Size of myocardial infarct measured using MRI

Dec 20 2004

In animal studies, researchers at Johns Hopkins have effectively used magnetic resonance imaging (MRI) to measure with 94 percent accuracy the size and amount of heart muscle damaged by a heart attack, known in medical terms as a myocardial infarct, or m.i., for short.

The Hopkins development, if confirmed in further pathology studies in humans, could standardize how physicians currently gauge the severity of a heart attack and a patient's chances for recovery. A variety of methods are currently used to determine the size of an infarct by MRI, such as visual cues, but these estimates have been shown in previous studies to overestimate damage by an average of 11 percent.

The researchers, whose findings will be published in the Journal of the American College of Cardiology online Dec. 21, hope to apply this information for determining more accurate dosing regimens for stem cell therapies currently under development, in which upwards of 200 million stem cells are injected directly into the damaged heart muscle.

"The size of a heart attack matters for determining how well a patient will recover from the trauma. Current methods for measuring the size of an infarct and assessing how much damage was done are highly subjective and arbitrary," said senior study author and cardiologist João Lima, M.D., associate professor of medicine and radiology at The Johns Hopkins University School of Medicine and its Heart Institute. "Indeed, a person who has suffered damage to more than 30 percent of the left ventricle of the heart is twice as likely to die within a year from the injury than someone who has suffered less damage, and bigger infarcts often require more aggressive drug therapy or, in the most severe cases, surgery to repair heart tissue or prevent further damage."

The Hopkins team measured the size of an m.i. in 13 dogs using eight different methods for analyzing pictures taken by a standard MRI, ranging from visual cues to varying strengths of a computer model called full-width at half-maximum. This computer method calculates the amount of damaged tissue by comparing MRI signal strength between damaged and undamaged tissue. Damaged heart tissue is denser than undamaged tissue because the muscle structure has collapsed, and MRI can distinguish between tissues of varying density.

The results of each method were then compared against precise measurements at autopsy. The size of an m.i. was measured as a percentage of left ventricular volume. All measurements using MRI were taken within 24 hours after heart attack to simulate "real life" conditions.

Results showed that the full-width-at-half-maximum method was superior to all other methods, at 94 percent accuracy, when the least strict computer modeling formula was used, set at only one standard deviation between density of damaged and undamaged tissue. Other, stricter formulas in current use were accurate to 85 percent or less, if at all. When visual cues, the most commonly used method, were used, infarct size was measured with 69 percent accuracy.

"Our hope is that these results will help establish clear and effective guidelines for measuring the size of an m.i., and this will improve a physician's ability to make an accurate prognosis," said lead study author and Hopkins cardiology research fellow Luciano Amado, M.D. "Cardiologists also need to accurately gauge the amount of damaged muscle as part of our preparations for future treatments that could possibly repair it, most notably stem cell therapies."

This six-month study was conducted between December 2001 and June 2002, with funding provided by the National Institutes of Health and Datascope Corp. Other Hopkins investigators involved in this research were Bernhard Gerber, M.D.; Gilberto Szarf, M.D.; Khurram Nasir, M.D., Ph.D.; and Dara Kraitchman, V.M.D., Ph.D., who helped design the study. Additional assistance was provided by Dan Rettmann, B.S.; and Sandeep Gupta, Ph.D., from GE Medical Systems, Waukeshas, Wis., who helped implement the computer software used for imaging analysis of infarct size by MRI.

MRI is a coiled, mechanical device for taking real-time, three-dimensional images inside the body, including organs, muscles and joints, without the need for invasive surgery. Unlike x-rays, there is no radiation involved. Instead, MRI uses radio-frequency waves and intense magnetic fields for obtaining images of the body's interior, as atoms in varying tissues and organs respond differently when excited by the magnetic field. This creates patterns that can be reproduced on a display screen.

According to the latest statistics from the American Heart Association, in 2001, there were an estimated 565,000 new cases of heart attack in the United States, plus an additional 300,000 cases of recurrent heart attack. Almost 185,000 of all heart attacks were fatal. The AHA also estimated that 7,800,000 Americans had suffered at least one heart attack. People who have had a heart attack have a sudden death rate that is four to six times greater than in the general population. About two-thirds of heart attack patients do not make a complete recovery; however, 88 percent of those under age 65 are able to return to work.