A characterization of cardiac-induced noise in R2* maps of the brain

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 therefore 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 3s -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. 50-60% of the overall cardiac-induced noise is localized near the k-space centre (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 characterisation 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.