How Does Computed Tomography Work?

A computed tomography (CT) scanner consists of an X-ray machine and a “receiver” that registers the images. The two parts of the scanner rotate around the patient, capturing signals from all directions. On this basis a computer calculates an image that looks like a transverse section through the X-rayed site. These three-dimensional images are used to judge the condition of the heart and the coronary arteries.

Today’s CT scanners can image the coronary arteries, which are only a few millimeters thick, with high resolution within one heartbeat. This is possible because of the complex radiological examination, which uses small X-ray doses, and by the patient being given an X-ray contrast medium.

The Examination

CT examination takes about 15 minutes. At the beginning an image is taken of the heart without X-ray contrast medium. This is used to determine the degree of calcification of the coronary arteries. This method, also known as calcium scoring, allows an initial estimate of the risk of arteriosclerosis. Next, the coronary arteries are viewed with the aid of contrast medium injected into the patient’s vein. This method is known as coronary CT angiography. To achieve good imaging quality the patient must usually take medication before the examination to enlarge the vessels (nitrate) and to reduce the heart rate (beta-blocker).

When is CT Necessary?

The cardiological professional societies recommend CT examination in patients with a low to moderate risk of coronary artery disease (CAD). For patients who are already known to have CAD or who have undergone a coronary artery bypass operation, CT is an alternative to the invasive heart catheter examination or can serve as an aid to making the decision whether heart catheterization is necessary. CT is not recommended for routine check-ups.

CT is also used at our hospital:

  • when the patient has a disease of the large (aorta) and small vessels, e.g. neck or leg vessels
  • before valve operations (transcatheter aortic valve implantation)
  • when the patient has metal implants in the body, to evaluate the cardiac anatomy and function
  • for follow-up examinations after heart transplantation
  • in patients with congenital heart defects

When Should CT NOT be Used?

CT examination is normally not performed when the patient:

  • has severe impairment of kidney function
  • has a nontreated overactive thyroid gland (hyperthyreosis)
  • is allergic to X-ray contrast medium
  • is pregnant.