What Does FEXI Measure in Neurons?

Abstract

Exchange between tissue compartments is crucial for interpretation of diffusion MRI measurements in brain gray matter. However, reported values of exchange time are broadly dispersed, about two orders of magnitude. We analyze the measurement technique called Filtered Exchange Imaging (FEXI) using numerical solution of Bloch--Torrey equation in digitalized neurons downloaded from NeuroMorpho.org. The FEXI outcome, which is the recovery of diffusion coefficient in cells with impermeable membrane is multiexponential, with the time constants defined by the eigenvalues of Laplace operator. Fitting the commonly used exponential recovery function results in a strong dependence of the apparent exchange time on the involved mixing time interval and the adjustment or fixation of the equilibrium diffusivity. To obtain an estimate of membrane permeability, we reinterpret previously published data on preexchange lifetime in neuronal cell culture. It results in 0.005 micrometer/ms. The corresponding exchange time is approximately 140 ms. We conclude that essentially shorter exchange times are due to fast geometric exchange inside the ramified cells.