Introduction : Injury to the white matter (WM) leads to severe functional loss in many neurological diseases such as stroke and multiple sclerosis. In-vivo neuroimaging techniques such as diffusion tensor imaging are becoming important tools for the diagnosis of WM abnormalities. However, these tools still present limitations and there is a crucial need for whole-brain ex-vivo complementary approaches to overcome them [1]. We propose a novel three dimensional (3D) imaging method to map the myelin fibers in the whole mouse brain with high throughput using in-line phase contrast X-ray tomography.Methods:Twelve C57Bl6 mice were included in the present study (stroke: N=3, focal demyelination: N=2, healthy&sham: N=7). The impact of fixation was examined in 5 of the healthy brains fixed either by paraformaldehyde (PFA) 4% or ethanol (range [25%-96%]). All subsequent brains were fixed with ethanol 96%. Imaging was performed on beamline ID19 at ESRF at 19keV as described in [2]. An indirect detection-based detector with a LuAg scintillator, standard microscope optics and a 2048x2048 pixel CCD camera was positioned 1-m from the sample to obtain phase contrast. The whole-brain data set was acquired at an isotropic pixel size of 7.5-μm. Acquisition time was 14 minutes per brain. Data were reconstructed as in [3].Results:In contrast to PFA fixation, ethanol >50% led to hyperintense WM fibers, allowing accurate 3D representation of main fiber tracts. Our imaging approach not only revealed the complexity of fiber orientations and intrication but also distinguished individual axons in the fiber bundle. Focal WM lesions were clearly depicted both at the macroscopic and microscopic levels.Conclusion:The proposed set-up permits brain-wide studies of fiber tracts and of their structural changes in diseases, using conventional animal models, minimal sample preparation, and with the potential to scan 30 brains in an 8-hours shift.Acknowledgments: This work was performed within the framework of the LABEX PRIMES (ANR-11-LABX-0063) of Université de Lyon, the ANR NanoBrain (ANR-15-CE18-0026-01) and was supported by the ESRF by allocation of beam time (project LS-2292).References :[1] Aswendt M and others. Mol Imaging Biol 2016. [2] Marinescu M and others. Mol Imaging Biol 2013;15(5):552-559. [3] Paganin D and others. Journal of microscopy 2002;206 (Pt 1):33-40.