Definition of Test
A computerised tomography coronary angiogram (CTCA) is a non-invasive method of imaging the coronary arteries. Unlike invasive coronary angiography, it does not require the use of catheters although does still involve X-rays. The test requires a 64 detector (or higher) CT scanner and involves an intravenous injection of an iodine-based dye to generate high resolution 3D images of the coronary arteries. The higher the number of detectors on the CT scanner, the better the image. The scanner takes multiple images of the heart from various different angles. The images obtained allow detection of fatty plaques or calcium deposits in the coronary arteries, which can result in coronary artery narrowing or blockages.
What the Test Involves
To obtain a clear picture of the coronary arteries in this test, it is important that the heart is not beating too quickly. Ideally, the heart rate should be 60bpm or less. Many patients will have been asked to take medication on the morning of the test to slow the heart down (eg beta-blockers, calcium-channel antagonists or ivabradine). On arriving for the test, the patient will be asked to change into a gown and an intravenous cannula inserted. Heart rate and blood pressure will be checked. If the heart rate is too fast, further medication will either be given by mouth or by injection into an arm vein, to slow the heart further. Once the heart rate is slow enough, the scan will be performed.
The patient will be asked to lie on a bed whilst ECG electrodes are placed on the chest. This allows the heart beat to be monitored during the test, and also allows us to ensure that we scan the heart during the same part of the cardiac cycle each time. The bed will then move into the scanner. See “Additional Information” for a patient video.
Most centres will start with an unenhanced scan (ie one that does not require the use of contrast to enhance the images). This is a very quick scan which detects the presence of calcium deposits in the coronary arteries (CT-calcium scoring). If there is a large amount of calcium in the coronary arteries, it can degrade the quality of the scan and make it non-diagnostic and for this reason patients with a very high calcium score may have the procedure terminated at this point. Calcium scoring is measured by the Agatston Score. A value of >400 is usually taken as ‘severe’ calcification and may result in the scan being terminated in favour of an alternative method of assessing the coronaries (eg an invasive coronary angiogram).
For those patients with a lower calcium score, an intravenous injection of an iodine-based dye is then given to help further visualise the coronary arteries. It is usually necessary to breath-hold for a few seconds during this part of the scan, to minimise movement of the heart and lungs. X-rays pass through the body and are picked up by detectors in the CT scanner, which then generates an image of the coronary arteries.
The scan itself usually takes 15 minutes or less, although there may also be some waiting about afterwards for monitoring if extra heart-slowing medication has been given.
Which patients might a CTCA be used on and why?
CT coronary angiography is best used as a ‘rule out’ test for coronary artery disease. The sensitivity for CT coronary angiography is high at 89% and the specificity even higher at 96%. The high level of specificity is supported by a high “negative predictive value” which indicates what a reliable test CT angiography is for ruling out coronary artery disease if the test is negative i.e. normal.
Common reasons for requesting a CT coronary angiogram include:
- For the evaluation of coronary artery disease in stable, low risk chest pain patients (as recommended by NICE Clinical Guideline 95) seen either in rapid access chest pain clinics, cardiology clinic or as inpatients
- To visualise coronary arteries in patients with heart failure of unknown cause.
- To evaluate coronary artery bypass grafts.
- To investigate patients who may have anomalous (abnormal) origins of coronary arteries
- To assess coronary arteries in patients with endocarditis and aortic root abscess, prior to valve surgery. In such patients, manipulation of an invasive catheter within the aortic root might be undesirable and a CTCA safer.
How is CTCA used to diagnose a cardiac condition?
Once the images have been acquired, they can be viewed on a computer by either a radiologist, a cardiologist or both. Post-processing allows 3D reconstruction of the coronary arteries to be performed. A report will then be issued to the requesting doctor. Patients are not usually informed of the result on the day of the test.
Benefits of CT-A over invasive coronary angiography:
- No requirement for invasive vascular access
- No risk of stroke/MI/vascular damage
Page last reviewed: 31 Jul 2020
