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
Maps of the MRI parameters R2* and magnetic susceptibility (π) enable the investigation of microscopic tissue changes in brain disease. However, cardiac-induced signal instabilities increase the variability of brain maps of R2* and π. In this study, we introduce ISME β a sampling strategy that minimizes the level of cardiac-induced instabilities in brain maps of R2* and π.
Methods
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 R2* and π maps acquired with ISME and with standard multi-echo trajectories in N=10 healthy volunteers. We investigate the effect of both trajectories on the spatial aliasing of pulsating MR signals and propose a weighted-least squares (NWLS) approach for the estimation of R2* that accounts for the increase of the residuals with echo time.
Results
ISME reduces the variability of R2* and π maps 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 R2* maps. The proposed NWLS approach for the estimation of R2* reduces the dependence of the fitting residuals on echo time and the variability of R2* by an additional 3/2/1% in the cerebellum/brainstem/whole brain.
Conclusion
ISME allows the mitigation of cardiac-induced signal instabilities in brain maps of R2* and π, improving reproducibility.
