CT Angiography
Coronary CT Angiography is Truly Revolutionary
Coronary CT angiography (CTA) is a relatively new test that has been made possible by the rapid advancement of CT technology in recent years. As discussed above, current CT scanners acquire images very quickly, allowing the scanner to freeze cardiac motion. Until the development of very fast modern CT scanners, coronary artery imaging with CT was not possible due to the fact that, because the heart is beating during the scan, the coronary arteries are moving during the study. Rapidly moving structures are very challenging to image effectively, particularly when the target structures are small. Modern CT scanners have the ability to scan rapidly using very thin slices, which maximizes the ability to see small detail. This capability has made it possible, for the first time, to directly visualize the internal portion of the coronary artery the lumen (Figure 4). Modern CT scanners can, therefore, create images that resemble those obtained with coronary artery catheterization (see below). However, unlike coronary artery catheterization, coronary CTA can also visualize the wall of the coronary artery, and the wall of the coronary artery is where atherosclerosis begins. So, it is now possible for physicians to directly visualize both the coronary artery lumen and wall for atherosclerotic plaque in a non-invasive fashion this capability is truly revolutionary and is changing the way physicians think about CAD. Coronary CTA allows physicians to create whole-heart images (Figure 5) and assess cardiac function by watching the heart contract in real time.
What is the benefit of coronary CTA over other techniques used to assess for CAD?
Early CAD detection. Coronary artery CTA has the ability to detect CAD at a far earlier stage than the perfusion techniques discussed above. Recall that, for a perfusion study (such as stress echocardiography, myocardial perfusion SPECT) to become abnormal, the coronary artery lumen must be narrowed by about 60-70%. This means that CAD is fairly advanced by the time it can be detected by these techniques. In contrast, coronary artery CTA can detect CAD when the coronary artery lumen is narrowed as little as 10-20% - the hope here is that detection at this stage would allow more aggressive treatment to halt the progression of CAD.
Detection of type of CAD. Recall from the discussion above that several types of coronary atherosclerotic plaques exist: calcified hard plaques, softer fibrous or lipid (fatty)-rich plaques, and mixed plaques. Perfusion techniques cannot distinguish among these types of plaques, nor can treadmill stress testing. However, it is clear that soft plaque - particularly those with abundant lipid or fat - are the plaques that are most prone to rupture and cause acute coronary syndromes. Detection of such plaques may be very important for directing therapy, and this is a very active area of research. As an example, consider the example of a patient with a calcium score of zero. From the foregoing, we know that such a patient has a very low likelihood of having a coronary event. However, the limitations of calcium scoring include the inability to visualize non-calcified plaque and the fact that a calcium score provides a measure of relative risk for coronary events compared to a reference population, but does not give specific anatomic information about the individual patient. Such a patient is presented in Figure 6. This patient had 2 major risk factors for CAD, but a calcium score of zero. Coronary CTA shows a single, noncalcified plaque in one of the major coronary arteries - the first portion of the left anterior descending coronary artery. Such a plaque, if it were to rupture, places a large amount of heart muscle at risk for damage. These plaques are potentially deadly and have been referred to as widow-makers for this reason. This plaque could not be seen with coronary artery calcium scanning and would not be detected with any perfusion or treadmill tests.
Detection of causes of chest pain or angina-like symptoms unrelated to CAD. Coronary artery CTA directly visualizes the heart, the tissues surrounding the heart, between the two lungs (called the mediastinum), the lungs themselves, the thin membrane lining the surfaces of the lungs (called the pleura), and the structures of the chest wall. Patients with chest pain or symptoms that resemble angina may be caused by derangements of any one of these anatomic regions. Furthermore, coronary CTA can detect potentially aggressive processes in these regions before symptoms develop.
Who should have a coronary artery CTA study?
This question is currently a very active area of research. Coronary CTA has been suggested by some to be a useful test for patients at low to intermediate risk for CAD who present with chest pain. A number of such patients currently undergo nuclear perfusion scintigraphy, and some of these patients will have abnormal or equivocal results. The latter group of patients may then undergo coronary artery catheterization (see below), only to be found to have no significant CAD. Such patients could be evaluated with coronary artery CTA - negative results exclude CAD with high confidence, and also have the capability to detect other causes of chest pain, such as lung disease. In contrast, patients at high risk for CAD who present with chest pain, or patients who are presenting with ACS are not appropriate referrals for coronary CTA. These patients should be evaluated by a cardiologist and usually require urgent coronary artery catheterization because the likelihood of obstructive CAD is high, and CAD can be treated during coronary artery catheterization procedures.
Other investigators have suggested that coronary artery CTA may be a good test for risk stratification for outpatients at low or intermediate risk for CAD. Negative coronary artery CTA results in such patients would conclusively exclude CAD. Some believe that positive results, particularly if predominantly soft plaque is identified, would be useful for justifying aggressive therapy.
Coronary CTA is also a very useful test for evaluating patients with inconclusive or unclear tests for CAD, such as treadmill tests or perfusion studies.
Coronary artery CTA also has value in assessing patients for congenital anomalies of the coronary arteries. Such patients are born with coronary arteries that have an unusual course or position that renders the patient at risk for heart-related chest pain or coronary events. Coronary artery CTA is the test of choice for such patients. Coronary artery CTA may also be useful for assessing patients with new-onset heart failure for CAD as the cause of the heart failure. Coronary artery CTA may also be employed to assess for cardiac valve and muscle function and to assess for masses in the heart or space surrounding the heart. Finally, coronary artery CTA may be used to assess patency of bypass grafts following coronary artery bypass grafting surgery.
Does the foregoing mean that coronary artery CTA is a good test for everyone? Absolutely not! Coronary artery CTA involves the injection of intravenous contrast (sometimes also referred to as dye) to opacify the arteries so that they may be visualized. The contrast used for coronary artery CTA, which is the same intravenous contrast used for other CT examinations and catheter coronary angiography, contains iodine and occasionally may be associated with allergic reactions and has the potential to worsen pre-existing disorders of kidney function. In a very small number of patients, such allergic reactions may be quite severe, even deadly. In would be inappropriate to expose a patient to such risks, no matter how small, if the information gleaned from the study would not be useful. Coronary artery CTA, like other CT examinations, uses x-ray radiation to create images of the human heart. X-ray radiation has the ability to harm human tissue, and may have the potential to cause cancers. This risk of cancer induction by x-ray radiation may be small, but cannot be assumed to be zero. Therefore, it is important that the information that may be gained by coronary artery CTA is worth the potential risks of a serious intravenous contrast reaction or radiation-induced malignancy.
