MD Antoine Bigourdan

PhD Fanny Munsch

PhD Pierrick Coupé

MD Charles R G Guttmann

MD Sharmila Sagnier

MD Pauline Renou

MD Sabrina Debruxelles

MD Mathilde Poli

MD Vincent Dousset

MD Igor Sibon

MD Thomas Tourdias
email: thomas.tourdias@chu-bordeaux.fr

Early fiber number ratio is a surrogate of corticospinal tract integrity and predicts motor recovery after stroke

Keywords: ischemic stroke, motor outcome, prediction, diffusion tensor imaging, corticospinal tract Corticospinal tract integrity and motor recovery

Background and purpose:

The contribution of imaging metrics to predict post-stroke motor recovery needs to be clarified. We tested the added value of early diffusion tensor imaging (DTI) of the corticospinal-tract (CST) towards predicting long-term motor recovery.

Methods: 117 patients were prospectively assessed at 24h-72h and 1 year after ischemic stroke with DTI and motor scores (Fugl-Meyer). The initial and final fiber number ratios (iFNr and fFNr) were computed as the number of streamlines along the affected CST normalized to the unaffected side and were compared to each other. The prediction of motor recovery (ΔFugl-Meyer) was first modeled using initial Fugl-Meyer and iFNr. Multivariate ordinal logistic regression models were also used to study the association of iFNr, initial Fugl-Meyer, age and stroke volume with Fugl-Meyer at 1 year.

Results:

The iFNr correlated with the fFNr at 1 year (r=0.70, p<0.0001). The initial Fugl-Meyer strongly predicted motor recovery (~73% of initial impairment) for all patients except those with initial severe stroke (Fugl-Meyer<50). For these severe patients (n=26), initial Fugl-Meyer was not correlated to motor recovery (R²=0.13, p=ns), while iFNr showed strong correlation (R²=0.56, p<0.0001). In multivariate analysis the iFNr was an independent predictor of motor outcome (β=2.601, 95%CI=0.304-5.110, p=0.031), improving prediction compared to using only initial Fugl-Meyer, age and stroke volume (p=0.026).

Conclusion:

Early measurement of FNr at 24h-72h post-stroke is a surrogate marker of CST integrity and provides independent prediction of motor outcome at 1 year especially for patients with severe initial impairment.

Introduction

Cerebral ischemic stroke is a major cause of motor impairment 1 with significant impact on the ability of patients to be self-sufficient in activities of daily living. Providing accurate prognosis of long-term motor impairment within the first few days following an insult is highly desirable (i) to correctly inform patients and caregivers, (ii) to rapidly anticipate the type, duration and goals of rehabilitation, (iii) to improve patient selection for clinical trials, such as those focusing on brain repair and reorganization.

Clinically, patient age and initial stroke severity are important predictors of motor disability in the medium to long term [START_REF] Weimar | Age and national institutes of health stroke scale score within 6 hours after onset are accurate predictors of outcome after cerebral ischemia: Development and external validation of prognostic models[END_REF][START_REF] Saposnik | Stroke prognostication using age and nih stroke scale: Span--100[END_REF] . Most patients exhibit a proportional recovery of approximately 70% of their initial motor impairment [START_REF] Prabhakaran | Inter-individual variability in the capacity for motor recovery after ischemic stroke[END_REF][START_REF] Winters | Generalizability of the proportional recovery model for the upper extremity after an ischemic stroke[END_REF] which therefore already conveys strong predictive information. Nevertheless this 70% rule fails in patients with initial severe deficit [START_REF] Prabhakaran | Inter-individual variability in the capacity for motor recovery after ischemic stroke[END_REF][START_REF] Winters | Generalizability of the proportional recovery model for the upper extremity after an ischemic stroke[END_REF] and the variability in medium to long-term outcome is not adequately explained by initial clinical severity alone for those patients. The inter-individual variability in the relation between initial impairment and subsequent recovery hampers accurate individualized prognosis [START_REF] Stinear | Prediction of recovery of motor function after stroke[END_REF] . While infarct volume could be helpful to improve prognostication [START_REF] Vogt | Initial lesion volume is an independent predictor of clinical stroke outcome at day 90: An analysis of the virtual international stroke trials archive (vista) database[END_REF] , its power as an independent predictor of clinical outcome is still debated [START_REF] Johnston | Validation of an acute ischemic stroke model: Does diffusion--weighted imaging lesion volume offer a clinically significant improvement in prediction of outcome?[END_REF] .

Stroke's impact on corticospinal tract (CST) integrity might constitute a more relevant determinant of motor impairment and recovery [START_REF] Puig | Acute damage to the posterior limb of the internal capsule on diffusion tensor tractography as an early imaging predictor of motor outcome after stroke[END_REF] . In line with this concept, Byblow et al. [START_REF] Byblow | Proportional recovery after stroke depends on corticomotor integrity[END_REF] recently demonstrated that CST integrity as assessed by the presence of motor-evoked potentials (MEPs) was a better predictor of the ability to recover compared to initial clinical impairment, thus improving the 70% rule. Furthermore, they also demonstrated the added value of fractional anisotropy (FA) measurements in the CST towards improving prediction of motor function recovery in combination with MEPs [START_REF] Byblow | Proportional recovery after stroke depends on corticomotor integrity[END_REF] . Measurement of CST integrity with diffusion tensor imaging (DTI) [START_REF] Ciccarelli | Diffusion--based tractography in neurological disorders: Concepts, applications, and future developments[END_REF] has been proposed both by assessment of decreases in FA distal to the stroke area, as well as by the quantification of a decrease in the number of CST fibers, as an index of fiber degeneration (Wallerian degeneration) [START_REF] Thomalla | Diffusion tensor imaging detects early wallerian degeneration of the pyramidal tract after ischemic stroke[END_REF][START_REF] Liang | A prospective study of secondary degeneration following subcortical infarction using diffusion tensor imaging[END_REF] . In most previous work, including the one by Byblow et al. [START_REF] Byblow | Proportional recovery after stroke depends on corticomotor integrity[END_REF] , these measurements were primarily obtained weeks or months after stroke [START_REF] Lindenberg | Structural integrity of corticospinal motor fibers predicts motor impairment in chronic stroke[END_REF][START_REF] Puig | Decreased corticospinal tract fractional anisotropy predicts long--term motor outcome after stroke[END_REF] precluding their use for early prognostication. At the subacute stage, Feng et al. [START_REF] Feng | Corticospinal tract lesion load: An imaging biomarker for stroke motor outcomes[END_REF] quantified the overlap of the acute stroke with a canonical CST derived from DTI of healthy controls instead of using DTI in patients. Referring to work by Puig et al. [START_REF] Puig | Wallerian degeneration in the corticospinal tract evaluated by diffusion tensor imaging correlates with motor deficit 30 days after middle cerebral artery ischemic stroke[END_REF] argue that in acute/subacute stage, DTI measurements along the CST might not accurately reflect CST damage, because degeneration takes time to develop and might not be detected by this MRI technique. Nevertheless, a decrease in FA or in the number of CST fibers has already been reported even after only few days following the insult with promising correlations with longer term outcome [START_REF] Devetten | Acute corticospinal tract wallerian degeneration is associated with stroke outcome[END_REF][START_REF] Maraka | Degree of corticospinal tract damage correlates with motor function after stroke[END_REF][START_REF] Moller | Dynamic changes in corticospinal tracts after stroke detected by fibretracking[END_REF][START_REF] Zhang | Pontine infarction: Diffusion-tensor imaging of motor pathways--a longitudinal study[END_REF] . Therefore, direct DTI measurements of the CST integrity rapidly after a stroke might be clinically feasible and could obviate the need for relatively complicated image analysis, which might suffer from image co-registration errors, and necessitates a canonical CST model derived from healthy volunteers. Here, we assessed the relationship between acute and chronic (1 year) tractographic analysis of the CST after stroke, under the hypothesis that DTI in the acute phase might provide a straightforward, quantitative and objective predictor of CST motor fiber integrity, and, ultimately, of the potential for motor recovery over that provided by the 70% rule for resolution of motor impairment. The primary aim of this work is to investigate the added value of early DTI, compared to established and easy-to-collect variables, such as age and bedside tests for initial motor impairment to improve the prediction of long-term motor recovery.

Methods

Study population

Patients admitted for an ischemic stroke between June 2012 and February 2014 were consecutively enrolled in a prospective cohort study. The study design included initial (between 24h-72h) and chronic (1 year) clinical and MRI post-stroke evaluations. The institutional ethics committee approved the study, and written informed consent was obtained for all patients from them or their relatives prior to inclusion. Primary inclusion criteria were: men or women, older than 18 years old, with a suspected clinical diagnosis of minor-to-severe supratentorial cerebral infarct (NIHSS between 1 and 25) 24 to 72 hours after the insult confirmed on trace diffusion MR. The exclusion criteria were: a pre-stroke functional deficit determined as a modified Rankin ≥1 or a previous stroke along the CST documented on imaging, an infratentorial ischemic stroke or haemorrhagic stroke, and patients presenting psychiatric disorders, pregnant women, coma and contraindications to MRI.

Clinical assessment

The main outcome was motor impairment at 1 year evaluated by the Fugl-Meyer score. Motor recovery was estimated by subtracting the initial (24h-72h) Fugl-Meyer from the chronic (1 year) scores (ΔFugl-Meyer=Fugl-Meyer recovery). The Fugl-Meyer score provides a detailed evaluation of motor impairment with inter-and intra-rater reliability close to one [START_REF] Gladstone | The fugl--meyer assessment of motor recovery after stroke: A critical review of its measurement properties[END_REF] . The motor function domains of the Fugl-Meyer score measure movement, coordination, and reflex action about upper extremity (shoulder, elbow, forearm, wrist, hand) and lower extremity (hip, knee and ankle) and range from 0 (hemiplegia) to a maximum of 100 points (normal motor performance). The patients were followed up to 1 year to allow recovery to plateau [START_REF] Verheyden | Time course of trunk, arm, leg, and functional recovery after ischemic stroke[END_REF] . All clinical evaluations were performed by a neurologist assisted by a trained physical therapist, without knowledge of MRI findings.

Magnetic Resonance Imaging

Image acquisition

MRI scans were acquired at the time of clinical evaluations (24h-72h and 1 year after stroke) by using a 3T Discovery MR 750w scanner (GE Medical System, Milwaukee, WI, USA) with a 32-channel phased-array head coil. The imaging protocol included a DTI sequence using dual echo-planar imaging (40 axial slices; repetition time: 15000 ms; echo time set to minimum; slice thickness: 3.5 mm; matrix: 160x160; FOV: 24 cm²; b values: 0 and 1000 s/mm² applied in 16 non-colinear directions, scan time: 4 minutes and 30 seconds).

Rationale for early DTI analysis

Wallerian degeneration distal from the primary lesion (typically within the brainstem) is not expected to be detectable during the first 72 hours after the insult [START_REF] Puig | Wallerian degeneration in the corticospinal tract evaluated by diffusion tensor imaging correlates with motor deficit 30 days after middle cerebral artery ischemic stroke[END_REF] . Nevertheless, FA decreases within the stroke itself from this early time point correspondingly with ischemic damage [START_REF] Sorensen | Human acute cerebral ischemia: Detection of changes in water diffusion anisotropy by using mr imaging[END_REF] . We reasoned that when tracking the CST from primary motor cortex to the brainstem, a significant reduction of FA within a stroke along the CST could interrupt the reconstructed streamlines, even in the absence of Wallerian degeneration. Therefore, although the "real" number of fibers may not yet be significantly decreased in <3 days MRI as there is not yet anatomical anterograde degeneration, we hypothesized that a measurment of so-called virtual fibers from DTI at an early time point could be a surrogate of the real fiber count at follow-up and, in turn, could be helpful for early prognostication of motor recovery.

Image processing and analysis

DTI inherently showed low signal-to-noise ratio (SNR) because scan time had to be short to be tolerated by most patients, especially in the acute/subacute phase of disease. We consequently used a new DTI denoising filter 25 in order to recover higher SNR and thus improve image quality and accuracy of diffusion parameters. Distortions induced by eddy currents and head motion were corrected by performing rigid registration of diffusion weighted images to b0 images [START_REF] Avants | A reproducible evaluation of ants similarity metric performance in brain image registration[END_REF] . The direction table was updated with the estimated motion matrices.

Stroke volumes were first calculated by segmenting each slice of the trace DTI from the initial MRI scan using a semi-automatic tool available in 3D slicer (http://www.slicer.org).

Then, a diffusion tensor model was fitted at each voxel using Trackvis (http://www.trackvis.org) to generate FA maps and to investigate CST integrity. The CST was identified using deterministic tractography between two distal seed-regions of interest (ROI): one in the primary motor cortex (M1) and its underlying white matter, the central sulcus serving as the posterior border, and the other in the anterior cerebral peduncles. This approach was applied as implemented in Trackvis, and ensures that supratentorial strokes will fall between the two seed ROIs. ROIs were placed by a specialized neuroradiologist symetrically, based on color-coded FA maps and trace DTI images using previously published and easy to determine landmarks 9, 14 (See Supplemental Figure 1 for illustration of ROI positions). Fibers which were not homogeneously blue-coded (i.e., superior-inferior fiber direction) and whose direction were not corresponding to the CST based on anatomic knowledge and a DTIderived atlas [START_REF] Wakana | Fiber tract--based atlas of human white matter anatomy[END_REF] were excluded by slight adjustenent of the seed-ROIs. Fiber tract propagation was terminated for FA<0.2 and angle<35° based on agreed-upon thresholds [START_REF] Puig | Acute damage to the posterior limb of the internal capsule on diffusion tensor tractography as an early imaging predictor of motor outcome after stroke[END_REF] . The total number of fibers connecting the 2 ROIs ipislateral to the stroke were normalized by the total number of fibers from the contralateral side in order to correct for age-related variations or pre-existing diffuse microstructural alterations. Whether this strategy perfectly correct the impact of leukoariosis in the ability to accurately quantify the CST integrity transcends scope of work and will be addressed in future analyses.We will refer to initial fiber number ratio (iFNr) for the 24h-72h MRI and to final fiber number ratio (fFNr) for the 1 year follow-up MRI.

Statistical analysis

To identify whether an early measure of fiber number could be a surrogate for real Wallerian degeneration at the chronic stage, we first assessed the correlation between iFNr and fFNr by using Spearman's rank order correlation.

Then, we tested the value of iFNr to predict motor recovery compared to the other predictors.

We reasoned that the first tests to predict motor recovery should be bedside tests. So we first tested the amount of variance (R²) explained by an initial Fugl-Meyer score with respect to motor recovery at 1 year with linear regression analysis. From this step, we identified a meaningful subpopulation (severely affected patients with initial Fugl-Meyer<50) whose motor recovery at 1 year was not predicted by initial Fugl-Meyer. A subsequent regression was therefore conducted in this subpopulation to test whether iFNr can explain more variance than initial Fugl-Meyer with respect to motor recovery at 1 year by comparing R². For these patient who are severely affected at baseline (initial Fugl-Meyer<50), we also performed receiving Operating Characteristic (ROC) analysis to test whether an iFNr cutoff might identify additional patients with a potential to recover by 70% or more despite their initial clinical severity.

The added value of iFNr compared to the other predictors was finally tested with multivariate ordinal logistic regression in the whole population. The dependent variable to predict was motor impairment at 1 year categorized as "severe" (Fugl-Meyer<50), "marked" (50≤Fugl-Meyer≤84) and "moderate-to-slight" (Fugl-Meyer>84) according to previosuly published thersholds [START_REF] Sanford | Reliability of the fugl--meyer assessment for testing motor performance in patients following stroke[END_REF] . The independent variables introduced in the model were initial Fugl-Meyer, age and stroke volume (model 1). The model was repeated with all the previous parameters plus the iFNr (model 2), and the overall discrimination of the two models was compared using an analysis of deviance table. To test whether prediction of the Fugl-Meyer impairment score would translate to prediction of motor disability, we finally conducted a binary logistic regression analysis including all previous parameters with the aim to predict the modified Rankin scale dichotomized at a threshold of 2 as outcome.

All statistical analyses were performed with the R software package (Version 3.0.1) and the GraphPad Prism version 6.0 software with a type I error set at α=0.05.

Results

Patient characteristics

Out of the first 230 patients screened between June 2012 and February 2014, we analyzed 117 patients with a complete set of data at 1-year follow-up at the time of analysis (see flowchart in Figure 1). Main characteristics of patients are summarized in Table 1.

Relationship between iFNr and fFNr

The denoising procedure [START_REF] Manjon | Diffusion weighted image denoising using overcomplete local pca[END_REF] provided improved image quality with 3 minutes of processing per sequence (Supplemental Figure 1) and allowed to quantify the fiber number with the same standardized procedure for each patient. The iFNr measured at 24h-72h was strongly correlated to the fFNr measured at 1 year (r=0.70, p<0.0001). Therefore, these results suggest that iFNr could be an early surrogate marker of the integrity of the CST one year later.

Prediction of motor recovery by using initial clinical severity

From the initial severity, we calculated the potential for motor recovery as the maximum Fugl-Meyer score (=100) minus the initial Fugl-Meyer 4 . In the whole population, the initial severity (and in turn the potential of recovery) was only moderately correlated with Fugl-Meyer recovery (R²=0.35, p<0.001, n=117, Figure 2-A). The residuals (difference between the observed recovery and the predicted recovery by using the linear regression model) showed substantial dispersion (heteroscedasticity) especially for the more severely affected patients with initial Fugl-Meyer<50 (and in turn a potential of recovery>50, Figure 2-B). To formalize the finding of more variable recovery in severe patients, we repeated the regression by excluding these patients (n=26). The overall fit of this new model was very good (R²=0.65, p<0.0001, n=91, Figure 2-C) and the residuals were highly concentrated around zero (Figure 2-D). The estimated regression model without the severe patients (Figure 2-C) showed that the remaining mild-to-moderate patients recovered approximately 73% of their maximum potential. Therefore, the initial Fugl-Meyer provided a strong estimation of the motor recovery except for the subpopulation with high initial impairment, defined as initial Fugl-Meyer<50, who demonstrated high variance in motor recovery.

Added prognostic value of iFNr compared to initial Fugl-Meyer

We investigated the value of the iFNr to predict motor recovery when initial Fugl-Meyer was less informative (initial Fugl-Meyer<50). There was no recurrent stroke on the 1 year MR scan that might have contributed to the poor motor recovery of some of these patients.

Interestingly, while initial Fugl-Meyer was not correlated with motor recovery for these severe patients (R²=0.13, p=ns, Figure 3, red), the iFNr correlated strongly with motor recovery (R²=0.56, p<0.0001, Figure 3, black). Some patients showed low initial Fugl-Meyer and low iFNr and didn't recover (Figure 3 and4-A). On the other hand, patients with low initial Fugl-Meyer, but preservation of some fibers based on the iFNr, showed substantial recovery (Figure 3 and4-B). Such clinical-to-DTI discrepancies are illustrated by the blue arrows in Figure 3. Using ROC analysis we showed that severe patients with initial Fugl-Meyer <50 can recover by 70% or more of their initial motor impairment if iFNr>0.26 (sensitivity=100%; specificity=83.5%; negative predictive value=100%; positive predictive value=72.7%; area under the curve=0.93±0.05; p=0.006). The supplemental figure 2 shows the recovery of these severe patients as a function of initial impairment and iFNr.

Added value of iFNr compared to other predictors

In multivariate ordinal analysis conducted on the whole population, initial Fugl-Meyer was the strongest predictor of motor outcome (model 1), higher initial Fugl-Meyer being associated with better recovery (β=0.083, 95%CI=0.059-0.115, p=<0.001, Table 2). When iFNr was added in the analysis (model 2), the predictive strength towards motor outcome (β=2.601, 95%CI=0.304-5.110, p=0.031, Table 2) was significantly increased compared to model 1 (p=0.026). These results on the overall population probably reflect the contribution of the more severe patients highlighted before. Such prediction of impairment also translated in improving the prediction of disability according to the mRS (see supplementary Table ).

Discussion

The iFNr measured by DTI between 24h to 72h after the insult provides an early surrogate of chronic CST fiber degeneration measured by the fFNr at 1 year follow-up. This biomarker provides significant independent added value towards predicting long-term motor impairment as measured with Fugl-Meyer compared to detailed clinical testing at baseline, age and stroke volume. This is especially relevant for the subpopulation of more severely affected patients who demonstrated highly variable motor recovery.

A decrease in FNr is supposed to reflect Wallerian degeneration of the CST but an accurate physiopathological interpretation early after stroke is difficult. Indeed, Wallerian degeneration takes time to manifest and one might argue that fiber degeneration is not yet present at 24h-72h after stroke onset. T2 hyperintensity along the affected tract is indeed only seen weeks to months after stroke [START_REF] Kuhn | Wallerian degeneration after cerebral infarction: Evaluation with sequential mr imaging[END_REF] and reduced FA along the CST distal to the stroke area has been shown to be significant at day 30 but not yet at day 3 [START_REF] Puig | Wallerian degeneration in the corticospinal tract evaluated by diffusion tensor imaging correlates with motor deficit 30 days after middle cerebral artery ischemic stroke[END_REF] . There are two alternatives to explain our measurement of reduced DTI fiber measurements at such an early time point. The first is that reduced iFNr reflects the beginning signs of actual histological fiber degradation that could be more or less acute depending on stroke severity [START_REF] Thomalla | Time course of wallerian degeneration after ischaemic stroke revealed by diffusion tensor imaging[END_REF] . In line with this interpretation, histological signs of Wallerian degeneration were already detectable in the brainstem 2 days after experimental stroke in rats [START_REF] Iizuka | Corticofugal axonal degeneration in rats after middle cerebral artery occlusion[END_REF] . In humans, a significant reduction of apparent diffusion coefficient in cerebral peduncles downstream of strokes with severe prognosis was reported on MRI performed 12 hours after insult [START_REF] Devetten | Acute corticospinal tract wallerian degeneration is associated with stroke outcome[END_REF] , and early signs of Wallerian degeneration were also observed with DTI [START_REF] Thomalla | Diffusion tensor imaging detects early wallerian degeneration of the pyramidal tract after ischemic stroke[END_REF][START_REF] Maraka | Degree of corticospinal tract damage correlates with motor function after stroke[END_REF] . Alternatively, although destruction of fiber structure is not yet evident histologically, the reduction of FA within a stroke located along the CST might influence the computation of tracking, and force streamlines to interrupt. FA values within ischemic strokes progressively diminish, especially after 24h (mean delay in our population=58 hours), although they might be elevated hyperacutely (<6h) [START_REF] Sorensen | Human acute cerebral ischemia: Detection of changes in water diffusion anisotropy by using mr imaging[END_REF][START_REF] Bhagat | Elevations of diffusion anisotropy are associated with hyper--acute stroke: A serial imaging study[END_REF] . The severity of cytotoxic edema, and the associated reduction in FA, might represent ischemic damage and determine whether fibers are tracked or not through a stroke along the CST (see figure 4), and ultimately whether fibers will degenerate or not, as well as a patient's potential for motor function recovery.

Further work is needed to shed more light on the specific histological correlates of iFNr which, nevertheless, appears to be clearly related to the degree of chronic CST damage.

Recently Feng et al. [START_REF] Feng | Corticospinal tract lesion load: An imaging biomarker for stroke motor outcomes[END_REF] used a metric called "weighted CST lesion load (wCST-LL)" to provide an early quantification of CST integrity similarly to our iFNr. The wCST-LL consists in quantifying stroke voxels that overlap a canonical CST derived from healthy controls [START_REF] Feng | Corticospinal tract lesion load: An imaging biomarker for stroke motor outcomes[END_REF][START_REF] Zhu | Lesion load of the corticospinal tract predicts motor impairment in chronic stroke[END_REF] .

While this approach is very interesting it requires projecting the patient's diffusion weighted images into the space of the canonical CST, thereby facing the challenge of spatial normalization of lesioned brains. In contrast, the iFNr could be calculated in clinical routine with minimal post-processing from a DTI that can be acquired within an acceptable scan time; denoising procedure [START_REF] Manjon | Diffusion weighted image denoising using overcomplete local pca[END_REF] being available to rapidly recover SNR if needed. Furthermore, overlap between stroke and the canonical CST with wCST-LL might not invariably be consistent with fiber degeneration depending on the severity of ischemia-related histological changes which can range from edema without irreversible destruction to full necrosis. iFNr might provide more specific information relative to fiber degeneration. Comparative work is required to address this question. The iFNr has nevertheless its own limitations. Contralateral alterations can confound the ratio in some patients with preexisting damage, but we chose it in order to reduce inter-individual and inter-scan variability. Vasogenic edema that peaks within the first week [START_REF] Lansberg | Evolution of cerebral infarct volume assessed by diffusion--weighted magnetic resonance imaging[END_REF] can also alter CST tracking in large strokes inducing mass effect and reduced anisotropy on a CST that is nonetheless intact, and that will remain intact at follow-up. As edema is reabsorbed, the number of fibers identified tractographically will be found to increase, as the tissue recovers part of its anisotropic architecture. These confounding factors might contribute to the imperfect relation between "true" fFNr at 1 year and iFNr during the sub-acute phase (fFNr being higher than iFNr). However, we consider this ratio as a realistic parameter of Wallerian degeneration as assessed by its capability to predict motor recovery.

Functional quantification of CST integrity by measurement of MEPs has also shown to improve predictive models of motor function recovery based on clinical measures 10 but DTI still adds predictive value when MEPs are absent.

Recent strategies to assess motor recovery recommend sequential identification of subsets of patients by combining predictors in a stepwise manner rather than considering them in isolation [START_REF] Byblow | Proportional recovery after stroke depends on corticomotor integrity[END_REF][START_REF] Feng | Corticospinal tract lesion load: An imaging biomarker for stroke motor outcomes[END_REF][START_REF] Krakauer | The proportional recovery rule for stroke revisited[END_REF][START_REF] Stinear | The prep algorithm predicts potential for upper limb recovery after stroke[END_REF] . In line with these strategies [START_REF] Byblow | Proportional recovery after stroke depends on corticomotor integrity[END_REF][START_REF] Feng | Corticospinal tract lesion load: An imaging biomarker for stroke motor outcomes[END_REF][START_REF] Krakauer | The proportional recovery rule for stroke revisited[END_REF][START_REF] Stinear | The prep algorithm predicts potential for upper limb recovery after stroke[END_REF] and in accordance with previous studies [START_REF] Prabhakaran | Inter-individual variability in the capacity for motor recovery after ischemic stroke[END_REF][START_REF] Winters | Generalizability of the proportional recovery model for the upper extremity after an ischemic stroke[END_REF] we found that most stroke patients exhibit a proportional motor recovery of approximately 70% of their initial impairment. This means that starting with bedside measures can already provide a strong indication of motor recovery. DTI could be reserved to resolve uncertainty for the subset of patients with more severe initial clinical impairment. Within these severe patients there were two distinct clusters: (i) patients with concordant low iFNr and low clinical score who didn't recover; (ii) patients with preserved iFNr despite low clinical score of patients who recovered substantially. Residual integrity of about 30% of the motor fibers (iFNr~0.26) was associated with motor recovery of 70% or more of the initial impairment despite the initial severity with a positive predictive value of 72.7%. There were still intermediate patients, whose recoveries were difficult to predict accurately (slight recovery despite low fiber ratio and low clinical scores). This might be due to the contribution of alternate motor fiber pathways, such as cortico-rubro-spinal and cortico-reticulo-spinal tracts [START_REF] Lindenberg | Structural integrity of corticospinal motor fibers predicts motor impairment in chronic stroke[END_REF] , or to brain reorganization 37 and compensation [START_REF] Zeiler | The interaction between training and plasticity in the poststroke brain[END_REF] . Alternate strategies to capture the residual ability to recover over that provided by the proportional recovery rule include taskrelated brain activation using functional MRI [START_REF] Marshall | Early imaging correlates of subsequent motor recovery after stroke[END_REF][START_REF] Zarahn | Prediction of motor recovery using initial impairment and fmri 48 h poststroke[END_REF] , MEPs from transcranial magnetic stimulation [START_REF] Byblow | Proportional recovery after stroke depends on corticomotor integrity[END_REF] or a combination of methods such as clinical evaluation, transcranial magnetic stimulation and DTI [START_REF] Byblow | Proportional recovery after stroke depends on corticomotor integrity[END_REF][START_REF] Krakauer | The proportional recovery rule for stroke revisited[END_REF][START_REF] Stinear | The prep algorithm predicts potential for upper limb recovery after stroke[END_REF] . These strategies might be helpful to further refine prognostic modeling in combination with the iFNr especially in patients with severe initial deficit.

Our study is not without limitations. While the initial sample size was relatively substantial, only 117 patients had a complete set of data and an even smaller number presented with severe stroke. Further confirmation by including a more significant proportion of severe patients will be needed. We used clinically available DTI acquisition and deterministic tractography. More advanced image acquisition and analysis models might be helpful, e.g. to track fibers at the crossing of the CST with the superior longitudinal fasciculus [START_REF] Lee | Have you ever seen the impact of crossing fiber in dti?: Demonstration of the corticospinal tract pathway[END_REF] . The reproducibility of the measures was not formally assessed, but the ROI positioning is straightforward with minimal training. Finally, the iFNr might suffer from a ceiling effect in the sense that it does not correlate linearly with the final outcome in patients with a benign disease course, which can anyway be predicted based on a good initial score.

These limitations notwithstanding, we identified a strategy applicable in clinical routine, before patient discharge from the hospital, which appears to significantly improve prediction of upper/lower limb motor impairment after stroke for patients with initially severe paresis. This strategy has the potential to significantly improve stroke patient management by aiding in determining goals, type and duration of rehabilitation. Furthermore, applying the proposed iFNr to stratify patients at inclusion in neuroprotective or neuroregenerative treatment trials, might help increase statistical power and accelerate discovery and validation of efficacy of novel approaches to stroke treatment.

Sources of funding

The study was supported by public grants from the French Agence Nationale de la Recherche within the context of the Investments for the Future Program, referenced ANR-10-LABX-57 and ANR-10-IDEX-03-02 and named "TRAIL" (Translational Research and Advanced Imaging Laboratory) and "CPU" respectively. The study was funded by a public grant from the French government (PHRC protocole hospitalier de recherche clinique inter-régional) funded in 2012. CRGG was funded by a visiting professorship grant from the Excellence Initiative (IdEx) of the University of Bordeaux.

Disclosures

None.

Figure 2 :

 2 Figure 2: Relationship between motor recovery and initial clinical severity

Figure 3 :

 3 Figure 3: Relationship between motor recovery, iFNr and initial severity

Figure 4 :

 4 Figure 4: Illustrative examples of iFNr and fFNr

Supplemental