Thin film high temperature superconductor RF coil and cryogenic preamplifier for low field MRI

The signal-to-noise ratio (SNR) in low field magnetic resonance imaging (MRI) of small fields of view is degraded by electrical losses in the RF coil conductors. The authors show that a 7 cm diameter high temperature superconductor (HTS) RF coil improves the 0.064 T (2.73 MHz) MRI SNR by 1.7× over the SNR from an equivalent copper coil. The HTS coil was made from an epitaxial Tl₂Ba₂CaCu₂O₈ thin film on a 7.5 cm diameter single crystal LaAlO₃ substrate. The coil´s windings and an interdigital capacitor were lithographically patterned on the wafer. A 1.1× improvement in SNR is further gained on the HTS coil when the room temperature low noise amplifier (LNA) is exchanged for a cryogenic LNA. Besides improving the SNR, the cryogenic JFET LNA also broadens the resonant band-width of the coil, providing easier tuning. The broadening is achieved by using a cooled resistive shunt feedback to the input impedance. The HTS coil and cryogenic LNA together provide a 1.9× SNR improvement over the room temperature equivalent sized copper coil and LNA