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Objective:
Automated exposure control (AEC) technology is a valuable tool for dose management; however, its use requires understanding by end users. The purpose of our study was to evaluate the positioning of patients receiving a CT of the thorax and measure the offset from isocenter and associated dose consequences. We further evaluated the effectiveness of an educational intervention to improve patient positioning performance.

Materials and Methods:
Using a Mercury 4.0 phantom (Sun Nuclear), we studied the AEC technology and the effect vertical ("Y") offsets (-60 mm to +60 mm at 10-mm increments) had on dose. Scout images were collected using PA projections followed by AEC helical scans of the phantom using the routine clinical chest protocol. We reviewed 60 patients (37 women, 23 men) who underwent CT of the thorax under various standard of care clinical protocols. Each exam was loaded into a visualization software, (Intellispace, CT Viewer, Philips Healthcare) to overlay a grid on the image. The anatomical center of the thorax was the carina. Vertical offset (mm) from isocenter was recorded. Baseline data for 60 patients was reviewed. An education session on AEC and its use was given to all technologists in the department. After two successive 3-month improvement periods, a post-education review and final review of 60 patients was performed.

Results:
The phantom study showed that above-center positioning produced a 14.36% decrease in dose from baseline (492 mGy/cm at isocenter versus 421.3 mGy-cm at +60 mm) and below-center positioning produced a 20.77% increase in dose (594.2 mGy/cm at -60 mm). Our initial evaluation showed 37 studies (62%) were vertically off-center by more than +/-20 mm. The average centering offset was -27.69 mm (-65 mm to +13 mm). The first post-education review (33 women, 27 men) showed 12 studies (20%) that were vertically off-center by more than +/-20 mm. Average centering offset was -6.53 mm (-32 mm to +31 mm), with statistically significant improvement using unpaired t test (p = <0.0001). The final review (31 women, 29 men) showed 13 studies (22%) that were vertically off-center by more than +/-20 mm. Average centering offset was -3.9 mm (-43 mm to 30 mm).

Conclusion:
Proper patient centering is essential when using AEC in CT because off-center patients are magnified or minified in the scout view. This results in erroneous determination of patient size, which can cause either excessive radiation exposure (over 20% increase) or inadequate image quality. Our results show that a simple educational review can significantly improve patient centering performance. Although we missed our goal of 85% improvement in examinations centered to +/- 20 mm from isocenter, we were able to achieve a rate of 78-80% of examinations centered within this range. Our study highlights the importance of proper patient centering in the use of AEC technology, and the benefit of understanding and education for the end user about the technology and how it affects the patients we serve.