Digital pelvic radiography: increasing distance to reduce dose.

PURPOSE: To investigate the effect of varying source-to-image distance (SID) on image quality and radiation dose using direct digital radiography (DR).
METHODS: An anthropomorphic pelvic phantom was positioned for a standard anteroposterior DR examination. The SID was initially set at 100 cm, and the tube potential remained constant at 80 kVp. Two exposures were taken at this SID, and the entrance surface dose (ESD) and effective dose (ED) were calculated. Images were sent to a reporting-grade picture archiving and communication system workstation for image quality analysis. The SID was then varied by 10-cm intervals, up to its maximum (80 cm to 147 cm), with 2 exposures taken for each interval. Grid usage also was varied to determine its effect on radiation dose and image quality. Image quality was assessed by 4 observers (2 images per SID interval) who were blinded to the measurements, using a rating system adapted from the European Guidelines on Quality Criteria for Diagnostic Radiographic Images.
RESULTS: Increasing SID results in lower ESD and ED compared with the standard 100-cm SID (147 cm: 2.56 mGy and 0.44 mSv, respectively, vs 100 cm: 3.00 mGy and 0.51 mSv, respectively; P<.05). Reduction of the SID resulted in an increased ESD and ED (80 cm: 3.79 mGy and 0.65 mSv, respectively; P<.05). Analysis of the resultant images demonstrated a statistically nonsignificant minor increase in image quality when increasing SID (P=.276). Image quality for 60 cm, 80 cm, 100 cm, 120 cm, 140 cm, and 147 cm was given a mean score of 17.5, 30, 31, 33, 33, and 32, respectively. Removal of the grid resulted in lower image quality but a significantly lower overall ESD and ED.
CONCLUSION: Significant reductions in ESD and ED can be achieved, without loss of image quality, with a larger-than-normal SID for AP pelvic DR examinations. ©2011 by the American Society of Radiologic Technologists.