Keywords: Microstructure, Relaxometry, T2* decayIn the absence of macroscopic magnetic field inhomogeneity, the decay of gradient-echo signals is generally observed to be approximately monoexponential, with time constant $$$T_2^*$$$. The monoexponential behavior is typically attributed to diffusion, which averages out the dephasing of spins due to microstructural sources of magnetic susceptibility. The rationale is that, over times long enough for spins to have explored the microstructure, the cumulants of the spins’ phase distribution should increase linearly with time. By means of a simple counterexample, we demonstrate that this assumption is incorrect, thereby calling into question the mechanism underlying the monoexponential decay of gradient-echo signals.
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