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For other articles and previous issues click here. May 16, 2005 Quantifying
Potential — New Studies Seek to Define CT’s Role in
Diagnosing Heart Disease The goal is to develop CT sensitivity and specificity and compare it with diagnostic catheterization so doctors will know how to appropriately use the technology. The new 64-slice CT scanners have sparked great interest among cardiologists and radiologists about the potential of coronary computed tomography angiography (CTA). Researchers have speculated about the possibility of CTA replacing diagnostic catheterization in many situations. Equipment manufacturers are understandably enthusiastic about the possibilities. “We feel 64-slice systems could be the major technology to replace catheterization,” says Robb Young, cardiac CT product manager for Toshiba Medical Systems Corporation, headquartered in Tustin, Calif. “But to prove something like that, you really need to do a large study.” Anecdotally, 64-slice technology has already demonstrated clinical advantages in facilities that have upgraded from 16- and 32-slice systems. However, as Young suggests, scientific validation requires clinical trials. The companies that developed these systems are working with researchers to perform those trials. Joao A.C. Lima, MD, director of cardiovascular imaging in the cardiology division at Johns Hopkins University School of Medicine in Baltimore, thinks the long-term implications of 64-slice technology will be significant. “The impact of the study findings may alter the practice of cardiovascular medicine comprehensively,” Lima says. “In turn, benefits gained by more effectively diagnosing coronary artery disease through 64-slice CT will no doubt have a positive economic impact on the way we treat one of leading causes of deaths in the world—heart disease.” Multicenter Comparison In the study, researchers will demonstrate how routine cardiac applications can be performed noninvasively using 64-slice technology. This will help determine whether the technology can replace diagnostic catheterization as the gold standard for some cardiac imaging. The goal, Young says, is to develop CT sensitivity and specificity and compare it with diagnostic catheterization so people will know how to appropriately use the technology. The study involves nearly 350 patients who will
undergo both CTA and catheterization. “The subjects are people
who would normally be going for a catheterization,” explains
Young. “But, for the purposes of the study, they will be getting
both a CT and a catheterization. Then, in a blinded study, researchers
will review and compare both.” “Between 20% and 40% of diagnostic catheterizations are negative, which means that the person diagnosed did not have to have an intervention,” Young says. “So, they underwent this risky procedure.” Young says that according to the American Heart Association, there is a 0.5% to 1% risk of death from catheterization. “That’s because you send a catheter up the leg. Death could come from something like a bleed-out of the femoral artery,” he says. “But, until now, there have been no other tools to image the coronaries except for catheterization.” Other Systems The research team is led by Dennis Foley, MD, professor of radiology at the Medical College and chief of digital imaging at Froedtert Hospital, and David Marks, MD, an associate professor of medicine at the Medical College of Wisconsin and director of the cardiac catheterization laboratory at Froedtert Hospital. Foley hopes 64-slice technology will enable physicians to non-invasively diagnose symptoms that today require catheterization. The LightSpeed VCT, which was first installed in the United States at Froedtert Hospital in June 2004, can scan the heart in roughly five heartbeats, or less than 10 seconds. The 64 slices are obtained in synchronization with a patient’s heart beat in 350 milliseconds. This speed allows users to capture a sharp, 3-D image of a patient’s beating heart without having the patient undergo angiography. Reducing Artifacts This is an important consideration, says Ackelsberg, as an accelerated heart rate can cause artifacts and reduce the sensitivity of the exam to stenosis. “The hope is that the shorter acquisition time will lead to a more robust exam and higher sensitivity and better positive and negative predictive values,” he adds. In the study, patients will first undergo a CT scan and then a catheterization. The researchers will compare the results in a blind study. The goal is to make sure the technology will be used in the correct fashion. “We will be able to determine, on an artery-by-artery and segment-by-segment basis, the level of agreement between the two exams using the coronary arteriogram as a gold standard,” explains Foley. “That is important because coronary arteries vary in diameter from the proximal to distal extent. So we would like to know how accurate we are for proximal vs. mid-vessel vs. distal vessel, and to what extent calcified plaque may cause some degree of early estimation of coronary artery stenosis.” Foley adds that there is another aspect of the study that, while it doesn’t involve comparison with angiography, could yield very important ancillary information. The researchers will look for low-attenuation plaque on the arterial wall. “Because of its low-attenuation nature, it is not seen on an arteriogram, but it is demonstrated on a CT scan,” he says. That could have significant prognostic value, Foley says. Patients who suffer heart attack without any preceding chest pain symptoms may have had plaque that underwent an internal hemorrhage and caused an overlaying intraluminal coronary artery thrombosis. “This plaque did not cause any symptoms because it wasn’t significantly stenotic,” he explains. “So that is one of the potential values of this technique, using CT as the recording medium above what can be obtained by a selective coronary arteriogram.” Economic Indication After the procedures, patients will then be followed for one year to see whether any cardiac events occur and determine whether CT would have predicted the event while the catheterization missed it. “It is a pretty exciting study,” says Ackelsberg. “We hope to show the relevance of CT as well as the potential economic benefits.” The main advantage of 64-slice CT, says Lima, is that it provides the thinnest slices. “This allows the CT to capture images of the coronary arteries,” he points out. “The thinner the image slice, the better the image quality. Using the Aquilion 64 CFX, we are able to acquire images as thin as 0.35 millimeter by using overlapping slices, which is a tremendous benefit for conducting cardiac CT studies.” In comparison with cardiac catheterization, the most obvious advantage of CTA is that it is noninvasive. The patient is given an intravenous injection and is then scanned, which only takes 5 to 10 seconds during a single breath hold. “It’s not only noninvasive, it’s quick,” says Young. “Patients can come in wearing their street clothes and be on and off the table in less than 10 minutes.” Shorter breath holds, which represent an advance over earlier generations of CT technology, are a huge advantage. “With a 16-slice scanner, breath holds are between 20 and 30 seconds, which can be pretty hard for some patients,” says Young. “Shorter breath holds mean that a greater number of heart patients will benefit from advanced cardiac CT diagnostic procedures,” adds Lima. “This is crucial for older and critically ill patients who were routinely excluded from these types of studies.” Shorter breath holds also mean less contrast usage.
Lima explains that a typical 64-slice cardiac study requires only
60 to 80 cubic centimeters (cc) of contrast, as compared with a
16-slice CT study that requires 140cc or greater. “That is
a huge advantage because people who have cardiac risk factors also
have contraindications for contrast,” comments Young. Also, with a shorter breath hold, a patient’s heart rate remains more stable. Thus, their entire heart can be imaged without artifacts. These advantages, Young says, combine to open up a large clinical population that can undergo imaging with a 64-slice scanner. Clinical Usage “As cardiologists, our focus at our center is coronary imaging. It’s not like a radiology center, where coronary disease evaluation is not the focal point. Our focal point is maximizing our ability to identify disease as precisely as possible,” says Lepor. “Having worked with both technologies, we feel that 64-slice CT has been able to achieve that.” Lepor thinks the new technology represents a significant advance in the center’s ability to care for patients who are at risk of coronary artery disease (CAD). In fact, it has life-saving implications, he says. “Since we know that over a million heart attacks occur per year, we know that we haven’t been doing the job of identifying CAD in an early enough phase.” Lepor points out that cardiologists have been relying on a relative risk model (the Framingham model) for heart disease. The 64-slice scanner represents a solution, he says. “Now, we finally have a tool that will take us from identifying relative risk to defining the actual presence or absence of the disease,” Lepor notes. “This will revolutionize our ability to identify these patients, treat them with aggressive risk and factor modification therapies, and certainly saves.” In a case-review paper, “The Emerging Use of 16- and 64-Slice Computed Tomography Coronary Angiography in Clinical Cardiovascular Practice,” Lepor and colleagues describe how multislice CTA can identify patients with both soft and hard plaque, which supplements information collected from an ordinary coronary calcium scan and classic risk factor assessment. They also outlined the following specific advantages of 64-slice CT technology that they observed: • thinner slices allow for more precise measurement of stenosis and visualization of smaller branch vessels; • ability to acquire high-quality images in obese patients; • shorter breath holds and acquisition times; • 30% reduction in the volume of contrast media; and • better characterization of soft plaque and less blooming artifact from arterial calcifications. The authors explain how clinicians now have the tools to identify the presence of CAD in the presymptomatic phase and to help identify the etiology of pain syndromes in patients presenting with atypical or obscure symptoms and who may be suffering from obstructive CAD, aortic dissection, pulmonary emboli, or other pathologic processes. “In our practice, it has certainly changed the paradigm with which we approach the patient at risk for CAD,” remarks Lepor. Best Role All four manufacturers of FDA-approved systems are involved in such studies. Three systems were discussed in this article. Philips Medical Systems, of Andover, Mass., chose not to discuss ongoing research with its Brilliance system until data becomes available. “Cardiac disease is a process that begins with the formation of soft plaque, and CT has the ability to see those soft plaques,” Ackelsberg points out. “So, the long-term hope is that CTA will reveal the development of arteriosclerotic disease earlier than the cath lab. There may not be stenosis yet, but there can be development of soft plaque, which could be either treated medically or by other means.” — Dan Harvey is a freelance writer based in Wilmington, Del. He is a frequent contributor to Radiology Today. |
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