Assessing and Correcting Respiration Induced Variation of B1 in the Liver

Introduction: It has been shown that the B1 inhomogeneities in high field systems (e.g.7T) can severely compromise image quality. B1 inhomogeneities are still significant at 3T, but generally do not destroy the ability to image. However, the variations in flip angle (and hence contrast) across the field of excitation can still cause significant problems, particularly with applications where absolute quantification is required. These observations have lead to the concept of B1 shimming, where multi-port transmit coils are driven in such a way as to correct the B1 inhomogeneity. The first generation of such approaches have generally assumed that the B1 shim is a temporally invariant quantity and depends only on the subject. However, there have been some recent observations ([1], [2]) which indicate that the B1 field is dependent on physiological parameters. Here we map the B1 field generated by a whole body transmit coil array during inhale and exhale breathholds across a number of subjects and assess the variability in B1 field within the liver (a target frequently imaged for quantitative analysis [3]) . We demonstrated that B1 shimming on an 8 channel body system could correct for variations in the B1 field between these two states.