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For other articles and previous issues click here. February 21, 2005 Emerging
Cardiac Tools Experts expect faster, better imaging equipment to transform how physicians diagnose heart disease. At RSNA 2004 this past December, Martin J. Lipton, MD, a professor of radiology at Harvard Medical School and staff radiologist at Brigham and Women’s Hospital in Boston, moderated a well-attended panel, “Cardiac Imaging in the 21st Century: Is Radiology Ready for Prime Time?” The consensus among the panelists: If radiologists are not ready, they ought to be. Lipton, who is educated in both cardiology and radiology, told the group that as scanners are becoming capable of producing high-speed, good-quality images of the heart and great vessels, interest in cardiac imaging is accelerating. MRI and CT have both rapidly gained ground in cardiac diagnostics. Thanks to impressive advances in spatial and temporal resolution and increased imaging speed, cardiac MRI is beginning to be used routinely at specialized heart centers to look closely at the structures and function of the heart and major vessels. Using state-of-the-art MRI scanners, physicians can examine the size and thickness of the heart’s chambers and determine the extent of damage caused by heart attack or progressive heart disease. “Stress testing using nuclear medicine has been the gold standard, but I think MR is reaching the point where it is equally competitive, if not better,” said Melvin E. Clouse, MD, vice chairman and director of research at Beth Israel Deaconess Medical Center in Boston and a professor of radiology at Harvard Medical School. Clouse was also a panelist during the RSNA focus session on cardiac imaging. Noninvasive procedures, MR and CT are also promising candidates for replacing invasive diagnostic coronary angiography used to detect coronary artery disease, which affects 13 million Americans and is a leading cause of death in this country. CT’s ability to detect coronary artery disease is one of the reasons why over the last 15 years cardiac CT “has gone from nowhere to now every cardiologist wants one,” said panelist Thomas Brady, MD, director of cardiovascular imaging and intervention at Massachusetts General Hospital (MGH) in Boston. Avoiding Catheterization MR and CT are gaining favor because they are noninvasive procedures and therefore are safer than catheterization. MR exams don’t expose the patient to ionizing radiation. Also with MR, the heart is not obscured from view by surrounding lungs, the breastbone, or the rib cage; the heart can be viewed from any angle. Not only is MR being used to determine the presence of plaque but also the type of plaque clogging arteries. Most heart attacks, Clouse said, aren’t caused by the slow buildup of plaque in the coronary arteries but by the sudden rupture of vulnerable plaque associated with an inflammatory reaction. Plaque has four main components: calcium, lipid, the fibrous cap, and thrombus. Because of their distinct compositions, each component looks different on the MR sequences used to image plaque. A number of researchers, including several at Beth Israel Deaconess, are studying whether those differences can be used to predict which plaque is stable and which is likely to rupture. “We have people who enter an arm of our study go for an MR to see if vulnerable plaque can be identified,” Clouse said. Investigators at Johns Hopkins University in Baltimore are also conducting extensive research in the diagnosis of coronary artery disease using MR, said David Bluemke, MD, PhD, associate professor of radiology and clinical director of MRI in Hopkins’ department of radiology. Bluemke was also on the RSNA 2004 panel. Emergency physicians are utilizing MR and CT to help physicians determine whether patients who show up in the emergency department (ED) have actually had a heart attack or are just suffering from a pulled muscle. Traditional electrocardiograms and blood tests can take up to four hours to confirm a heart attack. However, with cardiac MRI, physicians can determine whether there has been heart muscle damage in less than 40 minutes. While preliminary data in the acute chest pain setting are encouraging at MGH, additional studies on CT’s role in the detection of acute coronary syndrome are required, Brady said. Because a CT scan can be done so quickly, Brady predicts that someday the cardiac CT will be used in every ED to triage appropriate patients who present with acute chest pain. Confirming Heart Attack When a patient has had a heart attack, physicians need to know whether the damage to the heart muscle is permanent to determine the appropriate treatment. Having accurate viability information allows physicians to determine whether the patient will benefit from a revascularization procedure. PET is currently used to assess myocardial (heart muscle) viability, but experts believe MRI may prove a more robust technique. With MRI, radiologists can see and quantify the size and location and determine how much of the muscle wall was involved in the patient’s infarct and how much may still be viable and function if revascularized, Clouse said. Studies have shown that the areas of delayed enhancement exactly correlate with the infarcted region seen on pathologic slides, Bluemke said. Other studies have shown that if the region of enhancement on MRI is less than 50% of the wall thickness, revascularization of the segment is more likely to help the patient. MR is also being used more often to diagnose coronary artery disease in patients who are not good candidates for thallium stress tests, which use the radioactive substance injected into the bloodstream to see how well blood is flowing to the heart muscle. A thallium stress test often includes an exercise stress test on a treadmill or bicycle. MRI Stress Test The researchers reported in a 2002 issue of Circulation that MRI stress test proved to be an accurate predictor of heart disease. Among patients who had a negative MRI stress test, 97% were free of heart disease within the first year of testing. The biggest disadvantage with MRI is that claustrophopic patients may not tolerate being inside the magnet for the 30 minutes it takes to perform the test. Roughly 5% of patients are so claustrophobic that anxiety can sometimes trigger cardiac symptoms. Traditionally, patients with permanent pacemakers/internal cardioverter-defibrillators have not been able to undergo MRI because those devices’ function can be altered by the magnetic field. However, recent work at Johns Hopkins has shown that some patients can safely undergo MRI with certain pacemakers and implanted cardioverter-defibrillators. “This will overcome a major stumbling block for cardiovascular patients,” Bluemke said. Bluemke noted that reading cardiac MRIs requires experience. “There is a bit of a learning curve,” he said. “On the other hand, it’s not that steep. It’s not so complicated that people can’t learn it rapidly.” Some tests, such as wall motion abnormalities, may take more experience than others. “With wall motion abnormalities, that takes 100 to 200 cases to be very confident what you’re looking at,” Bluemke said. But in general, most radiologists are comfortable with cardiac MRIs and “understand what they mean,” he added. Like cardiac MR, cardiac CT holds tremendous promise in evaluating and treating cardiac diseases. CT is very good at detecting plaque. “We know CT can pick up soft plaque and calcified plaque,” Clouse said. “Hopefully, we’ll be able to use CT to find vulnerable plaque before it gets to the subcritical level and causes heart attacks.” Another advantage of CT, Clouse added, is that it’s easy to use and not as operator-dependent as some other imaging modalities. Like catheterization and echocardiograms, however, CT exposes the patient to radiation. Brady believes that “while we need to be judicious about the use of radiation, if a person has acute chest pain and we need to know what’s going on, the use of CT certainly can be justified.” Besides, he said, the likelihood of dying from a heart attack or other coronary disease is equal to, if not higher than, most cancers. CT/MRI Fusion As image quality and speed continue to improve, radiologists believe cardiac imaging’s role in diagnosing and treating heart disease can only grow. MGH’s Brady said that while CT image quality is very impressive today, “we can expect one more revolution in image quality in the next year or two.” Clouse compared cardiac MR and cardiac CT to two trains running on separate tracks: “One is ahead in one area and one is ahead in the other.” It’s going to take a few more years, he said, “to see how it’s going to all shake out.” With all the advantages of cardiac MR and CT, why haven’t they taken off yet? Brady believes the problem is that MR and CT are relative newcomers competing against very good modalities that “have been around for 20, 30, 40 years and that have benefited patients tremendously over the years.” They have to offer new and important information and at the same time save costs for physicians to change their referral patterns, he said. “If there is no existing tool and you bring a new tool in, everybody adopts it,” Brady said. “But when the existing set of tools is very good, it’s harder to adopt new innovations.” Diagnosing cardiac disease is complicated because it is a slowly progressive, indolent disease that progresses throughout life, Clouse said. A test for early or asymptomatic disease is different from one for advanced disease that produces symptoms. The coronary artery calcium score is an example of a test for early or asymptomatic disease so the physician can institute preventive measures. “When patients are symptomatic with far advanced disease, other tests such as stress testing with either radionuclides, echocardiography, or stress MRI are indicated,” Clouse said. “Patients who have had a heart attack [myocardial infarction] may benefit from myocardial viability studies to differentiate the areas of irreversible damage from viable muscle that can benefit from revascularization.” How quickly cardiac MR and CT will be adopted, Brady said, “depends on how good the radiologists doing the studies are at convincing the cardiologists that this is the way to go.” Frontline Tool Looking into the future, Michael Vannier, MD, FACR, professor of radiology and director of radiology clinical research at the University of Chicago, said, “It’s clear that cardiac imaging is going to remain important and be central to the management of all forms of heart disease.” Like his colleagues, Vannier does not expect MR or CT to ever replace the other modalities. “What we’ve seen up to now is every time a new modality is introduced, all of the other modalities continue.” The fact is, he said, each has its strengths and weaknesses. Also, “there is some diversity in the cardiac diseases among patients.” The challenge will be for cardiologists and radiologists to “match up the right modality with the right individual, and that’s something that will continue to evolve for the foreseeable future.” Patients will also have a say, Vannier said. He expects that most would rather have “an injection of a peripheral vein and lay in the CT scanner for a few seconds than have a hole punched in their artery in their leg so the physician can thread a catheter to their heart.” They’ll vote with their feet, he believes, and choose those centers that offer MR and CT. — Beth W. Orenstein is a freelance writer in Northampton, Pa. She is a regular contributor to Radiology Today. |
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