EVALUATION OF RADIO-FREQUENCY MICROCOILS AS NUCLEAR-MAGNETIC-RESONANCE DETECTORS IN LOW-HOMOGENEITY HIGH-FIELD SUPERCONDUCTING MAGNETS

We describe here experiments evaluating the performance of solenoidal radio frequency probes having submillimeter dimensions (microcoils) as detectors for liquid nuclear magnetic resonance (NMR) in very low- ho mogeneity (100 ppm/ cm) magnetic fields. Performance is based on the m easured H2O linewidth. A series of solenoidal microcoils having sample volumes 8, 53, and 593 nl were filled with distilled H2O and evaluate d for smallest obtainable unshimmed NMR spectral linewidths in a verti cal bore superconducting magnet, stabilized at 5.9 T (H-1 frequency525 0 MHz). The smallest microcoil (472 mu m diameter) gave a smallest H2O linewidth of 525 Hz, 25 times mm smaller than that from a standard 5. 7 mm probe. Linewidth increased approximately as the square root of sa mple volume. For comparison, shimmed H2O linewidths using the same mic rocoils in a high-homogeneity (0.1 ppm/cm) NMR magnet were also measur ed. Shimmed linewidths in the high-homogeneity magnet were two orders of magnitude smaller and exhibited a similar dependence on volume. The results demonstrate that by using microcoils the volume over which th e polarizing magnetic field must meet a specified homogeneity can be s ignificantly reduced, which would be advantageous for smaller, less ex pensive NMR systems. (C) 1998 American Institute of Physics. [S0034-67 48(98)01211-8].