Non-invasive Computed Tomography Angiography of the Coronaries - Is There Enough Evidence?

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Citation
ICR 2006;2006:1(1):23-24

Over the past decade, great strides have been made in the field of cardiac imaging, particularly in cardiac computed tomography (CT) to view the coronary artery lumen and plaque.1 Performance of cardiac CT angiography (CTA) has been challenging, given rapid cardiac motion, small vessel diameters, tortuous anatomical patterns and overlapping cardiac structures. Current 64- multi-row detector computed tomography (MDCT) systems have faster gantry rotation speed resulting in better temporal resolution and better z-axis spatial resolution made possible by thin collimations with extensive volumetric acquisitions. The current technique necessitates scanning with overlapping of slices in order to compensate for cardiac motion and 'virtually freeze' the heart by using segments from different heart beats in order to improve the effective temporal resolution. Technological improvements with cardiac CT have been advancing at incredible paces, with new equipment (with improved imaging capabilities) being introduced to the market almost every year. The 64-MDCT has transformed coronary CTA from an adjunct test to a robust clinical tool with a wide range of clinical applications.

Diagnostic Accuracy

One question that has been raised with cardiac CT is, is there enough validation to warrant clinical utility? There is not the wealth of data with CT as with nuclear or echocardiography; however, the data are quite consistent and supportive of a highly accurate test compared to invasive angiograpy. Leschka et al. showed that CTA with 64-MSCT has a sensitivity of 94%, a specificity of 97% and a negative predictive value of 99%. Raff et al. showed high quantitative and qualitative diagnostic accuracy of 64-slice multislice computed tomography (MSCT) in comparison with invasive coronary angiography in a broad spectrum of patients for detecting lesions >50%. Specificity, sensitivity and positive and negative predictive values for the presence of significant stenoses by patients were 95%, 90%, 93% and 93%, respectively. The negative predictive value for significant stenoses with CTA has been uniformly high (98-99%) using a 64-slice scanner.6-11 The high negative-predictive value of coronary CTA is useful in patients with atypical chest pain. As long as the image quality is diagnostic and all coronary segments are well seen, a 'negative' CTA using 64- MSCT rules out high-grade coronary stenoses with a very high degree of certainty (Figures 1 and 2).

Mollet et al. compared 64-CT and angiography; all segments, sensitivity and specificity of MSCT were 99% and 95%, respectively. Sensitivity and specificity for proximal, mid and distal segments were 100% and 97%, 97% and 94%, and 100% and 97%, respectively. In a study by Leber at al.,8 sensitivity for the detection of stenosis >75% was 80% and specificity was 97%. In comparison with intravascular ultrasound (IVUS), 46 of 55 (84%) lesions were identified correctly. Pugliese et al. demonstrated sensitivity, specificity and positive and negative predictive values of 64-slice CT on a per-patient basis results were 100%, 90%, 96% and 100%, respectively. Fine et al. studied 66 sequential subjects having both 64-slice CT and catheter angiography within 30 days. Accuracy results were 94% interpretable images, sensitivity 95%, specificity 96%, positive predictive value 97% and negative predictive value 92% for lesions >50%. These studies support 64-slice CT as a reliable diagnostic tool.

Prognostic Data with CTA

From an anatomical standpoint, CTA as a diagnostic tool should outperform nuclear perfusion testing. However, from a functional standpoint, the myocardial peformance index (MPI) is an established method for non-invasively assessing the functional significance of coronary stenoses and delivers valuable information for risk stratification. Patients with stable angina and normal MPI results have a low risk of death or fatal myocardial infarction and no intervention is required for these patients. Even in patients with documented coronary artery disease (CAD), normal MPI results have been shown to have similar low-event rates. MPI identifies myocardial perfusion defects, analysing the functional relevance of coronary artery stenoses and providing important information for clinical decision making. One study demonstrated significant improvement in diagnostic accuracy of CTA compared with nuclear testing in the same population. Further studies are needed, but the high accuracy of CT may argue for this to be the gatekeeper, with borderline results or non-diagnostic testing. It is not surprising that CTA outperformed MPI in the diagnosis of obstructive disease. The anatomic gold standard favours an anatomic test such as CTA. This study compared a well-established tool for the detection of myocardial ischemia, such as MPI, with a technique that provides anatomical information of the epicardial coronary arteries.

It has been suggested that MPI, due to the prognostic information available, should remain the gold standard for detection of CAD. Only study of CTA and prognosis has been reported. Plezel et al. evaluated the accuracy of CTA with 16-MSCT for determining significant CAD and a six-month prognosis and clinical outcome in a 'real world' clinical setting. The six-month follow-up in the 834 patients originally not referred for cardiac catheterisation showed two (0.2%) patients who subsequently had significant CAD. There were no cardiovascular deaths. However, there is vast prognostic information also available with coronary artery calcium scoring (CAC). The combination of CAC and CTA affords similar depth of prognostic information to MPI. CAC and CTA detection of CAD are not affected by left-bundle branch block, abnormal perfusion patterns from a dilated ventricle or breast/diagphragmatic artifacts.

Conclusions

CTA has become a robust modality to evaluate patients with CAD, with or without prior revascularisation (Figures 3 and 4). The ability to see the luminal stenosis, as well as the plaque burden, will make this a robust tool in the evaluation of CAD. Furthermore, there are two answers given with the results of a CT angiogram: is there clinically significant stenosis; and, is there clinically significant atherosclerosis? This can direct both further interventional care, as well as direct preventive care. This is a big improvement over current testing strategies, such as functional assessment, which will only answer the first question related to clinically significant stenosis. A normal nuclear test, while reducing risk in the next one to two years, does not assist in assessing mild stenoses or plaque burden.