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 *.
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.
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.
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.