Resolution Improvement in a High-Power Magnetic Resonance Imaging Gradient Power Amplifier

Abstract

High-resolution gradient power amplifiers are required to generate high-fidelity gradient fields in magnetic resonance imaging (MRI) systems. Various aspects of gradient power amplifiers have been the subject of research in the past years, however, systematic analysis and design methods for its resolution improvement have not been sufficiently addressed. This article addresses this research gap with comprehensive analysis and design methods for resolution improvement. First, a method for systematic resolution characterization of the MRI gradient power amplifier with gradient coil is proposed. Second, noise modeling with respect to the critical aspects in the system resolution degradation is developed and discussed. Third, the resolution improvement methods of bandwidth optimization, pseudorandom code modulation, and utilizing silicon carbide (SiC) switching power devices are proposed and analyzed, achieving a resolution at the one part in 10 6 level. Finally, the resolution improvement methods are experimentally validated in a 1000 V/400 A SiC gradient power amplifier. The experimental results show a 3.6 times reduction in the peak spurious signals and resolution improvement at all current levels and pulse lengths, compared to a conventional insulated gate bipolar transistor GPA. These results prove the validity of the proposed methods for resolution improvement in a high-power MRI gradient power amplifier, enabling future high-power and high-resolution amplifier designs.

Measurement System for Accurate Determination of Fast-Changing DC Currents Up To 900 A

“…As discussed in the introduction, the most stringent requirement of high-resolution amplifiers is current pulse reproducibility defined in Fig. 1 [1], [2], [3], [4], [5], [6], [7], [8]. This quantity can therefore be used as the key consideration in designing the measurement system.…”

“…The measurement system converts the current signal to an output voltage signal. Therefore, the current noise I nrms can be determined by measuring the voltage noise V nrms in the output, which can be derived as (5) where I FS denotes the full-span of the current generated by the device under test (DUT) and V FS indicates the full-span of the voltage signal sampled by the ADC. And the ratio of I FS and V FS can be seen as the scaling factor of the measurement system.…”

“…Since current noise and voltage noise are zero-mean processes, their rms values are equal to the standard deviation. Then, (5) becomes…”

“…An integration time T 0 of 10 ms is the most-often-used value in the application of high-resolution amplifiers [1], [2], [3], [4], [5], [6], [7], [8]. This value is further used in the performance estimation, resulting in an upper limit frequency of 100 Hz in (9).…”

“…In recent years, the rapid evolution of high-end systems, such as magnetic resonance imaging (MRI) systems and underwater detection systems, has increased the interest in reliable and accurate measurements of fast-changing dc currents generated by the high-resolution power amplifiers in these applications. These power amplifiers generate arbitrary fast-changing dc current waveforms of hundreds of amperes, with linearity and accuracy of a few parts per 10 3 [1], [2], [3], [4], [5], [6], [7], [8]. However, the most stringent requirement is the current pulse reproducibility, defined in Fig.…”