Highly Distributed RF Transmission with a 32-Channel Parallel Transmit System

Introduction: When RF signal gets transmitted (Tx) / detected (Rx), the B1 / B1 fields interact with the spin system of the imaged object, forming the basis of MR signal induction / detection. The concomitant E field meanwhile gives rise to RF loss in the object and dictates SAR / noise. An MR system implements / optimizes the RF electromagnetic fields for MR via the currents in an RF coil structure, by controlling their magnitudes, phases, temporal modulation and spatial distribution. Conventional Tx actively controls the temporal modulation only, with a single source driving a volume coil structure. Parallel Tx supports flexible spatiotemporal current pattern control with multiple sources driving a distributed RF coil structure, and promises to address major challenges facing conventional Tx in high field MR, including excitation profile degradation and SAR exacerbation. This study explores substantially distributed parallel Tx, aiming at its applications in reducing RF power dissipation in the subject / the coil structure, as well as in B1 shimming / accelerated parallel excitation of an arbitrarily oriented target slice or volume. We developed a cost effective 32-channel exciter module, RF coil arrays that support 32-channel parallel Tx and Rx, and 32 current-source amplifiers that facilitate coil current control. In the following we describe the development efforts and present initial phantom imaging results.