The Importance of Patient Positioning in Radiography When Utilising Automatic Exposure Control

Automatic exposure control (AEC) is a device used in radiographic practice which terminates a radiographic acquisition when a certain exposure threshold is reached. The AEC is commonly utilised for in-bucky imaging, such as for the chest, abdomen, pelvis and spine. There are many benefits to utilising the AEC where awareness and knowledge of the technology are thoroughly understood. Poor technique when utilising the AEC, however, will alter dose, and by extension, image quality. This study aimed to assess how changes in patient positioning when using AEC for chest, abdomen, lumbar spine and pelvic x-ray examinations affect patient dose and image quality?

An anthropomorphic phantom was marked every 1 cm from the ideal centring point in each direction; superior, inferior, left lateral and right lateral, for up to 5 cm of off-centring. Exposures were taken at each position and the tube current, air kerma and signal-to-noise ratios recorded for each exposure.

Air-kerma changes ranged from a maximum reduction of 6.06%, 74.72%, 76.07%, and 35.66% for the chest, abdomen, pelvis and lumbar spine, respectively, to a maximum increased air kerma of 50.09%, 8.00% and 10.32% for the chest, pelvis and lumbar spine. The abdomen did not demonstrate any increased air-kerma results. Statistically significant changes (p < 0.05) to dose were observed in 54 of the 80 (67.5%) separate recordings. Changes to signal-to-noise ratio varied from a maximum negative change of 49.14% to a maximum positive change of 25.10%.

Statistically significant differences in dose were commonly discovered for mis-positioning beyond 1-2 cm, which will likely occur within the clinical environment. Such variation in dose and signal-to-noise ratio identifies potential impacts to image quality and dose burdens to patients. The results highlight the need for careful consideration when using the AEC and the requirement for accurate positioning technique by the radiographer.