Magnetic resonance imaging of time-varying magnetic fields from therapeutic devices.

While magnetic resonance imaging of static magnetic fields generated by external probes has been previously demonstrated, there is an unmet need to image time-varying magnetic fields such as those generated by transcranial magnetic stimulators and radiofrequency hyperthermia probes. A method to image such time-varying magnetic fields is introduced in this study. This article presents the theory behind the method and provides proof of concept by imaging time-varying magnetic fields generated by a figure-eight coil inside simple phantoms over a range of frequencies and intensities using a 7T small animal MRI scanner. The method was able to reconstruct the three-dimensional components of the oscillating magnetic field vector.