This question came up earlier on another post and I thought it was worth bringing up again. Can I have an MRI with certain implants, or metal - and also what tests can you have?!
First of all..."Tell the technologist if you have any metal or electronic devices in your body, such as metallic joint prostheses, artificial heart valves, implanted electronic devices, cochlear implants or magnets in your dentures. The presence of metal in your body may be a safety hazard or affect a portion of the MRI image."
"Don't receive an MRI scan if you have an implantable cardioverter defibrillator or pacemaker. The strong magnetic field produced by the MRI unit may interfere with the function of these devices."
http://www.mayoclinic.com/invoke.cfm?id=SM00035
also see...
http://www.americanheart.org/presenter.jhtml?identifier=3005170
A lot of these metal implants were around before MRI's, so there are other tests that may be done in it's place such as ultrasounds, CAT Scans, Blood Tests and a few others, depending on the Dr. and the need.
Hugs, Teri 
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