February 7, 2003
Cardiac MRI Detects Heart Muscle Death
CHICAGO— Researchers from Northwestern University’s Feinberg School of Medicine have demonstrated that cardiac magnetic resonance imaging (MRI) technology can detect small areas of dead heart tissue that cannot be detected by commonly used imaging techniques.
Their finding is important because these small areas of muscle cell death, known as infarcts, can be early indicators of future, more severe heart problems.
In their study involving 91 patients with known or suspected coronary artery disease, the researchers found that the traditional nuclear imaging technique, single photon emission computed tomography (SPECT), detected only 53 percent of the microinfarcts that were detected by cardiac MRI. Additionally, 13 percent of study patients with microinfarcts were shown to have none when SPECT alone was used.
The spatial resolution of cardiac MRI is 60 times great than that with SPECT, allowing cardiac MRI to detect these microinfarcts.
Results of the cardiac MRI study were reported in the February 1 issue of the journal Lancet .
Feinberg School researchers involved in the study were Francis J. Klocke, MD, professor of medicine and director of the Feinberg Cardiovascular Research Institute; Robert O. Bonow, MD, Max and Lilly Distinguished Professor of Cardiology and chief of cardiology; Thomas Holly, MD, assistant professor of medicine; Michael Elliott, MD, assistant professor of medicine; Matthias Regenfus, MD; and Michele Parker. Investigators from Duke University collaborated on the study.
While both cardiac MRI and SPECT are extremely accurate in detecting large infarcts, the Northwestern study showed that only cardiac MRI systemically detected smaller infarcts that are missed by SPECT. In other words, the smaller the infarct, the more likely that SPECT will miss it.
Heart attacks—or myocardial infarctions—occur when blood flow to an area of the heart is cut off or blocked, depriving those muscle cells of required oxygen and nutrients. When the cells die, they tend to die from the inside of the heart’s pumping chamber and progressively outward. In a large heart attack, the area of cell death can cover the entire thickness of the chamber’s wall.
However, in microinfarcts, the cell death may only travel a short distance. Since the spatial resolution of SPECT is roughly equivalent to the thickness of the heart chamber wall, it can detect only those infarcts that have traversed a good portion of the chamber wall.
During a cardiac MRI examination, which is noninvasive and radiation free, a patient is guided through the cavity of a large doughnut-shaped magnet. The magnet causes hydrogen nuclei in cells to align, and when perturbed by radio waves, they give off characteristic signals, which are then converted by computers into three-dimensional images of the heart and its structures. While MRI technology itself is 20 years old, only in the past few years has technology improved to the point where accurate images of moving tissues can be taken.
SPECT technology creates a series of “slices” of the area to be studied, with a computer assembling the slices to create an image. Patients are usually given a radioisotope, which provides information about blood flow and metabolism of tissues being studied. In the study, both groups of patients were given a contrast-enhancement agent.
To verify the findings on human subjects, the researchers performed the same tests in animal models, some without heart disease and others with known disease; then they analyzed the heart tissue after testing. The scientists performed the confirming studies in the animals to ensure that the smaller areas detected by cardiac MRI were actual areas of cell death.
Results showed that cardiac MRI and SPECT detected infarcts involving greater than 75 percent of the chamber wall. However, for infarcts involving less than 50 percent of the chamber wall, cardiac MRI detected 92 percent, while SPECT detected only 28 percent.
The investigators said that further studies are needed to establish the role of cardiac MRI in the diagnosis of heart disease.
The Northwestern cardiologists estimate that about 30 percent of patients with heart disease find that conventional methods for imaging the heart fall short in providing accurate information by which to guide treatment. They said that MRI provides crisp, three-dimensional views of cardiac anatomy with no interference from adjacent bone or air.
The image quality of cardiac MRI also surpasses that of echocardiography, another common imaging technique. Cardiac MRI can better show physicians how well the heart muscle is contracting, as well as precisely reveal areas of damaged tissue.
Duke University collaborators were Robert Judd, Raymond Kim, MD, Anja Wagner, MD, and Heiko Mahrholdt, MD, who conducted the cardiac MRI study while on the faculty of the Feinberg School.
The study was funded by grants from National Institutes of Health, the Deutsche Forschungsgemeinschaft, and the Robert Bosch Foundation.