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For other articles and previous issues click here. July 11, 2005 Keeping
Score on Calcium Scoring Faster MDCT scans are changing the equipment, but the issues remain the same. A group of clinical studies now shows that multidetector computed tomography (MDCT) could be successfully used for coronary artery calcium scoring (CACS). Research shows that MDCT performs this application as well as the electron-beam computed tomography (EBCT) scanner designed specifically for heart studies. Questions still remain, however, about just how clinically useful CACS is in managing cardiac patients over the long-term. The presence of plaque—calcified deposits of fatty and cellular tissues—is a known risk factor for coronary artery disease (CAD), and CACS has a long history in the cardiology lab. Also called atherosclerosis, this plaque build-up leads to stenosis, the narrowing of the arterial pathways (lumens) through which blood circulates. For roughly 50 years, physicians have relied on angiography to help locate and identify these problems. By injecting a radiopaque contrast media into the heart during cardiac catheterization, angiography generates real-time fluoroscopic images of the heart’s anatomy and action. Angiography frequently is performed after the physician is already fairly sure of a diagnosis of CAD—in other words, for coronary intervention. However, approximately 30% of coronary arteriograms are normal, so these patients are subjected to the risks of an invasive study and an uncomfortable diagnostic procedure. Cardiac CT Patients wanting such screens generally have to pay for them out of pocket, though—insurers rarely reimburse for such EBCT use. That’s because in 2000, the American College of Cardiology (ACC) and the American Heart Association (AHA) jointly found “inconclusive risk-stratification evidence” for EBCT CACS screening.1 Subsequent statements continue to recommend against EBCT scanning either for detecting CAD or for predicting coronary events in asymptomatic or low-risk patients.2 There are other drawbacks as well. First, only one company, Imatron (now part of GE Healthcare), makes EBCT, so access is still limited. As of 2003, according to the AHA report Heart Disease and Stroke Statistics, there were only roughly 200 across the United States. Second, the Imatron machine costs roughly $2 million and the average scan costs the patient between $300 and $400 (in some areas, as much as $700). Third, EBCT results in a fairly high number of false-positive test findings—as high as 50% in some studies—among patients presenting with plaque but without significant stenoses.3 “Therein is one of the major problems with CACS,” notes Howard N. Hodis, MD, director of the Atherosclerosis Research Unit at the University of Southern California’s School of Medicine. “Calcium is not equivalent to plaque—if it was, there wouldn’t be such a high false-positive rate … in which coronary artery calcium may or not be associated with proven CAD.” Even so, unlike MDCT, EBCT now has nearly a decade of heart-specific protocol development and clinical use. EBCT vs. MDCT In comparison, a 16-slice MDCT system can cover the entire heart in approximately 15 to 20 seconds without gating. Explains Hodis, “With spiral CT’s continuous imaging, you can go back and select images captured during the quietest part of the cardiac cycle. Of course there’s a trade-off to being able to do that—you need to expose the patient to somewhat greater radiation since you are discarding many images to obtain the few that occur during minimal cardiac motion.” Exponential improvements in acquisition speed and resolution have significantly reduced that drawback. While it’s true that the 16-slice MDCT typically needs a 500-millisecond exposure time, whereas EBCT uses only 100 milliseconds, the newest 32- and 64-slice helical (spiral) MDCTs cut exposure time to 200 milliseconds for equally good imaging. MDCT can complete the total scan in as little as five to 10 seconds (in controlled studies). Using MDCT in sequential mode with cardiac gating allows limiting radiation exposure to no more than, and sometimes less than, the amount typically used in EBCT, says Hodis. “In fact, that’s the way we do cardiac CT.” That’s good news for the approximately 2,000 U.S. hospitals and imaging centers that already have MDCT and are eager to add a new use—and a new revenue stream.4 Even so, one key controversy surrounding CACS has been that there’s still no direct proof of a causal relationship between a high calcium score and stenosis—or between stenosis and cardiac events. It’s still unclear which patients with measurable calcium will die from CAD. As Hodis notes, “Not all plaques are calcified, at least as measured by CT, and not all calcification means plaque.” “The historic perspective was that stenosis caused by plaque eventually caused heart attacks,” says Martin J. Lipton, MD, FACR, FACC, of Brigham and Women’s Hospital’s radiology department and professor of radiology at Harvard Medical School. “But we now know that it’s more often a ruptured plaque, and the ones that rupture are usually soft. So the presence of soft plaque in a vessel that may not be particularly narrowed may be perhaps more important than the stenosis.” Both Lipton and Hodis emphasize that CT scanning for calcium is only a screening test. “CT scanning for coronary artery calcium is not an alternative to coronary angiography. The only acceptable diagnostic test to demonstrate the presence and location of coronary artery lesions is angiography,” states Hodis. “CT scanning can only tell you whether and how much calcium is present in the coronary arteries… Noninvasive testing cannot measure plaque burden in the arteries.” The fact is that most adults have some arterial calcium. By late middle age, physicians routinely expect to see specific levels of calcium build-up. Adding MDCT to the cardiology toolbox has only introduced more controversy to the question of CACS. Applying Meaning Given that, he says, “The question remains: Can CACS be reproducibly measured over time, and can CACS change be used to monitor the efficacy of antiatherosclerosis therapies?” He adds, “The other important question remains as to whether CACS and its modification have predictive value. What does it mean for outcomes?” He believes cardiac MDCT will certainly help accelerate finding the answers to these and related questions concerning the role of CACS in clinical medicine. Lipton acknowledges that when dealing with patients who score in the middle range for plaque, and who may have one or a few other risk factors, there’s still some controversy about how valuable calcium scores are in managing them over time. He says, “One thing is absolutely crystal clear: If you can measure the amount of calcium and quantitate it in a consistent, reliable way, [CT CACS] has a very high negative predictive value. We do know that when [CACS] shows no or very little calcium, at least in younger patients, the chances of them having an event in, say, the next 10 years is extremely low, somewhere on the order of less than two [percent] or less than 5%.” Such findings take on major significance for the kind of patients who will be most likely to seek out a cardiac scan. “The people who most want to know are those who’ve had a whole bunch of their family die young of heart attacks. [They may currently be asymptomatic] but still be at greater risk for hyperlipodemia and premature coronary artery problems. That whole range of intermediate risk patients could actually be high risk, especially given other factors,” says Lipton. Indeed, the ACC and AHA have noted (although, as Hodis points out, not endorsed) that serial screening for calcium progression may prove beneficial among specific patients.5 Gathering Data As a simplistic example (admittedly reducing a very complex issue), clinicians must ask, “How many people do you have to screen to detect one myocardial infarction? How many false positives are acceptable, since this can lead to more expensive and potentially invasive testing?” Just as important, “How many false negatives are acceptable? Many people who have extensive noncalcified atherosclerosis may not receive appropriate treatment, based on a low or no calcium score.” Hodis sums up, “In short, the data as yet do not support CT scanning for coronary artery calcium in mass screening.” More and longer epidemiological studies are vital before establishing protocols for use, says Lipton. “What you want to do is follow patients for some time and quantitate their improvement [as shown] by reduction of the plaque burden. But that will take a number of years to do … and there’s a big middle group of patients surrounded by controversy. How do you triage them? How does the calcium score help you?” Until the studies come out, we won’t have the answers, but, he says, “at least you can argue that [cardiac MDCT] is one of the most exciting fields in cardiovascular imaging. It’s captured the imagination of both cardiologists and radiologists.” Looking Forward Says Lipton, “Everybody feels they should be able to do this and read it themselves. We’re now able to look not only at the narrowings but to tell if vessels are narrowed more than 50%. And we’re now able to measure the size, shape, position, and to some degree the composition of the plaque. So cardiac MDCT is clinically viable… It opens the door for what really is a [noninvasive] diagnostic study of coronary arteriography.” Reinforced Findings Although CT angiography is far from ready to replace conventional catheterization, the authors wrote, “With rapidly improving technology, [it] may well evolve from a useful complement to invasive angiography to a clinically viable alternative.”6 These and subsequent similar findings may very well change, and eventually supersede, CACS, regardless of how or where it’s performed today. As Lipton says, “One of the major issues that radiologists are facing is that with these scanners, we no longer see just the angiogram as we used to, now we see the whole cross-sectional anatomy—the lungs, the bones, the tissues, the pericardium. So whoever does this work now needs to pay attention to incidental findings, such as a cancer of the lungs. And that affects the total treatment.” — J. K. Bucsko is a freelance writer and editor based in Westville, N.J. She is a frequent contributor to Radiology Today. Standardizing
Calcium Scoring By late 2003, the International Consortium for the Standardization of Cardiac CT, which included members from the four major scanner vendors (GE, Toshiba America, Siemens, and Philips) as well as radiologists and physicists, agreed on a new methodology for consistently measuring arterial calcification, regardless of whether an EBCT or a MDCT scanner is used. However, the group’s methodology relies on data drawn from EBCT studies. Doing so led to “the argument that the clinical meaning of coronary artery calcium determined using EBCT can’t be applied to [the same kind of] data collected with MDCT because it’s a different imaging machine, and therefore … will not be applicable,” says Howard N. Hodis, MD, director of the Artherosclerosis Research Unit at the University of Southern California’s School of Medicine. In his view, though, “that would be like saying that everything we understand about coronary artery stenosis as imaged by angiography on one machine has a different clinical meaning if that same [stenosis] is imaged on another piece of equipment. The growing data clearly support that MDCT can perform as well as EBCT. All the available data appear to show that there’s no real difference in the sensitivity of the measurement, [and that] the variability is as good if not better. So it doesn’t matter whether EBCT or MDCT is used to quantitate calcium as long as [the result] is reproducible in terms of sensitivity and specificity. Which,” he states, “it is.” Reproducibility is crucial to cardiac MDCT scanning, concurs Martin J. Lipton, MD, FACR, FACC, of Brigham and Women’s Hospital’s radiology department and professor of radiology at Harvard Medical School. While still controversial, the new scoring table makes it easier and more reliable to compare scans from different machines and vendors, but it also requires that each imaging facility calibrate its system very precisely. “Each lab has to be able to get the same answer in the same patient in about the same amount of time over repeated scans,” he says, but “with 64 slices and gating, we can get consistently good studies. Everyone agrees that, done well, [MDCT results] are reproducible.” More importantly, he adds, “we now know that calcium can have variable densities. The CT number gives a reference to whether this is mature, hardened plaque or young plaque that may never calcify. Or if it’s some calcium mixed up with soft plaque.” Those kinds of findings pave the way to further studies about how the heart works, which may be one of CT’s most valuable contributions. — JKB References 2. Prevention Conference V, U.S. Preventive Services Task Force; Consensus Statement From the Cardiac Imaging Committee, Council on Clinical Cardiology, and the Cardiovascular Imaging and Intervention Committee, Council on Cardiovascular Radiology and Intervention, American Heart Association; AHA Scientific Statement. February 2005. 3. Nallamothu BK, Saint S, Bielak LF, et al. Electron-beam computed tomography in the diagnosis of coronary artery disease: A meta-analysis. Arch Intern Med. 2001;161(6):833-838. 4. Rumberger JA. Clinical use of coronary calcium scanning with computed tomography. Cardiol Clin. 2003;21(4):535-547. 5. Taylor JA, et al. 34th Bethesda Conference: Executive summary—an atherosclerosis imaging techniques improve the detection of patients at risk for ischemic heart disease? J Am Coll Cardiol. 2003;41(11):1860-1862. 6. Hoffman MHK, Shi H, Schmitz BL, et al. Noninvasive coronary angiography with multislice computed tomography. JAMA. 2005;293:2471-2478. |
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