Susceptibility, echo time shifts, and T2* considerations for functional magnetic resonance imaging

Magnetic susceptibility differences exist near the interface of air/tissue in the ventral brain in fMRI (functional magnetic resonance imaging). These susceptibility differences will not only cause field inhomogeneity and its gradients which will result in image artifacts, but also cause shifts in the echo time for gradients echo acquisitions. The echo time shifts are caused by shifts in the effective k-space trajectory due to the gradients of the field inhomogeneity. Previous work has shown and validated methods for estimating the echo time shift based on the effective k-space trajectory. In this work, we demonstrate that accurate estimation of the R2* decay map (R2*=1/T2*) not only need to account for the field map and its gradients, but also needs to include the echo time shift. These changes in T2* sensitivity are directly related to changes in sensitivity in BOLD (blood oxygenation level dependent) fMRI studies.