Advances in multidetector CT technology over the past decade, including improvements in spatial and temporal resolution and the introduction of electrocardiographic gating, has made non-invasive visualisation of the coronary arteries feasible. The potential to obtain information non-invasively that is comparable to that provided by invasive coronary angiography has been a major driving force behind the rapid growth and dissemination of coronary CT angiography (CCTA).1 Non-invasive coronary imaging requires a CT system capable of acquiring motion-free, high-resolution images covering the entire heart in a single breath hold. Current-generation 64-detector row systems and more recently introduced CT scanners with 128-, 256- and 320-detector rows fulfil these requirements.1 Most UK radiology departments are in possession of such technology, and many can now provide, or are in the process of setting up, a cardiac CT service.2 In this issue of Postgraduate Medical Journal, Yerramasu et al provide a succinct review of the current status of CCTA technology and its application to the investigation of patients with coronary artery disease (CAD).3

Currently the main strength of CCTA is its extremely high negative predictive value whereby a normal study virtually excludes the possibility of significant CAD with a high degree of confidence (98–99%).4 5 This has been validated in several large clinical trials and has led to the introduction of CCTA into recently published UK guidelines for management of stable chest pain syndromes whereby CCTA is now advocated as a first line test in patients with a low pretest probability of CAD.6 It remains to be seen how widely these guidelines will be implemented and what impact they will have on patient management. Many CT departments across the UK already struggle to meet increasing demands from emergency departments and acute admission units as well as pressure to achieve cancer targets and reasonable waiting times for elective outpatient work. The requirement for on-demand CCTA, linked for example to a rapid-access chest pain clinic, may not be practical or achievable in many hospitals. Several preliminary economic studies have looked at the use of CCTA in the diagnostic evaluation of acute chest pain in the emergency department and suggest that a diagnostic strategy using CCTA may significantly decrease both length of stay and cost, particularly in lower risk patients.7–9 Such an approach has the potential to force a paradigm shift in acute chest pain evaluation, but outcome studies with larger cohorts and longer follow-up times are needed, and again the implementation of such a strategy in the UK would require major resource investment.1

As with any diagnostic test, CCTA has certain technical and patient-related limitations with which users should be familiar, as proper patient selection is important to maximise its diagnostic accuracy. Interpretation is intimately linked to image quality which is dependent on a steady heart rhythm as well as patient compliance with breathing instructions and a number of other factors including body mass index and scan protocol optimisation. Interpretation is challenging in patients with a high volume of calcified atherosclerotic plaque because of a tendency to overestimate disease severity due to calcium-related (blooming) artefact, particularly in smaller and more distal coronary segments.1 CCTA, as with any purely anatomical test (including invasive coronary angiography), is unable to predict the functional impact of a coronary stenosis—that is, the depiction of a stenosis does not imply myocardial ischaemia. Indeed, an analysis of studies comparing results of CCTA with nuclear myocardial perfusion scintigraphy found that only around half of anatomically significant (>50%) lesions had corresponding perfusion defects suggestive of ischaemia.10 A combination of non-invasive anatomical and functional imaging may therefore be the best strategy for identifying patients most likely to benefit from medical therapy (coronary atherosclerosis without ischaemia) or who may be candidates for coronary revascularisation (coronary atherosclerosis with ischaemia).11This further emphasises that the ability of CCTA to provide incremental diagnostic information that alters patient management is heavily dependent on pretest probability of disease, and clinicians should refer to recently published international referral criteria in this respect.

The field of CCTA is advancing at a tremendous rate, with technological evolution at times outpacing research evaluating its incremental benefits, and as a result many important questions remain as yet unanswered. For example, what is the optimal management strategy for incidentally detected non-haemodynamically significant coronary atherosclerotic plaques, and can CCTA provide any insight into prediction of subsequent plaque rupture and acute myocardial infarction?12 In the future, the ability of CCTA to provide such information not currently available from invasive angiography may forge a paradigm shift in how patients with atherosclerotic cardiovascular disease are classified and managed.1 CCTA, although still in its infancy, is already an incredibly powerful tool that is set to have a major impact on the management of CAD for years to come; clinicians must embrace this technology and understand its current limitations for facilitating optimal patient management.

Source: postgraduate journal of medicine