral microvasculature[END_REF][START_REF] Kunz | Cerebral vascular dysregulation in the ischemic brain[END_REF].

These anastomoses, which constitute the CC, provide a retrograde filling of the penetrating arterioles and of the microcirculation and, consequently, the maintenance of the cerebral perfusion during the occlusion of an artery [START_REF] Iadecola | Glial regulation of the cerebral microvasculature[END_REF].

It is possible to argue that the CC intervenes between a Macro-circulation, meant as the cortical cerebral circulation (the cortical and pial arteries and the penetrating arterioles) and a microcirculation, intended as the cortical network of capillaries which is responsible for autoregulation. Indeed, some Authors pointed out the CC as a "modulator" for the vascular autoregulation response and for some specific hemodynamic phenomena such as the neurovascular coupling [START_REF] Vasquez | Intracranial collateral circulation and its role in neurovascular pathology[END_REF].

When an infarcted territory is retrogradely perfused through the CC it can be maintained in a "stunned" condition and since the autoregulation can be assured by the CC this part of the brain can be considered as salvageable in case of recanalization of the occluded artery.

Oppositely, if the CC is not present, the territory which is not fed by the occluded artery will rapidly evolve to necrosis with a definitive loss of function of that area even in case of a complete recanalization of the occluded artery.

Indeed, is we consider that the recanalization rates and the capacity of clot retrieval have exponentially increased up to 90% of the cases, favorable clinical outcomes are still permanently observed up to 45-55%. The aforementioned "futile recanalizations" reflect the impairment between the technological development and the stagnation of the understanding of the physiopathology of AIS.

Therefore, CC has been proposed as a plausible explanation to this phenomenon, since the presence of this retrograde circulation allows the brain to be fed during the period of the occlusion of the cerebral artery and a certain level of cerebral perfusion can be maintained during the ischemia.

On the other hand, the lack of this CC is associated with a poor and critical perfusional status of the brain without a clinical benefit for patients treated by MT (Consoli A et al., 2016;[START_REF] Mangiafico | Semi-quantitative and qualitative evaluation of pial leptomeningeal collateral circulation in acute ischemic stroke of the anterior circulation: the Careggi Collateral Score[END_REF].

These concepts can be resumed in the definition of the "slow" and "fast progressors". Indeed, patients with good collaterals have been reported to achieve good clinical outcomes even in late temporal windows [START_REF] Liebeskind | Collateral Circulation in Thrombectomy for Stroke After 6 to 24 Hours in the DAWN Trial[END_REF] mainly because they harbored a good CC. The physiopathological explanation would lay in the effect of collaterals to maintain a level of cerebral perfusion during the ischemia, sustaining the ischemic penumbra and, therefore, the possibility to rescue the brain tissue from necrosis, even in patients treated in late windows, determining a slow progression of the ischemia (Rocha et al., 2017, Mohammaden MH et al., 2022).

Hemodynamics of cortical arteries and evaluation of cerebral blood flow velocity

The understanding of the hemodynamics of the cortical arteries of the brain is still limited. Pressure values and measurement of blood flow in this region still derives from measurements performed during microsurgical procedures after craniotomy and which were performed using microprobes in patients without LVOs [START_REF] Carter | Regional Cortical Blood Flow at Craniotomy[END_REF][START_REF] Bederson | Cortical Blood Flow and Cerebral Perfusion Pressure in a New Noncraniotomy Model of Subarachnoid Hemorrhage in the Rat[END_REF].

However, currently no data are provided in case of AIS and during an arterial occlusion, when we could assist to the activation of the cortical anastomoses which provide a retrograde cortical circulation.

Several papers focused on the issue of the measurement of blood flow velocity in cerebral arteries with non-invasive methods such as CT-Angiography or MR-Angiography [START_REF] Thierfelder | Color-coded cerebral computed tomographic angiography-Implementation of a convolution-based algorithm and first clinical evaluation in patients with acute ischemic stroke[END_REF][START_REF] Menon | Assessment of leptomeningeal collaterals using dynamic CT angiography in patients with acute ischemic stroke[END_REF][START_REF] Smit | Timing-invariant imaging of collateral vessels in acute ischemic stroke[END_REF], whereas the assessment directly performed on the DSA remains still limited [START_REF] Muehlen | Noninvasive Collateral Flow Velocity Imaging in Acute Ischemic Stroke: Intraindividual Comparison of 4D-CT Angiography with Digital Subtraction Angiography[END_REF].

Several groups focused on the correlation between the CC and perfusional imaging [START_REF] Galinovic | The ratio between cerebral blood flow and Tmax predicts the quality of collaterals in acute ischemic stroke[END_REF][START_REF] Cortijo | Relative cerebral blood volume as a marker of durable tissue-at-risk viability in hyperacute ischemic stroke[END_REF]) and more recently on the dynamic evaluation of collateral vessels, in particular using dedicated scores such as the Cortical Veins Opacification score (COVES), which were applied either on CT-Angiography or on MR/CT-Perfusion (Faizy et al., 2021;Faizy et al, 2021;[START_REF] Singh | Time-resolved assessment of cortical venous drainage on multiphase CT angiography in patients with acute ischemic stroke[END_REF].

Section II -PREFATORY RESULTS

PREFATORY RESULTS

The CAPRI Study

The impact of the CC for AIS had already been extensively investigated in the literature. Several

Authors had highlighted the unpredictable aspect of the development of these cortical anastomoses, since no correlations were found with age, sex or related vascular risk factors [START_REF] Lazzaro | The impact of diabetes on the extent of pial collaterals in acute ischemic stroke patients[END_REF]Arsava EM et al., 2014[START_REF] Christoforidis | Predictors for the extent of pial collateral recruitment in acute ischemic stroke[END_REF].

Although the concept of CC was anatomically clear, no explanation had been provided concerning the role of CC in the physiopathology and in the early 2010s the evolution of the perfusional assessment in the setting of AIS was starting to be settled.

It had frequently be recalled how CC would represent the expression of the perfusional status of the brain at the moment of the ischemia, suggesting the role of a vascular "reservoir" to supply a vascular territory which was not fed by the related occluded artery (Consoli A et al., 2016).

However, no clear proof of a correlation between CC and the perfusional status had been provided at that time.

After having published a novel grading scale for CC (the Careggi Collateral Score, CCS) directly based on the 2D-angiograms [START_REF] Mangiafico | Semi-quantitative and qualitative evaluation of pial leptomeningeal collateral circulation in acute ischemic stroke of the anterior circulation: the Careggi Collateral Score[END_REF], I had the possibility to coordinate a multi centric collaborative group that included five high-volume Italian centers focused on Interventional

Neuroradiology and a renowned European institution, the Karolinska Institutet in Stockholm.

This collaborative group aimed to investigate the correlation between patients with AIS of the anterior circulation that were screened by using CT-Perfusion, which provided perfusional color-maps showing the extension of the ischemic core (the brain part that would not have been rescued even in case of a complete recanalization) and of the ischemic penumbra (the salvageable cerebral tissue), and DSA at the moment of the procedure of MT.

The results of this analysis were published in the Journal of NeuroInterventional Surgery.

The main hypothesis was that high grades of CCS, meaning a good collateral status, were associated with favorable perfusional patterns, such as small ischemic cores and large ischemic penumbras.

This hypothesis was analyzed in the paper: CT perfusion and angiographic assessment of pial collateral reperfusion in acute ischemic stroke: the CAPRI study, published in the Journal of Neurointerventional Surgery in 2016.

The results of the CAPRI Study showed that a clear correlation between the collateral grade and the perfusional status of the brain could be established.

The perfusional status had been evaluated mainly on the CBV (Cerebral Blood Volume) color-maps which were a sensitive marker of the ischemic core, according to the literature [START_REF] Cortijo | Relative cerebral blood volume as a marker of durable tissue-at-risk viability in hyperacute ischemic stroke[END_REF] and the extension of the ischemic core was assessed using the ASPECT score.

Finally, we could observe that patients with a good collateral status were more likely to be associated with significantly higher rates of favorable clinical outcomes (Fig. 4). 13

The preliminary results at CIC-IADI Laboratory

After the publication of the CAPRI study, several papers focused on the correlation between collaterals and cerebral perfusion [START_REF] Galinovic | The ratio between cerebral blood flow and Tmax predicts the quality of collaterals in acute ischemic stroke[END_REF]Ginsberg MD, 2018[START_REF] Menon | Neuroimaging in Acute Stroke[END_REF].

Furthermore, in the last 5 years several technical developments were introduced in the study of cerebral perfusion through CT or MR Perfusion-Weighted Imaging (PWI) and a large number of automated softwares have been released in order to calculate rapidly the cerebral perfusional status.

These softwares, such as the RAPID ® or the OLEA ® software, allow to extrapolate a quantitative assessment of the ischemic core and the ischemic penumbra and to provide color maps almost in real time. On the other hand, no further advancements have been done concerning DSA, except for some sporadic attempts [START_REF] Muehlen | Noninvasive Collateral Flow Velocity Imaging in Acute Ischemic Stroke: Intraindividual Comparison of 4D-CT Angiography with Digital Subtraction Angiography[END_REF].

The main advantage of DSA is to provide a dynamic visualization of the intracranial circulation and of the CC thanks to the transit of the iodine contrast mean within the vessels. Contrast passes through the arteries, then to the capillaries and finally runs through the veins to be drained towards the heart and secondarily to the pulmonary circulation to be re-oxygenated. The algorithms used by the CT-Perfusion or MR-PWI are based on the same principle: after contrast injection, the transit time of the contrast itself is calculated and according to the different transit time in the cerebral areas a quantitative assessment is provided. Color-maps are generated according to the delay of transit of the contrast mean.

Intuitively, the transit time in a cerebral territory vascularized by an artery that is occluded will be much longer than the one measured in a territory where the feeding artery is patent. However, the presence of collateral vessels determines the maintenance of a certain amount of blood flow in the infarcted cerebral territory. This phenomenon is thought to support the concept of the ischemic penumbra: the cerebral territory receiving the blood flow through CC is maintained "stunned" and it remains salvageable if the occluded artery is properly recanalized.

However, it is known that several biochemical events occur at the level of the CC, such as in-situ thrombosis, platelets uncontrolled adhesion and distal migration of proximal emboli (downstream emboli). Therefore, one could argue that although a CC is present and extensively visible either on the non-invasive imaging methods (CT-or MR-Angiography) or directly on the DSA, the collaterals vessels could not be effective because of the aforementioned biological phenomena. Indeed, the thrombosis of these collaterals vessels would determine a slowdown in the CC, which could not provide the retrograde filling of the microcirculation (Fig. 3).

The concept of the effectiveness of CC represented the main target of my Master 2 project.

The aim of the Master 2 project was to focus on the analysis of the 2D angiograms acquired during the DSA, which represent the direct imaging method used to perform MT (Fig. 5). Indeed, basing on the assumption that the visualization of the CC could provide useful information about the extension of the collateral vessels but on necessarily on their effectiveness, the goal was to build up an algorithm of segmentation to be applicable directly to the 2D-DSA images in order to analyze the flow velocity within the CC, which could be considered as a surrogate of the effectiveness of these collateral vessels.

Since the setting of a complex algorithm requires a considerable time, we had decided to focus the Master 2 objective on the first step of the process: the acquisition of the angiographic data. 2D-DSA images are acquires by the angiographic machine while a bolus of iodine contrast mean is injected.

In order to analyze a dynamic process, such as the visualization of the CC, we argued that the standard acquisition protocols currently used in the clinical practice could not be sufficiently informative.

Methods

In order to properly analyze DSA images we had primarily defined an "experimental" acquisition protocol for the DSA images to be compared to a "standard" acquisition protocol which is already set on the angiographic machines. The standard clinical protocols are based on a 2 frames/second frequency with a variable number of frames during the time intervals of the acquisition. The "experimental" protocol was made up of a 6 frames/second frequency with a variable but higher number of frames acquired during the time intervals of the acquisition. The experimental protocol is currently already available in the basic setup of angiographic machines, but not systematically used in standard care methods. The acquisition is limited to a single angiogram and therefore there is no modification of procedural time and not significant differences in terms of radiation exposure. The objective is to find an acquisition method that can provide sufficient acquisition rate in order to provide information of the contrast behavior to reflex the pre-treatment CC functionality. The CC functionality is therefore evaluated by the enhancement speed of the CC area by the contrast agent.

Therefore, a first basic version of an algorithm for segmentation of vessels using Fussy C-means, which was capable to provide the perfusion-like curves according to the pixel opacification in the CC area, after the normalization of the results with the area of the involved brain hemisphere. A clinical analysis was performed in order to observe the impact of the visualization of CC on clinical outcomes, and in particular on the 3-months modified Rankin Scale (mRS, Table 1), a widely used scale to assess the functional independency at 3 months after the ischemic event.

Table 1. The modified Rankin Scale.

Imaging analysis aspects

The workflow of the technical process is summarized in Fig. 6. All the 2D DSA data were firstly manually segmented through the ITK-SNAP software (www.itksnap.org) [START_REF] Yushkevich | USse-guided 3D active contour segmentation of anatomical structures: significantly improved efficiency and reliability[END_REF]. I have performed the manual segmentation of the collateral region and of the hemisphere area for each patient, carefully identifying the collateral vessels on the first frame where these were visible until the last frame.

Indeed, since DSA is a dynamic imaging method the progression of the contrast agent is observed in different circulatory phases, passing through the artery (arterial phase), capillaries (parenchymal/ capillary phases) and the veins (venous phase). The contrast enhancement changes were computed based on the segmentation of CC arteries which led to a time-enhanced artery perfusion curve.

Results and perspectives

We have observed that the perfusional curves obtained through the post-processing of the experimental protocol at 6 frames/second were more precise and contributive as compared to those obtained with a standard 2 frames/second protocol.

Intuitively, the experimental protocol provided a higher amount of data in a definite time window, which represents a major advantage to analyze CC, since this type of collateral circulation is visualized during the whole angiographic run and in some cases all along the three different phases (arterial, parenchymal and venous). Since the basic version of the algorithm is sensitive to the opacification of the vessels and to the pixel density modifications over time, a 6 frames/second acquisition protocol is associated with a higher number of vessels and pixel modification to analyze.

This effect determines the achievement of more precise perfusional curves, which is particularly adapted for a dynamic circulation visible over different time points (Fig. 7). Furthermore, we had analyzed the clinical outcomes of the 10 patients included in the analysis. All the patients were completely or adequately recanalized, according to the modified Treatment In Cerebral Ischemia score [START_REF] Dargazanli | Modified Thrombolysis in Cerebral Infarction 2C/Thrombolysis in Cerebral Infarction 3 Reperfusion Should Be the Aim of Mechanical Thrombectomy: Insights From the ASTER Trial (Contact Aspiration Versus Stent Retriever for Successful Revascularization)[END_REF]) (mTICI score, Table 2). Although it is known that the recanalization grade is a strong predictor of a favorable clinical outcome, a certain number of patients do not reach functional independence after MT (about 45-55%) (Goyal M, 2016;van Horn N et al., 2021).

mTICI Description

Grade 0 No perfusion or anterograde flow beyond site of occlusion Grade 1 Penetration but not perfusion. Contrast penetration exists past the initial obstruction but with minimally filling of the normal territory Grade 2 Incomplete perfusion wherein the contrast passes the occlusion and opacifies the distal arterial bed but rate of entry or clearance from the bed is slower or incomplete when compared to non-involved territories

Grade 2a

Some perfusion with distal branch filling of <50% of territory visualized

Grade 2b

Substantial perfusion with distal branch filling of ≥50% of territory visualized

Grade 2c

Near complete perfusion except for slow flow in a few distal cortical vessels, or presence of small distal cortical emboli.

Grade 3 Complete perfusion of normal filling of all distal branches

In this context, it was very interesting to notice that after having analyzed the perfusional curves obtained, two definite clusters of patients were recognizable (Fig. 8): the upper cluster, which included those patients with good clinical outcomes and the lower cluster regrouping those patients with high mortality rates and poor clinical outcomes. The main difference in these two small cohort of patients was the perfusional curve obtained after the analysis of the CC. Hence it may be plausible to use the shape of the perfusional curve as predictors for patient outcomes These preliminary results determined the first step of a rigorous methodological approach to the setting of a segmentation algorithm dedicated to the analysis of CC in patients with AIS. Section III -PhD ENDPOINTS

CLINICAL RELEVANCE OF THE COLLATERAL CIR-CULATION AS A PROGNOSTIC FACTOR IN ACUTE ISCHEMIC STROKE

The role of CC in determining favorable clinical outcomes has been widely described in the literature. Several papers have shown how collaterals are associated with favorable clinical outcomes after endovascular treatment [START_REF] Bang | Impact of collateral flow on tissue fate in acute ischaemic stroke[END_REF][START_REF] Bang | Collateral flow predicts response to endovascular therapy for acute ischemic stroke[END_REF][START_REF] Ribo | Extending the time window for endovascular procedures according to collateral pial circulation[END_REF], Liebeskind et al., 2014, Consoli et al., 2016), by enhancing the results of the recanalization after MT [START_REF] Mangiafico | Effect of the Interaction between Recanalization and Collateral Circulation on Functional Outcome in Acute Ischaemic Stroke[END_REF][START_REF] Liggins | Interhospital variation in reperfusion rates following endovascular treatment for acute ischemic stroke[END_REF], increasing the exposure of the clot to the IVT (Seners P et al., 2019), limiting the ischemic vascular damage (Tong et al., 2018), encouraging the dislodgment of the clot [START_REF] Leng | Impact of Collateral Status on Successful Revascularization in Endovascular Treatment: A Systematic Review and Meta-Analysis[END_REF].

Furthermore, good collaterals have also been associated with a lower risk of hemorrhagic transformation after recanalization (Cao R et al., 2020;Hao Y et al., 2017), reducing the risk of growth and extension of the ischemic lesion as well as the risk of cerebral swelling and symptomatic intracerebral hemorrhages.

Considering the overall stroke population, it has been shown that about 45-55% of patients successfully treated by MT will not reach a functional independence (Goyal et al., 2016, van Horn et al., 2021). Among these, old patients, those with low baseline ASPECTS, those who are incompletely or who are lately recanalised and those where intraprocedural complications occurred are more likely not to benefit from MT. Patients with a good collateral profile are usually considered as the most suitable candidates for MT, since these are associated with less severe clinical conditions and with more limited ischemic lesions (Consoli et al., 2016). However, also patients with good collaterals may not benefit from MT, despite a favorable perfusional and clinical pattern, although a deep analysis of the factors which determine a poor outcome in this specific subgroup of patients has not been performed yet.

Therefore, this chapter was focused on the clinical relevance of the CC as a prognostic factor in AIS patients. In particular the aim was to investigate which factors can be associated with unfavorable outcomes in patients with a good collateral profile. This research question was approached in the UNCLOSE (UNfavorable CLinical Outcomes in patients with good collateral Scores study)

Study, a retrospective analysis of a cohort of patients with a good CC, submitted to the Journal of Neuroradiology.

Highlights •

A good collateral circulation is associated with favorable outcomes in patients with large vessel occlusions of the anterior circulation treated by mechanical thrombectomy. These patients are generally considered as the best candidates for the endovascular treatment.

•

Unfavorable outcomes may occur in almost half of the patients with good collateral circulation (47% in our series)

•

Predictors of unfavorable clinical outcome do not differ in patients with good collaterals as compared to those observed in the literature concerning patients with poor collaterals.

•

The presence of good collaterals may not be sufficient to achieve favorable outcomes in patients with large vessel occlusions of the anterior circulation.

•

Good collaterals should be considered as a supplemental reason to obtain fast and complete recanalizations.

Introduction

Good collaterals represent a hemodynamic reservoir for ischemic areas due to large vessel occlusions (LVOs). Their role has been widely described in the literature in terms of association with favorable clinical outcome, [START_REF] Gersing | Clinical Outcome Predicted by Collaterals Depends on Technical Success of Mechanical Thrombectomy in Middle Cerebral Artery Occlusion[END_REF][START_REF] Qian | A meta! analysis of collateral status and outcomes of mechanical thrombectomy[END_REF] of the modulation between "slow" and "fast progressors" [START_REF] Rocha | Fast Versus Slow Progressors of Infarct Growth in Large Vessel Occlusion Stroke: Clinical and Research Implications[END_REF][START_REF] Mohammaden | Characterizing Fast and Slow Progressors in Anterior Circulation Large Vessel Occlusion Strokes[END_REF] and of the reduction of the hemorrhagic transformation after mechanical thrombectomy (MT). [START_REF] Cao | Collateral Vessels on 4D CTA as a Predictor of Hemorrhage Transformation After Endovascular Treatments in Patients With Acute Ischemic Stroke: A Single-Center Study[END_REF] Therefore, patients with good collaterals are considered as the most sui-

Materials and Methods

Study cohort

All consecutive patients with acute ischemic stroke of the anterior circulation who underwent me-chanical thrombectomy from January 1st 2016 to December 31st 2021 at our institution were retro-spectively analyzed from a prospectively collected, web-based registry (ETIS Registry, NCT03776877).

Variables and definitions

In the present study, the following data were Ethical approval was obtained from the institutional review board, which waived for patient informed consent.

Inclusion and Exclusion Criteria

Inclusion criteria were: age ≥ 18, acute LVO in the anterior circulation (intracranial internal carotid and M1 or proximal M2 segments of the middle cerebral artery), and good collateral circulation which was defined as grades 3 and 4 of the American Society for Interventional and Therapeutic Neuroradiology/Society of Interventional Radiology (ASITN/SIR) scale. [START_REF] Higashida | Trial Design and Reporting Standards for Intra-Arterial Cerebral Thrombolysis for Acute Ischemic Stroke[END_REF] Patients with tandem or multiple occlusions, with significant pre-stroke disability defined as modified Rankin Scale (mRS) >2, and patients with incomplete and unavailable follow-up data were excluded.

Collateral circulation assessment

Interventional images were retrospectively re- Supplementary specific analyses, such as the subgroup of patients recanalized with mTICI 2c-3, were performed in order to help with the interpretation of the results.

Statistical analysis

Statistical analysis was performed using de-

Results

We reviewed 1419 patients with acute ischemic stroke due to LVO in the anterior circulation treat-ed by mechanical thrombectomy and 219

(females: 53.4%, mean age: 70.6 ±16 y.o.) patients were included in the final analysis after the application for the exclusion criteria. A flowchart of the study is provided in Fig. 1.

Kappa intra-and inter-observer agreement were respectively 0.61 and 0.69 per ASITN/ SIR grade and 0.72 and 0.82 per dichotomization (grades 1-2 vs 3-4). An unfavorable outcome was observed in 103 patients (47%). The results of the univariate analysis related to the prima-ry and to the secondary endpoints are summarized respectively in Table 1 and Table 2, whereas Ta-ble 3 reports the multivariate analyses. Supplementary analyses are reported in Supplemental Tables 123.

Primary endpoint

In our population, a statistically significant correlation with an unfavorable clinical outcome was observed for patients with older age (p<0.001), baseline mRS2 (p<0.001), higher baseline NIHSS (p<0.001), longer onset-to-recanalization (OTR, p=0.03) and "groin-to-recanalization" times (p=0.004). IVT administration (p=0.004), number of passes <3 (p=0.01), a partial mTICI score (TI-CI 0-2a; p=0.002), a higher 24h-NIHSS and lower 24h-ASPECTS (p<0.001) and sICH (p<0.001). The multivariate analysis showed an independent correlation between unfavorable outcomes and older age, higher 24h-NIHSS, lower 24h-ASPECTS, no-IVT administration and secondary transfers ("drip and ship" model).

Secondary endpoints

Factors associated with unfavorable outcome in patients with adequate recanalization are shown in Table 2 and did not differ from those identified for the primary endpoint at the univariate analysis, except for the OTR and the procedural time, which were not significantly associated.

In the sub-analysis performed on all-cause mortality at 90 days we observed that although older age (p<0.001), baseline mRS, baseline and 24h-NIHSS and 24h-ASPECTS (p<0.001),

IVT administra-tion (p=0.02) and adequate recanalization (p=0.01) were significantly correlated, only older age, 24h-NIHSS and 24h-ASPECTS resulted independent predictors at the adjusted multivariate analy-sis (Table 3).

Furthermore, sICH were significantly correlated to baseline ASPECTS, longer procedures, FPE, a higher number of maneuvers, recanalization failures and 24h-NIHSS, whereas only 24h-NIHSS, 24h-ASPECTS and PH1-PH2 grades proved a statistical correlation at the multivariate analysis.

Discussion

Patients with good collaterals could be described as the best candidates for mechanical throm-bectomy. However, a non-negligible part of patients treated by MT does not reach functional inde-pendence after the endovascular treatment, [START_REF] Van Horn | Predictors of poor clinical outcome despite complete reperfusion in acute ischemic stroke patients[END_REF] although patients with unfavorable outcomes show more frequently a poor collateral profile. [START_REF] Hassen | Inter-and intraobserver reliability for angiographic leptomeningeal collateral flow assessment by the American Society of Interventional and Therapeutic Neuroradiology/ Society of Interventional Radiology (ASITN/ SIR) scale[END_REF] In our cohort, age (OR: 1.05, IC95%[1.02-1.09], p=0.001) and higher 24hours NIHSS (OR: 1.23, IC95%[1.14-1.32],

p<0.001) were some of the main predictors of poor clinical outcome (Table 1), which is in line with previously published pa-pers. [START_REF] Arsava | The Detrimental Effect of Aging on Leptomeningeal Collaterals in Ischemic Stroke[END_REF] However, some Authors argued that age could have a different behavior which would be in favor of a preserved collateral status. [START_REF] Agarwal | Interaction of age with the ischaemic penumbra, leptomeningeal collateral circulation and haemodynamic variables in acute stroke: a pilot study[END_REF] The mean age of this cohort was 70.6 ±16 y.o., which could be considered slightly higher than in other acute stroke cohorts. We aimed to analyze the im-pact of other factors that could explain the unfavorable clinical results in this specific subgroup of patients with good collaterals.

Collaterals and recanalization

The adequate recanalization of the occluded artery is widely known as an independent predictor of favorable clinical outcome, although about half of the patients with adequate-tocomplete recanali-zation do not achieve a functional independence. [START_REF] Goyal | Endovascular thrombectomy after largevessel ischaemic stroke: a meta-analysis of individual patient data from five randomised trials[END_REF] Our results showed that an adequate recanali-zation grade remains a strong predictor of favorable clinical outcome in patients with good collat-erals. The interaction between the recanalization grade and collaterals had already been investigated pre-viously showing that collaterals enhance the effect of the recanalization, improving the clinical outcomes of recanalized patients. [START_REF] Leng | Impact of Collateral Status on Successful Revascularization in Endovascular Treatment: A Systematic Review and Meta-Analysis[END_REF] When we compared patients with favorable clinical outcomes and those with unfavorable ones, the rates of mTICI 2b-3 were high in both subgroups but significantly lower in the subgroup of patients with an unfavorable outcome (82% vs 95%, p=0.002; Table 1) independently from the endovascular technique used. Interestingly, neither the FPE nor the mTICI grade resulted as independent predic-tor of favorable outcome or a protective factor for mortality at the multivariate analysis (Table 3), however that could be explained by the selected population and the high recanalization rates in both subgroups. Thus, we observed similar results when we considered patients with mTICI 2c-3 grades (Supplemental Table 1).

These results are in line with the physiopathological concept that collaterals supply the microcirculation in the ischemic territory during the arterial occlusion, [START_REF] Kunz | Chapter 14 Cerebral vascular dysregulation in the ischemic brain[END_REF] maintaining a sort of "hemodynamic reserve". [START_REF] Consoli | CT perfusion and angiographic assessment of pial collateral reperfusion in acute ischemic stroke: the CAPRI study[END_REF] Other possible explanations of the effectiveness of the association between collaterals and recanali-zation could be related to the mitigation of the ischemic vascular injury or the better exposition to thrombolytics agents [START_REF] Seners | Better Collaterals Are Independently Associated With Post-Thrombolysis Recanalization Before Thrombectomy[END_REF] or to the higher chance of dislodgment of the clot. [START_REF] Leng | Impact of Collateral Status on Successful Revascularization in Endovascular Treatment: A Systematic Review and Meta-Analysis[END_REF][START_REF] Liggins | Interhospital variation in reperfusion rates following endovascular treatment for acute ischemic stroke[END_REF] Nevertheless, complete recanalizations may have a favorable impact on the collateral circulation itself, allowing the removal of the thrombotic material in the pial arterioles and in the capillary bed providing a proper supply of the microcirculation. [START_REF] Tong | Cerebral venous collaterals: A new fort for fighting ischemic stroke?[END_REF] Furthermore, we did not observe any differences concerning the type of anesthesia or the use of a balloon guide catheter (BGC) whereas the potential detrimental effect of general anesthesia was described in patients with poor or intermediate collaterals [START_REF] Liu | Adverse Outcomes Associated With Higher Mean Blood Pressure and Greater Blood Pressure Variability Immediately After Successful Embolectomy in Those With Acute Ischemic Stroke, and the Influence of Pretreatment Collateral Circulation Status[END_REF] and the BGC had previously been correlated with improved outcomes [START_REF] Blasco | Balloon guide catheter improvements in thrombectomy outcomes persist despite advances in intracranial aspiration technology[END_REF] unless a combined technique (stent retriever+aspiration) was used as a first-line strategy. [START_REF] Bourcier | Balloon Guide Catheter is Not Superior to Conventional Guide Catheter when Stent Retriever and Contact Aspiration are Combined for Stroke Treatment[END_REF] Moreover, intraprocedural complications were not associated with worse clinical outcomes since these were neither frequent nor clinically relevant in most of the cases. Indeed, mechanical vasospasm represented more than half of the reported complications without clinical consequences.

Collaterals and time

In our cohort, patients with unfavorable clinical outcomes were recanalized later with almost a 30-minutes difference in terms of onset-toreperfusion (299.4 vs 270.4 minutes, p=0.03; 1), which is in line with previously reported series [START_REF] Bourcier | More than three passes of stent retriever is an independent predictor of parenchymal hematoma in acute ischemic stroke[END_REF].

These results could suggest that despite good collaterals, a fast and complete recanalization must remain the goal of mechanical thrombectomy. Therefore, good collaterals should not be considered as a "time machine" that would provide a certain tolerance for long procedures.

The effect of the collateral circulation to maintain the cerebral perfusion during ischemia also in late temporal windows has already been described [START_REF] Liebeskind | Collateral Circulation in Thrombectomy for Stroke After 6 to 24 Hours in the DAWN Trial[END_REF] as well as its role of modulator of the progres-sion of the ischemia. [START_REF] Anadani | Collateral status reperfusion and outcomes after endovascular therapy: insight from the Endovascular Treatment in Ischemic Stroke (ETIS) Registry[END_REF] Moreover, in our population we did not observe any significant difference in terms of clinical outcome between patients treated before and beyond 6 hours (mRS0-2: 56% vs 44%, p=0.19, Supplemental Table 2) although higher recanalization grades were obtained before 6 hours, which supports the hypothesis of a neuroprotective effect of good collaterals on the brain tissue in both early and late windows.

Collaterals and thrombolysis

The role of intravenous thrombolysis on collateral circulation remains still debated. Some

Authors did not find any correlation between IVT and collaterals [START_REF] Anadani | Effect of intravenous thrombolysis before endovascular therapy on outcome according to collateral status: insight from the ETIS Registry[END_REF] while other papers had underlined the role of both fibrin [START_REF] Jeanneret | The Plasminogen Activation System Promotes Dendritic Spine Recovery and Improvement in Neurological Function After an Ischemic Stroke[END_REF] and platelets [START_REF] Xu | Antiplatelet Strategies and Outcomes in Patients with Noncardioembolic Ischemic Stroke from a Real-World Study with a Five-Year Follow-Up[END_REF][START_REF] Yao | Enhanced Procoagulant Activity on Blood Cells after Acute Ischemic Stroke[END_REF] in the development of the in-situ thrombotic phenome-na that can occur within the collateral vessels, particularly in the venous side. [START_REF] Tong | Cerebral venous collaterals: A new fort for fighting ischemic stroke?[END_REF] Intravenous thrombolysis (mainly Alteplase according to the inclusion period) was significantly associated with favorable clinical outcomes with a protective effect observed at t h e m u l t i v a r i a t e a n a l y s i s ( O R : 0 . 2 8 , IC95%[0.11-0.73], p=0.009). There is currently no evidence that IV thrombolysis may have an ef-fect on collateral circulation, and these results could be explained by the significantly higher rate of patients with mTICI 2b-3 among those who received IV thrombolysis prior to mechanical throm-bectomy (Supplemental Table 3). The SWIFT DIRECT Trial [START_REF] Fischer | Thrombectomy alone versus intravenous alteplase plus thrombectomy in patients with stroke: an open-label, blinded-outcome, randomised non-inferiority trial[END_REF] has recently shown that the asso-ciation IVT+MT provided better clinical outcomes compared to MT alone. However, in our cohort secondary transfers were significantly associated with unfavorable outcomes and it could be hy-pothesized that the transfer delays may be due to the IVT administration. Nevertheless, only 60% of the patients secondarily transferred to our institution had received IV thrombolysis at the Primary Stroke Centers (unpublished data).

Collaterals and hemorrhagic transformation

The rate of sICH was low (5.9%, 13/219) as one could expect in a selected population with good collateral scores. Indeed, poor collaterals are frequently associated with higher hemorrhagic rates after endovascular treatment. [START_REF] Cao | Collateral Vessels on 4D CTA as a Predictor of Hemorrhage Transformation After Endovascular Treatments in Patients With Acute Ischemic Stroke: A Single-Center Study[END_REF][START_REF] Hao | Predictors for Symptomatic Intracranial Hemorrhage After Endovascular Treatment of Acute Ischemic Stroke[END_REF] In this cohort, lower baseline ASPECTS, partial or unsuccessful recanalizations (mTICI 0-2a) and longer procedures were associated with a higher rate of sICH. Indeed, patients with adequate and almost complete/complete recanalization had significantly higher 24h-ASPECTS (p>0.001, Tables 23, Supplemental Tables 12). This result is in line with previously published papers [30] which highlighted a sort of protective effect of collaterals to slowdown the extension of the ischemia, although in this specific cohort of patients with good collaterals and adequate recanalization we observed an unfavorable outcome in 47% of the cases.

Limitations

The retrospective nature of the analysis represents the main limitation of this study. However, we acknowledge also the fact that since the collateral circulation was assessed using the ASINT/SIR [START_REF] Higashida | Trial Design and Reporting Standards for Intra-Arterial Cerebral Thrombolysis for Acute Ischemic Stroke[END_REF] scale and that the inter-and intra-observer agreement for this score could be quite poor, [START_REF] Hassen | Inter-and intraobserver reliability for angiographic leptomeningeal collateral flow assessment by the American Society of Interventional and Therapeutic Neuroradiology/ Society of Interventional Radiology (ASITN/ SIR) scale[END_REF] although in this cohort we considered a dichotomous parameter since we included only patients with ASITN/SIR grades 3 and 4 and an excellent inter-observer agreement was recorded.

Furthermore, although the predictors of unfavorable outcome did not differ from those observed in overall stroke populations and already described in literature, this study represents, to the best of our knowledge, the first analysis specifically focused of this subgroup of patients, providing find-ings that could be potentially hypothesis-generating. The local Institutional Review Board approved the data collection and analysis for this study.

Conclusions

[30]Broocks G, Kemmling A, Faizy 53) 72 ( 62) 44 ( 43) 0.004

Anesthesia protocol, N(%)

General anesthesia 38 ( 17) 16 ( 14) 22 [START_REF] Bourcier | More than three passes of stent retriever is an independent predictor of parenchymal hematoma in acute ischemic stroke[END_REF] 0.12 Conscious sedation 157 ( 72) 90 ( 78) 67 ( 65)

Local anesthesia 24 ( 11) 10 ( 9) 14 ( 14)

BGC used, N(%) 61 ( 28 8) 19 ( 10) 4 ( 13) 20 ( 10) 2 ( 15) 16) 7 ( 4) 8 ( 27) 12 ( 6) 2 ( 15)

42 2 0 13 (
Baseline NIHSS, median (IQR)

Discussion

The results of the UNCLOSE Study showed that even patients with good collaterals may not reach a favorable clinical outcome in terms, independently on the endovascular technique or the type of anesthesia used.

The overall results were partially similar to those reported in literature, while certain factors did not show the same results. Indeed, this was one of the first papers investigating this specific cohort of patients with good collaterals.

Nevertheless, we could observe poor clinical outcomes (mRS3-6) in 47% of the patients with good collaterals assessed through the ASITN/SIR score. Mortality rate was 13.6%.

The role of IV thrombolysis as a "protective" factor is in line with the current results reported by the recent trial SWIFT DIRECT, which showed that the association IVT+MT provided better clinical results than the MT alone (Fischer U et al., 2022). The interaction between IVT and collaterals will be discussed in the following chapter.

Furthermore, we could observe how patients successfully recanalized in late temporal windows (>6h) had the same rate of favorable clinical outcomes than those recanalized before 6h and these had also a better clinical evolution with lower 24h-NIHSS (11 vs 16, p<0.001).

These results are in line with the so-called « late window paradox » [START_REF] Albers | Late Window Paradox[END_REF], which explains how those patients treated in late windows are more selected through perfusional imaging in order to justify late treatments. For this reason, patients treated in late windows are associated with favorable outcomes, as it appeared in our cohort.

These data are particular interesting if we consider that:

this cohort is considered to be the most likely to achieve functional independence after MT these clinical results were independent from the type of endovascular technique or from the type of anesthesia used.

Nevertheless, if we had already observed that the endovascular technique seems not to have a direct impact on clinical outcome [START_REF] Lapergue | Effect of Endovascular Contact Aspiration vs Stent Retriever on Revascularization in Patients With Acute Ischemic Stroke and Large Vessel Occlusion: The ASTER Randomized Clinical Trial[END_REF][START_REF] Turk As 3rd | Aspiration thrombectomy versus stent retriever thrombectomy as first-line approach for large vessel occlusion 97 (COMPASS): a multicentre, randomised, open label, blinded outcome, non-inferiority trial[END_REF], controversial results

were published concerning the type of anesthesia and, in particular, a potential detrimental effect of general anesthesia on the collateral status (Fandler-Höfler S et al., 2020; [START_REF] Raychev | Physiologic predictors of collateral circulation and infarct growth during anesthesia -Detailed analyses of the GOLIATH trial[END_REF].

Although some Authors underlined the risk of a pressure drop in CC at the moment of the induction of the general anesthesia, it seems that the effect of general anesthesia could be more effective on poor collaterals, which would be more vulnerable than the good ones (Liu D et al., 2021).

Interestingly, the First Pass Effect was not significantly associated with a good clinical outcome, except for sICH. However, the hypothesis that CC could provide from one hand a higher resistance to ischemia also after the first manoeuver seems not to be supported. Indeed, we have observed that longer procedures with a higher number of passes were associated with unfavorable outcomes.

These results could raise some questions concerning the assessment and the prognostic value of the CC in patients with AIS.

Indeed, although the ASITN/SIR scale is probably the most used classification to assess the CC in AIS, the inter-and intra-observer reliability between different operators is quite poor (Ben Hassen W et al., 2019). Therefore, there is a growing need for a tool that could help the assessment of CC in a reliable and easy way.

On the other hand, one could argue that collaterals that are judged as good or excellent basing on the ASINT/SIR scale, could be ineffective.

Future perspectives

The CC represents a potential tool for patients' selection and prediction of clinical outcome and hemorrhagic risk for patients treated by MT for AIS.

Although CC has already widely proven to play a central role, together with the recanalization grade, in determining favorable clinical outcomes and providing useful prognostic information about hemorrhagic transformation, it is not rare to observe a sort of "mismatch" between the CC status and the final clinical result.

Nevertheless, as we observed in the literature, the current grading scale to assess CC seems not to provide a reliable evaluation which is very subjective according to the observers. It is also possible that a different type of analysis, bases on solid physiopathological assumption, could improve the reliability of the currently most used angiographic scale: the ASITN/SIR. This issue will be addressed in the next chapter, focusing on the critical analysis of the angiographic assessment of the CC. However, this classification presents some limitations. In particular, the assessment "per definition" of CC using ASITN/SIR is time-consuming and not easy-to-use. Moreover, the reported inter-observer agreement between different operators in the assessment of the ASITN/SIR score was far from being considered excellent (Ben Hassen W et al., 2019).

The subgroup of patients with good collaterals and unfavorable outcomes and the subjectivity of the assessment of CC could raise up some points of debate. The progression of the contrast agent through the cerebral circulation can be clearly divided into three different phases: arterial, parenchymal/capillary blush and venous. The transition between either two consecutive phases is therefore defined as a turning point of the two phases. In 2D pretreatment DSA, such turning points represent the frames of the angiograms where the transition from an angiographic phase to the following can be observed according to the opacification of the vascular structures (arteries, capillary blush, veins).

The turning points would help to differentiate the transition from the arterial to the parenchymal phase and from the parenchymal to the venous phase. Thus, the application of the turning points would help to separate and to differentiate the angiographic phases and it could provide the basis to "quantify" the desynchronization between the CC and the cerebral circulation.

The introduction of these two concepts seems to be a suitable approach to assess a dynamic process, such as the CC function.

Future perspectives

The detection of the CVP seems to increase the diagnostic performances of the currently most used classification for CC.

The introduction of the desynchronization and the turning point could represent an advancement in the understanding of the hemodynamics of the CC. Indeed, these concepts could help to "decompose" the assessment of the CC and could provide some measurable parameters in order to quantify the effectiveness of the collateral vessels and, hopefully, to calculate the flow velocity within the CC.

Furthermore, the main advantage of the angiographic analysis lays in the real-time imaging provided by the DSA. The assessment of the CC performed through the DSA would not be influenced by other delays, since it would be performed at the beginning of the procedure of Mechanical Thrombectomy.

Indeed, data that are currently reported in terms of correlation between the baseline extension of the ischemic area (i.e. DWI-ASPECTS or plain CT-ASPECTS or CT-Perfusion ASPECTS) and the angiographic evaluation of collaterals in the Angiosuite are definitely biased by the latency between the baseline neuroimaging and the beginning of the angiographic assessment (transfert time, setting, eventual general anesthesia etc…). As we have observed in the paper, it seems that the patency of the CVP decreases over time, which could be interpreted as a dynamic modification of the CC evolving towards a mechanism of in-situ thrombosis (Consoli A et al., The angiographic collateral venous phase: are good collaterals always effective? Supplemental Table 1, submitted to JNIS).

Moreover, the new trend that is currently under investigation in some randomized trials [START_REF] Pfaff | Direct Transfer to Angio-Suite Versus Computed Tomography-Transit in Patients Receiving Mechanical Thrombectomy: A Randomized Trial[END_REF][START_REF] Requena | Direct to Angiography Suite Without Stopping for Computed Tomography Imaging for Patients With Acute Stroke: A Randomized Clinical Trial[END_REF]) is to consider a direct access of stroke patients directly to the Angiosuite, waiving the preliminary selection process through the CT/MRI. According to this schema, DSA would assume a more central role in the identification of markers of effectiveness and prognosis rather than the current role.

The detection of the turning points, in particular, could be a substrate for automated algorithms that would be capable to reproduce the assessment of the transition of the angiographic phases and then to detect the presence of the CVP in the area of the CC.

Finally, the evaluation of a correlation with CT-Perfusion and MR-PWI are mandatory in order to strenghten the results of the critical analysis of the angiographic assessment of the CC, as well as a comparative assessment with the new imaging techniques such the evaluation of the profile of the cortical venous outflow assessed using the cortical vein opacification score (COVES).

All these findings could support the theory that the venous phase of the collateral circulation could represent the real modulator of the evolution of the ischemic process and that the representation of a good or of a poor collateral circulation could depend mainly on the effectiveness and permeability of the venous outlet of the CC rather than the extension of the arterial leptomeningeal anastomoses.

A TECHNICAL APPROACH TO COLLATERALS BASED ON PHYSIOPATHOLOGY: DEVELOPMENT OF AN AL-GORITHM FOR THE ANGIOGRAPHIC EVALUATION OF THE COLLATERAL CIRCULATION

The results of the clinical and critical analysis of collateral circulation, which were discussed in the previous chapters, raised several questions about the assessment of collateral circulation.

Indeed, the current methods to assess the CC may not provide a definite answer to the main physiopathological issues, such as the effectiveness of collaterals, the role of the venous phase of the CC and its capacity to properly sustain the cerebral perfusion although their extension.

Furthermore, the growing interest in a more rapid workflow for the management of acute ischemic stroke will put in a central position the role of the DSA in patients' assessment in the acute setting of AIS. This effort considers the direct access to the Angiosuite avoiding the intermediate diagnostic step based on the CT or MRI. However, this type of pathway needs to be further investigated and its applicability worldwide will be a matter of discussion considering the different geographical setting and the available human resources.

It would therefore be really advantageous if the pre-treatment DSA acquired in-situ can be used to provide indications of treatment strategies, such as selecting patients for MT, prognosis, strategies after the treatment, etc. This will in the end involves the development of a DSA-based dedicated algorithm to characterize the CC. This algorithm will be constructed according to the physiopathological assumptions that have already been discussed. To achieve this, the pre-treatment dynamic DSA series need to be quantitative analyzed. In particular, this analysis will try to look for the potential link between image features and the concept of deysnchronization between the cerebral and collateral circulation and the concept of the turning points.

Inspired by and as an extension of the M2 work, in this chapter we will first observe the time-density curve of the collateral region of the DSA data. Different from only a ROI based study, pixel-wise analysis will be also applied. Parametric maps and image based features will be computed and analysed, and coorelated with the radiological readings of these data. A tissue-specific (i.e. parenchyma artery, or veins) will then be made following the segmetion of vessels within in the collateral region as previously proposed in Fig. 6 (Section II -Chapter 2, p.33). FOCH centre in the ETIS Registry ( NCT03776877) were included in this analysis. Patient exclusion criteria was restricted to be minimum in order to analyse a mixed population of patients with good or poor collaterals, or age, therefore the timing, in particular the delay between the onset of symptoms and the beginning of the procedure of MT (onset-to-groin).

All patients were imaged through a 6 frames/second acquisition (as defined in Chapter 2) as the first angiographic run before starting the MT procedure.

Afterwards, a dedicated Quality Core Lab, which included two neuroradiologists not involved in the procedures, evaluated the quality of the images and stated whether to include the angiogram in the analysis. Motion artefacts represented the main reason for exclusion and 19 patients were excluded from the analysis.

Baseline and follow-up clinical data were prospectively collected from the ETIS Registry (NC-T03776877). The local Institutional Review Board approved the study and waived for written informed consent. All the data were stocked in a dedicated database and the images were anonymized and centralized inside the ArchiMed system hosted by the IADI laboratory, Nancy, France.

The DSA acquisition protocol

The adopted imaging protocol was the same as already used in the pilot study described in Chapter II (page 31). The Antero-Posterior view of the pre-treatment angiograms acquired with 6 frames/s were collected from the recruited population. As previously described, only the AP view was used for the analysis in order to avoid the overlap with the ACA territory in the lateral projections.

Indeed, it could be intuitive to observe that the flow density curves obtained by the segmentation of the vessels opacified using an "experimental" DSA acquisition protocol at 6 frames/second were more detailed that those obtained using a "standard" acquisition protocol at 2 frames/second. Both acquisition protocols are available on most of the angiographic machines although the standard acquisition protocol is set at 2 frames/second. This observation seems to be particularly relevant if we consider the dynamic behavior of the CC, since a higher number of frames provides more data in the same time interval, allowing the construction of a more precise curve.

A panel of variables have been studied to perform the critical analysis of the angiograms for the setting of the algorithm. These variables were summarized in the analytic plan that was set and named as "Image Feature Analysis (IFA)", which will be described later.

The analysis of a subgroup of patients imaged with both conventional clinical acquisition protocols

(2 vs 6 frames/second) showed the difference in the construction of the perfusional curves (Fig. 10), highlighting how the perfusional curves obtained through the experimental protocol were more informative and provided more data. In particular, the 6 frames/second protocol provided a higher number of images to be analysed in the late venous phases, which would determine a more reliable result overall in terms of detection of the collateral venous phase.

For these reasons, a 6 frames/second acquisition protocol was used.

Angiogram data analysis

In order to characterise the angiograms, especially the contrast enhancing time-course, we have thoroughly analysed the behaviour of the DSA series, not only by regions (cerebral, collateral region, watershed region, etc.) but also by pixel in order to understand the contrast behaviour by tissue.

Thus for each patient, their data have been divided into different regions and the contrast progression has been classified as described in the three-phase scheme already described in the previous chapters.

Spatial analysis the observed regions

We analyzed the imaging features in 4 types of regions: cerebral region, extended collateral region, the watershed regions and pure collateral regions (see details definition as below, Fig. 11), in order to observe the time course changes of contrast agent filling of collateral vessels in the cortical MCA region:

-Cerebral region: a ROI was drawn including the whole hemisphere perimeter including the tem- poro-basal pole, the inferior border passed through the ICA siphon.

-Extended-collateral ROI (large, red one) including the vascular territories of the ACA and the MCA, drawn from the midline and following the outer profile of the cortical convexity and the inner profile of the white matter territory, excluding the basal ganglia territory medially and the fronto-basal area inferiorly. A careful selection was performed excluding the superior sagittal sinus superiorly and the transverse sinus inferiorly in order to avoid a selection bias of these venous structures that could have influenced the modification of the pixel density. This ROI would provide data about the density curves related to both the cerebral and the collateral circulation.

-Pure collateral ROI (blue one, intermediate ROI), which included only the territory of the MCA. This type of ROI could limit the influence of the cerebral circulation (which was identified with the ACA territory, Fig. 9, Chapter 4, p.85) and provide data related to the density curve of the collateral area alone. The pure collateral ROI is overlapped with the extended collateral ROI.

-Watershed ROI (yellow one, more restricted), which was drawn at the level of the transition from the ACA to the MCA territory (watershed territory). This ROI could provide information about the density curve of the watershed territory, which can be considered as the area of activation of the collateral circulation in case of M1-MCA occlusions.

All the segmentation process in the initial phase was performed manually using the freeware ITK-SNAP. The three ROIs are overlapped.

Temporal analysis: the observed time window and key frames

According to the concept of the desynchronization, the collateral and the cerebral circulation have a different time window of observation. In the study both time windows were labelled and were identified through the first and the last frame of appearance of the opacified vessels. The first turning point corresponds to the transition between the arterial and the parenchymal phase and the second one to the transition between the parenchymal and venous phase. Some landmarks were used in order to define the turning points and are summarized as follows (Fig. 12):

- the ROIs (collateral and hemispheric), according to the Master2 protocol the first and the last frame of observation of the collateral circulation, in order to define the temporal window of observation the turning points, in order to compare the manual detection of the transition of the circulatory phases on the angiograms with the assessment performed by the algorithm. Two turning points were defined for both cerebral circulation and CC.

Semi-quantitative time-density curve analysis

Assessment of the ROI-based time-density curve analysis is done by computing the following parametric maps: Time-to-peak enhancement map (TTP), mean transit time map (MTT), and maximum enhancement projection map (MEP) as shown in Fig. 13. While interpretations were difficult to make on TTP and MTT data, MEP images resulted more indicative to give the first intuitive explanation of "good" and "poor" collaterals (named as Group-gc and Group-pc).

Fig. 13. ROI-based analysis of time density curves in patients with good collaterals (superior row) and poor collaterals (inferior row) according to MEP maps (first image of both rows) and TTP maps (second images of both rows).

Examples from the two groups (Patient TEST-002 and Patient TEST-041 representing good and poor collateral groups respectively) are illustrated in Fig. 13. It is most interesting to notice that in the MEP map, the collateral region of Group-gc contains more dense vessels and that we can observe an opposite situation in Group-pc. In the current calculation, the MEP is extracted basing on all the frames, and the current CC status (good or poor) is assessed by the arterial collateral status.

However, whether we consider also the venous phase or not, that would not lead to obvious differences in the MEP map.

Vessel complexity in the collateral region

In other to quantitatively depict the previous observation and also prove our hypothesis that the good/poor CC can be discriminated by the CC vessel complexity level, we used the metric called "fractal dimension", , a recent mathematical method which has been widely used to quantify retinal vessels and trabecular bone patterns in X-ray or CT [START_REF] Mainster | The fractal properties of retinal vessels: embryological and clinical implications[END_REF][START_REF] Jurczyszyn | The use of fractal dimension analysis in estimation of blood vessels shape in transplantable mammary adenocarcinoma in Wistar rats after photodynamic therapy combined with cysteine protease inhibitors[END_REF][START_REF] Majumdar | Fractal analysis of radiographs: assessment of trabecular bone structure and prediction of elastic modulus and strength[END_REF] where is the fractal dimension, is the length of box which creates mesh covering surface with examing pattern, is the minimal number of boxes which are required to cover examining pattern.

As previously observed in the previous section, the distal part of the pure collateral ROI provided the most relevant results in terms of differentiation between good and poor CC. A sub-analyis of this ROI was then performed. Therefore, we calculated the fractal dimension of three sub-regions of the pure collateral ROI (Fig. 14): 1. Pure collateral ROI (in red), 2. Watershed ROI (in yellow), 3. ROI 1 without ROI2 (in green). This was also because some of the patients, mainly those with poor collaterals, showed very dense watershed regions, although in the adjacent distal CC region only few vessels were observed. Thus it is necessary to make a thorough observation. The of this three ROIs in the two groups (Group-Gc and Group Pc) are plotted in Fig. 15A. Since the relative size of the collateral region within the cerebral may vary from patient to patient and that could introduce a bias, we also calculated the normalized fractal dimension by dividing the ratio of CC and cerebral areas (Fig. 15B).

D s D s D s = li m ε→0 logN(ε) log ( 1 ∕ ε ) D s ε N(ε) D s

Fig. 15. Boxplot of the fractal dimension in the three ROIs between the two groups (A) and the normalized fractal dimension between the two groups (B).

D s

According to the boxplot above, the normalized fractal dimension of the pure CC ROI ( ) seems to be a good candidate to separate Group-gc and Group-pc. An Anova test was then applied to the ( of the groups, leading to a p = 0.013 (p = 0.05 is considered as significant),showing the two groups are significantly different (Fig. 16).

Fig. 16. Anova plot showing the normalized fractal dimensions in both subgroups.

The Image Feature Analysis (IFA)

As aforementioned, a list of imaging features has been carefully chosen in order to characterise the CC regional intensity behaviour during the contrast progression through the different circulatiory phases of the DSA. Another objective of the IFA was to provide an intepretation of the physiopathological concepts that were introduced: the desynchronization and the turning points.

In either the CC ROI or the cerebral ROI, the evolution of the contrast trajectory is in general described as follows: the image intensity starts to decrease when the contrast starts to fill the arteries, and the anatomical information including the vessel networks starts to be enhanced.

The amount of information increases the number of vessels being enhanced, their sharpness increases until the maixum value is ahiceved and that will represent the end of arterial phase and the beginning of parenchyma phase, which corresponds to the first turning point. From that point, the contrast agent starts to leave the artery and to fill the capillary bed, where the perfusion to the parenchyma happens. This parenchymal phase causes the whole DSA image to become darker and darker, and the contrast between vessels and other tissues becomes less obvious.

n D s( pCC ) n D s( pCC )

However, due to the overlapping nature of DSA being a 2D-image, and to the delay between different regions (the cerebral and the CC region desynchronization as previously described), it may be the case that the parenchyma would be perfused progressively especially when the CC flow is slow.

After the DSA frame becomes the darkest and the less contrasted, the contrast agent starts to leave the parenchyma and entering the venous circulation. This represents the turning point from the parenchyma phase to the venous phase. At this time, the desynchronization between cerebral and CC regions would be the more obvious, with the worst case that before the end of the DSA acquisition which is defined as the full visualization of the dural venous sinuses (superior sagittal sinus and transverse sinuses), the venous phase of CC is still not shown.

The following image features are analysed: maximum histogram peaks change over time, the ROI intensity uniformity, mean and histogram skewness, and the entropy. As it was shown in Fig. 10, the IFA was based on several parameters:

The maximum histogram peaks at each frame in the CC and cerebral ROIs changes as the well as opacification when the contrast trajectory evolves. This curve starts to decrease when the contrast fills the arteries in the corresponding ROIs and starts to come back when the contrast leaves the ROI identifying a bi-phasic peak corresponding to the phase separation from the arterial to the parenchymal/capillary phase and from the parenchymal/capillary phase to the venous phase.

A particular mention will be done concerning the entropy. It is a scientific concept as well as a measurable physical property that is most commonly associated with a state of disorder, randomness, or uncertainty. The general definition of entropy can be found in the entropy focus criterion [START_REF] Atkinson | Automatic Compensation of Motion Artifacts in MRI[END_REF], which is used in order to favor the high contrast between the structures: However, entropy is also commonly described as a criterion of maximization in order to provide a noise reduction, according to a different formula, which therefore, this definition is a smoothness measurement:

E = - s ∑ j=1 B j B max ln [ B j B ma x ]
In the construction of the algorithm we defined the entropy focus criterion as Entropy-HC (as "high contrast") and the criterion of maximization as Entropy-SN (as "smoothness").

The assumption used here is that when the contrast agent enters the collateral circulation region, the information content in the collateral area will be perturbed due to the modification of the opacification and thus lead to a change of entropy.

When there is a transition through the different circulatory phases, such as from artery to parenchyma, and from parenchyma to the venous phase, a dramatic change on entropy can be found.

The analysis of these ROIs showed that the results were not significantly different (Fig. 17). Indeed, according to the IFA panel, the shape, the slope and the valleys of the perfusional curves obtained through the three different ROIs did not differ.

Fig. 17. The comparative analysis of the three different ROIs using the IFA panel. A fourth ROI is referred to the hemisphere (named as "brain" ROI), which is used for normalization of the curves.

No difference was observed in the behaviour of the perfusional curves.

Although no difference was observed in terms of behaviour of the perfusional curves, the "Extended collateral" ROI was chosen for the setting of the algorithm for three main reasons:

this ROI provides data from both types of circulations (cerebral and collateral), following the principle of the desynchronization; this ROI provides data for the detection of the venous phase, while the other two ROIs could miss this type of information being more limited;

-The extension of the ROI seemed to be the most reliable in order to perform the normalization with the hemispheric ROI in order to assess the real extension of the collateral area.

-In this context, we used entropy as a measurement of information content in a corresponding area.

-The hypothesis here is that the perfusion of phase contrast changes the information content of the collateral circulation area. The dramatic change of entropy value in this area should correspond to a local extreme (either a peak or a valley) in the function of entropy against time or the differential of this function. In the case of the DSA, after the contrast agent enters the ROI of the collateral circulation, the information content in the ROI changes, therefore the starting frame of the collateral circulation window corresponds to the inflection point of the entropy curve in Fig. 17 (Entropy HC graph). The peaks of the entropy curve may therefore indicates the turning points.

Therefore, by drawing the entropy change according to each frame, it may be possible to detect the following features:

1. the beginning of the collateral circulation window by observing the change of the shape of the entropy curve along the time 2. the turning points from the arterial phase to the parenchymal/capillary phase or from the parenchymal/capillary phase to the venous phase, according to the following considerations:

a.

The changing trend of the entropy curve will be altered, i.e. a local extreme (turning point) can be detected. More precisely, after entering the collateral area, the contrast agent starts to render the arteries. Thus, the appearance of this region starts to become more opacified from the initial flat grey with clear information of the arteries (activation phase). Therefore, following the first perturbation of the information content of this region, the entropy change within the collateral circulation area as well as the tendency of entropy change should be almost monotonic between the beginning of the collateral circulation perfusion window and the turning point from the arterial to the parenchymal phase. Indeed, the contrast bolus passing through the artery has highlighted the contrast between the structures. Thus, when all (or the largest part of) the arteries have been filled, the entropy within this region should be the smallest.

b.

When the bolus starts to spread into the parenchyma, the image starts to be blurred. Indeed, the contrast agent starts to leak into the parenchyma and smooths out the corresponding region in the angiogram. Therefore, the entropy curve will show opposite trend compared the one observed in the arterial phase. Such trend will stop when the parenchyma is fully perfused and the contrast starts to go to the viens. So, in that case, the turning point should correspond to the time when the entropy curve shows a local extreme, with a maximum value of the Entropy-HC.

For the Entropy-SN, the trend of the curve should be similar however it will be more evident concerning the transition from the parenchymal/capillary phase to the venous phase as it corresponds to the smoothest time of the whole perfusion state.

As far as the Point 1 is concerned, we found an excellent correlation between the beginning of the collateral phase identified by the algorithm and the one that was manually detected in almost all the current patients.

In this patient (TEST-002, Fig. 18), the two blue vertical lines correspond to the manual defined collateral circulation window, whereas the point on the red spot curve indicates the turning point of this curve (plot of Entropy), suggesting the transition of the phase from the arterial to the parenchymal phase and corresponding to the valley of this curve. This can probably be detected by looking at the peaks on the derivative of this entropy curve. Although we could achieve an excellent agreement in the detection of the frame corresponding to the beginning of the temporal window of observation (phase of activation), the end of the temporal window of observation was not easy to identify.

After the review of the patients for which it was not possible to identify the last frame of observation of the collateral circulation, we could conclude that this occurred almost in all patients with a poor collateral circulation, with a limited arterial phase and almost absent parenchymal and venous phase.

These results represented the base for the development of the different versions of the algorithm and also the first step of integration of the novel concepts that were introduced.

The goal of the algorithm will be to provide a real-time analysis of the CC through the identication of the markers of effectiveness, such as the presence of a collateral venous phase, and to assess the flow velocity in order to describe the hemodynamics of the collateral vessels.

The development of the algorithm was the object of a patent submission procedure, which is ongoing.

CONCLUSIONS AND FUTURE PERSPECTIVES

The role of the collateral circulation in AIS has been widely investigated in the past and recent literature.

Although several articles and studies found strong correlations between the collateral status and the clinical outcome of patients treated by mechanical thrombectomy, the physiopathological mechanism of collaterals remains poorly understood.

Recently, the hemodynamic features of these leptomeningeal anastomoses made the subject of further investigations, leading to a more dynamic vision of the collateral vessels in the physiopathology of acute ischemic stroke.

However, the current tools to assess collaterals seem to be still limited and focused on the extension of the collateral vessels and the introduction of novel concepts and parameters to analyze could provide more reliable and solid results.

Indeed, as we have observed in the previous chapters, patients harboring a good collateral circulation may not benefit from the endovascular treatment through mechanical thrombectomy, although these patients would represent the best candidates to achieve a good clinical outcome.

A partial response to this issue could be to find in the limited assessment of collaterals according to the currently used grading systems, such as the ASITN/SIR classification.

Furthermore, these results strenghten the concept that extended collaterals may not be sufficiently effective and that a physiopathology-centred assessment is crucial to better understand the role of collaterals in acute ischemic stroke.

Thus, the integration of specific data concerning the effectiveness of the collateral circulation could improve the overall assessment of these anastomoses.

The introduction of the concepts of the desynchronization between the collateral circulation and cerebral circulation, the turning points of the contrast trajectory phases, as well as the evaluation of the collateral venous phase could provide a more complete analysis of collaterals and, potentially, a substrate for a real-time assessment of collaterals in the angiosuite.

The development of an automated algorithm to characterise the hemodynamic behaviours and to characterize the effectiveness represented the final goal of the Thesis.

Fig. 1 . 6 Fig. 2 .Fig. 3 .Fig. 4 . 13 Fig. 5 . 22 Fig. 6 . 24 Fig. 7 .Fig. 8 .Fig. 9 .Fig. 10 .Fig. 11 .Fig. 12 .Fig. 13 .Fig. 14 .Fig. 15 .Fig. 16 .Fig. 17 .Fig. 18 .

 1623413522624789101112131415161718 Fig. 1. The Alberta Stroke Program Early CT Score scheme based on a CT scan……….…..…….6

2 .

 2 Une analyse critique des images artériographiques afin d'évaluer une corrélation possible avec les événements physiopathologiques des AVCi; 3. Le développement d'un algorithme de post-traitement dédié aux images d'artériographie en 2D. La thèse de sciences est constituée de trois sections différentes: vii -Section 1: INTRODUCTION Un chapitre introductif (chapitre 1) donnant un résumé du diagnostic, du traitement possible et les défis cliniques des AVCi. Cette partie introductive servira également à familiariser le lecteur avec le concept et la définition de la CC et son rôle dans les AVCi. Toutes ces notions sont développées plus en détail dans les sections suivantes; -Section 2: RÉSULTATS PRÉLIMINAIRES Ce deuxième chapitre (Chapitre 2) reprend les résultats préliminaires obtenus lors de mes expériences scientifiques antérieures dans le domaine de la CC au cours de mon programme de résidence et pendant le Master 2. -Section 3 : ANALYSE DES PARAMETRES D'EVALUATION DES OBJECTIFS Trois chapitres distincts (chapitres 3 à 5) sont consacrés au développement des trois paramètres d'évaluation susmentionnés de cette thèse. Le chapitre 3 est focalisé sur l'impact clinique de la CC en tant que facteur pronostique dans l'AVCi. Cet aspect a été analysé dans l'article: UNfavorable CLinical Outcome in patients with good collateral Scores: the UNCLOSE study. Consoli A, Pileggi M, Hasan A T M, Venier A, Sgreccia A, Pizzuto S, Coskun O, Di Maria F, Scarcia L, Lapergue B, Rodesch G, Bracard S, Chen B. Le papier a été soumis au Journal of Neuroradiology.

Chapitre 2 :

 2 RESULTATS PRELIMINAIRES Le Chapitre 2 résume les résultats préliminaires obtenus lors d'une étude antérieure cordonnées par le Doctorant et centrée sur la corrélation entre la classification artériographique de la CC et les images obtenues par scanner de perfusion, montrant une corrélation significative entre le degré de CC et l'extension de la lésion ischémique, notamment le core ischémique, observé sur le scanner de perfusion. Les conclusions de cette étude, l'étude CAPRI, publiée dans le Journal of NeuroInterventional Surgery, étaient en faveur d'une corrélation entre les deux méthodes d'imagerie. Ce travail a permis d'avancer dans la compréhension physiopathologique des AVCi en montrant l'importance de l'évaluation de la CC. Une autre partie de ce chapitre décrit les résultats d'une étude préliminaire effectuée pendant le Master2 au sein du même laboratoire. L'objectif était d'analyser le type d'acquisition artériographique à utiliser pour la construction d'un algorithme de caractérisation de la CC en utilisant directement les images artériographiques afin de permettre une évaluation en temps réel. Un protocole « standard » (à 2 images/seconde) et un protocole « expérimental » (à 6 images/seconde) ont été comparés. Les deux protocoles d'acquisition sont disponibles et installées dans toutes les machines d'artériographie, mais seulement le protocole standard est utilisé dans le cadre des procédures de TM. Les résultats de cette étude ont permis de conclure qu'un protocole d'acquisition à 6 images/ seconde est le plus adapté pour analyser la CC en raison de la quantité de données obtenus qui permettent d'analyser de manière plus spécifique la CC.

  pronostique d'un résultat clinique dans les AVCi traités par TM. En particulier, l'objectif était d'évaluer quels facteurs peuvent être associés à des résultats cliniques défavorables chez les patients présentant un bon profil collatéral. Cette question de recherche a été abordée dans l'étude UN-CLOSE (UNfavorable CLinical Outcomes in patients with good collateral Scores study), une étude rétrospective d'une cohorte de patients avec une bonne CC, soumise dans le Journal of Neuroradiology.

  : la CC et la circulation cérébrale sont visualisés en différentes phases de l'artériographie et ceux-ci pourraient être considérés comme « désynchronisés ». Il pourrait être intuitif que plus le délai entre l'opacification de la normale circulation cérébrale antérograde et l'opacification de la CC est court et plus la vitesse de flux sera élevée dans la CC et donc dans le territoire ischémique. En fait, la perméabilité du côté veineux de la CC permet d'éviter une stase sanguine dans les vaisseaux collatéraux et de maintenir un flux sanguin stable dans le territoire ischémique et la perfusion de la microcirculation. Turning points: La progression du produit de contraste dans la circulation cérébrale peut être clairement identifiée en trois phases différentes: artérielle, parenchymateuse/capillaire et veineuse. La transition entre deux phases consécutives peut être donc définie comme un tournant des deux phases (turning point). Les turning points aideraient à différencier la transition de la phase artérielle à la phase parenchymateuse et de la phase parenchymateuse à la phase veineuse. Ainsi, l'application des turning points aiderait à séparer et à différencier les phases artériographiques et pourrait fournir la base pour « quantifier » la désynchronisation entre la CC et la circulation cérébrale. Le développement de l'algorithme permettra à terme une analyse en temps réel de la CC en particulier par rapport à son efficacité (présence de la phase veineuse de la CC) ainsi que par rapport à la vitesse de flux. Ce type d'évaluation permettra d'ajouter des éléments hémodynamiques utiles pour la compréhension physiopathologique des AVCi et pour la gestion des patients pour diminuer le risque de transformation hémorragique identifiant les patients avec une CC inefficace. xv DEFINITION AND PRESENTATION OF THE THESIS Acute ischemic stroke (AIS) represent a life-threatening condition caused by the occlusion of a cerebral artery and determining an ischemic lesion which can evolve toward the necrosis of the brain if the blood flow is not rapidly restored. AIS is characterized by high rates of mortality and morbidity and the goal of the management of this disease in an acute setting is based on a rapid diagnosis and a proper choice of treatment. The new evidences brought by recent Randomized Controlled Trials (RCTs) in 2015 allowed the development and the widespread diffusion of Mechanical Thrombectomy (MT), a procedure per-

  be focused on the critical analysis of the CC according to the angiographic characteristics that were described in the paper: Angiographic collateral venous phase: a novel landmark for leptomeningeal collaterals evaluation in acute ischemic stroke. Consoli A, Pizzuto S, Sgreccia A, Di Maria F, Coskun O, Rodesch G, Lapergue B, Felblinger J, Chen B, Bracard S. Published in the Journal of NeuroInterventional Surgery (JNIS).The last Chapter will be dedicated to the development of a specific post-processing algorithm for the assessment of CC directly on 2D angiograms and to the use with a pre-existing algorithm. The technical workflow and the draft of the patent submission will be presented and discussed in this Chapter: Algorithm of characterization of collateral circulation "REACH" -Real-timeAssessment of Collateral circulation Hémodynamics in acute ischemic stroke)/Algorithme de caractérisation de la circulation collatérale REACH (REal-time Assessment of Collateral circula-

Fig. 1 .

 1 Fig. 1. The Alberta Stroke Program Early CT (ASPECT) score: a 10-points scale used to assess the extension of the ischemic lesion. The ASPECT score is widely used and it can be applied to both MRI and CT scan.

Fig. 2 .Fig. 2

 22 Fig.2.

Fig 3 .

 3 Fig 3. The anatomy of cortical pial arteries and of the microcirculation (adapted from Iadecola, C., Nedergaard, M. Glial regulation of the cerebral microvasculature. Nat Neurosci 10, 1369-1376

Fig. 4 .

 4 Fig. 4. The Careggi Collateral Score introduced in 2013 to assess the collateral circulation directly on Digital Subtraction Angiography (image from the Supplemental material of Consoli A, Andersson T, Holmberg A, Verganti L, Saletti A, Vallone S, Zini A, Cerase A, Romano D, Bracco S, Lorenzano S, Fainardi E, Mangiafico S; CAPRI Collaborative Group. CT perfusion and angiographic assessment of pial collateral reperfusion in acute ischemic stroke: the CAPRI study. The paper was published in the Journal of Neurointerventional Surgery(JNIS) in 2016. J Neurointerv Surg. 2016 Dec;8(12):1211-1216.. The score provided a semiquantitative evaluation of collateral vessels (in the anteroposterior projection) and a qualitative evaluation (in the lateral projection). In particular, the latter was assessed through the "suspended artery sign", first described in 2013 byMangiafico et al.

Fig. 5 .

 5 Fig.5. Screenshot from the title page of the Master 2 final thesis, focused on the assessment of flow velocity in cerebral cortical vessels directly on Digital Subtraction Angiography.

  Clinical data and the angiographic runs of 10 patients with AIS secondary to the occlusion of the M1 segment of the MCA treated by MT at the Foch Hospital have been selected for the purpose of the study. All the patients were imaged through the experimental acquisition protocol. In order to compare the two protocols, we have simulated the standard protocol by subtracting a number of frames proportional to the cadence of the angiographic protocol. Finally the algorithm evaluated the DSA series acquired with the two different protocols and provided the perfusional curves which were obtained.The local Ethical Committee at Foch Hospital had approved the use of the pseudonymized data of the patients and of the DSA data. Data corresponding to the two protocols lead to two different perfusional curves where the standard one shows an aliasing like behavior and might indicate different predictions of clinical outcomes.

Fig. 6 .

 6 Fig. 6. Post-treatment workflow. Phase 1: definition of the temporal window (time of interest, TOI) and of the collateral area (region of interest, ROI); Phase 2: segmentation of the opacified vessels in every frame of the TOI; Phase 3: production of the curve that plotted the number of enhanced vessels over time (perfusional curve).

Fig. 7 .

 7 Fig. 7. Results of the comparison of the perfusional curves obtained through the analysis of the angiograms provided by the experimental acquisition protocol (6 frames/second, blue dotted line) and the simulated standard one (after reconstruction at 2 frames/second, red dotted line).

Fig. 8 .

 8 Fig. 8. Graph showing the different behavior of the two identified clusters of patients according to the distribution of the perfusional curves after normalization (adapted and modified from the candidate's Master 2 project). The upper cluster (highlighted by the red rectangle) shows better clinical results (80% of favorable clinical outcomes and 0% of mortality) than the lower cluster (blue dotted rectangle: 25% of favorable clinical results and 75% of mortality). The main difference between thetwo clusters was based on the profile of the perfusional curve, which was more enhanced in the perfusional curves of the upper cluster, which showed a higher number of vessels opacified in the collateral area and therefore with effective collaterals.

  reviewed and analyzed: age, sex, cardiovascular risk factors, baseline modified Rankin Scale (mRS), baseline National Institute of Health stroke scale (NIHSS) score, presence of hemorrhage or acute FLAIR signal abnormalities at baseline CT or MRI, CT or DWI-Alberta Stroke Program Early CT Score (ASPECTS), arterial occlusion site, pa-tient's direct admission to our comprehensive stroke center or to a primary stroke center with sec-ondary transfer ("drip and ship"), time metrics (onset-to-groin -OTG, onset-to-recanalization -OTR, grointo-recanalization -GTR), intravenous thrombolysis administration, type of anesthesia, MT technique, number of passes, first pass effect (FPE) procedural complications, recanalization grade according to the modified Treatment in Cerebral Infarction (mTICI) scale, 24 hours-NIHSS and ASPECTS, hemorrhagic transfor-mation and symptomatic intracerebral hemorrhage (sICH) accord-ing to the ECASS definition and 90-days mRS score. Standard recanalization grades definitions were used: partial (mTICI 0-2a), adequate (mTICI 2b-3) and almost complete/complete (mTICI 2c-3). Patients at admission were mainly images by MRI (>90%).

  viewed by an interventional neuroradiologist and an interventional neurologist with ≥5 years of experience in the neurointerventional field.Collateral circulation was assessed on the prethrombectomy angiographic study, with injection from ipsilateral ICA (for M1 and proximal M2 occlusions) or contralateral ICA and posterior circulation (for intra-cranial ICA occlusions). The ASITN/SIR scale was used for the evaluation. A third neurointerven-tionalist with >10 years of experience reviewed all the angiograms and resolved the disagreements.Kappa inter-and intra-observer agreement was calculated per grade and per dichotomization (grades 1-2 vs grades 3-4). Patients without an adequate pre-thrombectomy angiographic study to assess collateral circulations were excluded.EndpointsThe primary endpoint was to investigate the factors associated with unfavorable outcomes defined as the lack of the achievement of a functional independence (mRS 3-6) at 90-days outpatient visit or telephone interview. Clinical evaluations were performed by certified stroke neurologists.Secondary outcomes included the analysis of those factors associated with: unfavorable outcomes in patients adequately recanalized (defined as mTICI 2b-3), all-causes mortality at 90days and post-procedural symptomatic intracerebral hemorrhage (sICH) in the overall population.

  scriptive evaluation with the median [interquartile] or mean ± standard deviation for continuous variables and frequency and percentage for categorical variables. Student's T-tests or Wilcoxon tests were performed for continuous variables depending to the number of patients in groups. For categorical variables, chi square test or Fisher's test were used when the expected value was less than 5. Those variables associated with the outcome in univariate analysis with a significance level below 0.2 were included in a multivariate logistic regression mod-el, which was adjusted for confounding factors. The odds ratios between factors and outcomes of their respective 95% confidence intervals were calculated. Stepwise backward multiple regression was performed to identify statistically significant predictors. All tests were bilateral with a 5% de-gree of significance. Statistical analysis was performed with the SAS 9.4 software.

  Unfavorable clinical outcomes were observed in 47% of patients following mechanical thrombecto-my despite good collaterals according to the ASITN/SIR classification. Although a good collateral circulation could allow to achieve functional independency also in patients treated in late temporal windows, partial recanalizations and longer procedures were associated with poor outcomes without resulting independent predictors, independently from the endovascular technique. FPE was not sig-nificantly correlated with clinical outcome. Intravenous thrombolysis increases the chance to achieve a good clinical outcome in patients with good collaterals without increasing the risk of hemorrhagic transformation.

  In particular: are good collaterals always effective? This research question represented the aim of the following paper published in the Journal of NeuroInterventional Surgery. the periphery of ischemic site, with persistence of some of the defect, and to only a portion of the ischemic territory 2 Rapid collaterals to the periphery of ischemic site, with persistence of some of the defect, and to only a portion of the ischemic territory 3 Collaterals with slow but complete angiographic blood flow of the ischemic bed by the late venous phase 4 Complete and rapid collateral blood flow to the vascular bed in the entire ischemic territory by retrograde reprfusion 54 Patients with anterior circulation stroke and LVO (M1/ICA terminus/tandem occlusions) in the study periodof collateral venous phase; mRS: modified Rankin Scale; PH1/PH2: parenchymal hematoma according to the ECASS-III scale

Fig. 9 .

 9 Fig. 9. The desynchronization of the collateral circulation (from "Lo studio dei circle collateral può sostituire le tecniche di imaging per identificare la penombra nella selezione dei pazienti da sottoporre a TM nelle finestre terapeutiche più lunghe?" -Arturo Consoli -Invited Conference at the 8th National Congress of the Italian Stroke Association, Verona -December 2021) . Digital Subtraction Angiography, antero-posterior projection, acquired in a patient with a left M1-Middle Cerebral Artery occlusion. Panel A shows the very early arterial phase of the angiogram: at this moment of the acquisition we can observe how the "normal" anterograde cerebral circulation (highlighted by the red triangle and corresponding to the territory of the Anterior Cerebral Artery) and the collateral circulation (blue triangle highlighting the collateral area) are desynchronized. Indeed in this early phase the collateral circulation is in the activation phase while the normal cerebral circulation is the arterial phase. Panel B shows the intermediate (capillary) phase in the normal cerebral circulation; in this phase the collateral circulation is in the arterial phase.

5. 1 .

 1 Materials: Study population and image acquisition protocol 5.1.1. The study population and image analysis One-hundred and eight patients with the occlusion of the M1-middle cerebral artery segment (M1-MCA) eligible to MT procedure with a M1-MCA occlusion between 2018 and 2021 from Hospital

Fig. 10 .

 10 Fig. 10. Comparative analysis between the standard acquisition protocol (2 frames/second) and the experimental protocol (6 frames/second). The analysis of the curves showed that the experimental acquisition protocol provided more informative curves since a larger number of data is available for the analysis of pixel density modification in the same interval time.

Fig. 11 .

 11 Fig. 11. Digital Subtraction Angiography, anteroposterior view showing the occlusion of the left middle cerebral artery. Depiction of the three ROIs that were compared for the analysis of the collateral circulation: the large ROI (red), the intermediate ROI (blue) and the critical ROI (yellow).

First

  turning point: first frame of observation of the parenchymal blush in the ACA territory Second turning point: first frame of visualisation of the superior longitudinal sinus on the midline -Collateral circulation First turning point: first frame of observation of the parenchymal blush in the CC ROI Second turning point: first frame of visualisation of the cortical veins, which have a straight course parallel to the visualisation of the cerebral sulci.

Fig. 12 .

 12 Fig. 12. Definition of the turning points in the cerebral and collateral regions according to the concept of the desynchronization

Fig. 14 .

 14 Fig. 14. Illustration of the three ROIs for fractal dimension calculation. The left image depicts the ROIs in a patient belonging to the Gc (good collateral group) and the right one is referred to a patient belonging to the Pc (poor collateral group).

Fig. 18 .

 18 Fig. 18. The Entropy plot in the collateral area. The blue lines identify the temporal window observation, manually defined. The blue triangle shows the turning point from the arterial to the parenchymal/capillary phase.

  luation artériographique de la présence d'une phase veineuse collatérale en rapport avec l'efficacité de la circulation collatérale (évaluation qualitative). Ce type de paramètre représente l'interprétation physiopathologique du rôle du système veineux et venulaire de la circulation collatérale dans le maintien de la perfusion cérébrale pendant l'occlusion artérielle et l'évolution de l'ischémie cérébrale. Le CVP a été comparé à l'évaluation faite par l'échelle ASITN/SIR et le CVP seul ainsi que l'évaluation composite de ces deux paramètres ont montré des meilleurs résultats en termes de corrélation avec les résultats cliniques.La dernière partie de la thèse est centrée sur le développement d'un algorithme de caractérisation de la circulation collatérale (algorithme ASCOT). L'algorithme permettra d'analyser en temps réel les images d'artériographie cérébrale en cours de traitement par thrombectomie mécanique et de fournir des données d'efficacité de la circulation collatérale qui pourront aider à élargir les connaissances physiopathologiques sur les AVCi, orienter les choix thérapeutiques intra-procédurales et de prise en charge post-procédurale.EnglishThe focus of this thesis is on the evaluation of collateral circulation in patients with acute ischemic stroke (AIS). The role of this collateral circulation has been investigated in the literature while the hemodynamic characteristics are still little known. Since 2018 there has been a growing interest around collateral circulation and several teams around the world have focused their efforts on understanding the pathophysiology of AIS as well as on new imaging methods to further describe the hemodynamic functioning of the collateral circulation.The objectives of the thesis were developed on three distinct axes: (i) the analysis of the clinical impact of collateral circulation in ischemic stroke, (ii) the critical analysis of collateral circulation assessment by cerebral angiography and (iii) the development of a real-time collateral circulation characterization algorithm for angiographic images.The clinical impact of collateral circulation is an essential step in the pathophysiological understanding. Several studies have shown significant correlations between good collateral circulation and favorable clinical outcomes and patients with good collateral circulation are considered the best candidates for mechanical thrombectomy procedures. In this thesis I present the results of a retrospective clinical analysis study (UNCLOSE study), targeted on this specific subgroup of patients with AIS and good collateral circulation evaluated by cerebral arteriography (ASITN/SIR scale) and treated by mechanical thrombectomy. The findings of this study showed an adverse clinical outcome in almost half of this subgroup (47%) despite the presence of good collateral circulation. The factors that may explain this type of results are not different from those already described in patients with insufficient or absent collaterals, with the exception of the lack of significance of First Pass Effect and a protective role of intravenous thrombolysis. In addition, these results focus on the modality of evaluation of collateral circulation by cerebral arteriography and the need to propose other evaluation parameters.A new evaluation parameter was described while proposing a critical analysis of the angiographical evaluation of collateral circulation: the angiographical venous phase of collateral (CVP, collateral venous phase). I present the results of a retrospective study of angiographic evaluation of the presence of a collateral venous phase in relation to the collateral circulation efficiency (qualitative evaluation). This type of parameter represents the physiopathological interpretation of the role of the venous and venular system of collateral circulation in maintaining cerebral perfusion during arterial occlusion and progression of cerebral ischemia. The CVP was compared to the ASITN/SIR assessment and the CVP alone and the composite assessment of these two parameters showed better results in terms of correlation with clinical results.The last part of the thesis focuses on the development of a collateral circulation characterization algorithm (ASCOT). The algorithm will enable real-time analysis of brain angiography images acquired during mechanical thrombectomy procedures and provide data about collateral circulation effectiveness that could help expand pathophysiology knowledge on AIS, and drive intraprocedural and post-procedural treatment choices.

Table 1 .

 1 The modified Rankin Scale………………………………………………………………………24

Table 2 .

 2 The modified Treatment In Cerebral Ischemia score (mTICI score)……………………….[START_REF] Xu | Antiplatelet Strategies and Outcomes in Patients with Noncardioembolic Ischemic Stroke from a Real-World Study with a Five-Year Follow-Up[END_REF]

Table 3 .

 3 TheASITN/SIR collateral grade………………………………………………………………..54

	vi

  Inspiré par et considéré comme une extension des travaux du Master2, dans ce chapitre sont décrites l'analyse des courbes temps-densité de la région de la CC extraites des données artériographiques. Différemment d'une étude basée exclusivement sur l'analyse d'une Région d'Intérêt (ROI), l'analyse en termes de pixels a été également appliquée. Une population de 108 pa-

	tients présentant une occlusion du segment M1 de l'artère cérébrale moyenne (M1-MCA), éligibles
	à la procédure de TM et traités entre 2018 et 2021 à l'hôpital FOCH a été analysée. Tous les pa-
	tients ont été imagés avec le protocole d'acquisition « expérimental » décrit précédemment (à 6
	images/seconde).
	Par la suite, un Core Lab Qualité dédié à analyser la qualité des images d'artériographie a été insti-
	tué, qui comprenait deux neuroradiologues non impliqués dans les procédures. Le Core Lab Qualité

Il serait donc très avantageux que les images artériographiques prétraitement acquises in situ puissent être utilisées pour fournir des indications par rapport aux stratégies de traitement, telles que la sélection des patients pour la TM, le pronostic, les stratégies post-traitement, etc. Ce concept a xiii été abordé par le développement d'un algorithme dédié pour caractériser la CC basé sur les images artériographiques. a évalué la qualité des images et a indiqué l'éligibilité pour l'analyse. Les artefacts de mouvement ont représenté la principale raison d'exclusion et 19 patients ont donc été exclus.

Un groupe de variables a été évalué pour effectuer l'analyse critique des angiographies et le paramétrage de l'algorithme. Ces variables ont été résumées dans le plan analytique qui a été établi et nommé « Image Feature Analysis (IFA) ».

  . In the former paper we introduced a new grading system based on the Digital Subtraction

	rough the Careggi Collateral Score (CCS), while in the latter we had observed that CC had a real
	impact on the clinical outcome of patients treated by MT.
	From 2011 to 2015 I have coordinated a collaborative group which included the Careggi University
	Hospital (Florence), the Lescotte Hospital (Siena), the Sant'Agostino Estense Hospital (Modena),
	the University Hospital of Ferrara and the Karolinska Institute (Stockholm). This collaborative
	group focused on the comparison between the angiographic assessment of the CC through the Ca-
	reggi Collateral Score and the perfusional aspect of the brain evaluated through the CT-Perfusion:
	the CAPRI Study (CT perfusion and angiographic assessment of pial collateral reperfusion in acute
	ischemic stroke: the CAPRI study). The retrospective analysis of our multicentric cohort led to the
	publication of the results of our paper on the Journal of NeuroInterventional Surgery, which is con-
	sidered one of the most valuable journals in the domain of Interventional Neuroradiology (Consoli
	A, et al. 2016). The CAPRI Study represented also the final thesis of my Residency Program.
	During the Master 2, in 2018, I had the chance to work on the CIC-IT/IADI Laboratory in Nancy,
	directed by Prof. Jacques Felblinger, under the close supervision of Dr Bailiang Chen and I have
	exposed my project to work on the imaging post-treatment in order to assess the qualitative aspect
	of CC directly on DSA. The preliminary work that I have conducted at the laboratory led to a basic
	version of an algorithm that segmented and evaluated the perfusional curves of the brain directly on
	the 2D angiograms that I had acquired using an experimental protocol at 6 frames/seconds.
	This experience represented a milestone in the construction of the project of the PhD thesis. Indeed,
	considering the initial results that we had observed I felt very confident to continue this project fol-
	lowing a more sophisticated roadmap. The team at CIC-IT/IADI Laboratory was very motivated as
	well. A strategy plan was built up in accordance with Prof. Serge Bracard, my PhD thesis Director,
	and Dr Mitchelle Bailiang Chen, my PhD thesis Co-director. Prof. Bracard and Dr Chen approved
	the subject of the PhD and the PhD thesis plan.
	COVID-19 pandemic has overturned the entire world during her last two years and that has affec-
	ted, at the very beginning, the workflow of my PhD project. However, we have rapidly thought
	about an alternative program and started weekly meetings with a remote work-based schedule. I had
	the possibility to continue the experimental and the clinical workflow at my institution, the Foch
	Hospital in Suresnes. I had the opportunity to work remotely with Dr Chen and we have mutually
	advanced in the development of a dedicated algorithm for the segmentation of the collateral vessels.
	Angiography (DSA) in order to provide a semi-quantitative and a qualitative assessment of CC th-
	xvi

submission Consoli A -CIC-IT IADI Laboratory. Algorithm of characterization of collateral circulation "ASCOT" (ASsessment of COllateral circulation hemodynamics in acute ischemic sTroke)/ Algorithme de caractérisation de la circulation collatérale "ASCOT" (ASsessment of COllateral circulation hemodynamics in acute ischemic sTroke)

	Invited Conference Speaker -National Congress Groupe de Réflexion sur la Cardiologie Interven-1. INTRODUCTION
	tionnelle (GRCI, Paris) -Prise en charge d'un AVC par un neuroradiologie interventionnel" -De-
	1. Published papers -Peer-reviewed Journals 1.1. Acute stroke -Overall background cember 2017
	Consoli A, Andersson T, Holmberg A, Verganti L, Saletti A, Vallone S, Zini A, Cerase A, Romano D, Acute stroke is considered as a life-threatening, emergency condition and it can be classified as:
	Bracco S, Lorenzano S, Fainardi E, Mangiafico S; CAPRI Collaborative Group. CT perfusion and Invited Conference Speaker -Annual National Congress of the Egyptian Stroke Conference (Cairo) acute ischemic stroke (AIS) and hemorrhagic stroke.
	angiographic assessment of pial collateral reperfusion in acute ischemic stroke: the CAPRI study. -"Mechanical thrombectomy or thrombus aspiration? Insights from the ASTER and ASTER2
	The paper was published in the Journal of Neurointerventional Surgery (JNIS) in 2016. J Neuroin-Trials" -December 2018 The former is secondary to the occlusion of a cerebral artery, caused by a clot with different sour-
	terv Surg. 2016 Dec;8(12):1211-1216. doi: 10.1136/neurintsurg-2015-012155. Epub 2016 Jan 22. ces:
	PMID: 26801947. Invited Conference Speaker -SLICE Course (Nice) -"Clot imaging with DSA" -October 2019
	Consoli A, Pizzuto S, Sgreccia A, Di Maria F, Coskun O, Rodesch G, Lapergue B, Felblinger J, Invited Conference Speaker -Italian Stroke Association (ISA) -"Lo studio dei circoli collaterali
	Chen MB, Bracard S. Angiographic collateral venous phase: a novel landmark for leptomeningeal puo' sostituire le techniche di imaging per identificare la penombra nella selezione dei pazienti da
	collaterals evaluation in acute ischemic stroke -The paper has been published in the Journal of sottoporre a trombectomia meccanica nelle finestre terapeutiche più lunghe?" -"Could the study of
	NeuroInterventional Surgery (JNIS) -2023 collateral circulation replace the imaging techniques to identify the penumbra to select patients for
	mechanical thrombectomy in late windows?" -Verona, December 2021
	2. Submitted papers -Peer-reviewed Journals Invited Conference Speaker -Annual Camp Base Neurovascular course (CBNV, Rome) -"How to Section I -INTRODUCTION
	Consoli A, Pileggi M, Hasan A T M, Venier A, Sgreccia A, Pizzuto S, Coskun O, Di Maria F, Scar-select patients for mechanical thrombectomy in acute ischemic stroke?" -September 2022
	cia L, Lapergue B, Rodesch G, Bracard S, Chen B. UNfavorable CLinical Outcomes in patients
	with good collateral Scores: the UNCLOSE study. -Submitted to the Journal of Neuroradiology
	3. Patent 4. Communications
	Invited Conference Speaker -Annual Camp Base Neurovascular course (CBNV, Rome) -"Angio-
	graphic and CT-Angiography assessment of collateral circulation" -September 2015
	xx xxi

Table 2 .

 2 The modified Treatment In Cerebral Ischemia score (mTICI score)

Table 1

 1

	). Sec-ondary transfers resulted signifi-
	cantly associated with unfavorable outcomes
	(OR: 4.41, IC95%[1.70-11.43], p=0.002) at the
	multivariate analysis whereas no differences
	were shown in terms of onset-to-groin. Howe-
	ver, if we consider only patients with almost
	complete/complete re-canalization we obser-

ved that the impact of the secondary transfers was not significant (Supple-mental Table

1

  ).

	in terms of procedure time, which was longer
	in patients with unfavorable outcomes (63.1 vs
	46.3 minutes, p=0.004), although no dif-feren-
	ces were shown in terms of first-line endova-
	scular technique or occlusion site. This datum
	could be explained by the higher number of
	passes performed in patients with poor clinical
	out-comes (2.7 vs 2, p=0.01, Table 1) or sICH
	(3.5 vs 2.3 minutes, p=0.01, Table
	The most significant difference was observed

Flowchart of the studyTable 1 . Overall study population data and univariate analysis (primary endpoint).

 1

	Proximal M2	40 (18)	27 (23)	13 (13)
	Side, N(%)
	Left	115 (53)	62 (53)	53 (51)
					0.76
	Right	104 (47)	54 (47)	50 (50)
	Baseline ASPECTS, median	8 (7-9)	8 (7-9)	8 (7-9)	0.1
	(IQR)
	Secondary transfer, N(%)	132 (60)	63 (54)	69 (67)	0.06
	Onset-to-groin, mean±SD	230.12±113.09	225.5 ±82.8	239.7 ±87.1	0.22
	Onset-to-recanalization,	288.3±117.3	270.4 ±93	299.4 ±96	0.03
	mean±SD
	Groin-to-recanalization,	53.8±41.1	46.3 ±32.7	63.1 ±47.6	0.004
	mean±SD
	IVT, N(%)	116 (
		Overall	Favorable outcome	Unfavorable	p
		(N=219)	(N=116)	outcome (N=103)
	Females, N(%)	117 (53.4)	56 (48)	46 (45)	0.59
	Age, mean±SD	70.6 ±16	65.3 ±16.9	76.6 ±12.9	<0.001
	Baseline mRS, N(%)
	0	181 (82)	105 (91)	76 (74)
	1	23 (10)	11 (9)	12 (12)	<0.001
	2	15 (7)	0	15 (14)
	Baseline NIHSS, median	15 (9-19.25)	12 (7-17)	18 (14-21)	<0.001
	(IQR)
	Occlusion site, N(%)
	ICA terminus	14 (6)	6 (5)	8 (8)
	M1-MCA	165 (74)	83 (72)	82 (79)	0.21
		40

T, et al. Effect of thrombectomy on oedema progression and clinical outcome in patients with a poor collateral profile. Stroke and Vascular Neurology 2021;6:doi: 10.1136/ svn-2020-000570.

Fig.1.

Table 2 -

 2 Secondary endpoints.

	PH1/PH2	30 (13.6)	10 (8.6)	20 (20)	0.02
	sICH, N(%)	13 (6)	0	13 (13)	<0.001
	Recurrent stroke, N(%)	11 (5)	3 (3)	8 (8)	0.09

Table 3 . Multivariate analysis for primary and secondary endpoints P value OR [IC 95%] Primary endpoint -Unfavorable outcome in overall population

 3

	Age	0.001	1,05 [1.02 -1.09]
	24h-NIHSS	<0.001	1.23 [1.14 -1.32]
	Secondary transfers	0.002	4.41 [1.70 -11.43]
	IVT	0.009	0.28 [0.11 -0.73]
	24h-ASPECTS	<0.001	2.42 [1.84 -7.21]
	Secondary endpoints
		Unfavorable outcome in adequately recanalised patients (mTICI
		2b-3)
	Age	0.0002	1.05 [1.03 -1.08]
	Baseline NIHSS	<.0001	1.15 [1.07 -1.22]
	IVT	0.022	0.41 [0.19 -0.88]
	Secondary transfers	0.005	2.95 [1.38 -6.33]
		Mortality
	Baseline mRS	0.003
	1 vs 0	0.534	1.57 [0.38 -6.47]
	2 vs 0	0.0006	10.67 [2.78 -41.02]
	Baseline NIHSS	0.003	1.17 [1.05 -1.27]
	mTICI (2B-3)	0.012	0.18 [0.05 -0.69]
		sICH
	Age	0.039	1.08 [1.07 -1.29]
	Baseline ASPECTS	0.005	0.53 [3.87 -106.34]
	First pass effect	0.033	0.10 [0.01 -0.83]

Table 2 . Subgroup analysis of good outcome according to the time elapsed be- tween onset of symptoms and recanalization.

 2

	OTR <6h	OTR >6h
	(N=176)*	(N=32)**

Table 3 . Subgroup analysis of recanalization rate according to the administra- tion of intravenous thrombolysis.

 3

	Use of BGC, N(%)		46 (26)	13 (40)	0.09
	First-line strategy
	Aspiration		116 (66)	14 (44)
					0.02
	Stent retriever/ Combined	60 (34)	18 (56)
	First pass effect, N(%)		80 (45)	5 (15)	0.001
	N° of passes, mean ± SD		2.14±1.2	3 ±1.8	<0.001
	Final mTICI 2b-3, N(%)		165 (94)	24 (75)	<0.001
	Final mTICI 2c-3, N(%)		135 (76)	15 (47)	<0.001
	Complications, N(%)		22 (13)	4 (12)	1
	24h-NIHSS, median (IQR)**	16 (10-20)	11 (6-18.5)	<0.001
	24h-ASPECTS, median (IQR)**	6 (5-8)	7 (6-8)	0.07
	sICH*, N(%)		11 (6)	2 (6)	0.9
	Stroke recurrence, N(%)		9 (5)	1 (3)	0.89
	mRS0-2, N(%)		99 (56)	14 (44)	0.19
	mTICI 0-2a	mTICI 2b-3	Tot.	P
	IVT-	17 (16%)	87 (84%)	104
					0.02
	IVT+	8 (7%)	110 (93%)	118
			50

IVT -= no administration of intravenous thrombolysis.

Table 3 .

 3 The American Society of Interventional and Therapeutic Neuroradiology/Society of Interventional Radiology (ASITN/SIR) Collateral grade

Table 1 . Subgroup analysis and multivariate analysis

 1

	Supplemental <60	60-90	>90
	N patients	41	39	120
	Age, mean ± SD	65.5 ± 17.2	69.8 ± 16.0	64.7 ± 15.4
	Baseline ASPECTS, median
	(IQR)	7 (5-8)	7 (5-8)	7 (5-8)
	CVP+, N(%)	23 (56.1%)	22 (56.4%)	45 (37.5%)
	ASITN/SIR >2, N(%)	22 (53.7%)	23 (59.0%)	68 (56.7%)
	mRS 0-2, N(%)	26 (63.4%)	19 (48.7%)	51 (42.5%)
	N passes, mean ± SD	2.29 ± 1.54 2.41 ± 1.67	2.78 ± 2.26
	Multivariate analysis: defined variable CVP+
		OR	IC95%	p
	ASITN/SIR >2	17.0	[6.3 -46.3]	<0.0001
	IV Thrombolysis	3.6	[1.5-9.0]	0.0053
	Onset to imaging			0.02
	180< vs <90	9.1	[2.0-41.7]
	90-180 vs <90	2.9	[0.98-8.53]
	Onset to groin			0.0006
	180-360 vs <180	0.1	[0.04-0.47]
	360< vs <180	0.03	[0.005-0.195]
		OR	IC95%	p
	Age	0.97	[0.941-0.996]	0.02
	CVP+	12.18	[5.25-28.28]	0.001
	Composite ASITN/SIR + CVP	6.56	[2.99-14.39]	0.001
	IV Thrombolysis	2.68	[1.17-6.14]	0.02
	mTICI 2b-3	5.21	[2.86-9.47]	0.006

  . An intuitive and practical understand of fractal dimension is that it describes how thoroughly it fills space. The calculation of fractal dimension here uses the FracLab

	(version 2.2, MATLAB toolbox; https://project.inria.fr/fraclab), a classical box counting method
	but can directly applied to grey-scale images. The equation used to calculate	is reported here be-
	low:

(3) 8 (10) 0.06 5 (3) 6 (21) 0.001 11 (6) 0 0.001

Acknowledgments

To Prof. Serge Bracard, for his continuous support and his clear supervision. He is an exemple of pea-

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Data availability statement

Data are available upon reasonable request to the corresponding author

CRITICAL ANALYSIS OF COLLATERALS ACCOR-DING TO DIGITAL SUBTRACTION ANGIOGRAPHY

In Chapter 3 the role of CC as a predictor of good clinical outcome for patients treated by MT for AIS secondary to LVOs was analyzed. Although the results is not significant, currently, the CC is considered as a relevant factor in determining the functional independence in AIS patients and it is considered as a co-factor together with the recanalization grade (mTICI score).

Indeed, as discussed in the previous chapters, the main role of CC is to modulate the evolution of the ischemic process, by providing a sort of "hemodynamic reserve" in case of arterial occlusion.

However, patients with good collaterals can also be associated with unfavorable outcome or, at least, not reach a functional independence.

Therefore, even those patients who are considered the best candidates to MT with the current criteria could not benefit from the endovascular treatment and in particular those with older age, without administration of IVT and treated with long procedure times (Consoli A et al., 2022 UNCLOSE).

The CAPRI Study (Consoli A et al., 2016), reported in Chapter 2, had shown how the CC assessed thought the CCS was significantly associated with the CBV assessed on the CT Perfusion imaging.

However, CCS is just one of the different classifications that have been proposed in order to assess the CC [START_REF] Mcverry | Systematic review of methods for assessing leptomeningeal collateral flow[END_REF]. More than 40 classifications have been proposed and about 20 are used in AIS patients.

The most used grading scale is the ASITN/SIR classification which describes a 5-grades scale for the assessment of collateral circulation (Table 3), based on an integrated semi-quantitative and qualitative evaluation of CC.

The different grades are calculated based on the peripheral or complete filling of the infarcted territory associated with the mention "rapid" or "slow" according to the difference of filling of the CC compared to the washout of the ACA (Higashida RT et al., 2003).

Supplemental Material

Supplemental Fig. 1. Flow chart of the study with eligibility criteria.

Discussion

The findings of this study showed how the CC should be assessed not only according to its extension and that we should consider also their effectiveness.

As it has been shown in the previous chapter, also patients with good CC (according to the ASINT/ SIR classification) may not reach functional independency although these may represent the optimal candidates to achieve favorable clinical outcomes.

In order to assess the effectiveness of the CC we have proposed the analysis of the CC based on the concept of the potency of the venous side of collateral vessels. According to this theory, the effectiveness of the CC would also relate to the effectiveness of the venules of the collateral circulation performing several tasks, such as the clearance of the downstream emboli, the maintenance of the blood flow and a preventive effect on platelet adhesion (Tong et al., 2018).

Furthermore, the patency of the venous side of the collateral circulation is related with the possibility for the CC to continue to "accept" the blood flow retrogradely from the pial circulation, avoiding a condition of stasis in the microcirculation with a subsequent risk of thrombosis.

The current study showed that the analysis of the venous phase of collaterals seems to refine the evaluation of CC. Although the ASITN/SIR-based analysis and the CVP-based analysis showed globally similar results, in line with the literature (Anadani et al. 2022[START_REF] Mangiafico | Effect of the Interaction between Recanalization and Collateral Circulation on Functional Outcome in Acute Ischaemic Stroke[END_REF], Liebeskind et al., 2022), it was possible to observe a stronger association between the presence of CVP (CVP+) and the measures of clinical outcomes (3 months mRS) or hemorrhagic transformation rather than the evaluation performed through the ASITN/SIR score.

Furthermore, the composite analysis that matched the combined evaluation of the CC through both the ASITN/SIR and the CVP provided solid results in terms of association with favorable clinical outcomes (OR: 6.56, IC95% [2.99-14.39]) and a lower risk of hemorrhagic transformation.

Interestingly, in the inclusion period of the study (2016-2021), we had to exclude about 400 patients from the analysis because of the lack of angiograms sufficiently durable to assess the venous phase of the collateral circulation. Although that could represent a potential limitation for the present study, this datum depicts the poor interest that has always been paid in the past decades to the venous side of the CC.

An external validation of these results will be mandatory in order to confirm these findings, which remain at the moment a hypothesis-generating tool.

Technical aspects

The CC should be considered as a dynamic circulation, retrogradely running from the neighbor vascular territories towards the territory of the occluded artery.

Digital Subtraction Angiography provides the visualization of the cerebral circulation through the opacification of the vessels thanks to the contrast agent and it could be considered particularly suitable to assess the CC.

Indeed, DSA provides a real-time imaging of the transit of the contrast agent, highlighting the different phases of the blood flow: the arterial phase, the parenchymal phase and the venous phase. Therefore, DSA is capable to provide a dynamic assessment of the CC as well.

During the PhD program, after having reviewed several angiograms and with the clinical experience as interventional neuroradiologist of the academic work on collaterals, I have been working on the introduction of two novel concepts in the assessment of the CC: the desynchronization and the turning points.

The desynchronization of CC depicts the different behavior of the CC as compared to the "normal" cerebral circulation, namely the anterograde circulation. Indeed, the CC is a retrograde circulation per definition and, therefore, the time of opacification is delayed than the one needed for the normal cerebral circulation (Fig. 9).

Consequently, the CC and the cerebral circulation are visualized in different phases of the angiograms and these could be considered as "desynchronized".

It could be intuitive that the shorter is the delay between the opacification of the normal anterograde circulation and the appearance of the CC the higher the flow velocity will be in the CC and then in the ischemic territory.

Theoretically, this feature could be interpreted as a marker of effectiveness of the CC. Indeed, according to the aforementioned theory by Tong et al. (Tong et al., 2018), the patency of the venous side of the CC allows to avoid a blood stasis in the CC and to maintain an effective blood flow in the ischemic territory and the perfusion of the microcirculation.

The efforts that have been done in the setting of this algorithm have been driven by the physiopathological assumptions concerning the collateral vessel in order to provide useful information about their hemodynamic features in order to facilitate their understanding.

The patent application, whether accepted, will enable the setting of multicentric evaluations based on the use of the algorithm as well as clinical trials to identify potential correlations with other imaging modalities (such as CT-perfusion or MRI-PWI).

Further studies centered on the clinical validation of the algorithm will be prepared and proposed including larger study populations.

The assessment of the venous phase of collateral circulation (Tong et al., 2018) will remain a core subject for the next years and the implementation of the algorithm and its automatization will represent the main mid-term goals to achieve.

The project of the Thesis has also considered the recent innovations and the growing interest in the assessment of the dynamic profile of the collateral circulation and, in particular, of its venous phase.

The preliminary results of the current work generated enthousiasm in the team and other future PhD students as well as Master2 candidates showed an interest to continue the development of the algorithm.

Furthermore, other colleagues form other specialities, such as cardiologists and neurologists, manifested their interest in the further evolution of the algorithm.

SUMMARY OF THE PHD THESIS