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 desig...