What is a CT coronary angiogram ?
Computed tomography coronary angiography (CTCA) uses X-rays to provide images of the coronary arteries and the heart muscle. Combined with an intravenous contrast material, CTCA provides high-resolution, three-dimensional images of the heart and the coronary arteries. These images show whether there is furring or blockage of the coronary arteries (arteriosclerosis).
What is a calcium score ?
Calcium is deposited during the process of furring of the arteries. Generally, a high calcium score is associated with a high risk of future heart attacks. Your GP may have requested a calcium scoring scan if you are thought to be at risk of developing coronary heart disease. A high calcium score carries a higher risk of developing heart attacks than a low score. A high score, however, does not give any information as to whether the coronary arteries are blocked or likely to be the source of a heart attack. On the other hand, a low calcium score is associated with a low risk of future heart attacks, it does not necessarily mean that the arteries are not furred up. This is why it is essential that you discuss the value of your calcium score with your cardiologist.
Coronary calcium is a marker for atherosclerosis (hardening of the arteries). The presence and amount of calcium detected in a coronary artery by the CT scan, indicates the presence and amount of atherosclerotic plaque. These calcium deposits appear years before the development of heart disease.
A calcium score is computed for each of the coronary arteries based upon the volume and density of the calcium deposits. This can be referred to as your calcified plaque burden. It does not correspond directly to the percentage of narrowing in the artery but does correlate with the severity of the underlying coronary atherosclerosis. This score is then used to determine the calcium percentile, which compares your calcified plaque burden to that of other asymptomatic men and women of the same age. The calcium score, in combination with the percentile, enables your physician to determine your risk of developing symptomatic coronary artery disease, and to measure the progression of disease and the effectiveness of treatment.
A score of zero indicates that there is no calcified plaque burden. This implies that there is no significant coronary artery narrowing and a very low likelihood of a cardiac event over at least the next 3 years. It does not absolutely rule out the presence of soft, non-calcified plaque or totally eliminate the possibility of a cardiac event.
A score greater than zero indicates at least some coronary artery disease. As the score increases, so does the likelihood of a significant coronary narrowing and the likelihood of a coronary event over the next 3 years, compared to people with lower scores. Similarly, the likelihood of a coronary event increases with increasing calcium percentiles.
What does a CTCA show?
A CTCA provides an image of coronary arteries and provides information about:
• Furring or blockage of the coronary arteries (arteriosclerosis)
• Calcification of the coronary arteries, which occurs when arteriosclerosis has been longstanding.
• Cholesterol build-up (‘soft’, or 'unstable' plaque).
• The degree to which arteriosclerosis narrows the coronary arteries.
• Structural abnormalities of the coronary arteries that you may have been born with.
Why have a CTCA ?
A CTCA provides very valuable information on the coronary arteries. The ultimate investigation for this, however, is a traditional coronary angiography using cardiac catheterisation. If the patient’s history is very suggestive of heart disease, it may be better to proceed directly to cardiac catheterisation, which will provide the work-up to angioplasty, stenting and coronary bypass operations.
A CTCA should be considered in the following circumstances:
• Chest pain with features that suggest a cardiac cause.
• Inconclusive stress test (treadmill, stress echocardiogram or nuclear stress test).
• A negative stress test but continuing chest pain.
• Unexplained breathlessness during physical exertion.
• Suspicion of an underlying coronary artery anomaly
• Investigation of coronary artery disease in patients undergoing non-coronary cardiac surgery.
A CTCA should NOT be undertaken in the following circumstances:
• Screening for coronary disease in patients at low risk.
• Without a clinical opinion from a cardiologist or a physician with knowledge and training in cardiovascular disease.
What does a CTCA involve ?
The CTCA takes less than 10 minutes to perform. You lie down on a special couch which takes you into the bore of the scanner. A small canula is inserted into a vein in your arm so that we can give the contrast substance that highlights the coronary arteries. Sometimes, we may need to give a beta-blocker intravenously to slow down the heart rate. You may be asked to lie still and to hold your breath at intervals. After contrast injection, some patients experience a metallic taste in the mouth or they may fell flushed. These effects are, however, short-lived. The scan will need to be analysed in in detail by the cardiologist. It is essential that the results are discussed with you afterwards.
Calcium score and CTCA references:
Greenland P et al. ACCF/AHA 2007 clinical expert consensus document on coronary artery calcium scoring. J Am Coll Cardiol 2007;49:378-402
Hendel RC et al. ACCF/ACR/SCCT/SCMR/ASNC/NASCI/SCAI/SIR 2006 appropriateness criteria for cardiac computed tomography. J Am Coll Cardiol 2006;48:1475-97.
Carr JJ, et. al., Evaluation of Subsecond Gated Helical CT for Quantification of Coronary Artery Calcium and Comparison with Electron Beam CT.; AJR 2000; 174: 915-921
A 3D reconstruction of a CT coronary angiogram
A CT scanner
Example of calcium score result