Measuring intra-axonal T 2 in white matter with direction-averaged diffusion MRI

To demonstrate how the T relaxation time of intra-axonal water (T ) in white matter can be measured with direction-averaged diffusion MRI. For b-values larger than about 4000 s/mm , the direction-averaged diffusion MRI signal from white matter is dominated by the contribution from water within axons, which enables T to be estimated by acquiring data for multiple TE values and fitting a mono-exponential decay curve. If given a value of the intra-axonal diffusivity, an extension of the method allows the extra-axonal relaxation time (T ) to be calculated also. This approach was applied to estimate T in white matter for 3 healthy subjects at 3 T, as well as T for a selected set of assumed intra-axonal diffusivities. The estimated T values ranged from about 50 ms to 110 ms, with considerable variation among white matter regions. For white matter tracts with primarily collinear fibers, T was found to depend on the angle of the tract relative to the main magnetic field, which is consistent with T being affected by magnetic field inhomogeneities arising from spatial differences in magnetic susceptibility. The T values were significantly smaller than the T values across white matter regions for several plausible choices of the intra-axonal diffusivity. The relaxation time for intra-axonal water in white matter can be determined in a straightforward manner by measuring the direction-averaged diffusion MRI signal with a large b-value for multiple TEs. In healthy brain, T is greater than T and varies considerably with anatomical region.