Design and experimental evaluation of a small-size, high-precision nuclear magnetic resonance probe for enlarged magnetic field measurement region on small-bore magnetic resonance imaging magnets

High-quality magnetic resonance imaging (MRI) relies on accurate magnetic field characterization; however, commercial nuclear magnetic resonance (NMR) magnetic field probes are often limited in boundary-accessible field mapping for bore-constrained magnets. In this study, a miniature free-induction-decay-based NMR magnetic field probe was developed, integrating a solenoid microcoil, a compact tuning/matching network, and a liquid sample unit, with a maximum width of ∼8 mm. Compared with a commercial NMR reference probe, the developed microprobe showed close agreement in single-point validation and yielded a mean absolute deviation of 0.27 μT in axial comparison measurements (corresponding to 0.039 ppm at 7 T), confirming its field-measurement accuracy and spatial tracking capability. Temperature-sensitivity tests over a range of 15-35 °C showed that the repeated-measurement standard deviation remained within 0.019-0.046 ppm, indicating good measurement stability. In wall-adjacent measurements inside a 172 mm-bore magnet, the maximum accessible trajectory diameter increased from 152 to 164 mm, corresponding to a 7.9% improvement in an accessible mapping range. Spherical-surface mapping of a 50 mm diameter spherical volume further yields a mean absolute deviation of 0.143 μT and a Pearson correlation coefficient greater than 0.99. These results demonstrate that the proposed microprobe provides a low-cost, stable, high-precision, and broadly applicable solution for extending accessible magnetic field mapping in bore-constrained MRI magnets.