ISME - Incoherent Sampling of Multi-Echo data to minimize cardiac-induced noise in brain maps of R 2 * and magnetic susceptibility
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Purpose
Brain maps of the MRI parameters R 2 * and magnetic susceptibility ( χ ) enable the investigation of microscopic tissue changes associated with brain disease in patient populations. However, maps of R 2 * and χ are computed from gradient-echo data acquired at multiple echo times and are affected by cardiac-induced physiological noise. In this study, we introduce ISME – a sampling strategy that minimizes the level of cardiac-induced noise in brain maps of R 2 * and χ .
Methods
Cardiac-induced noise causes exponential-like effects on the decay of the signal magnitude that vary across the cardiac cycle, increasing the variability of R 2 * and χ estimates. ISME uses phase-encoding gradients to shift the k-space frequency of the acquired data between consecutive readouts of a multi-echo train. As a result, the multi-echo data at a given k-space index is acquired at different phases of the cardiac cycle. We compare the variability of R 2 * and χ maps acquired with ISME and with standard multi-echo trajectories in N=10 healthy volunteers. We also investigate the effect of both sampling strategies on the spatial aliasing of pulsating MR signals in R 2 * and χ maps and propose a weighted-least square (NWLS) approach for the estimation of R 2 * that accounts for the increase in cardiac-induced noise with echo time.
Results
Compared to standard multi-echo trajectories, ISME reduces the variability of R 2 * and χ across repetitions by 25/26/21% and 24/32/23% in the cerebellum/brainstem/whole brain, respectively. With ISME, the spatial aliasing of pulsating MR signals is incoherent between raw echo images, leading to visually sharper R 2 * maps. The proposed NWLS approach for the estimation of R 2 * removes the dependence of the fitting residuals on echo time and brings an additional 3/2/1% reduction of the R 2 * variability in the cerebellum/brainstem/whole brain.
Conclusion
ISME allows the mitigation of cardiac-induced noise in brain maps of R 2 * and χ , improving reproducibility.
