Rapid T2 estimation with phase-cycled variable nutation steady-state free precession.

Variable nutation SSFP (DESPOT2) permits rapid, high-resolution determination of the transverse (T2) relaxation constant. A limitation of DESPOT2, however, is the presence of T2 voids due to off-resonance banding artifacts associated with SSFP images. These artifacts typically occur in images acquired with long repetition times (TR) in the presence of B0 inhomogeneities, or near areas of magnetic susceptibility difference, such that the transverse magnetization experiences a net phase shift during the TR interval. This places constraints on the maximum spatial resolution that can be achieved without artifact. Here, a novel implementation of DESPOT2 is presented incorporating RF phase-cycling which acts to shift the spatial location of the bands, allowing reconstruction of a single, reduced artifact-image. The method is demonstrated in vivo with the acquisition of a 0.34 mm3 isotropic resolution T2 map of the brain with high precision and accuracy and significantly reduced artifact.