1096 Simultaneous B0- and B1-map acquisition in a breath-hold for localized shim, frequency and RF power determination

Cardiac MR at 3 T is challenged by the increased inhomogeneity of the static magnetic field B0, especially when using balanced steady-state free precession sequences. One solution is to determine localized second order shim corrections and a localized on-resonance frequency F0 based on an acquired B0-map [ 1 ]. Furthermore, both numerical simulations [ 2 ] and measurements [ 3 ] have shown that also the transmit radiofrequency (RF) field B1 in the heart is more inhomogeneous at 3 T as compared to 1.5 T. Conventional methods to measure B1-maps, such as the dual-TR method [ 4 ], are rather lengthy. Recently, the saturated double angle method (SDAM) to acquire a B1-map covering the heart within a single breath-hold was introduced [ 5 ]. Applying SDAM, Sung et al reported a flip angle distribution from 34° to 63° across the entire left ventricle (LV) for a nominal flip angle of 60° [ 6 ]. This suggests that not only the B1 field over the LV is inhomogeneous by ± 30%, but that the average flip angle (power setting) is about 20% lower than the requested 60°. However, inadequate power settings may lead to signal reduction, changes in contrast, and eventually to biased quantitative measures.