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

Cardiac pulsation increases the noise level in brain maps of the transverse relaxation rate R₂ *. Cardiac-induced noise is challenging to mitigate during the acquisition of R₂ * mapping data because its characteristics are unknown. In this work, we aim to characterize cardiac-induced noise in brain maps of the MRI parameter R₂ *.

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 R₂ * maps computed across echoes. The area of k-space that contains most of the cardiac-induced noise in R₂ * maps was then identified. Based on these characteristics, we introduced a tentative sampling strategy that aims to mitigate cardiac-induced noise in R₂ * maps of the brain.

In inferior brain regions, cardiac pulsation accounts for R₂ * 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 R₂ * maps across repetitions by 11% in the brainstem and 6% across the whole brain.

We provide a characterization of cardiac-induced noise in brain R₂ * maps that can be used as a basis for the design of mitigation strategies during data acquisition.