Ans. is 'a' i.e. Automatic Cardioverter-defibrillators MR is considered among the safest imaging modalities for patients, even at very high field strengths, more than 3-4 tesla. But Ferromagnetic objects under magnetic field can be vulnerable to 4 adverse effects: Movement (causing structural injury), Current conduction (potentially causing electrical shock), Heating (possibly causing bum injury), and Artifact generation Serious injuries can be caused by attraction of ferromagnetic objects into the magnet, which would act as missiles if brought too close to the magnet. Ferromagnetic implants, such as aneurysm clips, may torque (turn or twist) due to the magnetic field, causing damage to vessels and even death. Metallic foreign bodies in the eye have moved and caused intraocular hemorrhage. Pacemakers and pacemaker leads are a contraindication, as the pacemaker can malfunction and cause arrhythmia or even death. However with growing expansion of MR, increasing number of implant medical devices are being MR safe. So newer pacemaker and aneurysm clips are being made which are MR safe. Absolute Contraindications for the MRI scan: Electronically, magnetically, and mechanically activated implants Ferromagnetic or electronically operated active devices like automatic cardioverter defibrillators Cardiac pacemakers- but there are pacemakers available from Medtronic which are safe in MRI environment but not so widely used. Metallic splinters in the eye Ferromagnetic hemostatic clips in the central nervous system (CNS) Patients with an implanted cardiac pacemaker have been scanned on rare occasions, but pacemakers are generally considered an absolute contraindication. Relative Contraindications for the MRI scan: Cochlear implants Other pacemakers, e.g. for the carotid sinus Insulin pumps and nerve stimulators Lead wires or similar wires (MRI Safety risk) Prosthetic heart valves (in high fields, if dehiscence is suspected) Hemostatic clips (body) Non-ferromagnetic stapedial implants Women with a first-trimester pregnancy Tattoos (only a problem in higher-strength magnetic field i.e. more than 3 tesla)