Magnetic Resonance Imaging System Based on Earth's Magnetic Field

Abstract

This article describes both the setup and the use of a system for magnetic resonance imaging (MRI) in the Earth's magnetic field. Phase instability caused by temporal fluctuations of Earth's field can be successfully improved by using a reference signal from a separate Earth's field nuclear magnetic resonance (NMR) spectrometer/magnetometer. In imaging, it is important to correctly determine the phase of the NMR signal. A reference signal of a fixed‐frequency oscillator cannot be used since the Larmor frequency changes with time, following temporal fluctuations of Earth's magnetic field. The reference signal frequency and phase, provided by a separate NMR spectrometer, change in the same way as Earth's field, creating thereby, a stable rotating frame of reference for the measured signal. In principle, excellent homogeneity of the magnetic field enables scanning of very large volume samples. Reduction in S/N ratio due to the weak magnetic field can be partly compensated by the receiving coil design and shielding of electromagnetic pick‐up in audio frequency (AF) range. The smallest voxel examined so far is on the order of 50 mm3. Unlike in the case of strong magnetic fields, detection and processing of low frequency signal are less demanding for the electronics. The techniques used and the results of measurements are briefly presented.