An Optimized 2 MHz Unilateral Magnet with a Large Homogeneous Sensitive Spot

We introduce an optimized design for a three-magnet array unilateral magnetic resonance (MR) device. The sensitive spot is 0.5 cm 3 , and begins roughly 0.5 cm from the magnet surface with a field variation of less than 1.5% of the B 0 resonant frequency. 3D simulation was used in conjunction with a trust-region optimization method to determine the optimal magnet geometry to achieve a large sensitive spot. A standard surface coil was used to excite and detect the MR signal from the sensitive spot. The array has dimensions of 8.4 × 7.4 × 4.1 cm and a mass of 0.74 kg. The surface of the magnets are shielded with a thin layer of copper tape to avoid acoustic ringing. Attenuation of the B 1 -field due to eddy currents in the copper sheet was reduced by displacing the coil from the surface, at the cost of working distance. The quality factor and B 1 -field attenuation due to eddy currents are explored experimentally by incrementally displacing the coil from the magnet surface. A minor reduction in working distance increases the sensitivity of the measurement. To assess device performance, T 1 , T 2 , T 1 – T 2 , and diffusion measurements were undertaken with a cod liver oil phantom.