Effects of motion on MRI signal decay from micron-scale particles.

Abstract

Transverse decay rate (R2∗) mapping is an established method for measuring iron overload in various biological tissues. Recently, R2∗ mapping was used to measure the mean 3D concentration distribution of micron-size particles dispersed in turbulent flows. However, some discrepancy was observed between the measured R2∗ and the expected decay based on existing theory. The present paper examines three flow-related mechanisms that could be responsible for this discrepancy. Computational simulations were used to study the effects of relative particle-fluid motion and preferential concentration by turbulence, while the effect of enhanced proton dispersion due to turbulence was examined via the existing MRI relaxation theory. Each flow phenomenon was shown to produce a different effect on the signal-time curve, as well as the extracted R2∗. Comparison to experimental data in a square channel flow showed that relative motion between the particles and fluid was the most likely cause of the discrepancy in the previous experiments; however, all three effects may be present in both medical and non-medical flows, and their differing effects on the MRI signal may eventually allow for their identification from MRI data.