Flexible Selection Field Generator Based On Adjustable Halbach Dipole Configurations

Abstract

In magnetic particle imaging, a promising high-performance diagnostic technique, spatial encoding is based on a selection field with a field gradient forming a field-free region. The here proposed field generator assemblies enable a complete 3D encoding requiring only one electromagnetic excitation coil, while the trajectory is generated by exploiting the use of mechanically moving double-layer permanent magnet arrangements, implemented as adjustable Halbach dipole configurations. A simulation study and demonstrator implementation were conducted to provide an understanding and assessment of the trajectories and associated scanner sequences. The study serves as a basis for evaluating the imaging capabilities of this field generator type. Magnetic dipole moments are used to approximate the far field of the magnets, and the scaled moments are organized with the appropriate angular position and orientation. The magnetic flux density of the field generator is computed for each discrete sequential step by superposition of all Halbach array fields, using a solid spherical harmonic expansion as efficient representation. For comparison purposes, a demonstrator, designed using finite-element methods, was constructed, fabricated, and measured using the same expansion. A maximum absolute deviation of 2.2mT was observed between measurements and simulation, while both confirmed a gradient strength of 0.8 T/m for the initial antiparallel dipole configuration. Although currently rather specific to the initial chosen configuration, the developed simulation environment provides a tool for the validation of possible field generators based on adjustable Halbach dipoles, with results that can be compared qualitatively and quantitatively with those of other simulation environments and the developed demonstrator.