Computed tomography coronary angiography: Is radiation a concern for the gold standard test for anomalous coronary arteries?

Abstract

Multi-detector computed tomography is now an established modality for the investigation of coronary artery disease.1,2^ 64-Slice multi-detector computed tomography is the standard requirement by the British society of cardiac imaging and can image the whole heart in four heart beats. The 320 slice multi-detector computed tomography can image the heart in one heart beat, but both technologies depend on B-blockers to slow the patient’s heart rate to 60 beats per minute to ensure excellent image quality and low radiation dose. The faster dual source multi-detector computed tomography is less dependent on B-blockers. The availability and ease of use of multi-detector computed tomography has meant that it is increasingly used in younger patients to rule out coronary artery disease or rule out anomalous coronary arteries. We consider the radiation associated with computed tomography coronary angiography and the available dose reduction techniques.

Multi-detector computed tomography (MDCT) is now an established modality for the investigation of coronary artery disease^1,2 and has become method of choice, for the investigation of anomalous coronary arteries, in young patients/athletes with exertional chest pain or exercise-induced syncope. Anomalous coronary arteries, which have a prevalence of 0.3% in the general population, are found in up to 20% of autopsy series of sudden death in athletes (Figure 1). Exercise testing, particularly in athletes is often negative.^1–3 MDCT provides three-dimensional images of the heart with excellent spatial resolution which demonstrate the coronary anatomy very clearly. MDCT is also a simple and fast test. However, there has been a concern about the radiation dose that these young patients would receive. When cardiac CT entered clinical practice in 2005, the radiation dose for a CT coronary angiogram was over 20 msv. The technological developments in cardiac CT since have focused on dose reduction techniques. The first and still the most widely used technique, to lower the radiation dose, was the ECG controlled tube current modulation (ECTCM). It reduced the X-ray tube current during systole, when coronary motion is high and the images are mostly unhelpful, and reduced the radiation dose by 25%. However, this is not sufficient. Einstein et al. ⁴ calculated the risk for future cancer from a CTCA with ECTCM (14 msv) in a 20 yr old woman, to be 1 in 219. Hence, it is critical to use all the additional dose reduction techniques now available, particularly in young patients. These include the use of the new iterative reconstruction technology to form the images, instead of the older filtered back projection, which reduces the radiation dose by 30%. This new technique, which also improves image quality, relies on the large computational capacity on the second generation 64 slice CT scanners. However, the most important way to lower the radiation dose is prospective ECG-triggering, where the acquisition is limited to a fraction of the cardiac cycle at end-diastole when coronary motion is minimal. This technique reduces the dose by 69%, but requires adequate B-blockade to slow the heart rate to 60 beats per minute. ⁵ The radiation dose can still be reduced by a further 31%, by reducing the CT X-ray tube voltage from 120 to 100 KVp, for non-obese patients, without loss of image quality. ⁶ Furthermore, as these mostly young/athletic patients are usually slim, they can be scanned at the even lower CT X-ray tube voltage of 80 KVp, reducing their dose by a further 30%. Utilising all the above techniques we have achieved radiation doses of 0.5 msv in our unit where we scan a number of young athletes (Figure 1). Finally, as it is just the origins of the coronary arteries that we are interested in, the length of the scan can be reduced to cover just the base of the heart, reducing the radiation dose even further.

Figure 1.

An image of an anomalous right coronary artery, with a slit like ostium, arising from the left coronary sinus and following an intra arterial course.

Using all these techniques would ensure that cardiac CT is not just the best diagnostic test for anomalous coronary arteries but that it is also a very safe test and radiation is not generally a concern at such low doses. For comparison, the average dose for nuclear myocardial perfusion scan is 15.6 msv, CT of the abdomen is 8 msv, CT of the chest is 7 msv, a simple abdominal X-ray is 0.7 msv and the chest X-ray is 0.02 msv. ⁷ It is critical that cardiologists and radiologists are both equally aware of the dose reduction techniques and that those techniques are utilised as much as possible particularly for patients being investigated for anomalous coronary arteries who are mostly young adults with a higher risk from radiation. ⁴

Figure 2.

An image of an ultra low radiation dose CT coronary angiogram demonstrating normal coronary anatomy in a young slim athletic patient acquired using very low tube voltage (KVp = 80).