Optimization of coupled Class-E RF power amplifiers for a transmit array system in 1.5T MRI
Aldemir, Muhammed Said
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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.