The system is for food that contains a lot of water.
The team said improving measurement field homogeneity will speed up its practical use.
Conventionally, Faraday coils have been used to detect magnetic fields but sensitivity decreases with decreasing magnetic resonance signal frequency.
A metallic contaminant detector using SQUID technology was previously described by researchers at Toyohashi Tech.
System components and noise
The system consists of an HTS-SQUID, an LC resonator, a CMSB, a cryostat, SQUID electronics, a measurement coil (Bm), three sets of gradient field coils (Gx, Gy, Gz), an AC pulse coil (BAC), a permanent magnet (Bp) at 1.1 T, and a nuclear magnetic resonance (NMR) spectrometer.
Detection area can be expanded using an LC resonator with the HTS-SQUID but a previous study found environmental magnetic noise became a problem because the system was in a magnetically shielded room (MSR) with the door open.
To overcome this, the team created a compact magnetically shielded box (CMSB), which has a small open window for transfer of a pre-polarized sample.
“When measuring the noise spectrum in the CMSB, white noise was reduced to 0.07 pT/Hz1/2, which was 2.5 times smaller than that measured in the MSR,” they said.
A two-dimensional (2D)-MR image is reconstructed from the grid processing raw data using the 2D fast Fourier transform method, which are taken in the CMSB.
Researchers said they got a clear image of a disk-shaped water sample, with an outer dimension closer to that of the real sample than in the image taken in the semi-open MSR.
Projected workflow
A sample is pre-polarized by the permanent magnet outside of the CMSB.
“When a trigger signal is supplied from the main pulse generator, the pre-polarized sample is then transferred to the appropriate position under the SQUID by compressed air, and exposed to Bm of 93.7 µT from the measurement coil in the z direction,” according to the study.
“Subsequently, by applying a gradient field and an AC pulse field, an MR signal is detected by the LC resonator, and detected by the SQUID. Finally, the MR signal is acquired by the [NMR].”
The sample is pre-polarized using the Bp for more than five second and then positioned under the SQUID within about 0.8 seconds.
“After the transfer, the gradient coil and AC pulse coil apply a gradient field of 27.7 µT/m…and a 90° pulse field BAC, respectively. The echo time (tE) of the 90° and 180° pulses is 500 ms. After the 180° pulse, the LC resonator becomes high Q-factor state by turning on the control signal of TTL logic.
“The sensitivity is improved and the detection area is enlarged at the resonant condition. Then, 512 points are acquired during an acquisition time (tacq) of 512 ms.
“In this study, radial scanning with a spin echo technique is used. The number of projection angles is 24, because the gradient field directions are rotated for 7.5° step-by-step to cover 180°.
The work was supported by The Knowledge Hub of Aichi, The Priority Research Project from Aichi Science & Technology Foundation.
Source: Physica C: Superconductivity and its Applications
“Ultra-low field MRI food inspection system prototype”
Authors: Satoshi Kawagoe, Hirotomo Toyota, Junichi Hatta, Seiichiro Ariyoshi, Saburo Tanaka