Exploring the Limits of RF Shimming: Single Slice and Whole Brain Field Optimizations at up to 600 MHz with Transmit Arrays of up to 80 elements
Mao W, Collins C, Smith M
Penn State University College of Medicine
Simulation and experiment have shown that separate control of magnitude and phase of individual coil drives in a transmit array can be used to provide significant improvement in homogeneity of RF fields, flip angles, and available signal intensity in high field MRI. Due to Maxwells equations and wavelength effects, however, there are fundamental limits of what homogeneity can be achieved by RF shimming alone with any number of coils. Here we investigate the possibilities of reducing inhomogeneities at very high frequencies with a large number of coils. With a large number of coils, results are impressive (though some limitations can be seen) even for whole-brain RF shimming at 600 MHz.