Optimization of coupled Class-E RF power amplifiers for a transmit array system in 1.5T MRI

Abstract

Radio Frequency (RF) field is generated mostly by linear RF power amplifiers in Magnetic Resonance Imaging (MRI). These amplifiers have relatively low efficiency and are placed far from the transmit coil in MRI system room. Additional cooling systems and long transmission cables increase the cost of MRI hardware. Coupled Class-E on-coil RF amplifiers for a Transmit Array System is proposed instead of linear RF amplifiers. Class-E RF amplifier is a nonlinear switching amplifier which has 100% drain efficiency ideally. State-space models for single Class-E amplifier and coupled Class-E amplifiers are derived and steady-state operation of coupled Class-E amplifiers is investigated. The state-space models are verified with the simulation results of the Class-E amplifier. Effect of coupling between channels on the output characteristics of the transmit system is observed. Instead of implementing decoupling methods, coupling is maintained and optimization of circuit parameters for coupled Class-E amplifiers is proposed to achieve high drain efficiency and high output RF power. Output power of 600 W and overall drain efficiency higher than 90% are achieved by two coupled Class-E amplifiers even with high coupling levels in the state-space model. Power and efficiency measurements are done with coupled amplifiers and MRI experiments are performed in 1.5T Scimedix MRI Scanner. In conclusion, coupled Class-E RF power amplifiers can be operated as on-coil amplifiers in a Transmit Array system with high output characteristics by optimization of circuit parameters.