Characterization of cardiac-induced noise in R2* maps of the brain

Purpose Cardiac pulsation increases the noise level in brain maps of the MRI parameter R2*. Cardiac-induced noise is challenging to mitigate during the acquisition of R2* mapping data because the characteristics of this noise remain largely unknown. In this work, we characterize cardiac-induced noise in brain maps of the MRI parameter R2*. Methods We introduce a sampling strategy that enables the acquisition of multi-echo data in several intervals of the cardiac cycle. From this data, we estimate the variability of brain R2* maps due to cardiac-induced fluctuations and identify the most sensitive area of k-space. From these characteristics, we derive a novel sampling strategy that successfully mitigates cardiac-induced noise in R2* maps of the brain. Results In inferior brain regions, cardiac pulsation accounts for R2* 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 and exhibit a reduced intensity during the first quarter of the cycle after the detection of the systole at the finger. 50-60% of the overall cardiac-induced noise is contained near the k-space centre (k < 0.074 mm−1), corresponding to 22% of voxels at 4mm resolution. The proposed cardiac noise mitigation strategy reduces the variability of R2* estimates across repetitions by 11% in the brainstem and 6% across the whole brain. Conclusion We provide characteristics of cardiac-induced noise in brain maps of the MRI parameter R2* that constitute a basis for the design of mitigation strategies during data acquisition.