Optimum Combination of Pickup Coil Type and Magnetically Shielded Room for Maximum SNR to Measure Biomagnetism

생체신호 측정을 위한 최대의 신호 대 잡음비를 가지는 검출코일의 형태 와 자기차폐실의 최적 조합

We have investigated the optimum combination of the environmental noise condition and type of SQUID pickup coil in order to obtain maximum signal-to-noise ratio (SNR). The measurement probe consists of 1st order gradiometer with pickup coils of 100 mm, 70 mm, and 50 mm baseline length, a 2nd order gradiometer with 50 mm baseline, and a magnetometer. The pickup coils are fabricated by winding Nb wire on a bobbin with 200 mm diameter. Noise and heart signal of a healthy male were measured by various SQUID sensors with different types of pickup coils in various magnetically shielded rooms (MSR), and compared to each other. The shielding factors were found to be 43 dB, 35 dB and 25 dB at 0.1 Hz for MSR-AS, MSR-BS, MSR-CS, respectively. White noises were $3.5\;fT/Hz^{1/2}$, $4.5\;fT/Hz^{1/2}$ and $3\;fT/Hz^{1/2}$ for the 1st order gradiometers, the 2nd order gradiometers, and magnetometer for all MSRs. SNR of the magnetometer was up to 56 dB in MSR-AS, while the 1st order axial gradiometer with 70 mm baseline length was up to 54 dB in MSR-BS. The 2nd order axial gradiometer with 50 mm baseline length of pickup coil was found to be up to 40 dB in MSR-CS.