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

Purpose: Cardiac pulsation increases the noise level in brain maps of the transverse relaxation rate R *. Cardiac-induced noise is challenging to mitigate during the acquisition of R * 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 *.

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 * maps computed across echoes. The area of k-space that contains most of the cardiac-induced noise in R * maps was then identified. Based on these characteristics, we introduced a tentative sampling strategy that aims to mitigate cardiac-induced noise in R * maps of the brain.

Results: In inferior brain regions, cardiac pulsation accounts for R * variations of up to 3 s 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 ). The tentative cardiac noise mitigation strategy reduced the variability of R * 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 * maps that can be used as a basis for the design of mitigation strategies during data acquisition.