Dr Carl Shakespeare consultant cardiologist  
       
 
   


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The Use and Indications of Cardiac CT Angiography:

 

Cardiac CT angiography has two separate types of application. The first involves Coronary Calcium Scoring to assess vascular risk, and the second is Coronary CT angiography to assess for coronary narrowings.

In patients that are less mobile, alternatives include other functional tests including myocardial perfusion imaging (a nuclear thallium scan see figure “nuclear scan”).The advantage of this test is that it has good prognostic value. Although it does not detect narrowings anatomically, it assesses the impact of narrowings, by assessing how much heart muscle is affected (affected muscle does not glow with radioactivity). In some ways this is more important than the anatomical interpretation. However, there can be issues in over-interpreting areas affected by ischaemia- so called “false positive tests”.

My major use of this test is in assessing patients who have had ischaemic heart disease who are due to have major surgery for other (non-cardiac) reasons. In the absence of detectable ischaemia indicates that surgery is low risk. My other main application is in patients who I feel there is equivocal narrowing of the arteries. A significant area of detectable ischaemia in the territory of the narrowing would indicate that the artery in question is likely to be the cause of symptoms.

Coronary Calcium Scoring and Vascular Risk:

This technique has gained acceptance as a method of assessing vascular risk. The commonest aggravants to cause damage to the vessel wall include the usual suspects: smoking, diabetes, hypertension, cholesterol and high blood pressure. In situations where blood vessels become damaged and weakened  by these substances deposition of atheroma occurs.

The body attempts to heal the damage by depositing calcium in the cracks. The more deposition, the more calcium deposition occurs, and this can be measured with low radiation dose CT. A score (Agatston Score) can be deduced that would indicate an age and sex adjusted risk of future events.
The technique requires no preparation. The patient lies in the scanner for only a few seconds to perform the study. There is very little likelihood of claustrophobia.

The chart to the right shows the degree of calcium deposition (Agatston Score) and increased coronary artery narrowing (significant % stenosis).

Coronary CT Angiography Angiography (CTA):


Although CTA is mostly used to evaluate coronary arteries, it is able to assess all cardiac structures, as well as the lungs, and blood vessels within the lungs (i.e. to exclude blood clots).

The image to the right shows a cardiac CT angiogram of a coronary artery.

In order to have the test, in most cases it is important to control the heart rate. Electronically, this helps freeze the heart motion. Patients are usually given a beta blocker drug before the test in order to get the heart rate around 60 beats per minute. This can either be given in tablet form an hour before the test, or via a vein just beforehand. Before the test is undertaken, a small cannula is introduced into a vein in the arm. Contrast is given via the cannula in a timed manner, usually with the breath held in. The actual scanning time is about 7 seconds, but the whole procedure takes about 10 minutes.

I have listed the most approved indications of this non-invasive technology:
  • Symptomatic patients with suspected coronary disease deemed of low to intermediate risk (pre-test probability).
  • Patients of high risk who refuse cardiac catheterisation.
    Although cardiac catheterization is a comparatively low risk procedure, it still carries a 1:000 risk of having a heart attack or stroke. Some patients still find this an unacceptable risk. CTA is an approved alternative.
  • Patients with a history suggestive of coronary disease and a negative/equivocal exercise test, myocardial perfusion scan, or stress echocardioigram.
    One of the difficulties with the above tests is the results can be equivocal. If the pre-test probability still remains low to intermediate, then CTA is the approved alternative.
  • Patients with suspected anomalous coronary arteries (incomplete coronary arteriogram visualization).
    In certain circumstances, the origin of the coronary artery may be compressed from cardiac structures outside heart (commonly the pulmonary artery). Patients can present with classical anginal symptoms at a young age, and have no coronary risk factors. Such patients are often falsely reassured. CTA can determine whether there is significant compression and what risk this may present.
  • Patients with recurrence of angina after CABG to assess coronary grafts.
    In patients with previous coronary artery bypass grafts, angiography is sometimes warranted for further symptoms. Conventional cardiac catheterization is usually carried out. As one would expect, procedure times are much greater and occasionally, the grafts are not always located. CTA is always able to detect these grafts.
  • Young patients due to have valvular heart surgery.
    In patients who are due to undergo open heart surgery to repair or replace a heart valve, surgeons usually request an evaluation of the coronary arteries. This is so that the coronary arteries can be treated at the same time if necessary. Of course in young patients, where the likelihood of coronary disease is low, then CTA provides a low risk non-invasive way of assessment.
  • Patients with heart failure (cardiomyopathy).
    Heart failure is a condition that occurs when heart muscle becomes damaged, so the heart becomes dysfunctional. There are many causes, the commonest of which in the Western world is due to coronary artery disease causing blockages. CTA is an approved method of assessing whether coronary disease is the likely cause.
  • Evaluation of cardiac mass with poor MRI/TOE images.
    The presence of an unexplained mass (lump) in the heart is most often suggested when an echocardiogram has been performed. A mass could either be a clot or a tumour. CTA provides an accurate means of detecting and diagnosing heart masses. In patients with a stroke suspected to be due to clot, CTA is an ideal method of detection.