A Cryogenic Tune and Match Circuit for Magnetic Resonance Microscopy at 15.2T

TL;DR

This study demonstrates that cooling only the tune and match circuitry in a cryogenic environment at 15.2T MRI significantly enhances SNR, enabling better imaging without extreme sample cooling.

Contribution

A novel cryogenic chamber design that cools only the RF tune and match circuitry, achieving substantial SNR gains while keeping the sample at ambient temperature.

Findings

SNR improved by a factor of 2 with cooled circuitry

Significant Q-factor increases observed for coils < 3 mm diameter

Resistance of components reduced by up to 45% at cryogenic temperatures

Abstract

Signal to noise ratios (SNR) in magnetic resonance microscopy images are limited by acquisition times and the decreasing number of spins in smaller voxels. Significant SNR gains from cooling of the RF receiver are only realized when the Johnson noise generated within the RF hardware is large compared to the electromagnetic noise produced by the sample. Cryogenic cooling of imaging probes is common in high field systems but proves difficult to insulate the sample from extreme temperatures. We designed a chamber to cool only the tune and match circuitry to show it is possible to achieve much of the available SNR gain available for cooled coils. We designed a microcoil circuit to resonate at 650 MHz for imaging on a 15.2 T scanner. Surface loops and solenoids of varying diameters were tested to determine the largest diameter coil that demonstrated significant SNR gains from cooling. A liquid N2 cryochamber was designed to cool the circuitry while leaving the RF coil in ambient air. As the cryochamber was filled with liquid N2, Q-factors were measured on the bench while monitoring the coil's surface temperature. Improvements of SNR on images of solutions were demonstrated via cooling the tune and match circuit in the magnet bore. At 650 MHz, loops and solenoids < 3 mm in diameter showed significant improvements in quality factor on the bench. The resistance of the variable capacitors and the coaxial cable were measured to be 45% and 32% of room temperature values near the Larmor frequency. Images obtained with a 2 turn, 3 mm diameter loop with the matching circuit at room temperature and then cooled with liquid nitrogen demonstrated SNR improvements of a factor of 2. By cooling the tune and match circuit and leaving the surface loop in ambient air, SNR was improved by a factor of 2. The results are significant because it allows for more space to insulate the sample from extreme temperatures.