Multislice cardiac CT: Who will benefit?
As technology offers more tools to help diagnose heart disease, you may need to adjust your approach. Here’s the latest on one of those new tools.
One of the greatest challenges facing primary-care clinicians involves working up symptomatic patients who are at intermediate risk for coronary artery disease (CAD). Newer noninvasive imaging modalities have become effective diagnostic tools for assessing occlusive coronary artery stenoses and have started to reshape traditional algorithms for CAD risk stratification. Multislice CT (MSCT) is the leader of these emerging technologies.
The challenge for primary-care clinicians, then, is to determine how MSCT fits into the algorithm they are currently using for determining CAD risk. This review will help identify appropriate indications for MSCT cardiac imaging in CAD risk assessment.
Establishing pretest probability for CAD Initial CAD risk stratification, essential when deliberating about any therapeutic approach, is based on clinical variables that are easily obtained from history—age, sex, symptoms, and the presence or absence of cardiac risk factors, such as hypertension, dyslipidemia, BMI >30, diabetes, tobacco use, and a significant family history. A well-documented history of prior MI or stroke is also important.
Tools, such as the Framingham Risk Score, and guidelines set forth by the National Cholesterol Education Program help incorporate these clinical variables into meaningful estimates of future CAD risks, such as MI and related deaths. Moreover, precise patient characterization of chest pain (when present) can also help lead to a more valid estimate of pretest likelihood for CAD, according to the Coronary Artery Surgery Study.
The summation of these clinical variables will help you derive a pretest probability for CAD and determine whether to start medical management, pursue further noninvasive diagnostic testing, or have the patient undergo invasive angiography with the possibility of revascularization.
Noninvasive testing is generally not recommended for patients who fall on either extreme of pretest probability for CAD. That is, patients with low pretest risk for CAD would not benefit from additional diagnostic testing, and there is the risk of a false positive. Patients with high pretest probability for disease run the risk of a false negative.
Noninvasive testing, then, would be best for those with intermediate pretest probability of disease. This group would include, for instance, middle-aged or older men and women with atypical chest pain. Various stress-testing methods are the most common noninvasive approach, but MSCT and other imaging tests are getting plenty of attention these days too.
The role of noninvasive coronary imaging MSCT offers increasingly detailed resolution of coronary anatomy. Meta-analyses show that MSCT provides both highly sensitive and highly specific identification of high-grade coronary stenoses. A source of some confusion in its early developmental years, MSCT, not cardiovascular MRI, is now preferred for noninvasive visualization of the coronary arteries.
A meta-analysis comparing the diagnostic capabilities of MSCT and MRI in the identification of significant luminal stenoses (≥50% narrowing) found that MSCT had a significantly higher sensitivity (85% vs. 72%) and specificity (95% vs. 87%) than cardiac MRI.1 In the clinical setting, cardiac CT, for both angiography and calcium burden assessment, produces images more quickly and is easier to use than cardiac MRI.
Despite rapidly increasing data on the accuracy and utility of the technique, current American Heart Association/ American College of Cardiology guidelines cite conflicting evidence and/or a divergence of opinion on MSCT’s usefulness and efficacy (class II recommendation) in the identification of coronary artery stenoses in symptomatic patients. Selective insurers consider the procedure experimental and will not cover its cost, frequently triggering frustrating and time-consuming appeals for all involved.
Curiously, reimbursement for invasive cardiac catheterization is rarely questioned, reflecting the current transitional period. Furthermore, the increasing accessibility and noninvasive nature of MSCT raise the possibility for clinical and financial misuse, allowing potentially for the unchecked proliferation of this modality.
In addition to addressing MSCT’s role in the existing algorithm for CAD risk and therapy, clinicians must consider current reimbursement schemes and, for the time being, generally incomplete cost-effectiveness data.
MSCT in calcium scoring
The newer MSCT machines, such as the 64-slice detector CT scanners, yield increasingly greater resolution by employing faster rotating gantry speeds and acquiring a greater number of thinner image slices simultaneously. Image-reconstruction software packages have also improved dramatically. Enhanced resolution today can allow for imaging of a heart beating in normal sinus rhythm without motion artifact. Image acquisition can be completed within approximately six heartbeats.
Although not yet widely available for everyday clinical use, the newly developed 256-slice detector CT scanners are already under active investigation at select academic university hospital and research centers. With their increased number of detectors, the 256-slice machines can cover four times the area in a single scan and can acquire a full image of the heart with just one heartbeat. The increased speed of the 256-slice machines renders motion artifacts less worrisome and allows patients with arrhythmias to be scanned.2
With MSCT, doctors can assess coronary artery calcium (CAC) and/or intraluminal coronary anatomy. Use of noncontrast CT to measure coronary artery calcified plaques (CACPs) is quick, now often taking <10 minutes, and does not, of course, require contrast injection. The amount of total calcium burden, often tabulated by either the Agatston or volume scoring methods, has been found to have direct correlation with atherosclerotic disease.
Studies also demonstrate that CACP is both independent of and incremental with traditional risk factors in the prediction of cardiac events. For instance, a patient with a high CAC score but an intermediate Framingham Risk Score can now be considered at high risk for cardiovascular disease (CVD)-related complications, such as MI or death.3 Both retrospective and prospective cohort studies suggest a direct proportional relationship between CAD risk and the amount of CACP.
A low CACP burden revealed by MSCT has been found to have high negative predictive value,4 suggesting that the presence of atherosclerotic plaque or obstructive disease is highly unlikely. While the utility of CACP evaluation in symptomatic patients is still under active investigation, studies using intravascular ultrasound imaging have documented that certain CACP patterns have strong correlation with culprit lesions in acute coronary syndrome.5
When would it be appropriate to use MSCT to calculate CACP burden? It seems that CAC assessment may be useful in symptomatic patients who have a low-to-intermediate pretest risk of CAD or in patients with equivocal stress-testing findings. However, most people of advanced age have some degree of CAC, potentially limiting the utility of this approach in the elderly.4
This test is currently best suited for further characterization of CVD risk for patients with an intermediate risk (i.e., 10%-20% 10-year risk) of a coronary event on the basis of traditional Framingham risk factors. Patients subsequently found to have higher CAC scores would therefore fall into a higher risk category, supporting the need to not only institute more aggressive medical therapy (e.g., LDL-lowering therapies), but also to consider additional diagnostic procedures.
MSCTs are also employed for angiographic visualization of coronary arteries after contrast dye injection (see Figure 1). Multiple studies have demonstrated that MSCT has a high negative predictive value.6 A CT coronary arteriogram (CTA) deemed to be normal can effectively rule out hemodynamically significant coronary artery stenoses.
CT angiography is better suited for the visualization and identification of proximal vessel disease, as the larger-caliber vessels are less subject to motion artifact. The distal, smaller-caliber vessels are more prone to motion artifact, often lowering their resolution in CTA. There are relatively limited but promising data on the use of CTA in evaluating the patency of coronary bypass grafts.7,8
Also under debate is what to do with a positive finding on CTA. As with invasive coronary angiography, the degree of luminal narrowing relies on visual estimate. Most current literature on CTA reports ≥50% narrowing to be a significant observation. However, stenoses <70% do not usually limit flow and often don’t trigger a trip to the catheterization lab. Consultation with a cardiologist or radiologist may be needed to consider whether patients with significant luminal narrowing on CTA would benefit from further invasive strategies.
While invasive coronary angiography routinely requires the direct input of a cardiologist, the training and qualification process necessary to read and interpret CTAs is undergoing active development. Because of the rapidly evolving nature of this modality, there is no formal curriculum for either radiology or cardiology trainees, nor is there a formal board-certification process.
When is it most appropriate to use CTA? Like CAC scoring, CTA can be employed to rule out significant stenoses in symptomatic patients who have an intermediate pretest risk for CAD. Interestingly, a recent study suggests that to reach a post-test probability of CAD <10% following a negative CTA, the initial pretest probability for CAD should be <50%. Securing a <5% post-test probability of CAD requires a pretest probability closer to 30%.9
These data prompted a commentary advocating conservative use of CTA.10 For instance, judicious indications for CTA would center on patients with nonspecific symptoms and equivocal results on functional noninvasive imaging, especially if the symptoms are recurrent and require multiple physician or emergency department visits.
In the practical clinical setting, CTA appears to be most useful for a patient with nonspecific or atypical chest pain and equivocal findings on functional stress testing or for a symptomatic patient with low-to-intermediate pretest CVD risk but a positive stress test. The importance of appropriate patient selection is reinforced by a recent analysis of cancer risk associated with radiation exposure from 64-slice CTA.11
As of the publication of this review article, we were unable to identify formal guidelines from primary-care organizations on the use of CTA. The previously referenced commentary is a particularly useful starting point worth further consideration.10
Conclusions and recommendations Traditional noninvasive diagnostic testing methods, such as ECG stress testing, stress echocardiography, or nuclear stress tests, continue to be the mainstay of a clinician’s arsenal for CVD risk stratification.
While MSCT offers significant resolution improvement over currently available noninvasive diagnostic techniques, this imaging modality should not be used as an initial screening tool for asymptomatic or occult CAD. Because of the relative novelty of the modality, multiple reproducibility and validation studies of calcium scoring and CT angiography are ongoing. Patients found to be high risk by clinical assessment or by functional stress testing should obtain further invasive testing, as noninvasive cardiac imaging would offer little additional helpful information. Patients at very low risk from CAD will also not benefit. (See “Limitations of MSCT.”)
However, for the patient with an intermediate risk of CAD or a CAD event following clinical assessment or noninvasive testing, MSCT offers incremental information beyond that provided by traditional stress-testing methods. When MSCT is well used, we anticipate it will lead to smarter, more efficient use of invasive procedures and medical and interventional therapies.
Dr. Lau is a fellow in cardiology at Mount Sinai Medical Center, New York City, and Dr. Steingart is chief of the cardiology service and director of nuclear cardiology, Memorial Sloan-Kettering Cancer Center, also in New York City.
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