S598 Abstracts graphical and temporal variations in the prevalence of mental disorders in suicide: systematic review and meta-analysis. J Af- fect Disord 190, 704–713. [2] First, M.B., Spitzer, R.L., Gibbon, M., Williams, J.B., 1995. Structured clinical interview for DSM-IV axis I disorders. New York State Psychiatric Institute, New York. [3] Dale, A.M., Fischl, B., Sereno, M.I., 1999. Cortical surface- based analysis. I. Segmentation and surface reconstruction. Neuroimage 9, 179–194. [4] Lee, Y.J., Kim, S., Gwak, A.R., Kim, S.J., Kang, S.G., Na, K.S., Son, Y.D., Park, J., 2016. Decreased regional gray matter vol- ume in suicide attempters compared to suicide non-attempters with major depressive disorders. Compr Psychiatry 67, 59–65. doi: 10.1016/j.euroneuro.2018.11.882 P.872 Study of corpus callosum cellular microstructure using diffusion microscopy MRI in subjects with autism spectrum disorders M.A. D’Albis 1,* , S. Sarrazin 1 , A. Lebois 2 , J.F. Mangin 2 , C. Laidi 1 , J. Boisgontier 2 , R. Delorme 3 , F. Bolognani 4 , S. Holiga 4 , J. Dukart 4 , C. Bouquet 4 , M. Ly-Le Moal 4 , A. Amestoy 5 , I. Scheid 1 , A. Gaman 1 , M. Leboyer 1 , C. Poupon 2 , J. Houenou 1 1 INSERM, Mondor University Hospital, Creteil, France 2 Neurospin, Atomic Energy Commission, Gif-Sur-Yvette, France 3 Institut Pasteur, Human Genetics and Cognitive functions Unit, Paris, France 4 Roche Innovation Center, Neuroscience- Ophtalmology and Rare Disease, Basel, Switzerland 5 Autism Expert Center, Charles Perrens Hospital, Bor- deaux, France Background: Autism Spectrum disorder (ASD) is a severe neurodevelopmental disorder affecting 1 to 1.5 % of the population worldwide. Alterations in brain white matter are central to its pathophysiology and symptoms [1]. Studies using diffusion MRI have thus consistently shown decreases of the fractional anisotropy (FA) in subjects with ASD, sup- posed to be associated with a decrease of the white matter integrity; this FA decrease has been most consistently observed in the corpus callosum (CC) [2]. However, frac- tional anisotropy is a non-specific index. Consequently, the cellular mechanisms underlying reductions of anisotropy in ASD remain unknown [3]. To investigate this, we used a new technique, microscopy MRI, which allows in vivo estimates of axon diameters and densities and fiber orientation dis- persion (ActiveAx model) [4]. To date, no such studies have been performed to compare white matter CC parameters between ASD subjects and controls in vivo. Methods: We included twenty-nine ASD subjects (age = 27.9 +/- 8.1) and twenty-one male controls (age = 32.2 +/- 10.8), all without intellectual disability. The mean age and total IQ were not statistically different. Data was collected with a 3T MRI (NeuroSpin, Saclay, France), with anatomical and specific multiple b multiple shell, diffusion weighted sequences (2 mm3, 30 directions, 4 shells b = 368 s/mm2, b = 580 s/mm2 b = 2400 s/mm2, b = 1795 s/mm2). First, the data analyses included detection and correction of artefacts, using Connectomist toolbox 2.0. Secondly, we used Freesurfer 6.0.0 to divide the CC into five parts (anterior, middle anterior, centrum, middle posterior and posterior). Finally, for each part, we extracted the mean generalized FA (gFA), the mean Orientation Dispersion Index (ODI), the mean Intra-Cellular Volume Fraction (ICVF, a proxy for axon density) and the mean axon diameter. Each step of the image analysis was visually checked for each of the subjects. An ANCOVA was performed with gFA, ICVF, ODI and axon diameter as a dependent variable, status as an independent variable and age in co-variable (uncorrected results for multiple tests). Results: Relative to controls gFA was significantly lower in subjects with ASD as concerns the anterior (p = 0.052) and posterior parts (p = 0.018) of the CC. We further found that ASD subjects displayed a statistically significant increase in ODI in these same parts (anterior p = 0.033, posterior p = 0.025). Regarding the ICVF, we found no differences in any part of CC between the two groups. As for the axon diameters, the analyses are in progress. Conclusion: Our study confirms the decrease of gFA in CC of subjects with ASD. This decrease is probably related to a de- crease of fiber coherence, as shown by the increase in ODI. This study is therefore the first to investigate CC in vivo at a cellular scale and to show the existence of cellular abnor- mality in CC of ASD subjects compared with controls. Our results support the usability of this innovative approach to provide better understanding of the pathophysiology in ASD. Disclosure statement: This work was financially supported in part by the Institut Roche, in part by the Investissements d’Avenir program managed by the ANR under reference ANR-11- IDEX-0004- 02. This project has also received funding from the European Union’s Horizon 2020 Frame- work Programme for Research and Innovation under Grant Agreement No 720270 (Human Brain Project SGA1)”. References [1] Geschwind, D.H., Levitt, P., 2007. Autism spectrum disorders: developmental disconnection syndromes. Curr Opin Neurobiol 17 (1), 103–111. [2] Travers, B.G., Adluru, N., Ennis, C., Tromp do, P.M., Des- tiche, D., Doran, S., Bigler, E.D., Lange, N., Lainhart, J.E., Alexander, A.L., 2012. Diffusion tensor imaging in autism spec- trum disorder: a review. Autism Res 5 (5), 289–313. [3] Le Bihan, D., Lima, M., 2015. Diffusion Magnetic Resonance Imaging: what water tells us about biological tissues. PloS Biol 13 (9). [4] Zhang, H., Hubbard, P.L., Parker, G.J., Alexander, D.C, 2011. Axon diameter mapping in the presence of orientation disper- sion with diffusion MRI. Neuroimage 56 (3), 1301–1315. doi: 10.1016/j.euroneuro.2018.11.883