A READOUT MAGNET FOR PREPOLARIZED MRI

Conventional MRI systems rely on large magnets to generate a field tha t is both strong and extremely uniform. This field is usually produced by a heavy permanent magnet or a cryogenically cooled superconductor. An alternative approach, called prepolarized MRI (PMRI), employs two separate fields produced by two different magnets. A strong and inhomo geneous magnetic field is used to polarize the sample. After polarizat ion, a weak magnetic field is used for readout. These fields can be pr oduced by two separate resistive electromagnets that cost significantl y less than a single permanent or superconducting magnet. At Stanford, the authors are constructing a PMRI prototype scanner suitable for im aging human extremities roughly 20 cm in diameter. With this system th e authors hope to demonstrate comparable image quality to MRI with red uced system cost. The authors' initial work on low-frequency reception indicates that it will be possible to obtain comparable image signal- to-noise ratio to an MRI scanner operating at the same polarizing fiel d strength. To reduce the capital cost of the system, the authors use resistive electromagnets. Here the authors discuss the full developmen t of the readout magnet including important design considerations, shi mming, and field plots. These encouraging results are an important ste p toward evaluating the cost effectiveness of PMRI.