Nonlinear droop compensation for current waveforms in MRIgradient systems

Abstract

Providing accurate gradient currents is challenging due to the gradient chain nonlinearities, arising from gradient power amplifiers and power supply stages. This work introduces a new characterization approach that takes the amplifier and power supply into account, resulting in a nonlinear model that compensates for the current droop. Methods:The gradient power amplifier and power supply stage were characterized by a modified state-space averaging technique. The resulting nonlinear model was inverted and used in feedforward to control the gradient coil current. A custom-built two-channel z-gradient coil was driven by high-switching (1 MHz), low-cost amplifiers (<$200) using linear and nonlinear controllers.High-resolution (<80 ps) pulse-width-modulation signals were used to drive the amplifiers. MRI experiments were performed to validate the nonlinear controller's effectiveness. Results:The simulation results validated the functionality of the state-space averaging method in characterizing the gradient system. The performance of linear and nonlinear controllers in generating a trapezoidal current waveform was compared in simulations and experiments. The integral errors between the desired waveform and waveforms generated by linear and nonlinear controllers were 1.9% and 0.13%, respectively, confirming the capability of the nonlinear controller to compensate for the current droop. Phantom images validated the nonlinear controller's ability to correct droop-induced distortions. Conclusion: Benchtop measurements and MRI experiments demonstrated that the proposed nonlinear characterization and digitally implemented feedforward controller could drive gradient coils with droop-free current waveforms (without a feedback loop). In experiments, the nonlinear controller outperformed the linear controller by a 14-fold reduction in the integral error of a test waveform.

Minimizing electric fields and increasing peripheral nerve stimulation thresholds using a body gradient array coil

“…To implement the investigated design, 288 gradient power amplifiers are required. We are in the process of designing low‐cost, custom‐built power amplifiers, and the methodology for using these amplifiers to drive the gradient array design has been outlined in the literature 34,42 . In this preliminary investigation, the DC resistance, wire spacing, connection cables, and heat dissipation in the windings have not been explored.…”

“…Here, we consider a total of 96 channels, consisting of both primary and shield coils, for each directional gradient. The method for driving the mutually coupled gradient arrays has been previously detailed in prior publications 34,42 . It involves using the array coil inductance matrix, including self and mutual inductances, to compute the necessary voltages to drive the array channels and achieve the desired currents.…”

“…The method for driving the mutually coupled gradient arrays has been previously detailed in prior publications. 34,42 It involves using the array coil inductance matrix, including self and mutual inductances, to compute the necessary voltages to drive the array channels and achieve the desired currents. The maximum self-inductance among the channels is 52 μH, and the maximum pairwise mutual inductance is 32 μH.…”

“…literature. 34,42 In this preliminary investigation, the DC resistance, wire spacing, connection cables, and heat dissipation in the windings have not been explored. The heating of the gradient coil (power deposition) is associated with the surface current density.…”