Improvement of Ultra-Low Field Magnetic Resonance Recordings With a Multilayer Flux-Transformer-Based High- $T_{\rm C}$ SQUID Magnetometer

We have developed a multilayer flux-transformer-based high-T_C SQUID (flip-chip) magnetometer that improves signal-to-noise-ratios (SNR) in ultra-low field magnetic resonance (ulf-MR) recordings of protons in water. Direct ulf-MR-based benchmarking of the flip-chip versus a standard planar high-T_C SQUID magnetometer resulted in improvement of the SNR by a factor of 2. This gain is attributable to the improved transformation coefficient (1.9 vs 5.3 nT/Φ₀) that increased the signal available to the flip-chip sensor and to the lower noise at the measurement frequency (15 vs 25 fT/Hz^1/2 at 4 kHz). The improved SNR can lead to better spectroscopic resolution, lower imaging times, and higher resolution in ulf-MR imaging systems based on high- T_C SQUID technology. The experimental details of the sensors, calibration, and ulf-MR benchmarking are presented in this report.