1615 Pelvic projection radiography: increasing the source image distance provides diagnostic images at a reduced dose

PURPOSE: To investigate the potential for reducing patient doses for pelvis projection-radiography by increasing the source–image distance (SID). METHODS: A tissue equivalent PIXY phantom was used to obtain anteroposterior (AP) projection radiographs of the pelvis at SIDs ranging from 90 cm to 130 cm using the same collimation size and exposure factors of 79 kVp and 12.5 mAs. The exposures for each SID were repeated 10 times to account for precision errors. A barracuda (MPD) solid-state detector probe was used to measure the entrance dose. The measurements were made using Siemens multix-TOP X-ray equipment and the images acquired using Konica CR. Image quality was measured (blinded) by three separate readers using an 18-point subjective scale. A Mann-Whitney U test was used to calculate the statistical differences between the entrance doses at the various SIDs. RESULTS: The entrance doses for SIDs at 90 cm, 115 cm and 130 cm were 1.63 mGy, 0.84 mGy and 0.61 mGy, respectively. These were statistically significantly reduced with increasing SID, with a p-value of <0.001. Using the 18-point subjective scale there were no significant differences between the diagnostic quality of the images at any of the SIDs. CONCLUSION: Increasing the SID for AP pelvis radiographs significantly reduces the entrance dose, while still providing diagnostic images within the range used in this study. The dose reductions are attributable to the inverse square law resulting in a lower entrance dose. Further research is required into whether reduced exposure factors may yield the same effect whilst maintaining diagnostic images.