The effect of source to image-receptor distance on effective dose for some common X-ray projections.

A number of studies on the effects of source to image distance (SID) on dose to the patient have been published. These generally assume that the X-ray beam is collimated to a rectangular region of fixed size at the entrance surface of the patient at each SID. This is poor radiographic practice. In this work, effective doses have been determined using the commercially available Monte Carlo simulation package PCXMC. Three collimation cases were considered, namely to regions of clinical interest at the entrance and at the centre of the patient, and a fixed beam size at the image receptor. For cases of collimation to a central region of interest or to the image receptor, the saving in effective dose at increased SIDs is modest.