68 2011
IMAGING & ONCOLOGY
however, it is clear that catheter angiography has a much longer pedigree and a much larger evidence base for clinical decision making founded on its results than does CTCA. So what role, if any, can CTCA play in the investigation of patients with suspected CAD?
One of the strengths of CTCA is its very high negative predictive value. If it does not demonstrate any significant stenosis, it is right almost 100 per cent of the time and the patient is very unlikely to have an adverse coronary event any time in the near future13
. One
of its weaknesses is that it tends to overestimate the degree of stenosis caused by large calcified plaques. As a patient’s pre-test probability of CAD increases, the likelihood that the patient will have extensive calcified atheromatous plaques also increases. The utility of CTCA decreases, not for the detection of coronary artery plaques, but for quantifying the severity of any resultant stenosis. A significant proportion of these patients may also require angioplasty or stent insertion, making catheter angiography a more logical first-line investigation.
The 2010 National Institute for Health and Clinical Excellence (NICE) guidelines on ‘Chest pain of recent onset: assessment and diagnosis of recent onset chest pain or discomfort of suspected cardiac origin’3
are mindful of these facts. They recommend that people
presenting with recent onset of stable chest pain who have a 10-30 per cent likelihood of having CAD should have a CT coronary artery calcification score. If this (Agatston score) falls between 1 and 400, they should then have a CT coronary angiogram. Clearly, they have chosen this group because the majority are likely to have normal or mildly abnormal coronary arteries. CTCA will confirm that most of these patients do not have significant disease and require no further investigation, while identifying the small proportion that do.
In practice, CTCA may be used more widely in patients with CAD. Recent guidelines from the USA2
considered 93 clinical scenarios and decided that it was appropriate to
perform CTCA for 35 of these indications, most of which relate to possible CAD. Direct comparison between these recommendations and those of NICE are difficult because they use different thresholds of pre-test probability to trigger different investigations. Broadly, the American recommendations suggest that CTCA is a reasonable test in a wider range of patients, including those in the 30-60 per cent pre-test probability range. NICE recommends functional imaging, such as a nuclear medicine myocardial perfusion scan, as the best first-line test for this scenario.
For patients with acute chest pain, NICE3 guidelines2
does not see any role for CTCA. The USA suggest that it may be appropriate for selected patients presenting with
acute chest pain, who have a low or intermediate pre-test probability of CAD. The main benefits in this group are lower investigation costs and reduced admission rates14,15,16
.
A related postulated use for ECG-gated CT is the ‘triple rule-out CT’. This is when clinicians find themselves uncertain whether a patient has myocardial ischaemia, pulmonary embolism (PE), or aortic dissection17
. No doubt this clinical dilemma does occasionally arise in practice. We have all seen aortic dissection in patients being scanned for possible
Figure 5. Maximum intensity projection image from a CTCA demonstrating the left anterior descending artery (yellow arrow) arising anomalously from the right coronary artery (green arrow).
Figure 6. Curved MPR from a CTCA demonstrating a severe stenosis in a vein graft to the right coronary artery, close to its origin from the ascending aorta (arrow).
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