Magnet and gradient coil system for low‐field EPR imaging

Air‐core coils are more practical than iron‐core magnets for producing the required magnetic field distribution for electron paramagnetic resonance imaging at frequencies of a few hundred megahertz. An air‐core, air‐cooled magnet/gradient coil system is presented in this paper, which is optimum in the sense that mathematically it produces fields, which for the number of coils specified, are as uniform (for the magnet) and as linear (for the gradient coils) as is possible over a spherical volume. The magnet is four‐coil (eighth‐order), the z‐coordinate gradient coil system is two‐coil, Maxwell (fifth order), and the transverse gradient coils are four‐coil, Anderson (fifth order). The design tradeoffs included homogeneity and linearity specifications, compatibility with access needed for physiological studies, power requirements, and provisions for rapid scanning. Incremental adjustments of the positions of the magnet coils, based on uniformity measurements, facilitated the meeting of design specifications at a reasonable cost. The 81 cm magnet can operate continuously at 90 G and can scan to 140 G. A 15 cm diameter working volume has a homogeneity of about 40 ppm. Three‐dimensional gradient coils are designed for 10 G/cm.