SQUID-detected magnetic resonance imaging in microtesla fields

The use of very low noise magnetometers based on Superconducting Quantum Interference Devices (SQUIDs) enables nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) in microtesla magnetic fields. An untuned superconducting flux transformer coupled to a SQUID achieves a magnetic field noise of 10(-1)5 T Hz(-1/2). The frequency-independent response of this magnetometer combined with prepolarization of the nuclear spins yields an NMR signal that is independent of the Larmor frequency wo. An MRI system operating in a field of 132 mu T, corresponding to a proton frequency of 5.6 kHz, achieves an in-plane resolution of 0.7 x 0.7 mm(2) in phantoms. Measurements of the longitudinal relaxation time T, in different concentrations of agarose gel over five decades of frequency reveal much greater T-1-differentiationat fields below a few millitesla. Microtesla NIRI has the potential to image tumors with substantially greater T-1-weighted contrast than is achievable in high fields in the absence of a contrast agent.