Years ago I needed an MRI for my left knee.  Before I got into the open MRI contraption, the MRI technician told me I had to remove my right above knee prosthesis.  Good thing!

According to the Princeton's MRI Certification Review Material, there are certain items or conditions that disqualify, with exceptions, an individual from participating in an MRI.  These items and/or conditions include:

• Pregnancy
• Aneurysm clip
• Cardiac pacemaker
• Implanted cardioverter defibrillator
• Electronic implant or device
• Magnetically activated implant or device
• Neurostimulation system
• Cochlear implant or hearing aid
• Spinal cord stimulator
• Insulin or infusion pump
• Internal electrodes or wires
• Bone growth/bone fusion stimulator
• Any type of prosthesis (eye, penile, etc)
• Heart valve prosthesis
• Artificial prosthetic limb
• Metallic stent, filter, or coil
• Swan-Ganz or Thermodilution catheter
• Joint replacement (hip, knee, etc)
• Aneurysm
• Seizures
• Stroke
• Parkinson's
• Meniere's Disease
• Any other neurological disease

The technical term from an MRI is magnetic resonance imaging.  Unlike an x-ray, an MRI uses magnetism and radio waves, which are sent through the body and create incredibly detailed pictures of a person's anatomy. 

The magnetic pull of an MRI can be so powerful that it can attract certain metals.  If a metal wrench is pulled into an MRI machine from only two feet away, that wrench will travel up to 40 miles per hour.  That goes the same for metal in or on a person's body.  People have actually been killed in situations like these, so it's important to be aware of what's around you, in you and on you.  Not only can an artificial limb become a dangerous flying object, microprocessor components (like the C-Leg) will be totally fried.