Development of Desktop MR Microscopes

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Compact MR microscopes were developed using yokeless permanent magnets. Two different size permanent magnets with 1.0 T central magnetic fields were used ; larger one : gap width ; 60 mm, weight ; 210 kg, size ; 36 cm x 32 cm x 28 cm, homogeneous regions ; 20 mm dsv (13 ppm), and smaller one : gap width ; 41 mm, weight ; 85 kg, size ; 27 cm x 24 cm x 18 cm, homogeneous regions ; 13 mm dsv (13 ppm). The most serious problem in the development of the MR microscopes was temperature drift of the central magnetic field mainly due to the large negative temperature coefficient (-1200 ppm/deg) of the residual magnetic flux density of the Nd-Fe-B magnetic material. Long-term changes of magnet temperatures and Larmor precession frequencies were measured under various thermal conditions of the magnets. As a result, we concluded that the internal NMR locking technique in a time-sharing mode of which time interval was several minutes could compensate the frequency drift and present high-resolution 3D images of chemically fixed animal samples with less than 100 microns cube voxel dimensions.