Improvement in B1+-homogeneity of 3T cardiac MRI in swine with dual-source parallel RF excitation

Background Conventional MRI scanners up to a magnetic field strength of 3T use an integrated birdcage quadrature coil to generate a radio frequency (RF) excitation field (B1+). At 3T, Sung et al. observed a flip angle variation ranging from 31 to 66% over the entire left ventricle (LV) in humans as well as a flip-angle distribution from 34° to 63, for a nominal flip angle of 60°. This not only demonstrates that the B1+ field over the LV is inhomogeneous but also that the average flip angle (RF power setting) can be 20% lower than desired. Such erroneous flip angles may lead to local signal reduction, artifacts, failure of magnetization preparation pulses, and eventually to biased quantitative measures. Recently, it has been shown that multi-channel transmit systems can be used to reduce these inhomogeneities in humans by the use of RF-shimming. Here, we quantify improvements in B1 +-homogeneity at 3T when using dual-source parallel RF excitation, and correlate results with animal size.