Multi-component apparent diffusion coefficients in human brain: Relationship to spin-lattice relaxation

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Abstract
In vivo measurements of the human brain tissue water signal decay with b‐factor over an extended b‐factor range up to 6,000 s/mm 2 reveal a nonmonoexponential decay behavior for both gray and white matter. Biexponential parametrization of the decay curves from cortical gray (CG) and white matter voxels from the internal capsule (IC) of healthy adult volunteers describes the decay process and serves to differentiate between these two tissues. Inversion recovery experiments performed in conjunction with the extended b‐factor signal decay measurements are used to make separate measurements of the spin‐lattice relaxation times of the fast and slow apparent diffusion coefficient (ADC) components. Differences between the spin‐lattice relaxation times of the fast and slow ADC components were not statistically significant in either the CG or IC voxels. It is possible that the two ADC components observed from the extended b‐factor measurements arise from two distinct water compartments with different intrinsic diffusion coefficients. If so, then the relaxation results are consistent with two possibilities. Either the spin‐lattice relaxation times within the compartments are similar or the rate of water exchange between compartments is “fast” enough to ensure volume averaged T1 relaxation yet “slow” enough to allow for the observation of biexponential ADC decay curves over an extended b‐factor range. Magn Reson Med 44:292–300, 2000.