Discovery Health answered:
The greatest risks presented by MRI scans involve magnetic objects.
Before allowing a patient or a member of the support staff into the scan room, he or she is screened for metal objects. This screening is not limited to what's in their pockets. Often, patients have implants inside their bodies that make it very dangerous to be near a strong magnetic field. Metallic fragments in the eye, for example, are very dangerous. Moving those fragments can cause eye damage or blindness. Eyes do not form scar tissue like the rest of the body does. A metal fragment that has been in the eye for 25 years is just as dangerous today as when it first entered the eye. No scar tissue holds it in place.
Patients with pacemakers cannot be scanned or even go near the machine because the magnet can make the pacemaker malfunction. Aneurysm clips in the brain also can be very dangerous. The magnet can move them, causing a tear in the very artery the clips were intended to repair. Some dental implants are magnetic as well.
On the other hand, most orthopedic implants, even if they are ferromagnetic, are fine. They are firmly embedded in bone. Metal staples in most body parts also are fine once they've been in a patient for a few weeks. Enough scar tissue will have formed to hold them in place. Every time medical professionals encounter patients with an implant or metallic object in their bodies, they investigate thoroughly to make sure it is safe to scan that patient. Some patients are turned away because it's too dangerous. If this happens, there is usually an alternative imaging method that can help them.
Doctors do not know of any biological hazards to humans from being exposed to the magnetic fields at the strength used in MRI imaging today. Still, most facilities prefer not to image pregnant women. There has not been much research in the area of biological effects on a fetus. The decision to scan a pregnant patient is made with consultation between the MRI radiologist and the woman's obstetrician. The benefit of a scan must outweigh the risk to the fetus and mother ? even if that risk is very small. Pregnant MRI technologists can remain at work in the department. In most cases, they are kept out of the scan room during their pregnancy.The greatest risks presented by MRI scans involve magnetic objects. Before allowing a patient or a member of the support staff into the scan room, he or she is screened for metal objects. This screening is not limited to what's in their pockets.... More