Systems and methods for sensing external magnetic fields in implantable medical devices

A system, comprising:

• a stimulation lead having a proximal portion and a distal portion;
• at least one electrode along the distal portion of the at least one lead;
• and
• an implantable medical device, coupled to the proximal portion of the lead, the implantable device including:
• a stimulator adapted to generate stimulation signals with appropriate signal characteristics to provide therapy;
• and
• a controller to communicate with the stimulator, the controller being adapted to deliver an electrical signal to the at least one electrode to provide therapy;
• a sensing circuit including at least one inductor having a magnetic core that saturates in the presence of a magnetic field having a prescribed flux density of a magnetic resonance imaging device;
• and
• an impedance measuring circuit connected to the sensing circuit, the impedance measuring circuit adapted to periodically measure impedance of the sensing circuit and to provide a signal to the controller when the impedance changes by a prescribed amount.