Getting the Phase Right: The Importance of Accurate Phase Definition in balanced Steady-State Free Precession MRI

Balanced steady-state free precession (bSSFP) MRI sequences provide images with a high signal-to-noise ratio and T2/T1 contrast1. Phase-cycled (PC) bSSFP, which refers to multiple acquisitions with different linear phase increments of the radiofrequency (RF) pulse, can be used for artifact removal and quantitative parametric mapping. For this application, the signal phase is an indispensable source of information. In contrast to early publications, contradictory mathematical descriptions of the PC-bSSFP signal model have been reported. While the magnitude is typically well described, the signal phase is often not directly derived from the Bloch equations, leading to inaccurate signal descriptions. This is particularly relevant for multi-compartment systems where bSSFP profile asymmetries can be observed. This letter emphasizes the importance of accurate phase definitions to describe the bSSFP profile, by providing an intuitive derivation and an experimental validation using single and two-compartment phantoms.