Compact low field magnetic resonance imaging magnet: Design and optimization

Magnetic resonance imaging (MRI) is performed with a very large instrument that allows the patient to be inserted into a region of uniform magnetic fi eld. The field is generated either by an electromagnet (resistive or superc onductive) or by a permanent magnet. Electromagnets are designed as air cor ed solenoids of cylindrical symmetry, with an inner bore of 80-100 cm in di ameter. In clinical analysis of peripheral regions of the body (legs, arms, foot, knee, etc.) it would be better to adopt much less expensive magnets leaving the most expensive instruments to applications that require the ins ertion of the patient in the magnet (head, thorax, abdomen, etc.). These "d edicated" apparati could be smaller and based on resistive magnets that are manufactured and operated at very low cost, particularly if they utilize a n iron yoke to reduce power requirements. In order to obtain good field uni formity without the use of a set of shimming coils, we propose both particu lar construction of a dedicated magnet, using four independently controlled pairs of coils, and an optimization-based strategy for computing, a poster iori, the optimal current values. The optimization phase could be viewed as a low-cost shimming procedure for obtaining the desired magnetic field con figuration. Some experimental measurements, confirming the effectiveness of the proposed approach (construction and optimization), have also been repo rted. In particular, it has been shown that the adoption of the proposed op timization based strategy has allowed the achievement of good uniformity of the magnetic field in about one fourth of the magnet length and about one half of its bore. On the basis of the good experimental results, the dedica ted magnet can be used for MRI of peripheral regions of the body and for an imal experimentation at very low cost. (C) 2000 American Institute of Physi cs. [S0034-6748(00)01102-3].