Fixation alters tissue properties significantly and in vivo vs ex vivo models should be adapted accordingly. Unlike in vivo studies of rodent gray matter (GM) where diffusion time-dependence D(t) was absent for t > 10 ms, allowing an interpretation of time-dependent kurtosis K(t) as resulting from inter-compartment exchange, here we show that ex vivo rodent GM displays marked D(t), with non-Gaussianity arising most probably from extracellular water. K(t) could thus result from combined effect of disorder and exchange. High-b data where extracellular water is preferentially suppressed may still enable the unconfounded estimation of exchange.
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