An Integrated Unipolar Ultrasonic Gradient for Mitigating PNS and Aliasing in Accelerated Silent MRI

Abstract

In this work, we propose an integrated unipolar ultrasonic gradient coil designed for head imaging at ultrahigh field (7T) to mitigate both neck/shoulder aliasing artifacts and peripheral nerve stimulation (PNS) encountered in accelerated silent MRI. Unlike conventional bipolar gradient coils, the unipolar design generates only one magnetic field lobe positioned away from the body, fundamentally eliminating aliasing caused by overlapping signal regions near the neck and shoulders. Additionally, the asymmetric field shape significantly improves PNS performance theoretically by eliminating maximum magnetic field peak within the patient’s body. The coil was optimized via a simulated annealing algorithm that balanced gradient efficiency, inductance, and linearity constraints in a compact plug-and-play design. Electromagnetic simulations demonstrate that the unipolar gradient field avoids backfolding artifacts and supports high acceleration factors through point spread function (PSF) reconstructions at a 20 kHz switching frequency. A prototype coil, with an efficiency of 0.25 mT/m/A and inductance of 263 µH, was manufactured and tested on a 7T system using both spherical and head phantoms. Experimental measurements showed minimal coupling with the existing RF coil (less than 3% deviation in B1-mapping), confirming that the unipolar gradient can be seamlessly integrated without specialized transmit/receive hardware. The measured field distribution closely matched theoretical predictions, and retrospective undersampling further verified the feasibility of highly accelerated acquisitions. Overall, this unipolar gradient coil addresses major limitations of conventional bipolar coil designs, providing a cost-effective solution for localized head imaging with reduced PNS risk and improved artifact suppression. Index Terms—Unipolar gradient coil, Aliasing, 7T Ultrasonic MRI, Accelerated MRI, Head imaging.