Nuclear magnetic resonance (NMR) signals are detected in microtesla fields.
Prepolarization in millitesla fields is followed by detection with an untuned de
superconducting quantum interference device (SQUID) magnetometer. Because the sensitivity
of the SQUID is frequency independent, both signal-to-noise ratio (SNR) and spectral
resolution are enhanced by detecting the NMR signal in extremely low magnetic fields,
where the NMR lines become very narrow even for grossly inhomogeneous measurement
fields. MRI in ultralow magnetic field is based on the NMR at ultralow fields.
Gradient magnetic fields are applied, and images are constructed from the detected
NMR signals.
