What do we mean by this? Well, the main magnet (B0) is always on. MR magnets are very strong. It will attract, with a strong force, many types of metallic objects. This can be very dangerous as they can become FLYING OBJECTS and accelerate towards the center of the magnet. Guess who is often in the center of the magnet...THE PATIENT! When uncertain, error on the side of caution and assume that a metallic object is not MRI compatible. Below are a few examples of MRI accidents. Fortunately no patients were in these magnets.
Whenever entering the MRI room as a healthcare provider you will should frisk yourself for a moment to think about what you have on you. Look for jewelery, a watch, keys, your pager, phone, and credit cards. These could become flying objects, cause burns, or cause permanent damage to the device. In the case of credit cards, they will be erased (unfortunately without erasing your bill!). Additionally, if you have personal medical devices or if you are at risk for having shrapnel near your orbit you should undergo an MRI safety screen.
Similarly, metallic objects inside a person, whether medical devices (aneurysm clip, pacemaker) or shrapnel from an accident or occupational hazard (veteran who was injured, metal worker) can be a safety hazard. First, these objects can heat up much like they would in a microwave. Have you ever put metal in the microwave accidentally (sparks!)? Second, they can move when they enter the strong magnetic field. This could cause vascular, nerve, or organ injury. This is of particular concern in the orbit where small metal fragments could be very close to the orbit, optic nerve, vessels, and extra-ocular muscles. Not all metals and not all devices cause a problem. For example, most orthopedic hardware and modern vascular clips are MRI safe. Although they are outside patients, tattoos can also become very warm with certain scans and may need to be handled specially.
Mechanical and electronic devices both internal and external to the patient can create danger as mentioned due to it becoming a dangerous accelerating projectile, migrate within a person (patient or employee), or heat up. Equally dangerous can be the malfunctioning due to the magnetic field. For example, while there are a few pacemakers that are MRI conditional, many conventional pacemakers are not. In theory, they may cease to function or potentially misfire.
The table below is a good summary of what items and medical devices are typically safe and unsafe with MRI. When in doubt, check it out or call radiology to inquire.
Some patients will have had or anticipate having difficulties with entering a small tube for an extended period of time. Often this can be solved with oral sedatives/anxiolytics but in rare occasions if the MR study is medically necessary general anesthesia can be administered.
For checking the MRI compatibility of medical devices, MRISafety.com is a valuable reference.