The Influence of Source-to-Image Distance on Effective Dose and Image Quality for Mobile Chest X-ray Imaging

Aim: To investigate the effect of source-to-image distance (SID) on effective dose and image quality at a fixed mAs. Furthermore, to determine whether effective dose varies as SID is varied after compensating mAs for the inverse square law. Method: A chest phantom with a simulated pathology was imaged at varying SIDs at a fixed mAs and kV. Observers visually compared the experimental images with a reference image using criteria divided into 3 categories: anatomy, noise and nodule image quality. A 2AFC program was used to display the images. A 5-point Likert scale was used ranging from ‘much worse’ to ‘much better’. PCXMC 2.0 84 was used to estimate effective dose. At each SID, a new mAs was calculated using inverse square law correction. Effective dose was estimated again. Results: Modal response for 2AFC visual evaluation of anatomy image quality was ‘equal’ for every SID, implying no observable change. Modal response for 2AFC visual evaluation of noise changed from ‘better’ to ‘worse’ as SID increased. Modal response for visual evaluation of nodule image quality was ‘worse’ at the smallest SIDs and ‘better’ at the largest SIDs. Decreasing SID from 190-100 cm: increased effective dose by 247%; compensating mAs reduced the effective dose by 36%. Conclusion: For variable SIDs used in this study, there is little perceived difference in anatomical image quality. Nodule image quality may decrease at shorter SIDs while effective dose increases. Therefore, when shorter SIDs are necessary due to lack of space, practitioners must be aware of patient dose.