- Risk groups for MRI scanning due to magnetic field or contrast agent: Metal in body: Pacemaker / Automatic Implantable Cardioverter Defibrillators / Implantable Cardioverter Defibrillators (coronary defibrillator), Nervus vagus stimulator, Artificial heart valve (depending on type), Metal clips on cerebral arteries or veins, Metal particles in eye, Port-a-cath, Metal stents, Hydrocephalic pump / insulin pump, Metal implants; f/e screws, prostheses, piercings.