December 28, 2004
MRI Helps Judge Heart Attack Size
Accurately measures extent of damage in dogs
HealthDayNews -- Magnetic resonance imaging (MRI) can help doctors accurately measure the size and amount of heart muscle damaged by a heart attack, at least in animals, a new study finds.
If the technique is borne out in further studies, the use of MRI could help standardize how doctors gauge the severity of a heart attack and the patient's chances for recovery. It could also help them decide the dosing of stem cell therapy, now under study to rejuvenate damaged hearts, the researchers said.
Currently, doctors use a variety of methods to determine the severity of a heart attack, including visual cues, but most methods are far from perfect.
The study findings appear in the latest online issue of the Journal of the American College of Cardiology.
In the study, Dr. JoÃ£o Lima and his colleagues at Johns Hopkins University measured the size of heart attacks in 13 dogs, using eight different methods for analyzing pictures taken by a standard MRI. The methods ranged from visual cues to varying strengths of a computer model that calculates the amount of tissue damaged by comparing signal strength of the MRI between damaged and undamaged tissue. Because damaged tissue is denser than undamaged tissue, the MRI distinguishes between the varying densities, Lima said.
The new study involved refinement of an MRI measurement technique that the Hopkins team has studied for more a decade, to better measure the severity of a heart attack. "We worked with the computer and taught it to do what the eye does," said Lima, senior study author and an associate professor of medicine and radiology.
The results of each of the eight methods were compared against precise measurements done during autopsies of the dogs. One MRI computer method, called full-width at half-maximum, was superior to all others, with 94 percent accuracy, Lima said.
"This is an important development," Lima said. "It has a lot of objectivity to it."
In comparison, visual cues gave just 69 percent accuracy.
Besides allowing a doctor to tell a heart attack patient how much heart tissue was damaged, the new method could also help determine more accurate dosing regimens for stem cell therapies under investigation, Lima said. "We are now developing stem cell [therapies] to cure the heart after heart attack, to regenerate heart tissue. For these methods to be evaluated correctly, one needs to know how much heart tissue was lost," he said.
The study results look promising, other experts said.
"What their data provides is further support for the use of MRI as an accurate, noninvasive technique for measuring infarct [heart attack] size," said Dr. Victor Ferrari, associate professor of medicine at the University of Pennsylvania School of Medicine.
Currently, methods such as echocardiogram (ultrasound imaging of the heart) are used to measure heart attack damage, Ferrari said, but each has its limits. An echocardiogram, for instance, can't differentiate between permanently and temporarily damaged heart tissue, Ferrari said.
While the use of MRIs after heart attack is growing, Ferrari said, it's not widespread.
"I think it's a good method and will add to clinical practice eventually," said Dr. Zi-Jian Xu, a cardiologist at Santa Monica/UCLA Medical Center in California. "I don't think it's going to be immediately used very widely, because although the information is very useful there are other ways clinically to assess the infarct size."
Xu calls MRI an emerging technique in the assessment of cardiac damage, "but how crucial this information will be in a clinical setting still needs to be tested."
To learn more about the warning signs of a heart attack, visit the American Heart Association.