Cryogenic Array Coil for Brain Magnetic Resonance Imaging

Combination of Magnetic Resonance Imaging (MRI) and Dissolution Dynamic Nuclear Polarisation (dDNP) promises faster diagnosis and treatment of cancer. To make the technology clinically viable, radio-frequency receivers with high Signal-to-Noise Ratio (SNR) are needed. This work investigates three aspects of radio-frequency receiver design: theoretical limits of preamplifier decoupling between receiver elements, matching network design, and cryogenic coil arrays. Preamplifier decoupling is a technique used widely to avoid de-tuning coil elements in coil arrays. To investigate preamplifier matching/decoupling, the quantitative relation between preamplifier Noise Figure (NF) and preamplifier decoupling is established using preamplifier parameters. Closed-form design equations for matching circuits with arbitrary preamplifier input impedance are derived. This theory relaxes restrictions on preamplifiers and greatly improves design flexibility. The theory is confirmed by numerical examples and experimentally tested in preamplifiers at 32.13 MHz. The potential of SNR improvement of a single coil is investigated, and compared with the SNR loss of different matching network designs. A mouse coil is matched by two types of matching networks at room and cryogenic temperatures, and compared in regard to output SNR. It is found that loss of matching circuits have higher influence on the SNR than the temperature of coils and matching circuits. Experiments reveal that an improper design of matching network reduces SNR by 7.9 dB, whereas cryogenic cooling only compensates for 2.0–2.6 dB. This stresses the importance of careful matching network designs. Finally, a cryogenic 14-channel array for human brain imaging is constructed to investigate SNR improvement of coil arrays by cryogenic cooling. A cryostat for human head that offers reliable cooling for approximately 1 h and decent thermal insulation is built. Electronic circuits are adapted to cryogenic temperature and strong static magnetic field operation. The MR image SNR when using the array at cryogenic temperature is found 27%–168% higher than at room temperature, with SNR improvement 47% at the image centre. Thus, it is demonstrated that SNR can increase by cryogenically cooling an array.