Data acquisition strategies to reduce cardiac-induced noise in brain maps of R2* and magnetic susceptibility
Purpose: Cardiac pulsation increases the noise level in brain MR images. Maps of the transverse relaxation rate R2* and magnetic susceptibility (QSM) are particularly affected by cardiac-induced noise as they are computed from gradient-echo data acquired at multiple echo times. Here, we introduce two data acquisition strategies to mitigate the impact of cardiac-induced noise in brain maps of R2* and QSM. Methods: The proposed strategies are based on the higher level of cardiac-induced noise near the k-space centre. Using a pseudo-spiral sampling trajectory, the first strategy allows for the acquisition of a specific number of averages at each k-space location, set from the local level of cardiac-induced noise. The second strategy synchronizes the acquisition with the cardiac cycle in real time. We compared in 10 healthy volunteers, the variability of data acquired across 4 repetitions and in the same session, using both strategies and with a standard linear trajectory. Results: Compared to linear sampling, the pseudo-spiral trajectory reduced the variability of R2* and QSM maps across repetitions by 26/28/22% and 19/18/16% in the brainstem/cerebellum/whole brain, for a 14% increase in scan time. The pseudo-spiral sampling also reduced the level of aliasing artifacts from pulsating blood vessels. The cardiac-triggered trajectory did not reduce the variability of R2* or QSM maps. Conclusion: Pseudo-spiral k-space trajectories can be designed to mitigate cardiac-induced noise in brain maps of the MRI parameters R2* and QSM. Synchronization of the acquisition with the cardiac cycle in real time did not lead to any reduction in cardiac-induced noise.