N Ignatiev

A Fedorova

A Rodin

L Zasova

F Montmessin Service

T Encrenaz

E Lellouch

T Fouchet

Lesia

V Formisano

M D Smith

D Titov

M Tschimmel

L Maltagliati

WATER IN MARS ATMOSPHERE: COMPARISON OF RECENT DATA SETS

Introduction:

Water vapour, a minor constituent of the Martian atmosphere characterized by only trace content (few to one hundred precipitable microns in column), plays a major role in Martian climate. Water cycle is characterized by a strong sensitivity to variation of the orbital parameters. There are also strong indications that due to long-term accumulation the atmospheric water might form major geomorphologic features of the Martian surface, i.e. the asymmetric polar caps. Faint water ice clouds are believed to affect meridional water transport, as well as dust cycle and thermal regime of the atmosphere [START_REF] Clancy | Water vapor saturation at low altitudes around Mars aphelion: a key to Mars climate?[END_REF]. An assessment of such fine processes could be provided by sophisticated General circulation models of Mars (MGCMs), allowing to vary orbital and climatic parameters. Tuning of these models is only possible with detailed experimental data sets. Therefore, the understanding of the complex interaction of the atmospheric water with the Martian climate system and the surface relies upon experimental data.

Modern climatology of Mars atmospheric water and aerosols is based on MGS/TES, measurements performed in thermal spectral range during three full Martian years [START_REF] Smith | Interannual variability in TES atmospheric observations of Mars during 1999-2003[END_REF]. A good knowledge of the temperature profile is required for the retrieval, and comparison to climate models suggests that TES sometimes overestimates water abundance. Viking epoch water climatology is based on MAWD data (Jakosky and Farmer, 1982) taken in the near infrared. These data are sensitive the aerosol contents, and are sometimes underestimated [START_REF] Fedorova | MAWD observations revisited: seasonal behavior of water vapor in the martian atmosphere[END_REF]. There are physical reasons to have different values at different wavelengths: vertical distribution (H2O vertically mixing ratio is non constant), and scattering by aerosol. Spectral resolution and other instrumental peculiarities affect error bars on the quoted values and could introduce some biases. Finally, retrieval techniques and even spectroscopic databases are perfectible.

Mars Express, a unique versatile atmospheric mission, carries three instruments capable to measure atmospheric water vapour from near-IR to thermal IR spectral ranges. The simultaneous use of these data should allow to validate both techniques, and, moreover, to obtain some additional information on the vertical distribution of water vapour. More information on water vapor is to be extracted from Mars Express limb and solar occultation measurements like it was done during Phobos-88 project [START_REF] Rodin | Vertical distribution of water in near-equatorial troposphere of Mars: water vapor and clouds[END_REF].

Some comparisons of data on the first Mars Express conference in ESTEC showed a substantial disagreement in the data sets even within PFS measurements, and also among SPICAM-PFS (near IR) and OMEGA-SPICAM-PFS. Few comparisons with TES were by far not sufficient to conclude on the subject. Only one paper on Mars Express water vapour measurements is published so far (Encrenaz et al., 2005). We went on with the effort and have mounted an International Team in the International Space Science Institute in Bern, named as the title, and encompassing the coauthors of the present abstract. The Team has gathered in Bern in November 2005, the next meeting scheduled for February 2006. The presentation will summarize the first results of this study.

Water vapour data sets:

We most extensively compare the measurements of Mars Express instruments:

-PFS measures in both IR spectral ranges with very good spectral resolution of 1.5 cm -1 (at 2.56 µm and 1.38 µm in the near-IR (SW channel), and at 30 µm in thermal-IR (LW channel), and its data should be self-consistent. Simultaneously PFS measures thermal structure (crucial for thermal-IR retrievals) and atmospheric dust content (needed for near-IR) -SPICAM IR measures similarly to MAWD at 1.38 µm with good spectral resolution (3.5 cm - 1 ) and acceptable S/N. Algorithms to account for aerosol are to be implemented. We count to use simultaneous aerosol data from PFS spectra. SPICAM IR measures H2O in nadir, at limb and in solar occultations.

-OMEGA contributes to the subject with spatially resolved measurements in near-IR, in particular at 2.56 µm with much lower spectral resolution but good S/N. OMEGA data validated with other measurements. Limb data are of spe-cial interest because of their excellent spatial resolution.

There are also different research groups performing Mars Express data analysis is slightly different ways. PFS H2O data are treated independently by two groups in IKI/IFSI and in LESIA, and OMEGA data are treated also independently in LESIA (Encrenaz et al., 2005) and in MPS Lindau.

TES on Mars Global Surveyor -thermal emission spectrometer with spectral resolution of 5-10 cm -1 measured water vapour at 30-50 µm in thermal-IR, and thermal structure started in 1997, and ended in Aug 2004 [START_REF] Smith | Interannual variability in TES atmospheric observations of Mars during 1999-2003[END_REF].

Preliminary comparison of Mars Express and MGS TES water vapour results analysis, as has been discussed at ESTEC Mars Express conference has shown large disagreement (Figure 1). PFS thermal IR data were generally in good agreement with SPI-CAM 1.38-µm measurements; TES data indicating 25-50% larger values, while PFS 2.56-µm measurements giving significantly larger results. Furthermore, PFS LW analysis by different groups revealed some significant discrepancies. OMEGA results from analysis of 2.56-µm absorption band by Lindau group (not shown in the Figure 1) are comparable in magnitude with PFS SW values.

Ground-based observations:

We also try to summarize the available groundbased observation of martian water vapour, which coincide with TES and Mars Express measurements. Unfortunately, there is a very little coincidence:

-Centimeter (22 GHz) VLA Mars water vapor measurements during 2003 opposition Ls=265-290° [START_REF] Butler | New VLA observations of Mars atmospheric water vapor[END_REF] were preliminary compared to TES results, and revealed good match. VLA Mars water limb emission maps in the fall of 2004 (after the favorable opposition) taken with the intention of comparing to TES are to be processed. Future observations are planned for 2007 (MRO validation).

-A series of near-IR (870 nm) high-resolution spectroscopy measurements was performed in 1996-1999 in Arizona [START_REF] Sprague | Water vapor abundances over Mars north high latitude regions: 1996-1999[END_REF], and compared to TES measurements (Sprague et al., submitted). These observations were discontinued after 1999.

Differences in retrieval methods:

We have compared the different approaches by different groups to data analysis. The main differences are identified below.

Synthetic spectra computation: Spectroscopic data two main spectroscopic databases are used, HI-TRAN-2004 [START_REF] Rothman | The HITRAN 2004 molecular spectroscopic database[END_REF], and GEISA-2003[START_REF] Jacquinet-Husson | The 2003 edition of the GEISA/IASI spectroscopic database[END_REF]. To our understanding, GEISA-2003 data for the H2O bands of interest correspond to earlier version of HITRAN database. A significant difference is found for some strong H2O lines in the 1.38-µm band, measurements and HITRAN-2004 indicating 25-40% weaker lines than indicated in HITRAN-2001 andGEISA-2003 databases. The overall number of lines in the 1.38-µ m band in new database is twice as larger w.r.t. the older one. This difference, however is not expected to have a significant impact on comparison, because the newer database is used for the 1.38-µm retrieval, and there are no reported difference between the databases in the thermal IR range. The 2.56-µm band is yet to be compared.

Line broadening in CO2 is important. Although synthetic spectra calculated with different broadening are almost undistinguishable, slight difference in line widths appears to have a significant impact on retrieved H2O amount. Previously used broadening factor in CO2 of 1.3 w.r.t. the air (i.e. each database value for broadening is multiplied by 1.3) results in too narrow lines. Another possibility to cope with broadening is calculation used for Venus' spectra analysis [START_REF] Pollack | Near-infrared light from Venus' nightside -a spectroscopic analysis[END_REF] and the references therein) and later calculations by [START_REF] Gamache | CO2-broadening of water-vapor lines[END_REF] result is noticeably larger absorption lines (e.g, in the 1.38-µm band the average broadening factor is about 1.8). A frequently adopted factor for broadening is 1.5. Comparing broadening is not straightforward (Fig. 3). In spite of differences, on the average the factor of 1.5 or Gamache procedure lead to very similar results for the end-to-end water retrieval. All groups compute spectra line-by-line, with subsequent construction of look-up tables, in slightly different manners; with the exception of OMEGA treatment by LESIA group, who computed band absorption, and used curves of growth (Encrenaz et al., 2005).

Atmospheric model:

The knowledge of the temperature profile is essential, in particular for thermal IR H2O band. TES and PFS LW channel use consistent temperature re-trievals from the CO2 16-µm band. All teams working with NIR H2O bands use LMD MGCM predictions [START_REF] Forget | Improved general circulation models of the Martian atmosphere from the surface to above 80 km[END_REF] from EMCD, although the use of coinciding PFS profiles when available looks more appropriate. Water vapor saturation altitude is defined from the temperature profile (and depends on water contents). Below the saturation level water vapour in the atmosphere is assumed uniformly mixed; above we can assume it as constrained by saturation, or allow some degree of supersaturation, or imply zero value with no visible impact on the retrieved results. The MPS Lindau group ignores saturation both in PFS and OMEGA retrievals.

Pressure is defined from EMCD, sometimes with refinement of EMCD 3.5°x5°bins with MOLA data and subsequent integration according to an instrument FOV.

Solar spectrum:

The use of an appropriate solar spectrum is important for analysis of NIR spectra, in particular with high spectral resolution, because Fraunhofer lines mix up with H2O absorption features. PFS team has made a significant effort to construct such a spectrum (Fiorenza and Formisano 2005). SPICAM team is using so far [START_REF] Kurucz | The solar spectrum: atlases and line identifications[END_REF] spectrum having disadvantages of being theoretical within the H2O bands, and undersampled for our purpose (1-cm -1 bins).

Correction for dust and aerosol:

Aerosol, suspended in the atmosphere diffuse a fraction of solar light, reflecting it backwards to an instrument, and therefore in the analyzed NIR spectrum the imprint of the water vapour band structure is superimposed by continuum aerosol spectrum. If albedo of the surface or the calibration of the instrument are not perfectly known, the spectrum interpretation indicates less water in the atmosphere. This effect is well known since long time, first analyzed by Davies and Wainio (1981) for MAWD/Viking polar data. The accurate account for aerosol is computation consuming, and the totality of MAWD data was reanalyzed only recently [START_REF] Fedorova | MAWD observations revisited: seasonal behavior of water vapor in the martian atmosphere[END_REF]. The impact of aerosol is estimated in the above two papers. It is insignificant for the atmospheric dust load below τ=0.3 and airmass below 3, and reaches a factor of 3 to 4 for τ>1 and large arimasses. Thermal IR water vapor data are much less sensitive to atmospheric dust, unless its load is extremely high.

Mars Express NIR data are so far analyzed with no account for the aerosol, but such an analysis is planned to be implemented soon. For most of compared measurements dust load and airmass do not lead to correction larger then 20-30%, present discrepancies among different measurements being far beyond this value.

Preliminary results:

Preparing for the first meeting the teams have aligned the processing methods, in particular such critical parameter in computation of synthetic spectra as line broadening (1.5 broadening factor for CO2, or Gamache et al., 1995 data), PFS low-level data processing etc. Cross-checking of synthetic spectra computation and even end-to-end water vapour retrievals between teams was performed. As a result, we have more consistent picture with much less (but still significant) discrepancy among PFS LW and SW channels (Cf. Figs 4 and1). OMEGA values falling somewhere in between and frequently follow PFS LW. In rare cases OMEGA shows values above PFS SW. This alignment in processing method was, however fatal for SPICAM 1.38-µm measurements, which have previously shown a good much to PFS LW, now being far below all other measurements. It should be noted that internal calibration of SPICAM was refined, and a narrower function describes now the spectral resolution. One hypothesis might be that Gamache factors are overestimated for the 1.38-µm band, another is that there are instrumental biases specific to SPICAM.

The considerable differences among PFS SW and 2.56-µm band measurements by PFS and OMEGA are yet to be explained. For the future we plan to complete the comparison for several orbits, and to conduct multiparameter analysis seeking for correlation with altitude, surface albedo etc. OMEGA analysis by MPS team is to be compared with other data. The progress status of this work will be presented on the workshop.

Figure 1 .

 1 Figure 1. Intercomaprison of different vapour measurements from two Mars Express orbits, and the corresponding MGS TES measurements.

-

  IRTF TEXES high-resolution imaging spectroscopy at 8 µm in June 2003 (Ls=206°) were found to be consistent with TES results(Encrenaz et al., 2005). Water vapor is retrieved from HDO lines. Next observation was planned for Dec 2005. Summarizing ground-based results, so far there is no processed observations coinciding with Mars Express measurements. Ground-based IR results were found to be consistent with TES taking into account error bars. NIR results, though corrected for aerosol effect indicated significantly smaller water vapour values than TES in most of cases (Fig.2).

Figure 2 .

 2 Figure 2. Comparison of high-resolution groundbased IR water vapour measurements to coinciding TES data (figure adopted from Sprague et al., submitted).

Figure 3 .

 3 Figure 3. Comparison of H2O line broadening in CO2 from different sources for some lines in 2.56-µ m band. Green triangles correspond to computation with early version of R. Gamache data provided by R. Tipping.

Figure 4 .

 4 Figure 4. Comparison of water vapour measurements by three Mars Express instruments along orbit 1023. Crosses: PFS LW; x -(red and green) PFS SW by different teams; diamonds -OMEGA; asterisks -SPICAM.

Figure 5 .

 5 Figure 5. Same as in Fig.3 showing only PFS measurements compared to older version of SPICAM processing.


Nefeli Valyraki

Adrien Goujon

Marjorie Mateos

Adrien Lecoeuvre

MD-MSc Augustin Lecler

PhD Igor Raynouard

Candice Sabben

Michael Obadia

Julien Savatovsky

Dr Pierre Seners
email: pierre.seners@gmail.com

Sophie Rushton-Smith

MRI Spot Sign In Acute Intracerebral Hemorrhage: An Independent Biomarker of Hematoma Expansion and Poor Functional Outcome

Keywords: Cerebral Hemorrhage, Magnetic Resonance Imaging, Stroke, Prognosis

BACKGROUND:

In acute intracerebral hemorrhage (ICH), the prognostic value of the MRI spot sign on hematoma expansion (HE) and poor functional outcome is poorly known.

METHODS:

We retrospectively included patients admitted over a 4-year period for an acute ICH in a single institution using MRI as the first-line imaging tool. The presence and number of MRI spot signs on contrast-enhanced T1-weighted imaging was evaluated by one neuroradiologist, blinded from outcomes. The primary outcome was HE, defined as >6mL or >33% ICH volume growth from initial MRI to 24-48 h follow-up imaging; the secondary outcome was poor 3-month modified Rankin score (4-6).

RESULTS:

Overall, 147 patients were included, and 62% had a spot sign. Among the 130 patients with follow-up imaging, 24% experienced HE. HE occurred in 6%, 21% and 43% patients with 0, 1 and ³2 spots, respectively (P<0.001). The MRI spot sign was independently associated with HE (adjusted OR,]; P=0.008), with a dose-dependent effect. The negative and positive predictive values of the spot sign for HE were 0.94 and 0.35, respectively. Poor functional outcome occurred in 27%, 32% and 71% patients with 0, 1 and ³2 spots, respectively (P<0.001). In multivariable analysis, the presence of ³2 spots was independently associated with poor functional outcome (adjusted OR,]; P=0.024).

CONCLUSION:

The MRI spot sign is an independent biomarker of HE, and the presence of ³2 spots is independently associated with poor 3-month outcome. The lack of spot sign is highly predictive of a favorable evolution.

INTRODUCTION

Acute intracerebral hemorrhage (ICH) represents 10-20% of all strokes, and leads to high mortality rates and severe disability in most survivors [START_REF] Feigin | Worldwide stroke incidence and early case fatality reported in 56 population-based studies: a systematic review[END_REF][START_REF] Van Asch | Incidence, case fatality, and functional outcome of intracerebral haemorrhage over time, according to age, sex, and ethnic origin: a systematic review and meta-analysis[END_REF]. Hematoma expansion (HE) is common in the acute phase of ICH, occurring in 20-40% of patients, and is strongly associated with poor clinical outcome [START_REF] Demchuk | Prediction of haematoma growth and outcome in patients with intracerebral haemorrhage using the CT-angiography spot sign (PREDICT): a prospective observational study[END_REF][START_REF] Salman | Absolute risk and predictors of the growth of acute spontaneous intracerebral haemorrhage: a systematic review and meta-analysis of individual patient data[END_REF][START_REF] Davis | Hematoma growth is a determinant of mortality and poor outcome after intracerebral hemorrhage[END_REF][START_REF] Dowlatshahi | Defining hematoma expansion in intracerebral hemorrhage: relationship with patient outcomes[END_REF]. Although the benefits of therapies targeting HE have currently not been demonstrated, stratification of HE risk following initial imaging could influence the triage and intensity of monitoring of ICH patients and improve their prognosis [START_REF] Greenberg | Guideline for the Management of Patients With Spontaneous Intracerebral Hemorrhage: A Guideline From the American Heart Association/American Stroke Association[END_REF]. Moreover, accurate prediction of HE may be useful to inform relatives about the patient's prognosis, and could help in the design of randomized trials testing interventions to limit HE.

The spot sign, defined as enhancing foci on imaging within acute ICH corresponding to active bleeding, is an important predictor of HE [START_REF] Demchuk | Prediction of haematoma growth and outcome in patients with intracerebral haemorrhage using the CT-angiography spot sign (PREDICT): a prospective observational study[END_REF][START_REF] Dowlatshahi | Evolution of computed tomography angiography spot sign is consistent with a site of active hemorrhage in acute intracerebral hemorrhage[END_REF]. Using computed tomography angiography (CTA), an independent association between spot sign and HE has been consistently found in several studies, regardless of the spot sign definition, the timing of CTA acquisition (arterial, venous or parenchymal phase) and the definition used for HE [START_REF] Demchuk | Prediction of haematoma growth and outcome in patients with intracerebral haemorrhage using the CT-angiography spot sign (PREDICT): a prospective observational study[END_REF][START_REF] Salman | Absolute risk and predictors of the growth of acute spontaneous intracerebral haemorrhage: a systematic review and meta-analysis of individual patient data[END_REF][START_REF] Romero | Prospective validation of the computed tomographic angiography spot sign score for intracerebral hemorrhage[END_REF][START_REF] Ciura | Spot sign on 90-second delayed computed tomography angiography improves sensitivity for hematoma expansion and mortality: prospective study[END_REF][START_REF] Brouwers | Clinical applications of the computed tomography angiography spot sign in acute intracerebral hemorrhage: a review[END_REF][START_REF] Rodriguez-Luna | Venous phase of computed tomography angiography increases spot sign detection, but intracerebral hemorrhage expansion is greater in spot signs detected in arterial phase[END_REF]. Consequently, the recent guidelines from the American Heart Association/American Stroke Association state that CTA may be reasonable to identify patients at risk for subsequent HE [START_REF] Greenberg | Guideline for the Management of Patients With Spontaneous Intracerebral Hemorrhage: A Guideline From the American Heart Association/American Stroke Association[END_REF]. However, few data exist regarding the predictive value of the spot sign evaluated on MRI [START_REF] Schindlbeck | Spot Sign in Acute Intracerebral Hemorrhage in Dynamic T1-Weighted Magnetic Resonance Imaging[END_REF][START_REF] Murai | Magnetic resonance imaging-documented extravasation as an indicator of acute hypertensive intracerebral hemorrhage[END_REF], which is the first-line imaging tool in some countries for acute neurological deficits. In the modern era, only one small-scaled, single-center study has shown an association between MRI spot sign and discharge modified Rankin Scale (mRS) score [START_REF] Schindlbeck | Spot Sign in Acute Intracerebral Hemorrhage in Dynamic T1-Weighted Magnetic Resonance Imaging[END_REF], but its association with functional outcome assessed at a later time-point and with HE is still unknown.

Here, we tested the hypothesis that the presence of a spot sign evaluated on contrast-enhanced T1weighted imaging is an independent biomarker of HE and poor functional outcome in patients with acute ICH.

METHODS

Study Design and Data Sources

We retrospectively collected the data from all consecutive patients with acute ICH admitted within 24 h from last seen well in our institution between January 2018 and December 2021, a comprehensive stroke center in Paris, with available baseline MRI including a contrast-enhanced T1weighted sequence. Acute phase management was done according to European guidelines, including blood pressure target below 140/90 mmHg [START_REF] Steiner | European Stroke Organisation (ESO) guidelines for the management of spontaneous intracerebral hemorrhage[END_REF][START_REF] Sandset | European Stroke Organisation (ESO) guidelines on blood pressure management in acute ischaemic stroke and intracerebral haemorrhage[END_REF]. Patients with secondary ICH such as trauma, underlying neoplasm, or hemorrhagic venous infarct were excluded. Patients who underwent surgery for hematoma evacuation before 24 h follow-up imaging were excluded from the primary outcome analysis (HE) but were included in the secondary outcome analysis (3-month mRS). Conversely, patients without an available 3-month mRS score were excluded from the secondary analysis, but were included in the primary outcome analysis. Our analysis was conducted according to the Strengthening the Reporting of Observational Studies in Epidemiology criteria for observational studies [START_REF] Von Elm | The Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) Statement: guidelines for reporting observational studies[END_REF]. The research was approved by the Rothschild Foundation Hospital review board. Formal informed consent was waived by the ethics committee owing to the retrospective design.

Clinical Data

The following variables were collected: age, sex, vascular risk factors, prior vascular events, prestroke antithrombotic medication, National Institute of Health Stroke Scale (NIHSS) score on admission and at 24 h, systolic and diastolic blood pressures on admission and hourly during the first 24hrs, and pre-stroke and 3-month modified Rankin Scale (mRS) score. We also collected the date and time of last seen without neurological deficit, time of symptoms onset (or symptoms discovery in case of unwitnessed stroke onset) and baseline MRI. On admission blood sample, we collected platelets count, prothrombin ratio, international normalized ratio (INR), activated partial thromboplastin time ratio and fibrinogen level.

Radiological Data

Imaging Protocol

MRI is systematically implemented as the first-line imaging tool in acute stroke suspicions in our center, except in the case of MRI contraindication or severe agitation. The acute stroke protocol, which is similar for all patients, is performed on a 3T MRI scanner (Philips 3T Ingenia Elition, Best, the Netherlands) included diffusion-weighted imaging, intracranial T1-weighted time-of-flight magnetic resonance angiography, 3D fluid-attenuated inversion recovery, susceptibility-weighted imaging (SWI), and a contrast-enhanced turbo spin echo T1-weighted imaging. The MRI parameters were as follows: 3D turbo spin echo T1 postcontrast (repetition time/echo time/turbo spin echo factor=450 ms/27 ms/25; field of view 230´203 mm, matrix 232´203, slice thickness 1.3 mm, compress sense factor 10; acquisition duration 56 s), 3-dimensional SWI (repetition time/echo time=30/7, 13.5, 20, 26.5 ms; field of view 235´174mm, matrix 336´194, slice thickness 2 mm, compress sense factor 8; acquisition duration 54 s). The gadolinium injection was done with a bolus of Gadobutrol 0.1 mL/kg. The delay between contrast agent injection and the beginning of the T1weighted imaging was between 150 and 180 seconds. Noncontrast CT was regularly performed immediately after MRI to obtain a reference CT at baseline, and this subset of patients was used to compare ICH volume between the two imaging modalities (see below). Follow-up imaging (CT or MRI) was routinely scheduled »24 h from baseline imaging to evaluate HE.

Imaging Analysis

To optimize homogeneity in radiological evaluation, one neuroradiologist (AG) reviewed all baseline MRI using a structured form, blinded to clinical and radiological outcomes. The following variables were collected: (1) hematoma topography, categorized as deep, lobar or infratentorial; (2) associated intraventricular hemorrhage; (3) microbleed presence and location; (4) extent of white matter hyperintensities, using the Fazekas classification; and (5) presence of the spot sign and the number of spots on contrast-enhanced T1-weighted imaging. The spot sign was defined according to 2 criteria:

(1) spot-like or serpiginous high signal intensity >1.5 mm in at least one dimension located within the margin of the hematoma and without connection to an outside vessel; and (2) no hyperintensity at the corresponding location on nonenhanced T1-weighted time-of-flight magnetic resonance angiography.

Typical cases with and without the spot sign are shown in Figure 1. To assess reproducibility of spot sign evaluation, another neuroradiologist (MM) reviewed a random subset (n=50) of baseline MRIs, and a consensus was made for discrepant cases. Three months apart from spot-sign evaluation, the second neuroradiologist performed the volumetric hematoma measurements on baseline MRI (SWI), acute noncontrast CT (in the subset of patients with acute CT acquired immediately after MRI), and 24 h CT or MRI (SWI), using the Carestream Vue PACS software, blinded from clinical outcomes and spot sign status.

Study Outcomes

The primary outcome was substantial HE, defined as an absolute ICH volume growth >6 mL or a relative growth >33% from initial MRI to 24 h follow-up imaging [START_REF] Demchuk | Prediction of haematoma growth and outcome in patients with intracerebral haemorrhage using the CT-angiography spot sign (PREDICT): a prospective observational study[END_REF]. In patients for whom HE was evaluated from baseline MRI to 24 h CT, a 11% correction of baseline MRI ICH volume was applied to account for the MRI-to-CT volumetric overestimation observed in the subset of patients with both acute MRI and CT (see Results). The secondary outcome was poor functional outcome, defined as a mRS score of 4-6 at 3 months considering the overall poor functional outcome in patients with acute ICH.

Statistical Analysis

Continuous variables are described as mean±standard deviation or median (interquartile range [IQR]), as appropriate, and categorical variables as numbers and percentages. Interobserver agreement for spot sign presence and number of spots was measured using Kappa and overall weighted Kappa statistics, respectively. Univariable relationships between HE and baseline characteristics were assessed using Student's t test or the Mann-Whitney U test for continuous variables, and the Chi-square test of Fisher's exact test for categorical variables, as appropriate. To adjust for potential confounders, multivariable binary logistic regression analysis was performed using a forward stepwise selection procedure with HE as the dependent variable. Covariates candidates into the multivariable regression model were significant at a threshold of P<0.20 in the univariable analysis. Covariates were assessed for collinearity and interaction effects. The same procedure was repeated with poor functional outcome (mRS 4-6) as the dependent variable. Last, sensitivity, specificity, and positive and negative predictive values of the MRI spot sign to predict outcomes were calculated. All tests were 2-tailed, and the threshold for statistical significance was set to P<0.05. Statistical analyses were conducted using SPSS 28.0 (IBM, Armonk, NY) and R version 4.0.4 (R Project for Statistical Computing).

RESULTS

Study Population

During the study period, 194 patients with acute ICH were admitted in our institution. Among these, 147 fulfilled the inclusion criteria (see Figure 2 for exclusion reasons). The comparison of included vs excluded patients is provided in Table 1. Excluded patients had higher rates of atrial fibrillation and pre-stroke anticoagulant therapy, and higher baseline NIHSS than included patients, but the two populations were otherwise similar. Included patients had a median age of 66 (IQR, 53-80) years, 50 (34%) were female, median NIHSS was 13 (IQR, 8-19) and 33 (22%) patients had unwitnessed symptoms onset. Median last seen well-to-imaging time was 136 min (IQR, 100-289), symptoms discovery-to-imaging time was 111 min (IQR, 85-169), 99 (67%) patients had a deep ICH, and median ICH volume was 14 mL (IQR, 6-30). A spot sign was present in 91 (62%) patients. The number of spots was 0, 1 and ³2 in 56/147 (38%), 31/147 (21%) and 60/147 (41%), respectively. The comparison of the spot-sign positive and negative patients is provided in Table 2. Patients with vs those without the spot sign were older, more frequently had antiplatelet therapy, had more severe NIHSS, earlier last seen well-to-imaging delay, and larger ICH volume. There was an excellent interobserver concordance for the presence of the spot sign (Kappa, 0.96 [95% CI, 0.88-1.00]) and the number of spots (overall weighted Kappa, 0.95 [95% CI, 0.90-1.00]).

Primary Outcome Analysis (HE)

One-hundred and thirty patients were included in the primary outcome analysis (see Figure 2 for exclusion reasons). Follow-up imaging was CT in 128 patients and MRI in 2 patients; median baseline-to-24 h follow-up imaging time was 23 h (IQR, 18-28). Fifty-nine patients had a noncontrast CT acquired immediately following the acute MRI (median MRI-to-CT delay, 25 min [IQR, [START_REF] Sandset | European Stroke Organisation (ESO) guidelines on blood pressure management in acute ischaemic stroke and intracerebral haemorrhage[END_REF][START_REF] Von Elm | The Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) Statement: guidelines for reporting observational studies[END_REF][START_REF] Hallevi | The spot sign in intracerebral hemorrhage: the importance of looking for contrast extravasation[END_REF][START_REF] Murai | Spot Sign in Acute Intracerebral Hemorrhage in Dynamic T1-Weighted Magnetic Resonance Imaging[END_REF][START_REF] Delgado Almandoz | The spot sign score in primary intracerebral hemorrhage identifies patients at highest risk of in-hospital mortality and poor outcome among survivors[END_REF][21][22][23][24][25][26][27][28][29][30][31][32]), with a median absolute ICH volume difference between CT and MRI of -1 mL (IQR, -2, 0) and a median relative ICH volume difference of -11% (IQR, -22, -3). Consequently, in an attempt to compensate for such ICH volume difference between MRI and CT, a -11% correction of baseline MRI ICH volume was applied for the HE evaluation in patients with a 24 h follow-up CT, with 31 (24%) patients experiencing HE. Poor 3-month functional outcome was significantly more frequent in patients with, as compared to those without, HE (22/31 [71%] vs 39/97 [41%], respectively; P=0.003).

Univariable Analysis

The univariable comparison of patients with vs without HE is presented in Table 3. HE was associated with older age, higher NIHSS, larger ICH volume and presence of the spot sign. HE occurred in 3/50 (6%), 6/29 (21%) and 22/51 (43%) patients with 0, 1 and ³2 spots, respectively (P<0.001; Figure 3A). HE volume increased with the number of spots, being 0 mL (IQR, -1, 1), 1 mL (IQR, 0, 3) and 3 mL (IQR, 1, 9) in patients with 0, 1 and ³2 spots, respectively (P<0.001). The sensitivity, specificity, and negative and positive predictive values of the spot sign for HE were 0.90 (95% CI, 0.74-0.98), 0.47 (0.37-0.58), 0.94 (0.83-0.99) and 0.35 (0.25-0.46), respectively. The mean systolic and diastolic blood pressure over the first 24hrs was similar in both groups (

Secondary Outcome Analysis (3-Month mRS 4-6)

One-hundred and forty-five patients were included in the secondary outcome analysis (see Figure 2 for exclusion reasons), of whom 67 (46%) had a poor 3-month functional outcome.

Univariable Analysis

The univariable comparison of patients with vs without poor functional outcome is presented in Table 5. Patients with poor functional outcome were older, had more vascular risk factors and previous stroke, higher NIHSS, higher fibrinogen level, larger ICH volume, more frequently intraventricular hemorrhage, microbleeds and a spot sign. The 3-month mRS range according to the number of spots is presented in Figure 3B. Poor functional outcome occurred in 15/55 (27%), 10/31 (32%), and 42/59 (71%) patients with 0, 1 and ³2 spots, respectively (P<0.001). The sensitivity, specificity, and negative and positive predictive values of the spot sign for poor functional outcome were 0.78 (95% CI, 0.66-0.87), 0.51 (0.40-0.63), 0.73 (0.59-0.84) and 0.58 (0.47-0.68), respectively.

Multivariable Analysis

After stepwise variable selection into the multivariable model using the variables with P<0.20 from Table 5, there was a nonsignificant trend towards an independent association between the presence of the spot sign and poor 3-month functional outcome (Table 4, Model 1). However, there was a significant independent association between number of spots and 3-month poor functional outcome, after adjustment on age, NIHSS, diabetes, microbleeds and previous stroke (Table 4, Model 2). The results were very similar if ICH volume was forced into the multivariable models (data not shown).

DISCUSSION

This novel study aimed to evaluate the association between the MRI spot sign evaluated on a contrastenhanced T1-weighted sequence and HE and poor functional outcome in patients with acute ICH. Three salient findings emerged. First, the MRI spot sign was associated with an increased risk of HE.

Second, the risk of HE increased with higher spot-sign burden. Third, the presence of 2 or more MRI spots was independently associated with poor 3-month functional outcome.

The prevalence of the spot sign was 62% in our cohort, which is higher than in the two previous single-center studies reporting spot sign on contrast-enhanced T1-weigthed MRI in patients with acute ICH (33% and 49%, respectively) [START_REF] Schindlbeck | Spot Sign in Acute Intracerebral Hemorrhage in Dynamic T1-Weighted Magnetic Resonance Imaging[END_REF][START_REF] Murai | Magnetic resonance imaging-documented extravasation as an indicator of acute hypertensive intracerebral hemorrhage[END_REF]. The discrepancy may be explained by the use of a very low-field magnet in the former study (0.2T vs 3T) [START_REF] Murai | Magnetic resonance imaging-documented extravasation as an indicator of acute hypertensive intracerebral hemorrhage[END_REF], the different types of contrast-enhanced T1weighted sequence used [START_REF] Schindlbeck | Spot Sign in Acute Intracerebral Hemorrhage in Dynamic T1-Weighted Magnetic Resonance Imaging[END_REF][START_REF] Murai | Magnetic resonance imaging-documented extravasation as an indicator of acute hypertensive intracerebral hemorrhage[END_REF], and by an unusual low baseline NIHSS (median 6.5 vs 13 in our dataset) and long symptoms onset-to-imaging delay (7.2 h vs 2.3 h) for an acute ICH population in the latter study [START_REF] Schindlbeck | Spot Sign in Acute Intracerebral Hemorrhage in Dynamic T1-Weighted Magnetic Resonance Imaging[END_REF]. The prevalence of the CTA spot sign is also highly variable across studies (range, 18-60%) [START_REF] Demchuk | Prediction of haematoma growth and outcome in patients with intracerebral haemorrhage using the CT-angiography spot sign (PREDICT): a prospective observational study[END_REF][START_REF] Salman | Absolute risk and predictors of the growth of acute spontaneous intracerebral haemorrhage: a systematic review and meta-analysis of individual patient data[END_REF][START_REF] Romero | Prospective validation of the computed tomographic angiography spot sign score for intracerebral hemorrhage[END_REF][START_REF] Ciura | Spot sign on 90-second delayed computed tomography angiography improves sensitivity for hematoma expansion and mortality: prospective study[END_REF][START_REF] Brouwers | Clinical applications of the computed tomography angiography spot sign in acute intracerebral hemorrhage: a review[END_REF][START_REF] Rodriguez-Luna | Venous phase of computed tomography angiography increases spot sign detection, but intracerebral hemorrhage expansion is greater in spot signs detected in arterial phase[END_REF], the most influential factors likely being variability in the time from ICH onset to imaging and variations in time to CTA acquisition after administration of the contrast bolus [START_REF] Ciura | Spot sign on 90-second delayed computed tomography angiography improves sensitivity for hematoma expansion and mortality: prospective study[END_REF][START_REF] Rodriguez-Luna | Venous phase of computed tomography angiography increases spot sign detection, but intracerebral hemorrhage expansion is greater in spot signs detected in arterial phase[END_REF][START_REF] Hallevi | The spot sign in intracerebral hemorrhage: the importance of looking for contrast extravasation[END_REF]. Indeed, several CT studies have shown that the spot sign is a dynamic entity, being more prevalent in CTA acquired at the venous or parenchymal phase than in CTA acquired at the arterial phase [START_REF] Ciura | Spot sign on 90-second delayed computed tomography angiography improves sensitivity for hematoma expansion and mortality: prospective study[END_REF][START_REF] Rodriguez-Luna | Venous phase of computed tomography angiography increases spot sign detection, but intracerebral hemorrhage expansion is greater in spot signs detected in arterial phase[END_REF][START_REF] Hallevi | The spot sign in intracerebral hemorrhage: the importance of looking for contrast extravasation[END_REF].

To our knowledge, ours is the first study to show an independent association between the MRI spot sign and HE or poor functional outcome. In the 1990s, Murai et al. described a similar association using a 0.2T MRI, yet their results were not adjusted on important confounders such as baseline clinical severity or ICH volume, limiting their conclusions [START_REF] Murai | Magnetic resonance imaging-documented extravasation as an indicator of acute hypertensive intracerebral hemorrhage[END_REF]. More recently, another group did not find an association between MRI spot sign and HE, likely due to the small sample size (n=25) [START_REF] Schindlbeck | Spot Sign in Acute Intracerebral Hemorrhage in Dynamic T1-Weighted Magnetic Resonance Imaging[END_REF].

They reported, however, a significant association with discharge mRS, but did not adjust the results on potential confounders [START_REF] Schindlbeck | Spot Sign in Acute Intracerebral Hemorrhage in Dynamic T1-Weighted Magnetic Resonance Imaging[END_REF]. In our study, the negative predictive value of the spot sign was high for both HE and poor functional outcome (0.94 and 0.73, respectively), but the positive predictive value was low (0.35 and 0.58), indicating that a good radiological or clinical evolution is highly likely in the absence of the MRI spot sign, yet its presence does not reliably predict either HE or poor clinical outcome. The high sensitivity and low specificity are consistent with CTA studies evaluating spot sign at the venous phase or later, as done here on contrast-enhanced T1-weighted MRI. Using earlier, such as arterial, CTA acquisitions significantly reduces the sensitivity of spot sign but increases its specificity for HE prediction [START_REF] Ciura | Spot sign on 90-second delayed computed tomography angiography improves sensitivity for hematoma expansion and mortality: prospective study[END_REF][START_REF] Rodriguez-Luna | Venous phase of computed tomography angiography increases spot sign detection, but intracerebral hemorrhage expansion is greater in spot signs detected in arterial phase[END_REF]. The precise pathophysiological processes underlying the MRI spot sign are uncertain and may differ for CT, since MRI and CT are based on entirely different physical principles. In contrast to CTA, there is no contrast enhancement within the vessel in contrastenhanced T1-weighted MRI. Hence, contrast-enhanced T1-weighted can only detect leakage of the contrast agent from an injured vessel, which may correspond to leakage from an active arterial source but also venous stasis or blood-brain barrier disruption [START_REF] Schindlbeck | Spot Sign in Acute Intracerebral Hemorrhage in Dynamic T1-Weighted Magnetic Resonance Imaging[END_REF][START_REF] Murai | Spot Sign in Acute Intracerebral Hemorrhage in Dynamic T1-Weighted Magnetic Resonance Imaging[END_REF]. As far as we know, the MRI spot sign has never been studied on post-contrast arterial magnetic resonance angiography, and a direct comparison between CTA and MRI spot sign is lacking. Further studies are warranted. Importantly, we found that the risk of HE was higher in the case of high spot sign burden (³2 spots), in line with previous studies using the CTA spot sign [START_REF] Romero | Prospective validation of the computed tomographic angiography spot sign score for intracerebral hemorrhage[END_REF][START_REF] Delgado Almandoz | The spot sign score in primary intracerebral hemorrhage identifies patients at highest risk of in-hospital mortality and poor outcome among survivors[END_REF]. This dose-dependent relationship may help further to stratify the patients at risk of poor radiological evolution. However, while the presence of 2 or more spots was associated with a poor 3-month functional outcome, there was little difference in the prevalence of a poor functional outcome when comparing 0 and 1 spot sign, which might be due to a lack of power.

Our findings expand the clinical interest of the spot sign to the MRI population, which is the first-line imaging tool in some countries for suspicion of acute stroke. Stratifying the patients at high risk of HE or poor functional outcome using clinical and radiological biomarkers has several potential interests.

First, it may help the triage and intensity of monitoring of ICH patients in centers where MRI is used as first-line imaging tool. For example, the clinical management of acute ICH patients without the spot sign (i.e., at very low risk of further deterioration) may reasonably be done in a primary stroke center without an on-site neurosurgery, whereas the monitoring of patients with ³2 spots may be more reasonable in comprehensive centers. Second, it is valuable to inform relatives about the patient's prognosis on admission. Last, it may have potential implications for clinical trial design in patients with acute ICH because the success of future treatments aiming to prevent HE may depend on accurate determination of patients at risk of HE (e.g., selecting the patients with ³2 MRI spots). For these reasons, adding a contrast-enhanced T1-weighted sequence to the standard MRI protocol may be of interest for centers using MRI to measure acute neurological deficits. The guidelines from the American Heart Association/American Stroke Association state that CTA may be reasonable to identify patients at risk for subsequent HE, and some centers with MRI as first-line imaging tool routinely perform a CT/CTA immediately following the acute MRI. As our results suggest that the MRI spot sign has a good predictive value for HE, repeating CT/CTA following an acute post-contrast MRI may not be warranted.

Our study has several limitations. First, roughly one-quarter of the patients with acute ICH admitted during the study were excluded, mainly because of the lack of a contrast-enhanced T1-weigthed sequence. The contrast-enhanced T1-weigthed sequence was mostly withheld to shorten the MRI duration in patients with severe condition or agitation, which induced a selection bias towards less severely affected patients. Second, our study is single center and had a moderate sample size. An external validation in a larger dataset is warranted. Third, we compared ICH volume on baseline MRI baseline with follow-up CT in most patients. However, the CT-to-MRI absolute difference in ICH volume measurement evaluated in the subset of patients with both CT and MRI on admission was small (median of -1 mL), and was taken into account in our analyses of HE. Last, the precise ICH topography (i.e., according to brain regions eloquence) was not retrieved in our dataset, while it may be a relevant predictor of 3-month functional outcome. However, eloquent brain region evaluation is not always straightforward on visual imaging analysis. Moreover, the involvement of eloquent brain region is well captured by the admission NIHSS, which was included in our multivariable models. The reasons for exclusion are presented in Figure 2.

a: first blood pressure measured after arrival, in the imaging room. a: first blood pressure measured after arrival, in the imaging room.

Figure 1 .

 1 Figure 1. Illustrative Cases With and Without the MRI Spot Sign

Figure 2 .

 2 Figure 2. Study Flow Chart

Figure 3 .

 3 Figure 3. Hematoma Expansion and 3-month Modified Rankin Scale Score and According to Number of Spots

Table 3 ,

 3 systolic blood pressure: median= 138 mmHg [IQR, 132-144] vs 140 mmHg [IQR, 136-145] in the no HE vs HE groups, respectively, P=0.34; diastolic blood pressure: median= 78 mmHg [IQR, 71-85] vs 80 mmHg [IQR, 65-85], respectively, P=0.88). the presence of the spot sign was independently associated with HE (OR, 6.15 [95% CI, 1.60-23.65]; P=0.008; adjusted on NIHSS and ICH volume;

	Multivariable Analysis
	After stepwise variable selection into the multivariable model using the variables with P<0.20 from

Table 4 ,

 4 Model 1). The odds of HE increased with the number of spots (

Table 4 ,

 4 Model 2).

Table 1 . Comparison of Included and Excluded Patients.

 1

	Included	Excluded	P value
	N=147	N=47

Table 2 . Comparison of Patients With and Without Spot Sign.

 2

	Spot sign negative	Spot sign positive	P value
	N=56	N=91

Categorical variables are expressed as numbers (%) and continuous variables as median (IQR) or mean (SD). ICH indicates intracerebral hemorrhage, NIHSS, National Institute of Health Stroke Scale.

editorial assistance and was funded by the authors.

CONCLUSION

The spot-sign evaluated on a contrast-enhanced T1-weighted MRI sequence is independently associated with HE, and the presence of ³2 spots is an independent predictor of poor 3-month outcome. Given its high negative but low positive predictive values, the MRI spot sign may be of particular clinical interest when absent, being predictive of a good evolution. † Hematoma expansion was defined as an absolute ICH volume growth >6 mL or a relative growth >33% from initial MRI to 24 h follow-up imaging. In patients for whom HE was evaluated from baseline MRI to 24 h CT, a 11% correction of baseline MRI hematoma volume was applied, based on the median volume overestimation on susceptibility-weighted imaging observed in the subset of patients with both acute MRI and CT (see results). a: first blood pressure measured after arrival, in the imaging room.


Charles Graziani
email: charles.graziani@univ-amu.fr

Mathieu Nespoulous
email: mathieu.nespoulous@univ-amu.fr

Renaud Denoyel
email: renaud.denoyel@univ-amu.fr

Stephan Fauve
email: fauve@lps.ens.fr

Christian Chauveau
email: christian.chauveau@cnrs-orleans.fr

Luc Deike
email: ldeike@princeton.edu

Mickaël Antoni
email: m.antoni@univ-amu.fr

A new experimental set-up for aerosol stability investigations in microgravity conditions

Keywords: droplets, microgravity, microscopy, tomography, Von Karman swirling flow

des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

An aerosol is a multiphase system in which at least one of the phases is in a dispersed form. These phases can be liquid or solid and their simultaneous evolution within the surrounding gas involves mechanisms at the molecular and macroscopic scales. Like emulsions or nanofluids, aerosols are dispersions with large exchange surfaces which give them specific physico-chemical properties, making them central to fundamental and applied fields. Aerosols can be of natural or anthropogenic origin. Fog, dust, clays, forest exudates are natural examples while smog, fine particles produced by internal combustion engines or building heating devices are all anthropogenic.

Aerosols are complex media, sensitive to temperature and pressure conditions, sometimes chemically active.

They are naturally subject to the influence of gravity, which determines the settling speed of the particles they contain [START_REF] Gunn | The terminal velocity of fall for water droplets in stagnant air[END_REF] [START_REF] Best | Empirical formulae for the terminal velocity of water drops falling through the atmosphere[END_REF] . Aerosols are used in the pharmaceutical [START_REF] Davidson | Measurement of the Raman spectra and hygroscopicity of four pharmaceutical aerosols as they travel from pressurised metered dose inhalers (pMDI) to a model lung[END_REF] and cosmetic industries (inhalers, sprays) [START_REF] Márquez-Sillero | Determination of parabens in cosmetic products using multi-walled carbon nanotubes as solid phase extraction sorbent and corona-charged aerosol detection system[END_REF] in metallurgy (spray drying) [START_REF] Du | Enhancing the Thermal and Upper Voltage Performance of Ni-Rich Cathode Material by a Homogeneous and Facile Coating Method: Spray-Drying Coating with Nano-Al2O3[END_REF] and in agriculture (pesticides) [START_REF] Caratelli | A paper-based electrochemical device for the detection of pesticides in aerosol phase inspired by nature: A flower-like origami biosensor for precision agriculture[END_REF] . From an academic point of view, they are still the subject of intense research activity. The description of their evolution is for example a major issue in climatology, especially for the understanding of cloud microphysics [START_REF] Lohmann | Total aerosol effect: radiative forcing or radiative flux perturbation?[END_REF][8] [START_REF] Remer | Global Aerosol Climatology from the MODIS Satellite Sensors[END_REF] . This problem motivated CNES to support the development of a new experimental facility for conducting experiments in parabolic flight conditions (Figure 1).

A relevant study of cloud microphysics requires well controlled conditions, especially in temperature and pressure. In clouds, droplets are made of water and constitute metastable systems produced by a rapid condensation (actually liquefaction) in a gaseous medium in which the relative humidity is sufficiently high.

Despite their small size, cloud droplets are subject to the acceleration of the earth's gravity field. Their limiting velocity in the atmosphere was measured more than 70 years ago with values typically between 1 cm/s and 100 cm/s for radii ranging between 1 and 100 µm [START_REF] Bagnold | The physics of Blown Sand and Dust and Desert Dunes[END_REF] . Other measurements carried out under well controlled experimental conditions (20°C, 1 atm, 50% relative humidity) show that droplets of 80 µm diameter exhibit a drainage velocity that reaches 150 cm/s [START_REF] Gunn | The terminal velocity of fall for water droplets in stagnant air[END_REF] . Empirical laws have been proposed and give values in good coherence with these observations and with Stokes law when the diameter of the drops becomes lower than 50 µm [START_REF] Best | Empirical formulae for the terminal velocity of water drops falling through the atmosphere[END_REF] . Measurements have confirmed these settling velocities and demonstrated the key contribution of gravity in the evolution of aerosols.

In the context of cloud microphysics, this raises the question of scale separation. The characteristic times associated with the interaction between water droplets, such as coalescence, can be very short when compared to those associated with gravity. When droplets radii reach few micrometers, gravity becomes dominant and complicates studies of their mutual interactions. Performing experiments in microgravity is therefore essential for a detailed understanding of the mechanisms governing the evolution of aerosols. The reduced gravity of parabolic flights has the great advantage of decoupling buoyancy from capillary phenomena and, in the case of aerosols, of making possible an original description of the interaction mechanisms involved in a droplet population. In such conditions, it becomes indeed possible to measure the damping time of the flows created during the aerosol formation or the evolution of the droplet size distribution.

The aerosols investigated hereafter consist of populations of water droplets with radius of a few micrometres evolving in air. They are thus assimilable to fogs. From an experimental point of view, the air used is not inseminated with solid particles [12] . The one used in the experiments is not purified and therefore naturally contains particles favouring heterogeneous nucleation. This is the reason why the term 'aerosol' will be used hereafter to designate the populations of droplets under consideration. Investigations on such a 'canonical' family of aerosols is seen here as essential to improve the understanding of their physical properties. The main goal here is the production of aerosols sufficiently stable for the realization of reference experiments to bring new elements in the understanding of (i) the dynamics of evaporation/condensation of droplet populations, (ii) the coalescence of droplets in turbulent flows, (iii) the influence of relative humidity in nucleation phenomena. Three parabolic flight campaigns have been carried out, all aiming at validating the technical options implemented and at establishing the good feasibility of the experiments. So far, they demonstrated the possibility of producing populations of droplets by a protocol involving a fast compression of the air followed by a fast expansion (see sections 1 and 4) and validated aerosol observability by optical tomographic microscopy. The key point here was to ensure that the fast flows created by the compression/expansion sequence were damped over times smaller than the driving times of the physical phenomena governing the evolution of the aerosol when evolving in microgravity. The continuous phase being air, the appearance of long-lasting transient regimes was to be feared. Such behaviors would have been incompatible with an analysis by optical tomographic microscopy due to coherence loss between successive tomographic images. Preliminary estimates based on the numerical integration of a simplified model (a hard sphere with given initial velocity evolving in a continuous viscous phase) suggested damping times of the order of a few seconds. But this basic model was not accounting for important parameters such as the multiplicity of neighboring droplets, the finite geometry of the experimentation chamber (Figure 2), the pressure difference used for the expansion, etc. The parabolic flight experiments conducted in the frame of this work allowed to validate the dimensioning of the different elements of the equipment as well as the protocols to be implemented for the creation of the aerosols. They also evidenced typical damping times which turn out to be shorter than few seconds. Beyond these times, aerosols show up as droplet populations whose movements are only due to the residual acceleration of the flight conditions and not to the initial compression/expansion procedure. Sequences of about 15 seconds of reduced gravity were carried out during which the aerosols present a good stability thus allowing to track the evolution of several hundreds of droplets.

Experimental cell and aerosol production protocol

Aerosols are produced from moist air with a pneumatic circuit consisting of two chambers (Figure 2): a cylindrical experimentation chamber (EC, diameter 3 cm, height 6 cm) inside which droplets are generated and an expansion-compression chamber (ECC, volume of about 170 cm 3 ) acting alternatively as a gas reservoir or an expansion tank (Figure 2 and Figure 3). EC and ECC are initially at room temperature and pressure. They both contain air which may or may not be saturated with water. A membrane pump is used to circulate the air from the ECC into the EC. Then, by means of a system of valves, a fast expansion between the EC and the ECC is achieved. This expansion leads to a temperature drop inside the EC, which results in a decrease of the equilibrium saturation vapour pressure. An excess of water vapor is created that condensates into micron-sized droplets by nucleation [START_REF] Vilsanen | Homogeneous nucleation rates for water[END_REF][14] [START_REF] Miller | Homogeneous nucleation rate measurements for water over a wide range of temperature and nucleation rate[END_REF] . Each experiment follows the same protocol: (i) closure of all valves, (ii) opening of TEV, (iii) starting up the pump for pressure increase in the EC up to a target value, (iv) closure of TEV, (v) few seconds delay for temperature relaxation, (vi) opening of PEV for air expansion and droplet formation, (vii) scanning tomography with image recording. The interest of this protocol is to maintain temperature and pressure in the EC before and after droplet formation almost constant and thus to avoid the implementation of an accurate thermal control of the whole set up which would be, from an instrumental point of view, highly challenging. This protocol takes only a few seconds and has the great advantage of limiting heat transfers especially with the walls of both EC and ECC.

A temperature measurement device composed of a type K thermocouple driven by an Agilent 34972A LXI is implemented inside the EC. The pressure inside the EC and the ECC are recorded with two pressure sensors (KELLER PAA-33X). A hygrometric sensor is mounted inside the EC (Sensirion SEK-SHT35). It also gives the temperature which allows a redundancy of the measurements. All the above sensors are placed at selected locations to provide measurements as accurate as possible without being too intrusive.

Optical tomography microscopy and imaging technique

Optical tomography microscopy in transmission mode is used to observe the droplets once formed and to track their evolution. This technique consists of acquiring images with a microscope with moving object plane. It has the advantage of enabling analyses in the visible spectrum and, in the particular case of optically transparent droplet populations as the ones investigated here, to allow in-depth analyses. The acquisition of tomographic scanning sequences allows a 3D reconstruction and thus a precise follow-up of the evolution of droplets over time. This technique is well known and has been for long used in medical fields [START_REF] Arridge | Optical Tomography in Medical Imaging[END_REF][17] [START_REF] Kamilov | Learning Approach to Optical Tomography[END_REF] and, more recently, to describe emulsions [19][20] . An Optronis CP70-1HS-MC-1900 grabbing camera is used and mounted on a motorized translation stage (Figure 4). It allows a frequency acquisition up to 2000 fps in full frame that ensures reasonable coherence between all successive images. The microscope traveling distance is 2 mm with travel speed up to 10 mm/s. A tomographic shot (i.e. a single scan forwards or backwards) therefore lasts 0.2 s and generates 400 images of about 1 Mb each. The distance between two successive images is typically 5 µm.

Tomography is applied perpendicularly, at ±1 mm on both sides of the symmetry axis of the EC and almost at its center. The overall analyzed area corresponds to a volume of about 2 mm 3 . Images are encoded on 8 bits and correspond to a field of view of 1280 µm by 860 µm (Figure 5). They all present a sufficient contrast (on at least 128 gray levels) for reliable post-processing. Vignetting may remain despite the optical quality and the collimation of the incident light beam. It is eliminated by subtracting a reference image acquired before the aerosol formation. The identification of the droplets and their location in the images is performed by thresholding predetermined gray levels (about ten). An image of the contours generated by each droplet is then obtained (Figure 6). The spherical geometry of the droplets and the fact that they are distant from each other produces typical circular contours, which is very helpful for the accuracy and CPU time effort of image post-processing. The principle of the imaging technique is to first detect the location of the center of each of the contours (Figure 6(b) and (d)), regardless of whether the corresponding droplet is focused or not, and then to construct, from the set of the centers of all the droplets and all the images of a given scanning shot, the individual optical axis of each droplet (Figure 7). The quality of the optics and image contrasts allows the detection of droplets with a radius down to 2 µm. Optical axis are processed one by one in a way to determine the droplet location in the scanning direction. This process is iterated for all the droplets (Figure 8). Optical axis construction requires coherence between consecutive images and thus an aerosol evolving on slow characteristic times when compared to those of the image acquisition system. This constraint is not satisfied right after droplet formation because of the fast flows generated by the initial expansion. As these flows decay after a few seconds (see below), sequences of several tens of thousands of coherent images are produced and therefore usable for post processing. Figure 7 represents all the optical axes for a single tomographic sequence. It shows hundreds of optical axes and illustrates the accessible precision of the analyses. This imaging technique is further used to obtain 3D representations (Figure 11). The identification of the optical response produced by each droplet leads to optical axes of length depending upon their diameter. The larger the radius of droplets, the longer their optical axis. Figure 8 illustrates this fact in a representation showing the radius obtained from the optical response of each droplet. When the working plane of the microscope is in the vicinity of a droplet location, the measured apparent radius reaches a minimum value. This property is used in the last phase of the image analysis procedure to estimate the radii of the droplets. The apparent radius of a given droplet is obtained from the value at half-height of the radial average of the gray levels of its optical response. This radius is larger when droplet is out of focus and minimal when it is in focus (Figure 8(b)). Its actual radius is finally computed from the minimum of a quadratic fitting of all the apparent radii when running along its optical axis. At this minimum contrast is the sharpest. Beside droplet radius, this procedure also determines the location of the droplet along its optical axis. This technique may however be too rough regarding to the comparatively large fluctuations shown Figure 8

Set up for Von Karman swirling flows and droplet coalescence investigations

The EC is equipped with two coaxial counter-rotating rotors consisting of smooth flat disks of 29 mm diameter facing each other. They're used to generate a Von Karman swirling flow within the EC [START_REF] Zandbergen | Von Karman Swirling Flows[END_REF][22] [START_REF] Labbé | Study of the von Karman flow between coaxial corotating disks[END_REF] . When activating the rotors, the air close to the disks is driven radially outward by the centrifugal force. This generates an axial flow toward the disks along their axis and a recirculation along the opposite direction along the EC lateral boundary. Therefore, a radially inward flow is generated in the midplane between the disks such that there is a stagnation point at the center of the EC. It is expected that the droplets, being subject there to the azimuthal velocity shear, are entrained by the air flow and accumulate in the vicinity of this stagnation point. The main interest of setting the droplets in motion in such a flow is to force them closer to each other. Such experiments are actually expected to help to understand coalescence of droplets within clouds [START_REF] Dufour | Thermodynamics of Clouds[END_REF] as they will give access to the evolution of the density (actually the liquid fraction) and the mean size of the droplets in the vicinity of the stagnation point. Investigations of the influence of the properties of the Von Karman swirling flows (VKSF), that can involve more or less turbulent regimes depending on the Reynolds number, are also planned.

The motion of the rotors is achieved with two FAULHABER servomotors (4221G 024EXTH) equipped with an encoder (IE3-2024) each controlled by a motion controller (MC5005 S RS) allowing regulation of the rotation frequency (motors are shown in Figure 2 but not in Figure 3). Rotors are both attached to a relay magnetically driven by a motion transmitter connected to the motors allowing rotation frequencies in the range 1 to 75 Hz. The sealing is obtained by a flange which separates the relay from the EC and thus from the outside. Magnetic neodymium segmental arc magnets provide the magnetic coupling. The main advantage of this configuration is that it allows for optimal sealing since no direct drive shaft has to be used.

Ground experiments

Ground experiments are essentially aimed at qualifying and understanding the response of experimental devices and to ensure control of experimental conditions in terms of pressure, temperature and relative humidity.

For all experiments presented in this section, initial pressure and temperature are ambient ones while relative humidity (H) ranges between 2% and 96%. The aerosol formation protocol (section 1) and the optical tomography analysis are synchronized, which allows for a fast triggering of all devices (actually with a single 'click'). This is necessary because of the fast gravity drainage of the droplets and, for parabolic flights, the limited duration of the microgravity periods.

Figure 9 displays the temperature and pressure evolution when producing an aerosol with a target pressure of 2000 mbar. Reference time (t=0) is set at the opening of PEV (step (vi)). As expected, two jumps in both temperature and pressure obviously associated to step (iii) and (vi) show up. Temperature relaxes back to its initial value sufficiently fast for both steps which confirms that air, within the EC, keeps close to its initial value despite droplet formation. In other words, forming droplets is not expected to yield significant heat transfers with the boundary conditions (in particular the walls of EC). It is therefore reasonable to assume that aerosol production can be achieved, at first approximation, under quasi-isothermal conditions. The typical temperature relaxation time, t relax , as a function of relative humidity H is plotted in Figure 10 for 108 experiments and four target pressures (2000, 2100, 2200 and 2300 mbar). t relax is defined as the time required for the system to recover 95% of its initial temperature after step (vi). This figure shows that the higher the relative humidity, the higher the relaxation time. This is a straightforward consequence of the larger thermal heat capacity of water loaded air. The increasing trend of t relax as a function of H is therefore consistent with thermodynamics and a straightforward consequence of the fact that equilibration time of a system increases with its thermal capacity. The set of experiments of Figure 10 also provides fundamental information about droplet formation. Image analysis (not shown here) indicates indeed that within the pressure ranges investigated here, no droplets are formed when H < 40%. It also appears that the overall number of droplets (resp. the typical radius of the droplets) increases (resp. reduces) when increasing the target pressure. This turns also consistent with thermodynamics but, droplets getting drained very fast due to buoyancy forces, reliable quantitative investigations are not possible.

Microgravity preliminary results

All the results presented in this section were obtained from three CNES parabolic flight campaigns carried out in October 2020, March 2021 and March 2022 in Bordeaux on board the Airbus A310 0G managed by Novespace. Aerosol formation follows the same protocol as for ground experiments. The pressure in the EC is well controlled and fixed at 1 atm. It is thus slightly higher than that of the cabin. The conditions in temperature are determined by the temperature in the cabin and are thus not controlled. It tends to increase as the experiments are carried out but remains within the range of 18 and 28°C, over the duration of each flight. This temperature drift obviously results from the activity of the people and the numerous experiments running onboard. Given the precision of the measurements and the way experiments are planned for each parabola sequence, this problem is not expected to significantly affect the accuracy of the experimental results.

The first fundamental question that experiments had to answer was the possibility of producing aerosols and to verify that they were stable enough to maintain coherence between the tomographic images for relevant image post-processing. All experiments have clearly demonstrated the possibility of producing droplets. The continuous phase being air, fast turbulent transient regimes occur right after expansion (step (vi)). This makes tomographic microscopy analysis potentially ineffective, as image coherence is essential. Fortunately, it turns out that the rapid flows created by the initial expansion are damped out within typically 4 to 7 seconds. Beyond this time, droplets exhibit small amplitude movements only due to the residual acceleration of the parabolic flight conditions. Periods of about 15 seconds of reduced gravity are thus accessible during which their evolution can be studied under scientifically relevant conditions. Figure 11 shows an example of a 3D reconstruction of a single tomographic shot and illustrates the achieved level of precision. Tomographic shots are processed one by one to identify each individual droplet. This process is iterated to investigate the properties of the droplet populations generated for each experiment. Figure 12 shows the evolution their number (N) as a function of time for four replica experiments and two values of relative humidity: water saturated (H=100%) and sub-saturated (H=70%). The expansion phase starts at t = 0 and is operated at the very beginning of the parabola microgravity phase. The gray shaded area indicates the time period within which image reconstruction quality is poor due to the lack of coherence between successive images. Here, it lasts for typically 7 s. Beyond this time, water-saturated experiments show droplets persisting for the remaining duration of the parabolas while they have all disappeared after typically 11 s when H=70% [25][26] .

These results clearly demonstrate the possibility to measure the number of formed droplets in microgravity conditions. This is an essential condition for assessing the feasibility of aerosol investigations over longer periods of time. They further indicate that evaporation of droplets in the sub-saturated regime is a very fast phenomenon as it takes place in less than 0.1 s. Such investigations could for sure complete the very rich literature focused on the evaporation of sessile droplets in microgravity [START_REF] Brutin | Sessile Drop in Microgravity: Creation, Contact Angle and Interface[END_REF][28] [29] . Furthermore, fundamental questions such as how quickly droplet populations evaporate as relative humidity conditions change or the precise number of droplets (e.g. liquid fraction) remaining after expansion become quantitatively accessible. But Figure 12 raises practical questions that have yet to be resolved. The experiments show indeed poor reproducibility. Broader ensemble of data have to be collected to reduce the statistical variability of probing a small volume. Another explanation could be associated to unexpected differences, up to couple of seconds, in the duration of step (v) of aerosol formation. This problem results from the lack of precise synchronization in the operation of the pump (for pressure increase in the CE) and the valves opening/closing (for aerosol formation).

The consequences of this uncontrolled delay are most likely differences in the pressure drop during aerosol formation and thus modified heat exchanges from one experiment to another. This instrumental problem has been solved and forthcoming parabolic flight experiments are expected to show a significantly improved reproducibility. Parabolic flights have demonstrated that it is possible to generate stable aerosols only subject to the residual acceleration of flight conditions (Figure 11). Another fundamental point to check here is the ability of the rotor device presented in section 3 to generate a VKSF within the EC and to set the overall aerosol in motion. The rotors have a smooth surface (not equipped with blades) and the question is whether it is possible or not to make droplets move and, if set in motion, to determine the time required for the establishment of a VKSF. Tuning parameter is the rotation frequency (noted f hereafter) imposed on the rotors. In practice, it is not possible to have a global view of this flow as only a very limited volume in the vicinity of the center of the EC is analyzed (section 2). But, in view of its structure, this volume is expected to contain the stagnation point in the vicinity of which the droplets are expected to gather. This phenomenon is unfortunately not observable in the experiments because the periods of microgravity are too short and the positioning of the rotors most likely lack of precision.

Still, the droplets, once formed, can be used as tracers. The analyzed area being in the center of the EC, it is reasonable to consider that the VKSF is established in the whole EC once they're set in motion (actually transported by the VKSF).

The experiments are focused on the measurement of the drag time (t dr ) corresponding to the time lag between rotor activation and droplet transport by the VKSF. The protocol is similar to that of section 1 with the following additional step: after 7 seconds of an ongoing parabola, as the droplets are at rest, the rotors are activated. The drag time t dr is evaluated by a direct observation of the movement of the droplets. Its value is determined when they appear to exhibit a coherent motion over a distance larger than 100 µm. Such movements are easy to identify by eye and cannot be the consequence of residual acceleration. Results are plotted in Figure 13 and indicate that 1/(f×t dr ) increases linearly with f in the investigated frequency range (6 ≤ f ≤ 15 Hz). It extrapolates down to zero for f ≈ 1.5 Hz which is consistent with the fact that no motion is observed for the duration of the parabolas when f < 4 Hz. This corresponds to a Reynolds number of the order 400 (Re = 2fR 2 /where ≈ 1,5 10 -5 m 2 .s -1 , is the kinematic viscosity of air and R ≈ 1.45 cm de rotor radius) for which it is known that the first instabilities of the VKSF take place. For smaller values the flow is laminar and the motion develops on a diffusive time scale (2R) 2 /60 safter the rotors are set into motion. This turns longer than the duration of the parabolas. No flow is therefore observable. The red squares are the average value for each replica series.

Conclusions

The developments carried out within this work for the study of the populations of water droplets in microgravity required the implementation of specific instrumental techniques. Parabolic flight experiments were performed to verify their relevance. For the formation of the droplets, a protocol consisting of compressing and then expanding the air in a cylindrical experimentation cell was implemented. This protocol is short enough to assume droplet production to be a quasi-isothermal process. Preliminary parabolic flight experiments have established the possibility to quantitatively describe droplet populations under microgravity conditions. The flows generated by the initial expansion fade away in few seconds. Beyond this time, as residual acceleration is small, the droplets are almost at rest. Optical microscopy tomography in transmission mode is used to reconstruct the droplet populations in 3D and to follow their evolution in time over more than 15 seconds of microgravity. As the thermodynamic conditions are controlled, the evaporation dynamics of large droplets can be investigated. This work also shows how droplets can be set in motion by creating a Von Karman flow. The instrumental developments will be continued in the next future. Important improvements remain to be undertaken for the control of the relative humidity and the thermal regulation of all the elements containing the air used for the droplet formation.

Figure 1 :

 1 Figure 1 : Overall view of the rack used for the parabolic flight experiments.

Figure 2 :

 2 Figure 2 : Schematic view of the experimental set-up.

Figure 3 :

 3 Figure 3 : Photograph of the EC assembly. LED (foreground), EC (foreground), microscope (background). In this picture the motors used for Von Karman swirling flow generation are not mounted.

Figure 4 :

 4 Figure 4 : View of the microscope and optical tomography system.

Figure 5 :

 5 Figure 5 : Typical image of a population of droplets. Droplet show up as black spots.

Figure 6 :

 6 Figure 6 : Image after background subtraction (a) and after treatment (b). The black and red arrows indicate two droplets and their signature after treatment. (c) and (d) correspond to a zoom on a droplet and both illustrate the gray levels produced by a droplet of about 10 µm in diameter. The black point is the center position obtained when averaging over all the contours.

Figure 7 :

 7 Figure 7 : Left : Schematic illustration of droplet tracking. The two horizontal lines correspond to the optical trajectory of two droplets indicated by the blue arrows. The decimation of 10 images is used. The actual distance (resp. time interval) between each image of this figure is thus about 50 µm (resp. 5 ms). The coordinates (x,y) are in the plane of the images and z indicates the position of the microscope working plane. Right: Treated tomographic sequence. The orientation of the optical axes along the z the direction is a signature of the aerosol stability and good image coherence.

  (b). Accurate measurements of droplet size distributions will therefore require further efforts and most likely the implementation of refined techniques such as FFT-based analysis.

Figure 8 :

 8 Figure 8 : (a) Zoom into the domain 400<x<800 (µm), 400<y<800,600<z<1600 (µm) of Figure 7. The apparent radii of the droplets are shown as circles. (b) Evolution of the apparent radius of the droplet in the red frame of (a) as a function of the position of the working plane.

Figure 9 :

 9 Figure 9: Evolution of pressure and temperature as a function of time inside the EC. Reference time t=0 is set at the opening of the PEV (step(vi)).

Figure 10 :

 10 Figure 10 : Time required for temperature stabilization after expansion as a function of H. The legend indicates the target pressure range in mbar at step (iii) (compression phase). For these experiments, temperature is 24°C ± 2°C.

Figure 11 :

 11 Figure 11 : 3D reconstruction of an aerosol. Droplets are black spots. Their radius is multiplied by a factor 10 for visibility. About 24 hours CPU are necessary for such a reconstruction on a basic work-station. This experiment was performed in the CNES 59 th parabolic flight campaign (parabola 57, September 2020) with and EC diameter is 4 cm and height 4 cm.

Figure 12 :

 12 Figure 12 : Evolution of the number of droplets as a function of time for target pressure 2000 mbar and temperature 22 °C. Top: H=100%. Bottom H=70%. The gray shaded area corresponds to the transient evolution phase. Vertical red dashed line indicates evaporation time. For parabolas P53 and P54 microgravity duration was shorter than expected. These experiments were performed in the CNES 59 th parabolic flight campaign (September 2020). For both figures, EC diameter is 4 cm and height 4 cm.

Figure 13 :

 13 Figure 13 : 1/(f×tdr) as a function of rotation frequency f. Five replica experiments are achieved for each experiment (small gray diamonds).

Acknowledgements

Authors gratefully thank CNES, in particular Christophe Delaroche and Laurent Arnaud, CNRS and Région Provence-Alpes-Côte d'Azur for important financial support, ESA -European Space Agency, within the project ESA/EVAPORATION (ESA Contract Number 4000129506/20/NL/PG) and Yvan CECERE from MADIREL for technical support.