Image quality and radiation exposure of coronary CT angiography in patients after coronary artery bypass graft surgery: influence of imaging direction with 64-slice dual-source CT.

BACKGROUND: The evaluation of native coronary arteries (NCAs) as well as coronary artery bypass graft (CABG) patency after surgery is essential. However, NCAs are often blurred in the craniocaudal scan direction because of long scan time with 64-slice CT.

OBJECTIVE: The purpose of the study was to determine the effect of scan direction on image quality and radiation exposure in assessment of NCAs and CABGs.

METHODS: Retrospective analysis of 191 consecutive individuals undergoing coronary CT angiography to evaluate CABG patency using 64-slice dual source CT. A retrospectively ECG gated spiral acquisition protocol with ECG based tube current modulation and automatic adjustment of tube current to a reference of 320 mAs ("CareDose 4D") was used. Tube current was 120 kVp. Scan direction was either cranio-caudal (CRC, n = 98) or caudo-cranial (CRC, n = 93) and the scan volume covered the entire course of all bypass grafts. Independent investigators determined quantitative image quality of the coronary arteries by evaluating contrast-to-noise ratio (CNR), radiation exposure by comparing the effective dose, and qualitative image quality through a 5 point rating scale.

RESULTS: Quantitative image quality was not significantly different for the two groups except for the CNR of the right coronary artery which was significantly higher in patients with caudio-cranial scan direction (P = .0007). The qualitative image quality of the CaC group also was better for both NCAs and CABGs (P = .002 for NCAs and <.001 for CABGs), mostly because of the lower frequency of respiration artifacts on coronary arteries of the CaC group (P = .005). As an effect of automatic tube current adjustment, radiation dose was lower in patients with caudo-cranial scan direction (6.8 mSv vs. 9.6 mSv, p < 0.0001).

CONCLUSION: In patients with coronary bypass grafts imaged by 64-slice dual source CT with spiral acquisition and automated tube current adjustment, a caudo-cranial scan direction results in improved image quality and reduced radiation exposure.