A characterization of cardiac‐induced noise in R 2 * maps of the brain

Abstract Purpose Cardiac pulsation increases the noise level in brain maps of the transverse relaxation rate R 2 *. Cardiac‐induced noise is challenging to mitigate during the acquisition of R 2 * mapping data because its characteristics are unknown. In this work, we aim to characterize cardiac‐induced noise in brain maps of the MRI parameter R 2 *. Methods We designed a sampling strategy to acquire multi‐echo 3D data in 12 intervals of the cardiac cycle, monitored with a fingertip pulse‐oximeter. We measured the amplitude of cardiac‐induced noise in this data and assessed the effect of cardiac pulsation on R 2 * maps computed across echoes. The area of k‐space that contains most of the cardiac‐induced noise in R 2 * maps was then identified. Based on these characteristics, we introduced a tentative sampling strategy that aims to mitigate cardiac‐induced noise in R 2 * maps of the brain. Results In inferior brain regions, cardiac pulsation accounts for R 2 * variations of up to 3 s −1 across the cardiac cycle (i.e., ∼35% of the overall variability). Cardiac‐induced fluctuations occur throughout the cardiac cycle, with a reduced intensity during the first quarter of the cycle. A total of 50% to 60% of the overall cardiac‐induced noise is localized near the k‐space center (k < 0.074 mm −1 ). The tentative cardiac noise mitigation strategy reduced the variability of R 2 * maps across repetitions by 11% in the brainstem and 6% across the whole brain. Conclusion We provide a characterization of cardiac‐induced noise in brain R 2 * maps that can be used as a basis for the design of mitigation strategies during data acquisition.