A magnetometer is provided comprising an atomic vapor in an enclosure, a source of light for preparing the vapor into a state exhibiting electromagnetically induced transparency, a first laser beam passing through the atomic vapor, a phase detector for detecting changes in phase of the first laser beam, and a controller which controls the light source and laser beam and receives the information detected by the phase detector in order to compute from those changes in phase a magnetic field strength in the presence of a selected background magnetic field of at least 0.001 T. Operation in the presence of a background field helps make this magnetometer suitable for diagnostic imaging applications.