Introduction
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 (1, 2). The
radiation dose delivered during coronary CT angiography using retrospective gating is necessarily
high, because only part of the total radiation (about 20%,
3) 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.
