Inside-out NMR with two concentric ring magnets

We report the design and the implementation of an inside-out NMR sensor that produces a large sensitive region with substantially uniform magnetic field at a remote location. The construction using a pair of ring magnets is simple yet provides multiple benefits, including large sample volume, operation with low RF power, and the ability to measure samples with long T₂ and high diffusivity. A palm-size inside-out NMR sensor (57 mm OD × 29 mm height, 420 g including the housing and the coil PCB) was built with inexpensive magnets. The sweet spot is located ∼5 mm above the magnet surface with ∼4 mm width and ∼5 mm height assuming t₁₈₀ = 18 μs. The field strength at that point is 0.16 T and achieved SNR ∼23 per two scans when operated with ∼10 W peak RF power. Its quasi-uniform B₀ around the saddle point allows the measurement of T₂ = 1.5 s with a 100 μs echo time.