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The diffusion phenomenon is the random translational motion (Brownian motion) of molecules in matter. Diffusion depends on the type of molecule and its environment and is typically described by a coefficient, known as the diffusion coefficient. Diffusion-weighted 1H MR imaging allows in vivo measurement of free water diffusion coefficient in human brain and, as importantly, permits generation of diffusion coefficient brain maps that help localize derangements in specific brain regions. In the human brain, the diffusion of free water is influenced by several factors, including cellular structure, myelin, and circulation of blood in capillary networks.1 Hence, what can be measured in the image voxel, usually on the order of millimeters in dimension, is an apparent diffusion coefficient (ADC) that accounts for the effect of these factors on the diffusion of free water in vivo.2
Recent observations have suggested that the diffusion of free water in the extracellular compartment is faster than in the intracellular compartment because of the hindering effects of cellular structure and that the ADC measurement reflects a weighted average of the contribution from both compartments.3 Another observation …
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