Calibration Tools for RF Shim at Very High Field with Multiple Element RF Coils: From Ultra Fast Local Relative Phase to Absolute Magnitude B1+ Mapping

Introduction: At very high magnetic field, strong B1 heterogeneities occur in human MR images as RF wavelength becomes equal to, or smaller than, the size of the imaged target, interacting with lossy and dielectric properties of biological tissues(1,2). Different RF Shim techniques can be used in order to mitigate B1 inhomogeneities, which have considerable differences in their respective hardware and B1 calibration requirements, especially with large numbers of elements in a transmit coil array(3). In this study we propose a comprehensive B1 phase shim strategy, considering gradual solutions for increasing B1 Shim demands and for different hardware setups as shown in Table 1. We introduce fast B1 calibration tools utilizing some linear algebra formalism with transmit coil arrays, ranging from simple (relative B1 phase) to more demanding measures (absolute B1 magnitude). We show that local B1 Phase Shim can be performed in less than one minute with a 16 elements transmit coil array, with dramatic gain in local B1+ magnitude.