A permanent magnet assembly for magnetic resonance imaging (MRI) has been developed using a high energy Nd-Fe-B sintered (NEOMAX) magnet. There are three types of magnet assembly for MRI: super-conductive, resistive conductive and permanent magnets. The permanent magnet type is superior to the others in some points. One is the small leakage around the magnetic circuit. The leakage flux is so small that it can be installed in an ordinary hospital without any special shieldings. Although the magnetic circuit has a self shielded configuration, there exists some low level of leakage flux. This leakage field is calculated by using a 3-dimensional integral equation method. This method is well adapted for the solution of these problems. It is found that the measured results agree fairly well with the calculated results, and that there exists two types of leakage flux paths. To decrease the leakage from the top and bottom plates, permanent magnet materials are arranged on the top and bottom plates so as to cancel that leakage. It is shown in consequence that the 0.1 mT leakage flux is reduced to about half using this method.
Sakurai et al. (Sat,) studied this question.
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