This study aimed to determine the optimum source to image distance (SID) for key radiographic projections, in diagnostic radiology. Three secondary aims were to determine the effect of changes in SID on the amount of scattered radiation at the image receptor, to determine the effect of changes in SID on the radiation dose to the patient and to assess any effects of changes in SID on the quality of the images. The study was performed using Monte Carlo simulation, employing EGSnrcMP and PCXMC. The Monte Carlo codes were benchmarked against independent experimental studies and shown to give...