NMR apparatus compensated for primary field changes

Claim 1

An NMR diagnostic imaging apparatus comprising means for positioning a field intensity measuring object of a given shape and having known properties in the vicinity of a subject to be examined such that said field intensity measuring object and said subject are not overlapped at least in one direction of projection; means for applying a primary magnetic field to said subject and to said object; means for applying RF pulses that give nuclear magnetic resonance to the nuclei of atoms constituting the tissue of said subject and of atoms of said object; means for applying gradient magnetic fields to project an NMR signal of said nuclei of atoms at least in one direction and an NMR signal of said object; means for phase-detecting and monitoring the NMR signals projected by said gradient magnetic field applying means and representing the subject and the object; and arithmetic means for constructing an image of the tissue of the subject from an output signal from said monitoring means; wherein said means for phase-detecting and monitoring said NMR signals comprises means for detecting the phase of the NMR signals at different time intervals and comparing the phase with the RF pulses, means for monitoring the Larmour frequency of the nuclei of atoms in said object, and means for selectively controlling the oscillation frequency of the RF pulses to conform to the Larmour frequency when the phase is detected whereby the variations in the primary magnetic field have no effect on the constructed image.