What are the radiation risks from coronary computed tomography angiography (CCTA)?

Updated: Dec 21, 2017
  • Author: Eugene C Lin, MD; Chief Editor: Eugene C Lin, MD  more...
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Answer

Attention has been drawn to the risk of cancer from computed tomography. In these discussions, coronary computed tomography (CT) angiography (CCTA) is often cited for its high radiation dose. For example, in a review by Smith-Bindman et al, [48] it was estimated that 1 in 270 women, and 1 in 600 men, who underwent CCTA at age 40 would develop cancer from that scan. However, these figures do not take into account recent technological developments that can greatly decrease the radiation dose from CCTA.

There are several things that are important to note. First, the radiation dose from CCTA is highly variable and is largely dependent upon the specific equipment and techniques used. For example, one study suggests that use of a prospectively triggered technique reduced the risk of cancer by 87%, compared to a retrospectively gated technique. [49] Therefore, it is very important for the practitioner to understand ways of reducing the radiation dose to the patient. Other examinations that might be used instead of CCTA, such as cardiac nuclear medicine studies, have relatively high radiation doses. In addition, noninvasive examinations such as CCTA have the potential to reduce the use of coronary catheterization, which is invasive and involves relatively high radiation doses. In a study of 398,978 patients who underwent elective coronary catheterization, [50] only 37.6% had obstructive coronary disease.

Examinations that do not utilize ionizing radiation, such as stress echocardiography, could also be considered. A decision analytic model suggests that stress echocardiography followed by CCTA if needed is most appropriate for evaluation of patients with a pretest probability for coronary artery disease of less than 20%, while CCTA alone is more appropriate for intermediate-risk patients. [51]

Radiation doses for CCTA studies, if performed with retrospective gating in helical mode, are typically relatively high. Radiation doses are high because data are acquired throughout the cardiac cycle, and with the fast gantry rotation required for high temporal resolution in CCTA, a low pitch (table travel per gantry rotation/collimation) is required in order to avoid gaps in the data. If data are acquired throughout the cardiac cycle, the table should not move more than the beam width during one cardiac cycle. In particular, slow movement is required with fast gantry rotation; otherwise, all phases of the heart at a specific location will not be seen by the detector. As pitch is inversely related to radiation dose, a low pitch results in a high radiation dose. [40]

The reported radiation doses for CCTA vary depending on the specific technology and techniques employed. With retrospectively gated single-source 64-slice CT, the reported effective radiation doses have ranged from 9.5 to 21.4 mSv. [3] However, using many of the technologies and techniques discussed below, it is possible to lower the dose to less than 5 mSv, and doses less than 1 mSv are currently possible in some patients. For comparison, the average yearly background radiation dose is around 3 mSv, and a chest x-ray dose is 0.05 mSv. [15] Depending on the technique, CCTA may have a higher or lower effective dose than conventional coronary angiography (3.1-9.4 mSv). However, the dose from a cardiac single-photon emission computed tomography (SPECT) scan performed using technetium-99m is typically high (8 -17.5 mSv).

A variety of methods exist for decreasing radiation dose from cardiac CT. [52] In general, radiation dose from a CT scan can be reduced by reducing tube current, reducing tube voltage, or increasing pitch.


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