ISME-incoherent sampling of multi-echo data to minimize cardiac-induced noise in brain maps of R 2 * $$ {R}_2^{\ast } $$ and magnetic susceptibility.

Purpose: Maps of the MRI parameters $R_2^{*}$ and magnetic susceptibility ( $\chi$ ) enable the investigation of microscopic tissue changes in brain disease. However, cardiac-induced signal instabilities increase the variability of brain maps of $R_2^{*}$ and $\chi$ . In this study, we introduce incoherent sampling of multi-echo data (ISME)-a sampling strategy that minimizes the level of cardiac-induced instabilities in brain maps of $R_2^{*}$ and $\chi$ .

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 are acquired at different phases of the cardiac cycle. We compare the variability of $R_2^{*}$ and $\chi$ 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 approach for the estimation of $R_2^{*}$ that accounts for the increase of the residuals with echo time.

Results: ISME reduces the variability of $R_2^{*}$ and $\chi$ 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 $R_2^{*}$ maps. The proposed weighted least-squares approach for the estimation of $R_2^{*}$ reduces the dependence of the fitting residuals on echo time and the variability of $R_2^{*}$ 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 $R_2^{*}$ and $\chi$ , improving reproducibility.