Static susceptibility effects in balanced SSFP sequences.

The steady state of balanced steady-state free precession (b-SSFP) sequences in presence of field inhomogeneities can be expressed in terms of a rapidly convergent series, which contains the characteristic function of the susceptibility related phase variations. As long as the intravoxel standard deviation sigma of the accumulated phase per repetition time (TR) is of order 1 (radians) or lower, the signal at TE=TR/2 behaves like a spin echo (SE), in accordance with recent observations. The signal decays quadratically around the maximum near TR/2 with a curvature, which solely depends on sigma and not on T'2. For larger sigma, a qualitative change toward a gradient echo (GRE) occurs and the signal shows characteristics of an usual FID. With increasing off-resonance the width of the SE regime decreases.