R2* decay is nontrivial in the presence of magnetic microstructure; the logarithm of the signal is approximately quadratic over short times (the static dephasing regime) and asymptotically linear at long times (the diffusion narrowing regime). The transition can, in principle, provide an estimate of the characteristic length scales of the microstructure. Using Monte Carlo simulations of water molecules diffusing through a magnetic field perturbed by magnetic microstructure, we explore the regime of validity of the weak field approximation, in which the signal can be calculated analytically. We also illustrate how the signal behavior changes beyond the weak field limit.
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