In order to encode spatial information onto the signal from processing nuclei for MRI, the magnetic field strength is made a function of position and time with the application of Gradient fields throughout the imaging process. Gradient fields are typically switched on and off thousands of times in the acquisition of a single image. The switching of the gradient coils (or magnets) in the presence of the large static magnetic field, B0, creates large forces and the loud noise associated with MRI. The rapidly-changing magnetic fields during switching also induce unwanted electrical fields throughout the human body. These electric fields have been known to simulated nerves resulting in sensory perception (feeling a buzzing sensation, or even pain) or in involuntary muscle contraction. Fortunately these effects tend to occur at the surface of the body and anything truly detrimental - such as induction of cardiac contraction - is extremely unlikely.
We have used a variety of methods to calculate the electric fields induced within and surrounding the human body during switching of the gradient fields. There are still a lot of questions to be answered and experiments to be performed before it is clear how well these can be used to predict peripheral nerve stimulation (PNS) in MRI.
