A near-magnet, wide band-gap Gallium Nitride transistor based class-D current driver was developed for MRI shimming applications to improve the magnetic field homogeneity in magnetic resonance imaging systems. Requirement specifications for the voltage, current and the number of channels were derived from analyzing optimized shim coil designs based on the whole human-brain magnetic field maps. The developed shimming power amplifier has current/voltage capability of 15A/24V is suitable for both static and dynamic shimming. The high switching frequency of 625 kHz simplifies the ripple current filter design and makes the high bandwidth applications feasible (larger than 20kHz). An analog proportional-integration (PI) controller was used for easy tuning to different loads. Corresponding damping optimized air-core filter was implemented avoiding any ferrite cores to make the amplifier placement near the main magnet feasible. Due to the high efficiency of the switched-mode class-D design, the dimensions of shimming amplifier could be made small, which makes the developed amplifier suitable for applications requiring the high numbers of shimming channels, such as matrix shimming coils.
The near-magnet topology eliminates the bulky and costly wall-mounted shimming filters and greatly reduces the length of the cables, while simultaneously reducing the power requirements for the shimming coils power supply. The developed near-magnet shimming amplifier reduces the drive system cost and reduces the space requirement for high-order non- spherical harmonic (SH) shimming coils, which makes the high-performance multi-channel shimming affordable for most research systems. This is expected to accelerate the translation of a great number of cutting-edge scientific developments to wider application fields and eventually to the clinical routine.
The devised requirement specification of optimized shim coil design specifically for whole human-brain can guide the MR device manufactures and OEM suppliers to make the corresponding product lines quickly available on the market.
