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

Purpose To demonstrate how the T 2 relaxation time of intra‐axonal water ( T 2a ) in white matter can be measured with direction‐averaged diffusion MRI. Methods For b‐values larger than about 4000 s/mm 2 , the direction‐averaged diffusion MRI signal from white matter is dominated by the contribution from water within axons, which enables T 2a 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 2e ) to be calculated also. This approach was applied to estimate T 2a in white matter for 3 healthy subjects at 3 T, as well as T 2e for a selected set of assumed intra‐axonal diffusivities. Results The estimated T 2a 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 2a was found to depend on the angle of the tract relative to the main magnetic field, which is consistent with T 2a being affected by magnetic field inhomogeneities arising from spatial differences in magnetic susceptibility. The T 2e values were significantly smaller than the T 2a values across white matter regions for several plausible choices of the intra‐axonal diffusivity. Conclusion 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 2a is greater than T 2e and varies considerably with anatomical region.