A constant gradient unilateral magnet for near-surface MRI profiling

The design and construction of a unilateral NMR (UMR) magnet assembly for near-surface 1D profiling is presented. The arrangement consists of a single permanent magnet topped with a shaped iron pole cap. The analytically determined profile of the pole cap shapes the field over the magnet, giving a constant gradient of 31 G/cm over a 8 mm depth at a 1H frequency of 4.26 MHz in a spot ∼5 mm wide. The moderate gradient allows 1D profiling of planar samples with a frequency encoded spin-echo experiment. The curvature of the magnetic field limits the available resolution to 100’s of μm. The device is suitable for profiling planar samples in which a coarse resolution but large spatial extent is desired.