Over the last five years, the speed and resolution of mutislice Computed Tomography (CT) has advanced to the point where it can now be used for accurate cardiac evaluation1. Calcium scoring, a popular method of cardiac evaluation using Electron Beam CT technology (EBCT), has been re-evaluated using spiral CT technology, and has been validated for risk assessment of patients’ long-term cardiovascular outcomes and in varied patient ethnicities2-4. Calcium scoring, a gated non-contrast examination of the heart using cardiac CT, is a quick and safe way of evaluating for the presence of calcified atherosclerotic coronary plaque. The largest drawback of calcium score is that while a positive test confirms the process of atherosclerosis, a negative score does not completely rule out the disease. While high calcium scores can assess general risk of coronary events, they cannot accurately predict individual coronary stenosis1,5(Figure 1).
Multislice Computed Tomographic Coronary Angiography (CTCA), unlike calcium scoring, allows for the evaluation of the soft as well as the calcific plaque within the coronaries. The technique is similar, but the addition of iodinated contrast during the scan, and higher resolution imaging allows for more anatomic evaluation of the coronary structures and the surrounding cardiac anatomy5(Figure 2).
Prior to the advent of 16 slice scanners, it was difficult to evaluate the heart, much less the coronary arteries, due to the constant motion of the beating heart. With a 16-slice scan, patients would typically have to hold their breath for 25 to 30 seconds for the entire heart to be imaged. This was difficult for some patients to do, and there was acceleration of heart rates towards the end of the scan. In addition, it was difficult for the patient to remain movement free for such a long time.
Sixty-four slice scanners, which were largely available starting in 2006, allowed for acquisition of heart scans in only few seconds, and at higher heart rates than were previously possible. With the larger (approximately 4 cm) arrays in 64-slice scanners, the heart could typically be scanned in its entirety within five or six heart beats. This eliminated almost all of the problems with breath holding, and there was a much shorter time that patients needed to remain motionless. The additional benefit was that a wider range of heart rates were acceptable for scanning.
There has been a great deal of publication on 64-slice scanners with regards to accuracy of the evaluation of coronary disease, as well as its utility in evaluating the larger structures of the heart. The majority of recent evaluations suggest that the sensitivity and specificity for 64-slice scanners is exceptionally high, typically in the 90-95% range. Currently, most research and clinical trials are being performed on 64-slice technology, and the majority of scans being performed clinically are using this technology.
From a patient’s perspective, having a CTCA or calcium score is a relatively simple and painless process. Calcium scoring is slightly less involved than a CTCA, since no contrast is used, and no intravenous access is necessary. ECG leads are placed on the patient’s chest, and a 2.5 to 3mm thickness scan is performed through the level of the heart, gated to the patient’s heart beat. There is no post-imaging recovery time, and the patients are able to leave immediately. A preliminary report is often given to the patients before they are discharged. The final report is generated only after the review of the final images by the reading physician. Images are sent to a workstation for initial processing by the imaging technologist and then reviewed by the reading cardiologist or radiologist. For patients, the total time this process takes, including paperwork1, is less than 30 minutes.
For patients receiving a CTCA, the process is similar. Outpatients are typically given oral beta blockers two to three days prior to and on the day of the scan. They are asked not to eat six hours prior to scan. Once in the imaging suite, an intravenous line is placed in an antecubital vein. Telemetry is connected to ECG leads which are placed on the chest, but as lateral as possible to avoid interference with the scan. Patients who have significant renal insufficiency are typically not candidates for this test, although a mild amount of renal insufficiency is not enough to disqualify them. With 64-slice scanners, there is less of an issue related to heart rate, and patients with heart rates up to 80 bpm are still able to be imaged with excellent image quality. Patients with irregular heart rates (e.g. atrial fibrillation or a large amount of ectopy) are not good candidates for coronary imaging, although it is still feasible to evaluate the larger structures of the heart.
The CTCA scan itself consists of a series of pictures including an optional initial calcium score. A scout scan it used to localise the borders of the heart, and then a test bolus (typically 20 cc of contrast material) is used to localise the left main coronary and estimate the transit time of the contrast from the IV to the coronary arteries. The full bolus (typically 80 cc of contrast material) is injected at a rate of 3 to 5 cc / second. The patient is asked to hold their breath for the duration of the scan, which lasts 5 to 10 seconds, depending of the area of the scan. Once the scan is complete, the patient is allowed to leave. Images are sent to the reading workstation, and the reading physician evaluates the raw data, reconstructed images, and generates a final report. For patients, the process (including paperwork) takes less than one hour.
In comparison with its invasive counterpart, CTCA is much safer. The only risks are related to the contrast dye, which is potentially toxic to the kidneys, and the theoretical risk of the dose of ionising radiation. For patients with normal renal function, the contrast load is very safe. Only about 100 to 120 cc of contrast is given during a typical cardiac CT. The radiation is variable, but typically ranges from 10 to 16 mSv for a gated study6,7.
The application for multislice cardiac CT generally falls into two categories:
Evaluation of the coronaries and evaluation of the non-coronary cardiac structures (Table 1). The evaluation of coronaries is mostly done to the patients who suffer from chest pain and have a low to moderate risk of flow limiting coronary disease. It is always better to send patient for an invasive angiogram if there is a very high pre-test probability of significant coronary disease. Many patients sent for CTCA have had a prior stress test which was thought equivocal or negative, but in patients who had an invasive angiogram it is deemed overly aggressive. In this subset of patients, CTCA can define those patients in whom it is unnecessary to proceed with further testing8.
In high-risk asymptomatic patients, there is still potential use for CT to identify patients who have very premature or sub-clinical atherosclerosis9. This type of evaluation is currently not being reimbursed by insurance companies, and is often paid for by patients out-of-pocket.
Coronary anomalies, while uncommon, are potentially lethal. Certain subsets of these patients are at high risk for sudden cardiac death, and so it is helpful to evaluate the course of the anomalous arteries (image-anomaly Figure 3) CTCA is considered the gold standard for the evaluation of the coronary tree for the presence of anomalous coronaries10. While this is its strongest indication, these patients represent a much smaller group than the patients who are suspected of having flow-limiting coronary disease.
A large group of patients are now receiving cardiac scans as part of the preoperative evaluation for electrophysiology procedures such as pulmonary vein ablations for atrial fibrillation (Figure 4). In our centre, as with many others, this is becoming the standard of care prior to this type of invasive treatment11,12. A growing number of patients in whom re-operative coronary artery bypass is being contemplated are receiving CTCA to evaluate patency and location of grafts (both readily determined utilising CT) prior to repeat median sternotomy13-15. Some patients are now receiving CTCA instead of invasive angiography prior to non-coronary cardiac surgery in whom the pre-test probability of disease is relatively low16-18. The most significant change from 16 to 64-slice scanners was the reduction of acquisition time, while the resolution of the scans was essentially unchanged. Beta blockers, which are less necessary with 64-slice scanners, are still typically used as they improve the overall image quality and allow for larger radiation dose modulation.