Radiation dose is becoming a major issue for contrast-enhanced cardiac multislice CT (coronary CT angiography), since 64-slice CT is providing promising results in the evaluation of coronary arteries. The radiation dose delivered during coronary CT angiography using retrospective gating is necessarily high, because only part of the total radiation (about 20%) is used to reconstruct the image. The “useful” radiation is that delivered during a temporal window of one phase of the cardiac cycle (for example, mid-diastole). This window is determined by the rotation time of the scanner, which is about half the rotation time necessary for monophasic reconstruction.
In daily practice, the CT dose index (CTDI) for coronary CT angiography may reach or pass 100 mGy, with a dose-length product (DLP) of up to 2000 (100 mGy x 20 cm) when the entire thorax is scanned without the use of dose-sparing tools. Thus, the effective dose may reach 40 mSv (corresponding to a DLP of 2000 mGy cm), which causes concerns regarding breast exposure to radiation in women. In cardiac patients who are repeatedly exposed to various sources of medical radiation (e.g. nuclear studies and conventional angiography), the radiation exposure risk becomes certain when the cumulated dose exceeds 200 mSv. Due to these high radiation levels, it is essential to minimize the radiation dose associated with cardiac CT examinations. Radiologists should be increasingly careful about radiation dose levels, and should attempt to use dose-savings algorithms whenever possible.
There are different ways to lower radiation doses, including optimization of exposure time and scan coverage, use of ECG to reduce tube current, and adapting protocols to the patient’s anatomy.