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 th e measurement of bone mineral in the human wrist and heel. The spectro meter uses a relatively low-cost, medium-size, permanent magnet employ ing neodymium-iron-boron magnet material. Passive shimming by the appr opriate addition of steel was enhanced by the use of computer programs which utilized features of ''expert systems'' and ''fuzzy logic.'' Pa ssive shimming procedures, previously developed in shimming a whole-bo dy permanent magnet and summarized in a large database and data files, were used in the shimming procedure. The type of shim required and it s location were predictable. In some cases, the size could be approxim ated 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 f ield-width inhomogeneity factor of the spectrometer magnet was improve d from 3196 ppm to 127 ppm in an earlier shimming procedure. The latte r figure corresponds to a full bandwidth of 1362 Hz at the frequency o f the spectrometer (5.688 MHz). In studies of excised thoracic vertebr ae at 3.52 T (60.7 MHz), the spectral linewidth of bone was about 6000 Hz. In another shimming procedure, where the additional thin faceplat e on the bottom pole was absent, a full field-width of 197 ppm was obt ained.