More than 2 million Americans have implanted pacemakers and defibrillators to keep their hearts beating well or to protect them against sudden death from an irregular heart rhythm.

However, people with these implants are barred from MRI (magnetic resonance imaging) tests commonly used to diagnose a variety of serious diseases, including cancer. The high-energy radio waves emitted by MRI machines can heat the devices enough to burn surrounding heart tissue and also confuse their electronic functioning.

To study the effect of MRI waves on pacemakers and defibrillators, researchers at Johns Hopkins first subjected 24 pacemakers made after 1996 and defibrillators manufactured after 2000 to very high doses of MRI energy to determine if they became dangerously hot.

The next step looked for other safety issues when these devices were implanted into animals. The Hopkins team has now followed special procedures to safely carry out MRI examinations in more than 100 patients with implanted pacemakers or defibrillators.

MRI exams were done only in people with the devices the researchers had already tested. They reprogrammed the devices and turned off the shocking action of the defibrillator during the 30- to 60-minute scan. Even though they also cut the strength of the MRI electromagnetic field in half, the tests provided accurate information about all conditions outside the chest and for 93 percent of those involving the heart or upper body.

The researchers have provided a list of the pacemakers and defibrillators with satisfactory MRI testing. They warn that MRIs should only be undertaken in people with these specific implanted devices by physicians with special training in MRI safety, and with MRI scanners of the same type and magnetic strength as those used at Hopkins.

Meanwhile, efforts continue to help widen the use of MRI technology in patients with pacemakers and defibrillators that do not fit the criteria of the Johns Hopkins studies. Researchers at Metronic, Inc., have asked the FDA for permission to test a pacemaker designed to be compatible with MRI scanning. Workers at the University of Pittsburgh are testing shields to cover the chests of patients during MRI scans.

Such possible advances are eagerly awaited by the thousands of people whose implanted devices ban them from much-needed MRI scans.