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CT Exams and Coronary Calcium Scoring

Computed Tomography in Evaluating Coronary Artery Disease & Atherosclerotic Plaque

Computed tomography, or CT, is a technology that employs x-ray radiation to create anatomical images of practically any region of the body. X-rays are a form of electromagnetic radiation, much like radio waves, microwaves, ultraviolet light, and even visible light. X-rays are generated in a specialized tube and then directed at the patient. X-ray detectors on the side of the patient opposite the x-ray generator collect and record the x-rays that pass through the patient. The degree of absorption of x-rays is different for different tissues in the body, so different amounts of x-ray radiation will pass through the patient and be recorded by the detector. A powerful computer compiles the differential x-ray absorption and reconstructs the density of the tissue that must have been present to allow the degree of absorption for a given location in space. This process occurs millions of times, and in so doing creates a "density map" of a patient - this is the x-ray image.

The CT scanner was developed in the early 1970s. These early CT scanners were very slow compared to today's powerful machines. The original CT scanner developed by the men credited with the invention of CT, British engineer Sir Godfrey Hounsfield of EMI laboratories in London, England, and Allan Cormack of Tufts University, took hours to acquire a single image, or "slice," and days to reconstruct the acquired raw data into an actual image. Today's machines acquire multiple individual slices, up to 64 with more modern, advanced machines, with some prototype scanners capable of acquiring 256 slices with a single turn, or rotation, of the x-ray tube, which may occur in as fast as one-third of a second. The raw data acquired may be reconstructed into visible images in a matter of a few seconds.

So how can CT be used to assess for CAD? There are two main ways CT scanning of the heart is used to evaluate patients for suspected heart disease - non-contrast CT scanning to assess for coronary calcification, and contrast-enhanced CT scanning to directly visualize the coronary arteries. Each will be discussed below.

Coronary Calcium Scoring


Calcium Scoring Brochure (pdf)

Coronary calcium CT scanning is based on the principle that some atherosclerotic plaque is calcified - so-called "hard" plaque - and calcium is not present within the wall of a normal coronary artery. Anatomic regions that have calcification tend to absorb x-rays quite well, so they appear relatively bright on CT scanning. So, it is possible to detect calcified coronary atherosclerosis with CT scanning. Coronary calcium scanning is performed without using the intravenous contrast injection required for coronary CT angiography or catheter coronary angiography (see below). Patients lay flat on the CT scanner table and breath-hold for the length of the scan, which is a matter of seconds. The CT scan is acquired in synchrony with the patient's heartbeat to freeze cardiac motion and allow clear visualization of any coronary calcium that may be present (Figure 3). A computer calculates the total amount of calcium present and reports the value as a "score." Thousands of patients have been scanned in this fashion and their scores collected, creating a large body of data regarding the coronary artery calcium scores in the general population for a given age, gender, and race. So, it is possible to generate a coronary calcium score for a patient and compare that score to other patients of the same age, race, and gender to create a percentile rank for that calcium score. For example, a white male of age 60 with a coronary calcium score of 30 is in the 50th centile of coronary calcium scores for that age, gender, and race, whereas a white woman of the same age with the same calcium score of 30 is in the 79th centile of coronary calcium scores for woman of the same age, gender, and race. On line tools are available to calculate percentiles for coronary calcium scores given the score, race, gender, and age of the patient (http://www.mesa-nhlbi.org).

What is the relevance of detecting coronary artery calcium? Once coronary artery calcium is seen, the presence of CAD is confirmed. Furthermore, the amount of coronary calcification provides a good burden of the overall burden of coronary atherosclerotic disease. In fact, the coronary calcium score provides additional information regarding the likelihood of future coronary events independent of the traditional Framingham risk factors. In contrast, a coronary artery calcium score of zero is a very strong predictor that a patient will not have a coronary event in the near future. Often, physicians classify coronary artery calcification scores based on the amount of cardiovascular risk associated with the score range.


  1. Coronary calcification score of zero: risk of obstructive CAD is <1%

  2. Coronary calcification score 1-100: mild atherosclerosis. Average risk for CAD. Risk for coronary event over next 3-5 years is about 1.9x that of a patient with a calcium score of zero.

  3. Coronary calcification score 101-400: moderate atherosclerosis. Moderate risk for CAD. Risk for coronary event over next 3-5 years is about 4.3x that of a patient with a calcium score of zero.

  4. Coronary calcification score >400: severe atherosclerosis. High-risk for CAD. Risk for coronary event next over 3-5 years is about 7.2x that of a patient with a calcium score of zero.

  5. Coronary calcification score 1000: severe atherosclerosis. Very high-risk for CAD. Risk for coronary event over next 3-5 years is about 10.8x that of a patient with a calcium score of zero.


Patients with a coronary calcium score of 0 have a very low risk of CAD, even if patients are symptomatic - the likelihood of CAD in such patients is 1% or less. The annual risk of any coronary event in a patient with a calcium score of zero is very low, on the order of 0.4% or less. In contrast, compared to patients with a zero calcium score, patients with any coronary calcium have about a 4-fold higher risk of coronary events in the next 3-5 years.

Coronary calcium scanning has limitations. One main limitation is that coronary calcium scoring is not site-specific for obstructive coronary artery disease. This means that where calcium is seen is not necessarily the most severe site of CAD, nor does the presence of coronary calcium absolutely mean that obstructive CAD is present. In fact, there is approximately 5 times as much non-calcified plaque as there is calcified plaque in most patients, so sites of narrowing may, or may not, be related to where calcified plaque is located. Also, as discussed above, coronary events are often related to soft, "vulnerable" plaque, and such plaques are often not calcified and thus not visible at all on non-enhanced coronary artery calcium studies. The presence of coronary calcium correlates with overall atherosclerotic burden and is a strong predictor of future coronary events, but it can't tell if a coronary artery is narrowed nor can it determine which artery has the most severe narrowing. So how do doctors use coronary calcium scanning? Many doctors use coronary calcium scanning in patients who are at intermediate risk for CAD. Coronary calcium results can be used to place such patients into high or low risk categories, which will alter the treatment plan and goals. Coronary artery calcium scanning, however, does not usually add much management-altering information for patients already at high or low risk for CAD. Occasionally, some physicians will use coronary calcium scanning in patients with chest pain that is not typical for angina. A calcium score of zero in such a patient would make the likelihood of CAD as the cause of the patient's pain very low, and the treating doctor would probably direct his or her attention to non-cardiac causes of chest pain. On the other hand, if coronary calcium were found in such a patient, CAD may not be excluded as a cause for that patient's chest pain, although the likelihood of obstructive CAD would still be low if the patient's coronary calcium score is 1-100.