We model the magnetic microstructure of tissue by a colloidal suspension of spherical beads, whose susceptibility differs from that of the surrounding medium. We show that length scale and susceptibility differences of the microstructure can be quantified with good accuracy by imaging the suspension with a multiple gradient echo sequence and fitting the signal as a function of echo time to the results of Monte Carlo simulations. By comparison, the second-order cumulant expansion used in deriving analytic expressions for the signal is a poor approximation at long echo times, even when the dephasing is relatively small over the correlation time.
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