Reducing Noise Induced by Cardiac Pulsatility in Brain Maps of R₂* and Magnetic Susceptibility Using Tailored k-space...

Maps of the transverse relaxation rate R₂* and magnetic susceptibility (χ) are computed from gradient-echo data and are sensitive to signal instabilities induced by cardiac pulsation. Here, we introduce two k-space sampling strategies that aim to mitigate the impact of cardiac-induced noise in brain maps of R₂* and χ. The proposed strategies are based on the higher level of cardiac-induced noise near the k-space centre compared to the periphery. Using CArtesian trajectory with Spiral PRofile (CASPR), the first strategy allows for the acquisition of a specific number of averages at each k-space location, derived from the local level of cardiac-induced noise. The second strategy uses cardiac triggering to synchronize the acquisition near the k-space centre with the cardiac cycle in real time. We compared the variability across four repetitions of R₂* and χ maps computed from data acquired using both strategies and with a standard linear trajectory. Data were acquired in 10 healthy volunteers. Compared to linear trajectory, CASPR reduced the variability of R₂* and χ maps across repetitions by 22% and 16% across the whole brain, reaching over 30% in inferior brain regions, for a 14% increase in scan time. CASPR also reduced the level of aliasing artefacts from pulsating blood vessels. Cardiac triggering did not reduce the variability of R₂* or χ maps. CASPR can be designed to mitigate cardiac-induced noise in brain maps of the MRI parameters R₂* and χ. Synchronization of data acquisition with the cardiac cycle did not reduce the level of cardiac-induced noise.