A portable, ultra-Low cost NMR device

Abstract

Nuclear magnetic resonance (NMR) provides powerful measurements that remain inaccessible in many applications due to the instruments' size and expense. Recent research efforts have focused on creating handheld devices with lower resolution but greatly reduced cost. Persistent challenges include implementing a miniature magnet with sufficiently homogeneous magnetic field, and isolating the weak NMR signal from the powerful excitation pulses. In this thesis, we demonstrate a magnet design and experimental technique to address these needs. A significant cost for a small NMR magnet is associated with the extensive labor for assembly and correction of field variations. To alleviate this difficulty, we optimized and constructed a self-assembling NMR magnet. The palm-sized assembly, called a shim-a-ring, had a mass of 418 g. The magnetic field strength was 0.48 T, large enough to perform spectroscopy.

To ease the process of correcting the field, we propose an active shim system, which would eliminate much of the labor required with other strategies. Electromagnetic shims were optimized to correct 14 lower-order spherical harmonics with minimal power consumption. When comparing the efficiency of the shims to the correction needed in the shim-a-ring magnet, the required current was found to be too large for steady-state operation. In short experiments, however, the strategy was shown to be feasible, with heat dissipation causing only a negligible temperature change. Stochastic excitation provides a low-power alternative to standard NMR techniques. With the pulse amplitudes reduced by orders of magnitude, isolating the signal from the excitation is much less challenging. Experiments performed in the shim-a-ring magnet demonstrated this benefit.

Although the magnetic field variations were too large for spectroscopy, the initial amplitude of the impulse response was proportional to the number of resonant nuclei in the sample, called the spin density. The ratio of water and heavy water contained in a sample was characterized using this technique.