Static magnetic fields as low as 0.75 mT1 to 1.7 mT have been reported to cause deflection, programming changes and closure of the reed switch which converts the pacemaker to asynchronous mode, in some pacemakers. For these reasons cardiac pacemakers are an absolute contraindication to MRI. As of early 2004, there had been 13 pacemaker related deaths1. In virtually every case, the patient apparently entered the MR environment without the staff knowing a cardiac pacemaker was present.
Note: there are an estimated 10 million MRI scans made per year.
1Online column, Frank Shellock, Jan 2004.