64-Slice Cardiac CT Scanning
Cardiac Computed Tomographic Angiography, or CCTA, is an exciting new development in the diagnosis of coronary artery disease (CAD). CT uses x-ray radiation (“regular” x-rays) to create images.
CT technology, however, uses an x-ray tube that rotates around the patient, passing x-rays through the patient at numerous angles. A powerful computer is used to reconstruct the absorption of these x-rays within the patient, which creates a three-dimensional map of internal tissue.
CCTA takes this process one step further by acquiring these data in time with the motion of the heart. It synchronizes the imaging while simultaneously recording the heart’s electrical activity by means of an electrocardiogram. Intravenous contrast (or x-ray “dye”) is injected into a peripheral vein. The resulting images of the heart are capable of “freezing” heart motion and providing images of superb clarity, all in a matter of seconds. Heart function, such as the strength and timing of contraction of the heart muscle and the opening and closing of the heart valves, can also be evaluated.
CCTA adds yet another powerful tool for the evaluation of various diseases of the heart. These include coronary artery disease, diseases of the heart muscle (so-called cardiomyopathies”), valvular abnormalities, diseases of the external lining of the heart (referred to as “pericardium”), masses involving the heart, and congenital heart disease.
The appropriate indications for CCTA are still evolving as through research is performed experience with the technique. Currently, CCTA is most commonly used in patients:
- Who have symptoms that suggest the possibility of coronary atherosclerosis
- Who are undergoing assessment for possible coronary atherosclerosis
- Who have technically limited or indeterminate study results
- Who are suspected of abnormal coronary artery course or position (usually referred to as coronary artery anomaly)
- Who have congenital cardiovascular diseases
CCTA may occasionally be used outside these indications for selected cases. It is generally avoided for patients who are highly suspected of having a heart attack (myocardial infarction) – such patients typically require urgent treatment and delay in such treatment must be avoided.
In addition to CCTA, assessment of suspected coronary artery atherosclerosis can be performed using coronary calcium scoring CT. This type of CT scan is also timed with the heart beat to provide optimal image quality, but intravenous contrast (“dye”) injection is not required. Coronary calcium CT scanning is based on the notion that some atherosclerotic plaque is calcified – so-called “hard” plaque – and calcium is not present within the wall of a normal coronary artery. Tissue that is calcified tends to absorb x-rays quite well, and thus appears relatively bright on CT scanning. So, it is possible to detect calcified coronary atherosclerosis with CT scanning. Calcium scoring CT scanning is often performed to create on overall estimate of the likelihood of a major cardiac event occurring within a given time frame, typically a one- or 10-year period. Coronary calcium scoring CT scanning often adds greatly to this effort, especially for patients who are found to be at intermediate risk for coronary artery disease. These patients have a yearly chance of a major cardiac even of 0.5 – 2%.
Calcium scoring CT in these patients can often reclassify them as high risk if significant amounts of calcium are found. These patients require more aggressive therapy to prevent heart attacks. On the other hand, if no calcium is found, the patient may be re-classified as low risk for coronary artery disease.
Ultimately whether or not coronary calcium scoring CT scanning or CCTA is appropriate for a given patient depends on a multitude of factors, all of which need to be individually weighed for each patient. Occasionally more than one test may be required to provide definitive answers.