Accuracy, Safety and Application of Coronary Computed Tomography Angiography

Accuracy, Safety and Application of Coronary Computed Tomography Angiography

Chun-Leng Lim Michael
Asia-Pacific Cardiology Volume 1 - Issue I
Published: December 2007
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The main triggering factor for onset of acute coronary syndromes (ACS) following a prolonged period of coronary atherosclerosis is the rupture of an atherosclerotic coronary plaque followed by localised coronary thrombosis and/or spasm.1 The major factors that predispose the rupture of a vulnerable plaque are a relatively large lipid core, a thin cap and an accumulated macrophage content. None of these determinants of plaque rupture is related to each other or to the severity of luminal stenosis.1 Plaque rupture typically occurs at milder stenoses with 40–60% diameter narrowing or less.2-6 These mild stenoses generally do not give rise to symptoms or ischaemia on treadmill testing.7-9

The degree of stenosis in focal plaque segments on coronary angiograms does not correlate with the risk of plaque rupture. Plaques that have developed more recently are lipid-laden, cause minor luminal narrowing and are more likely to rupture than older, hardened plaques with more severe luminal narrowing.10-12 Therefore, it is not surprising that 65% of stenoses associated with subsequent myocardial infarction have <50% luminal diameter narrowing and 85% have <70% diameter narrowing.2,10-12

Conventionally, non-invasive methods for the detection of coronary disease using stress treadmill testing, stress echocardiography and myocardial perfusion scans provide indirect means of diagnosing significant coronary disease where the coronary luminal diameter narrowing is >70%. They are unable to detect the vulnerable plaques that do not cause any significant functional stenoses but that account for the large numbers of patients who eventually develop ACS. Coronary computed tomography angiography (CCTA) is one of the newest modalities of coronary artery imaging and is having a major impact on the assessment of coronary artery disease.

CCTA is performed by imaging the coronary arteries with a multidetector CT (MDCT) scan. Multiple submillimetre detector elements mounted on a gantry with a subsecond gantry rotation time allow high-resolution axial images of the coronary arteries to be recorded while the patient holds his or her breath. A 16-channel MDCT scanner will have a lower volume coverage than a 64-channel MDCT scanner. Hence, the breath-holding duration for a 16 MDCT CCTA is between 20 and 25 seconds, and for a 64 MDCT CCTA it is between seven and 10 seconds.

Accuracy of Coronary Computed Tomography Angiography
With such a wide choice of non-invasive cardiac imaging modalities, it is important to assess each new modality on its own merits. CCTA must be able to demonstrate superior accuracy compared with current modalitites if it to be used routinely. Conventional non-invasive modalities for coronary artery assessment are performed to detect the likelihood of significant flow-limiting lesions, but provide no information as to the presence of vulnerable plaques and are unable to detect the presence of sub-clinical atherosclerosis that may predispose future cardiac events. The results of these tests may be influenced by gender, cardiac rhythm, inability to exercise and the number of vessels involved. Treadmill testing, stress echocardiography and myocardial perfusion scans have sensitivity/specificity/ accuracy of 68/77/73%, 85/84/87% and 89/80/89%, respectively.13

Many centres have demonstrated that CCTA has a high sensitivity and specificity for detection of coronary artery disease compared with invasive coronary angiography. In our centre, we were able to demonstrate a sensitivity of 99%, specificity of 98%, positive predictive value (PPV) of 94% and negative predictive value (NPV) of 99% for the detection of significant coronary artery stenoses.14-18 This compares very favourably with the other conventional modalities of non-invasive cardiac imaging. Hence, among the non-invasive tests CCTA has the highest specificity and sensitivity for non-invasive detection of coronary artery disease.

It is also consistent in most of the published papers that CCTA has a high NPV.14-18 The PPV of CCTA varies from centre to centre as different protocols and post-processing methods are used. Initially, there is a tendency towards overdiagnosis, as there is a learning curve for CCTA. However, with proper patient preparation, experience, optimal protocols and appropriate post-processing techniques, a high PPV is achievable.18

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