LAUSR

Abstract Understanding diffusion‐weighted MR signal in brain white matter (WM) has been a long‐sought‐after goal. Modern research pursues this goal by focusing on the biological compartments that contributes essentially to the signal. In this study, we experimentally address the apparent presence of a compartment in which water motion is restricted in all spatial directions. Using isotropic diffusion encoding, we establish an upper bound on the fraction of such a compartment, which is shown to be about 2% of the unweighted signal for moderate diffusion times. This helps to eliminate such a compartment that have been assumed in literature on biophysical modeling. We also used the diffusion decay curve obtained from the isotropic encoding to establish a lower limit on the mean diffusivities of either of intra‐ or extra‐axonal compartment as a function of their relative water fraction. HighlightsIsotropic diffusion measurement shows an absence of still water compartment in brain white matter.The lower limit on the trace of intra‐ and extra‐axonal compartment was estimated.Orientation dispersion of axons and glial processes have to be accounted to fit isotropic measurement.