Three-Dimensional Nonaxisymmetric Pole Piece Shape Optimization for Biplanar Permanent-Magnet MRI Systems

The pole pieces of a permanent-magnet system for magnetic resonance imaging (MRI) are optimized for maximum magnetic field homogeneity in the imaging volume. Axisymmetric and nonaxisymmetric optimized pole piece designs based on the same geometrical parameterization are presented. The optimization algorithm demonstrated is based on field calculations by the three-dimensional finite-element method and takes into account the nonlinearity of the magnetic materials involved. The necessity of the nonaxisymmetric design in obtaining suitable field homogeneity over a large imaging volume is demonstrated through comparison of various pole piece designs for a novel biplanar permanent-magnet assembly with reduced pole dimensions as required in the development of an integrated linear accelerator and MRI system for real-time image-guided adaptive radiotherapy. The sensitivity of the field inhomogeneities to geometrical variations in the nonaxisymmetric design surface is explored and statistical parameters quantifying this sensitivity are approximated.