1. A method of dynamically controlling an implanted medical device located within a patient's body in the presence of a magnetic resonance imaging (MRI) field, the method comprising: detecting the presence of an MRI field within the body; measuring the response of an excitation voltage or current applied to an implantable lead in the presence of the MRI field; comparing the measured response obtained in the presence of the MRI field with a predicted response outputted by a lead and tissue reference model, the model including one or more model parameters stored within a memory unit for modeling the lead and body tissue within the body; and dynamically modifying a voltage or current applied to the lead based at least in part on the predicted response outputted by the model.