Simultaneous brain and cervical spinal cord MP2RAGE for T1 measurement: robustness and sensitivity for tissue modification assessment in multiple sclerosis in a multicenter context

Abstract

International audienceIntroductionRecent optimisations of T1 quantification through magnetization‐prepared two rapidacquisition gradient echoes (MP2RAGE; Marques et al. 2010) allow to perform both brainand cervical spinal cord acquisitions simultaneously with good trade-off between acquisitiontime, robustness and accuracy (Rasoanandrianina et al. 2019; Forodighasemabadi et al.2021). This sequence is of particular interest to investigate tissue microstructuralmodifications in pathologies such as multiple sclerosis (MS) (Demortière et al. 2020;Mchinda et al, 2021). In order to spread out the use of the MP2RAGE sequence, weevaluated the reproducibility and variability in two different centres.MethodsThe data included in this work were collected in the context of the multicentric MSTRACTS(NCT04220814), OSV-IRM (NCT05107232) and T1-M3C-SEP (FLI-RE2) studies. Sixhealthy controls (HC; F/M: 4/2, mean age 38.9 years) were scanned 3 times each(separated sessions), in two different centres both equipped with 3T Siemens scanners(Prisma with 20 channels in centre 1, Vida with 64 channels in centre 2 ). Additionally 26 HC(centre 1/2: 20/6) were scanned one time (F/M: 19/7, mean age 39.2 years). The sameacquisition protocol was performed in both centres and included MP2RAGE and B1 mapacquisitions covering both brain and cervical spinal cord (cSC). The acquisition parameterswere previously described in (Rasoanandrianina et al. 2019; 4000ms TR, 243×300mm² FOV,176 slabs, 6/8 partial Fourier (PF) factor 0.9×0.9×1mm 3 voxel size, TI1/TI2 = 650/2000ms,ɑ1/ɑ2=4/5°, GRAPPA 2). After B1 correction (Massire et al. 2016), mean T1 values wereextracted in different regions including brain white matter (bWM), deep grey matter (dGM)and cortical grey matter (cGM; all computed using CAT12 [(Ashburner and Friston 2000)])and all cSC segments (computed using the SCT toolbox [(De Leener et al. 2017)]). Weevaluated the variability between centres and subjects using linear mixed-effects modelswith subject as random effect and centre as fixed effect. The coefficients of variation (CV)and the intraclass correlations (ICC) of between-session and between-participant variabilitieswere computed according to Combès et al. (2019). In order to interpret these results withrespect to potential application in MS pathology, we also reported exploratory analysesbased on the extraction of T1 values in the same regions for 5 MS patients (centre 1/2: 3/2,same acquisition protocol and image processing) without cSC lesions.ResultsFor the whole dataset collected in HC, the mean (and standard deviation) T1 values in thebrain were 1281.5 (28.8), 1176.5 (20) and 823.9 (21.1) ms for cGM,dGM, and bWM,respectively and were ranging from 921 (22.6) to 954 (30.5) ms over the 7 cSC segments(see Fig. 1)For the brain, we observed evidence of centre differences for the three regions (all p<.01).Nevertheless, the estimated differences between centres were low, ranging from 4.71 (bWM)to 25.31 (dGM) ms (ie. 0.57 to 1.98% of the mean). Between-participant CV were 2.1, 1.7and 1.8%, and between-session CV were 0.2, 2.2 and 0.5% for bWM cGM and dGM,respectively. Between-session ICC were .01, .61 and .06 for the same regions.For the SC, we observed evidence of centre differences for all vertebrae (all p<.05), exceptC4, C5 and C7 (p=.149, .163, .062, resp.). The estimated mean differences were also low,ranging from 9.6 (C5) to 20.2 (C1) ms (ie. 1.03 to 2.15%). To simplify the results, T1 valuesfrom C3 to C5 levels were averaged. In this region, between-participant andbetween-session CV were 1.5 and 1.6%, while between-session ICC was .53.MS patients showed a mean T1 value increase ranging from 19.5 (cGM) to 44.2 (dGM) msfor the brain, and from 14 (C7) to 122.7 (C3) ms for the cSC compared to the mean in allHC. Fig. 2 shows that, for each region, the majority of patients (coloured triangles) havehigher T1 values than the third quartile of HC.DiscussionOur results showed that, even if differences exist between the two centres, the variability islow, especially for bWM (0.57 %) and central cSC segments (1.03 %). Moreover, the T1variability is primarily explained by between-participant variability for the brain and by bothsession- and participant-variabilities for cSC. The differences between scanners were foundto be less important than the differences observed between HC and MS patients with nocSC lesions. Overall, our results highlight the multicenter robustness of simultaneous brainand cervical spinal cord acquisition and its potential for further applications in multicenter MSstudies to assess regional tissue impairment