Cardiac Computed tomography (CCT) has the potential to revolutionise the practice of cardiovascular medicine. This technology allows assessment of several different parameters of heart disease including coronary calcium scores, non-invasive coronary angiography, and some assessment of myocardial perfusion and function. 

Although CCT has come to prominence only relatively recently, the stunning clarity of the images has generated considerable excitement in both the medical and the lay communities.

Cardiac CT image showing the  coronary arteries. The LAD is highlighted on the volume rendered (coloured) image and displayed on the right in a multiplanar reformat.

Technology and Protocol 

In order to provide interpretable images of the small, rapidly moving coronaries, an imaging technique must have high spatial resolution (to clearly define vessels that are frequently less than 3mm) and high temporal resolution (to freeze cardiac motion).

Temporal resolution is achieved by ‘gating’ the images to the cardiac cycle with the use of ECG monitoring (to control for cardiac motion) while asking the patient to breath-hold during the scan (to obviate diaphragmatic motion). Although the heart moves rapidly during systole, there is usually a motion free period during diastole that is suitable for acquiring images. The final picture is made up from images which have been acquired over several heart beats, that are then combined into one data set. Hence patients with irregular rhythms, or rapid heart rates, may not produce optimum pictures, and an ectopic beat may render some images uninterpretable. It is routine to reduce a patient’s heart rate prior to CCT by giving oral or intra-venous beta-blockers.

Multi-slice CT scanners are typically referred to as being 4, 16, or 64 ‘slice’. The ‘slice’ actually denotes the number of detectors present in the scanner. A 64 detector CT will often provide better pictures than a 16 detector CT because it will complete the scan in a shorter time. The current optimum resolution is around 0.5mm in all directions. Because the images are acquired faster by modern scanners with a higher number of detectors, the scan duration is reduced. This means that patients are asked to hold their breath for around 12-20 seconds with a 64 slice machine, as opposed to 30-40 seconds with a 16 slice An additional benefit of the shorter scan time, is that often less radiation and contrast are required. Although there was enthusiasm several years ago for electron beam CT (EBCT) due to its high temporal resolution, this technology has been largely superseded by the higher spatial resolution and multiple applications of modern multi-slice machines.

During a CT angiogram the patient has contrast injected via a vein in their arm. It is important that the heart and coronaries are full of contrast during the entire scan, and around 70-100mls of contrast will be required. There is a significant radiation dose from a cardiac CT (approximately 5 – 12 mSv), but this must be balanced against the radiation that would be received in alternative investigations. An invasive coronary angiogram delivers between 4.6 - 15.8 mSv, and a SPECT scan gives between 5.6 – 11.8 mSv per injection depending on the isotope used.

Coronary Artery Calcium (CAC) Scoring

The presence of calcium in the coronary arteries is intimately associated with atherosclerotic plaque (1;2), and appears to be a predictor of cardiac events, independent of the other standard risk factors such as hypertension and smoking (3;4). CAC scores can be obtained with a very short (5 second), low radiation, non contrast scan, and they have been proposed as a method for screening the population for coronary disease.

A protocol for quantifying and scoring the amount of coronary calcium was developed by Agatston, and the eponymous score allows individuals to be placed in age and sex matched quartiles (5). Those in the highest quartile are more likely to have future cardiac events than those in the lowest quartile, and this may prove useful in the further classification of risk in those patients who are otherwise classified as ‘intermediate’. However calcium presence signals old, stable plaque, whereas it is the recent, uncalcified plaque that gives rise to acute coronary syndromes. CAC relies on the fact that the presence of the former increases the likelihood of the latter too. Another important use of the coronary artery calcium score is the high negative predictive value of a zero score, which is consistent with a risk of coronary events of <0.1% per year (6). However the exact clinical applications of CAC scores remain to be determined.

CT Coronary Angiography (CTCA)

Traditional non-invasive methods of assessment for coronary artery disease, such as stress echo or nuclear SPECT scans rely upon demonstrating the effects of coronary stenosis in terms of diminished perfusion or function. CTCA however, like invasive coronary angiography, allows visualisation of the coronaries, including their course, calibre and lumen (7). The impressive colour pictures generated are ‘volume rendered’ images (Fig.1), which prove helpful for demonstrating the course of coronaries (which is useful for anomalous vessels, or of grafts prior to re-do CABG), but are less useful for showing stenoses. Intra –coronary lesions are best viewed on axial slices, or images which have been reformatted across multiple planes (Fig.2). Recent studies suggest that, if clear images are obtained, then the sensitivity and specificity of CTCA is in a range above 90% (8;9). However while CTCA can reliably identify the presence of a stenosis, it is much more difficult to quantify the severity of the lesion (10). As a 40% stenosis would be unlikely to require intervention, but a 70% stenosis almost certainly would, CTCA is better at predicting the presence of luminal disease, than the likelihood of requiring intervention. A further drawback is that often some coronary segments will not be evaluable due to suboptimal image quality or heavy calcification. This does not matter much in a study that is assessing the ability of CTCA to define stenosis on the basis of individual coronary segments, but when it happens to an individual patient it means an inconclusive test.

Other Applications

Rather than simply reconstruct the images at one point of the cardiac cycle, to assess the coronaries, multiple reconstructions can be performed to allow visualization of cardiac motion. This produces images similar to echo or cardiac MRI, which allow quantification of cardiac function, and wall motion. It is also possible to demonstrate perfusion defects on CT, which usually represent areas of prior infarction.

Conclusions

As older CT systems are replaced, cardiac capable scanners are appearing in many hospitals. However, if reliable results are to be obtained from this complex technique, it is essential that the doctors who wish to perform and interpret these scans, be they cardiologists or radiologists, undergo the appropriate training. Guidelines for training have already been established (11). Although many cardiologists are very enthusiastic about the technology, many others are skeptical (12). It is important to remember that the chest X-ray was opposed by many leading physicians, including Osler, who believed that clinical examination was superior, and it took nearly 30 years for the chest x-ray to become an established clinical test (13). Hopefully the cardiac CT will prove its benefit in clinical trials, and become part of the diagnostic armamentarium in the near future.