Off-resonance-dependent slice profile effects in balanced steady-state free precession imaging.

The purpose of this study was the simulation and measurement of balanced steady-state free precession (bSSFP) slice profiles for a detailed analysis of the influence of off-resonance effects on slice profile shape and bSSFP signal intensity. Due to the frequency response function of the bSSFP sequence, measurements that are not on-resonance result in broadened effective slice profiles with different off-resonance-dependent shapes and signal intensities. In this study, bSSFP slice profile effects and their dependence on off-resonance were investigated based on bSSFP signal simulations of phantom data as well as blood and tissue. For a better assessment of the similarity of measured and simulated slice profiles the field map was integrated in the slice profile simulations. The results demonstrate that simulations can accurately predict bSSFP slice profiles. Both measurements and simulations indicate that there is a substantial increase in signal intensity close to the banding artifacts, i.e., at spatial locations with off-resonance frequencies corresponding to a dephasing/TR = +/- pi resulting in signal void (bands). For routine bSSFP imaging, off-resonance-dependent slice broadening may thus result in a substantial difference between nominal and true slice thickness and lead to spatially varying slice thickness and signal intensities across the imaging slice.