A gate modulated digitally controlled modified class-E amplifier for on-coil application in MRI

Abstract

The switch-mode RF power amplifiers, known for their high output power capability and good efficiency, have proved valuable for on-coil applications in MRI hardware. The class-D and class-E amplifier topologies have been demonstrated to be promising candidates to replace the conventional inefficient linear RF power amplifiers used in MR hardware which are placed away from the scanner room. Conventionally, the amplitude modulation of the output waveform in such switch-mode RF power amplifier applications is achieved either by implementing an amplitude modulation block at the drain of the amplifier, or by encoding the amplitude modulation information in the phase of the carrier signal at the gate of the amplifier. Both these approaches require additional hardware, thus increasing the cost and complexity of the system. Considering the aforementioned background, a novel technique of modulating both the amplitude and frequency of the output waveform, without the need for any additional hardware other than the driver circuitry for the amplifier itself, is presented and implemented for class-E amplifier topology. At 64 MHz (1.5 T), the analytical models of the amplifier for both the switch-on and switch-off cases are first derived and implemented in software. The period of the digital carrier signal at the gate of the amplifier is then divided into k bits, where k is greater than 2. It is then noted that both the amplitude and frequency of the output waveform can be controlled by altering this digital input in a certain manner. For a typical 2 ms 1.5 T MRI RF pulse, the digital carrier bitstream would consist of k × 128000 bits. This would require testing 2k×128000 bitstream combinations to achieve the desired output waveform, requiring infeasible computational power. It is however shown that by intelligently programming the bitstream patterns for a selected number of periods, and by repeating those patterns for a chosen duration of time, the desired amplitude and frequency modulation of the output waveform can be achieved. The normalized root mean square error (NRMSE) for a 2 ms sinc pulse designed using such an approach is calculated to be 11%. The designed bitstreams are tested on hardware as well, both in bench-top and MRI experiments. Bench-top experiment results correlate well with the software predictions. The amplifier shows a peak drain efficiency of 89% at 50 W input power. The MR images obtained at 50 W input power using a 2 ms sinc pulse designed using the presented approach show no artifacts. The ultimate goal of the current research is to design a 32-channel transmit array coil for the MRI, capable of delivering a total of approximately 10 kW output power. Each amplifier element should therefore be able to deliver about 300 W output power. In this regard, further research needs to be conducted to achieve such output power level using the presented modulation approach. Nonetheless, the approach is general and can be implemented to other switch-mode RF power amplifier topologies as well. It promises to provide a performance equivalent to the other modulation approaches while reducing the overall cost and complexity of the system at the same time.