Main Magnetic Field and Safety

The main magnetic field of a 1.5 T magnet is about 30,000 times the strength of the earth's magnetic field. It is strong enough to pull fork-lift tines off of machinery, pull heavy-duty floor buffers and mop buckets into the bore of the magnet, pull stretchers across the room and turn oxygen bottles into flying projectiles. Deaths have occurred from trauma as a result of these effects. Smaller objects such as pagers, bobby pins and pens have been known to be pulled off the person carrying them.
The strong field also effects common devices such as pacemakers and watches. The magnetic reed switch in modern pacemakers is disturbed by strong magnetic fields resulting in possible deleterious effects to the patient with one implanted. Mechanic watches will "freeze up" in a strong field, sometimes permanently.
Many intracranial aneurysm clips are ferromagnetic and as a result experience a torque or twisting in a magnetic field. Not everyone with an aneurysm clip experiences a fatal hemorrhage when placed in a magnet, but several cases have been reported.
Some types of heart valves (e.g., Star-Edwards) are torqued in a magnetic field: however, this torque is less than the stresses that occur normally as a result of blood flow. Therefore heart valves are now considered not to be an absolute contraindication for MRI.
More of an annoyance than a safety problem is the ability of the magnetic field of a MRI machine to erase the information contained on the magnetic strip on ATM and credit cards. This may occur a short distance inside of the scanner room of a MRI machine.
Some metallic objects that are usually safe near an MRI machine are gold jewelry and eyeglass frames.