The MRI signal in white matter tissue is composed of constituent signals from two different water pools: intra/extracellular water and myelin water. When using a multi spin-echo acquisition, the relaxation rate measured in each pool has been shown to depend on the orientation of the containing white matter fibre with respect to the main magnetic field. Here, we show an additional influence of the acquisition echo spacing on the apparent intra/extracellular rate relaxation rate, and using numerical simulation, we provide evidence that decoherence mediated by diffusion through local field inhomogeneities created by blood vessels cannot explain the observed orientation dependence.
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