Static susceptibility effects in balanced SSFP sequences

Abstract

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 σ 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 σ and not on T′2. For larger σ, 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. © 2006 Wiley-Liss, Inc.