Published: Oct. 17, 2006
Updated: Oct. 28, 2010
At the age of 33, St. Louis Cardinals pitcher Darryl Kile was young, fit, and had just passed a physical and EKG. So--though he had a family history of heart disease--Kile’s sudden death in June 2002 from a heart attack came as a shock to his family, colleagues, and fans alike. As the Washington Post wrote shortly afterward, Kile's case exemplified "one of the biggest frustrations in cardiology": the fact that for an unlucky quarter-million Americans each year, a fatal heart attack will be their first and only clear symptom of heart disease.
A new diagnostic technology being studied at Duke may soon change that: cardiovascular magnetic resonance imaging, or cardiovascular MRI. The technology can detect tiny areas of tissue damaged by transient, undiagnosed heart attacks that standard imaging techniques cannot. Sometimes called "silent" heart attacks, these events may be symptomless, or masquerade as a cold or other seemingly unrelated symptom.
In a study of patients with cardiac risk factors but no history of heart attack, Duke cardiologist Raymond Kim, MD, and biomedical engineer Robert Judd, PhD, discovered that a third of them did show evidence of heart attack with MRI. A subsequent study by the team found that the traditional nuclear imaging technique--single photon emission computed tomography, or SPECT-- detected only 53 percent of the microinfarcts (the medical term for the subtle tissue damage caused by these transient “mini” heart attacks) that were detected by cardiac MRI. In fact, when used alone, SPECT completely missed previously diagnosed microinfarcts in 13 percent of the study patients.
"Unlike risk factor measures such as high blood pressure or high cholesterol, this is actual, direct evidence of cell death from heart disease,” says Kim. "If you have one of these events, we believe your risk for having another heart attack is quite high, and you should probably be evaluated and treated as if you’ve already had a noticeable heart attack."
Duke is home to one of the country's first and only centers dedicated to cardiovascular MRI. At the heart of the Duke Cardiovascular Magnetic Resonance Center (DCMRC) are two state-of-the-art MRI scanners ($2.6 million each) specially designed to capture images of the heart. MRI shows details of heart structure, function, and blood flow with unrivalled crispness--and, unlike the more standard imaging technology, echocardiography, it can view the heart from any angle, without interference from bone, lung, or air.
Magnetic resonance imaging has been a staple of medical diagnostics since the late 1970s. During an MRI examination, which is non-invasive and radiation-free, a patient is guided through the cavity of a large doughnut-shaped magnet. The magnet causes hydrogen nuclei in cells to give off characteristic signals, which are then converted into three-dimensional images of the heart and its structures. The technology uses powerful magnets that cause hydrogen nuclei within the body's molecules to vibrate or resonate, emitting radiofrequency energy. Even tiny differences in tissue change the rates at which this energy is emitted. The MRI machine detects these energy emissions and converts them into visible images, making it possible to see and evaluate many conditions, including very early stages of disease that were previously difficult or impossible to identify.
Despite the powerful benefits it offers other medical specialties, the use of MRI in cardiovascular medicine was limited by the technical difficulties of capturing a clear image of a beating heart. Over the past few years, however, a new form of high-speed MRI has been developed, allowing images of the heart to be captured in real time. Displaying the function, structure, and blood flow of the heart and surrounding blood vessels, these images facilitate a comprehensive evaluation of cardiac health. Cardiovascular MRI offers significant advantages over other imaging methods. It produces remarkably clear, complete, and detailed three-dimensional images of cardiac anatomy without interference from adjacent bone or air.
Those images can be used to accomplish a broad variety of diagnostic tasks. Cardiovascular MRI can identify damage from a heart attack, diagnose certain congenital cardiovascular defects, and evaluate disease of the larger cardiac blood vessels. It can be used to perform a real-time, minimally invasive stress test or to evaluate how well a heart is responding to treatment. All parts of the cardiovascular system can be imaged, making MRI especially useful in complex cases. The technique can be combined with other studies to help physicians predict which areas of the heart will respond best to bypass surgery, angioplasty, or revascularization. And MRI is as benign to the patient as it is powerful for physicians: Unlike most other non-invasive cardiac imaging tests, MRI does not expose the patient to potentially harmful radiation.
Its unique attributes make MRI especially valuable in evaluating congenital heart disease in both adults and children. "You can see things you can't see with echocardiography," says Stephen Sanders, MD, chief of pediatric cardiology. "It has a remarkable potential for use in young children, allowing us to image abnormal arteries in a painless, non-invasive way."
Duke cardiologist Eric Velazquez, MD, head of the Cardiac Diagnostic Unit, has turned to MRI numerous times to help him target treatments for patients. "Echo provides good general information, but the data it offers may not be sufficient," he says. "MRI is more specific, allowing you to quantify exactly how big the heart is, how thick the walls are, and whether there is evidence of irreversible damage. That information improves our ability to make the right clinical decisions."
And that's just the beginning. Soon, researchers think, MRI could be used to guide minimally invasive surgeries, replace many current techniques that are invasive or involve radiation, and perhaps even detect imminent heart attacks.
Says Judd, who co-directs the DCMRC with Kim, "The idea behind this center is to advance the field by improving cardiovascular imaging techniques, educating physicians about when MRI can be helpful, and making available those clinical applications that are ready for prime time."