Highly efficient 300 W modified class-E RF amplifiers for 64 MHz transmit array system
Zahra, Fatima Tu
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The conventional MRI system uses high power linear RF amplifiers which are placed away from scanner room, have low efficiency, high cost and need a cooling system. We aim to use parallel transmit system with on-coil amplifiers that has shown to be advantageous in improving B1 field homogeneity, slice selectivity and SAR reduction. In this work the on-coil class-E RF switching amplifier is suggested for MRI to decrease the cost and complexity of the current system while improving performance. The amplifier is digitally controlled and for pulse generation purpose supply modulation is used. The transmit coil acts as the load network of the amplifier so the need for matching circuit is eliminated. The amplifier has an output power of 300 W with maximum efficiency of 92%. The efficiency does not drop below 75% in 1 MHz bandwidth. The performance of amplifier at high temperature is also evaluated and it is established that at low duty cycles due to high efficiency no cooling system is required but for high duty cycle applications a cooling system might be needed for the uninterrupted operation. The ultimate goal of this research is to design a 32-channel transmit array using on-coil amplifiers, as a step forward towards this goal a prototype for two channels is designed. Our results depict that the behavior of the class-E amplifiers under coupled operations is acceptable. The dual-channel prototype was tested on Scimedix 1.5 T and no artifacts were observed in the images due to the presence of amplifiers near transmit coils inside the bore. To sum up, the class-E amplifier is proved to be a favorable candidate for on-coil applications in RF excitation due to its small size, reduced complexity and high efficiency.