Cardiac Computed Tomography: Aiming at a Moving Target

Due to constant and rapid improvement of computed tomography (CT) technology, it has become possible to visualize the small and rapidly moving structures of the heart. Indeed, cardiac imaging has been the major driving force of hardware development in CT, prompting equipment manufacturers to provide a continuous increase in spatial and, mainly, temporal resolution. Under certain circumstances, spectacular images of the heart can be obtained, including visualization of the coronary arteries. Indeed, cardiac CT imaging has started to penetrate clinical cardiology and is used with increasing frequency throughout the world. Although assessment of cardiac morphology and imaging of ventricular function are relatively straightforward, coronary artery visualization remains challenging. In spite of the impressive capabilities of the most modern CT hardware, the small dimensions of the coronary arteries, paired with their often rapid motion, can easily result in artifacts that may prevent interpretation or cause false-negative or, much more frequently, false-positive interpretation of coronary artery stenosis.

In this setting, a group of experts representing several professional societies have compiled the impressive "2010 Expert Consensus Document on Coronary CT Angiography." They have achieved a very comprehensive clinical perspective that is easy to read. This document provides some technology background and a summary of currently available scientific data with coronary CT angiography being the major focus. A possible disappointment to some may be that the authors do not provide clear categories of when coronary CT angiography is considered clinically reasonable or appropriate and when it is not (as, for example, has been done in the "Appropriateness Criteria" for cardiac CT and magnetic resonance imaging that were published in 2006 [1] and for which an update is expected soon). However, they do indicate that the high negative predictive value of coronary CT angiography may be helpful to rule out coronary artery stenosis in selected situations, stating that "In most circumstances, a negative coronary CT angiogram rules out significant obstructive disease with a very high degree of confidence." For patients with acute chest pain, the document states that "...a negative coronary CTA improves diagnostic certainty for ruling out significant coronary artery stenosis." Justifiably, the authors are very reluctant about a potential utility of coronary CT angiography in patients after coronary bypass surgery, indicating that because of the often severe coronary calcification in postbypass patients, "the difficulty of accurately assessing the native vessels is an important limitation for the clinical use of coronary CTA." On the other hand, the authors are surprisingly positive about the use of CT imaging in patients who have coronary artery stents, stating that "...in a patient known to have larger stents and whose clinical presentation suggests low to intermediate probability for restenosis, 64-channel coronary CTA may be a reasonable alternative to invasive angiography to rule out significant in-stent restenosis, presuming high-quality image quality can be obtained." No consensus was obtained in three areas: Is it required to interpret noncardiac incidental findings in CT data sets acquired for cardiac imaging? Can CT angiography be useful in asymptomatic individuals? Is a "triple-rule out" scan for emergency room patients with acute chest pain a reasonable strategy? For all of these questions, the evidence base was not sufficient to reach a conclusion.

Some readers of this document may be surprised by the fact that it does not provide more specific indications, giving clear guidance as to the patients and clinical situation in which coronary CT angiography is currently indicated. The authors of this document had a difficult task: They needed to avoid statements that are too positive, to prevent uncritical and prematurely widespread use of a technology that may not be ready. On the other hand, making the statement too negative might prevent applications that are useful and quite reasonable even today, in this relatively early stage of development. In fact, the document remains a bit vague because, quite literally, it deals with a "moving target": the rapid coronary artery motion poses a tremendous challenge to hardware technology and scan protocols, and in a broader sense, the groundwork upon which to base this document is also constantly changing, with a complex interplay of ongoing technology development, continuous growth of the evidence base, and shift of clinical focus in the workup of coronary artery disease. This creates a number of issues:

First, technology is improving at an unprecedented pace. Sixty-four-slice, single-source CT was considered "state of the art" only 2 or 3 years ago. Meanwhile, 256-slice systems, 320-slice systems and dual-source CT systems have become available and substantially improve coronary artery visualization in many patients. It is hard for clinical studies to keep up with the improvements in technology; by the time a larger trial is designed, implemented, performed, evaluated, and published, the utilized technology is already considered outdated and uncertainty as to the incremental value of newer scanner generation exists.

Second, and this is a necessary consequence of the lengthy and careful writing process of consensus documents that include a large number of authors, repeated critical reviews, and numerous steps of approval, the evidence base is growing and changing while the document is being created. Hence, for example, the recently published initial trials of myocardial perfusion imaging by CT are not covered in the document.[2,3,4] Some new data that might have provided substantial support to specific indications of coronary CT angiography (such as the Computed Tomographic Angiography for the Systematic Triage of Acute Chest Pain Patients to Treatment trial of acute chest pain patients, which has so far only been presented in abstract form) were not yet available during the writing period. Other recent data, such as the sobering analysis of the ability to detect in-stent stenosis by CT angiography in the Core64 multicenter trial [5], might have led to a more pessimistic conclusion in the consensus document. Not unexpectedly, the need for an update is evident even as the expert consensus document is being published.

Third, there has been a shift of our entire understanding of coronary artery disease. A few decades ago, anatomy was the primary focus, and the detection of coronary artery stenosis was considered the "holy grail" of cardiology. Meanwhile, cardiologists operate in a much more complex scenario, where plaque type and burden seem more important than luminal stenosis for prognostic assessment and functional measures, such as perfusion and fractional flow reserve, seem more important than the mere degree of luminal narrowing for decisions about revascularization. This is both a problem and a tremendous potential for CT imaging: Traditionally, CT angiography is targeted to identify coronary artery stenosis, and even though there is some very preliminary data about the ability of CT to analyze myocardial perfusion [2,3,4], it cannot currently be considered to provide the required information to fully assess the hemodynamic relevance of a stenotic coronary artery lesion. On the other hand, CT angiography has the potential to provide information about coronary atherosclerotic lesions that extends beyond the degree of luminal narrowing. This includes quantification and, to a certain degree, characterization of plaque, with substantial implications for risk stratification. However, imaging of plaque requires maximum image quality that cannot be achieved in all patients and is certainly not yet an application with clinical utility.

Finally, there is an extremely important aspect that is hard to quantify or measure. Cardiac CT imaging, and especially coronary CT angiography, not only requires up-to-date equipment, but also specific competence and expertise. Because most potential clinical applications are at the very border of what is technically possible, only carefully and expertly performed examinations provide the diagnostic accuracy that is reflected in clinical trials and that provides the basis for clinical indications that may be considered. In fact, even the previously published competence statements [6] seem to incompletely reflect the amount of training that is necessary to achieve the highest possible diagnostic yield.[7] Patients that are considered for coronary CT angiography must be carefully selected, data acquisition must be performed thoroughly and with expertise, and interpretation requires specific training and considerable experience. Of course, this holds true for all imaging modalities. However, CT imaging is relatively new to the community, and the often spectacular images are tempting to both physicians and patients. We need to constantly remind ourselves that using this test in the wrong patient or trying to interpret data sets of insufficient quality will be detrimental and needs to be avoided.

So, where does the current evidence base and this expert consensus document leave us with respect to clinical applications for coronary CT angiography? Due to the numerous influences outlined above, there is no single good answer and the use has to be carefully considered on an individual basis. As clinicians, we deal with a wide spectrum of patients: On one end of the spectrum, the patient with typical symptoms and a strongly positive stress test, hence a very high likelihood of one or several flow limiting coronary artery stenosis. At the other end, a patient whose symptoms may be less clear, and potentially with an ambiguous or even negative stress test. Although CT does not seem to currently be a good option for the former situation, it may well be useful if it can help avoid invasive testing in the latter, but it should not be performed just because "it is there." For any patient in between, the decision whether or not CT angiography should be performed, by necessity, is more difficult. Ultimately, choice of diagnostic testing must be made on a patient-by-patient basis and it needs to factor in whether the patient is a "suitable candidate" for coronary CT angiography, with a low and regular heart rate, ability to follow breathhold commands, and preferentially with a reasonable body mass. Of course, improved technology, growing clinical evidence, and more widely disseminated expertise with cardiac CT imaging will change and probably broaden potentially useful indications. For a while to come, we will be aiming at a moving target.