Nuclear magnetic resonance (NMR) signals are detected in microtesla
fields. Prepolarization in millitesla fields is followed by detection
with an untuned dc 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.
