Design and construction of a second generation high-resolution MPI field free line scanner

There are two classes MPI selection fields: Field Free Point (FFP) magnets and Field Free Line (FFL) magnets [1, 2, 3]. A FFL selection field has three distinct benefits over a FFP selection field, namely (a) improved spatial resolution, (b) improved speed, and (c) improved SNR which scales with the number of projections [4]. Spatial resolution improves because the FFL is isotropic and hence uses the saturation magnetization of the pole material more efficiently. For example, a high strength small animal selection field magnet built with NdFeB permanent magnets or iron would be 10 T/m × 5 T/m × 5 T/m in a FFP magnet versus 10 T/m × 10 T/m in a FFL magnet. Imaging speed for the FFL is improved because the signal is integrated along the FFL axis, allowing for acquisition of projection images more than 100-times faster. This increase in imaging speed can also be traded for sensitivity through averaging, giving the FFL an order of magnitude sensitivity improvement for a given scan time. Three-dimensional imaging can be done using projection reconstruction [4] or iterative reconstruction techniques [3].