The excellent soft tissue contrast of diffusion‐weighted magnetic resonance imaging, DW‐MRI has made it an invaluable technique especially in oncological assessment bringing hope in distinguishing between brain abscesses and necrotic and cystic neoplasms. However, a clear characterization of brain tumors and the associated pathologic structures is still a challenge. In this study, a tissue model is proposed to interpret the water diffusion behaviour in white matter based on the measured apparent diffusivities and their volume fractions. Measurements were performed on a 1.5T clinical scanner (Siemens). The protocol included: DTI measurements (b=0 and 500s/mm2), DW images (96 b‐values ranging from 0 to 10,000s/mm2 with diffusion gradient rotated in the x‐z plane, NEX=6, TE=200ms) and 96‐echoes, non‐linearly sampled using T2‐sequence. All decay curves measured in the splenium agreed well with the assumed bi‐exponential function: [formula omitted]. The fast water pool, ADCfast had a long apparent T2 relaxation rate while the slow water pool, ADCslow had a shorter T2 constant. The volume fraction fslow correlate with the volume fraction directly estimated from the T2 spectrum. Markedly anisotropy was observed in ADCfast and fslow and more subtle in ADCslow. Evaluation of water diffusion in normal appearing corpus callosum is clearly assisting the peculiar properties of water in biological system, invoking long‐range hydration structures modulated by the concentration of lipid‐protein complexes in the membranes, cellular size and tissue architecture. ADCslow originate from the water in the hydration layers, as ADCfast the rest of the tissue bulk water. The membrane and associated hydration layers form a barrier to water diffusion perpendicular to them since diffusion within the hydration layers is anisotropic. This model is very sensitive to cellularity and further to a brain tumor grade; cell swelling and increased density of membranes enlarge fslow resulting in a decreased ADCs linked to cell proliferation.

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Journal Medical Physics
Olariu, E., & Cameron, I. (2009). SU‐FF‐I‐123: Clinical Value of Diffusion‐Weighted MRI in White Matter in Vivo. In Medical Physics (Vol. 36). doi:10.1118/1.3181244