SYSTEMATIC PASSIVE SHIMMING OF A PERMANENT-MAGNET FOR P-31 NMR-SPECTROSCOPY OF BONE-MINERAL

A low-field (0.33 T, 5.688 MHZ) P-31 NMR spectrometer was built for the measurement of bone mineral in the human wrist and heel. The spectrometer uses a relatively low-cost, medium-size, permanent magnet employing neodymium-iron-boron magnet material. Passive shimming by the appropriate addition of steel was enhanced by the use of computer programs which utilized features of ''expert systems'' and ''fuzzy logic.'' Passive shimming procedures, previously developed in shimming a whole-body permanent magnet and summarized in a large database and data files, were used in the shimming procedure. The type of shim required and its location were predictable. In some cases, the size could be approximated by scaling down shims used in the whole-body permanent magnet. In other cases, size had to be determined by trial-and-error. The full field-width inhomogeneity factor of the spectrometer magnet was improved from 3196 ppm to 127 ppm in an earlier shimming procedure. The latter figure corresponds to a full bandwidth of 1362 Hz at the frequency of the spectrometer (5.688 MHz). In studies of excised thoracic vertebrae at 3.52 T (60.7 MHz), the spectral linewidth of bone was about 6000 Hz. In another shimming procedure, where the additional thin faceplate on the bottom pole was absent, a full field-width of 197 ppm was obtained.