Diffusion time dependence of the apparent diffusion tensor in healthy human brain and white matter disease

The diffusion time dependence of the brain water diffusion tenser provides information regarding diffusion restriction and hindrance but has received little attention, primarily due to limitations in gradient amplitude availa ble on clinical MRI systems, required to achieve short diffusion times, Usi ng new, more powerful gradient hardware, the diffusion time dependence of t enser-derived metrics were studied in human brain in the range 8-80 ms, whi ch encompasses the shortest diffusion times studied to date. There was no e vidence for a change in mean diffusivity, fractional anisotropy, or in the eigenvalues with diffusion time in healthy human brain. The findings are co nsistent with a model of unrestricted, but hindered water diffusion with se mipermeable membranes, likely originating from the extracellular space in w hich the average extracellular separation is less than 7 microns. Similar f indings in two multiple sclerosis plaques indicated that the sire of the wa ter diffusion space in the lesion did not exceed this dimension, (C) 2001 W iley-Liss, Inc.