Methods and apparatus for determining the location of a magnetic probe within the anatomy of a patient by means of ESR

A magnetic probe 1 for use with a magnetic resonance imaging apparatus comprises an electron spin resonance (ESR) magnetometer. The position of the probe 1 tracked by measuring the resonant frequency of an ESR sample in the probe. The resonant frequency is measured in the presence of a gradient sequence which employs three orthogonal, linear gradients. By determining the resonant frequency of the ESR sample in the presence of each of the gradients, the position of the probe can be determined. The probe may be placed on or in an object, and the position of the probe may be displayed on a desired magnetic resonance image of the object.