Prospective compensation of second-order concomitant fields in a high-performance gradient system using a second-order harmonic shim coil.

Abstract

Purpose: The use of high-performance gradient coils results in stronger spatially dependent second-order concomitant magnetic fields, which can lead to signal dropout, blurring artifacts and phase errors that become more significant at locations farther from the gradient isocenter. A correction coil-based method for prospectively compensating second-order concomitant fields in higher-performance gradient systems is described.

Methods: An insertable, axially symmetric second-order field coil to prospectively correct for second-order concomitant field-induced phase errors on a high-performance head-only gradient system at 3.0T was developed. The efficacy of the second-order concomitant-field correction was demonstrated in phantom and healthy volunteer scans using 2D phase contrast (PC) and spiral gradient echo (GRE) imaging.

Results: By employing the correction coil, there was a significant reduction in second-order concomitant field-induced blurring in the 2D spiral images, and reduction in phase errors and signal degradation in the GRE PC images. In the single-sided PC, the z- and radially-directed second-order concomitant phase accrued in the coronal and axial PC acquisition was reduced by 100% and 83%, respectively. Signal enhancement up to 968.9% was obtained in the two-sided PC acquisitions. In spiral GRE images, blurring was reduced by ˜40.2% at 60 mm from the gradient isocenter in a phantom. Correspondingly, the reduction in concomitant field-induced blurring in in-vivo spiral GRE images was noted with the correction coil.

Conclusions: The described second-order correction coil insert prospectively compensates erroneous phase accruals due to second-order concomitant fields on a high-performance gradient system at the source, complementing or replacing software corrections/compensations during image reconstruction.