Adaptive Radiofrequency Shimming in MRI using Reconfigurable Dielectric Materials

TL;DR

This paper introduces a novel, cost-effective method for adaptive B1+ shimming in MRI using reconfigurable dielectric materials with switchable connections, improving field homogeneity at high field strengths.

Contribution

It presents a new approach employing switchable high permittivity materials for local B1+ modulation, demonstrated through simulations and phantom experiments at 3T.

Findings

Achieved up to 11% B1+ modulation in simulations.

Demonstrated up to 11.6% modulation in phantoms.

Observed up to 6% modulation at the human heart depth.

Abstract

Inhomogeneity of the transmitted radiofrequency field B1+ is a major factor hindering the image quality in Magnetic Resonance Imaging (MRI) at high field strengths. Here, a novel approach is presented, to locally modulate the B1+ utilizing an array of high permittivity materials with switchable connections. A 3$\times$3 array of barium titanate suspension elements was constructed, with two PIN diode-based switchable connectors per element. Electromagnetic simulations were performed to determine configurations that produce strong B1+ modulation. Remote B1+ field switching was tested in a disk- and and a torso-shaped phantom at 3T by applying different bias voltages to the PIN diodes. The attained B1+ modulation was assessed at various switching pattern positions and various depths within the phantoms. The configuration with the strongest effect size has produced up to 11% modulation in simulations at 15 mm depth, with excellent translation properties. The effects were successfully replicated in phantoms, with a 5 V bias voltage producing up to 11.6$\pm$0.2% modulation. At the relative depth of the human heart, up to 6% of modulation was observed in the torso phantom. The presented method may provide a promising direction for cost-effective, and adaptive B1+ shimming without changes to the scanner hardware.