An objective approach to including the entire femur on one antero-posterior projection

Several studies have shown that in trauma imaging, a full length antero-posterior femur on one image is desirable. An increased source image distance (SID) is required, and radiographic approaches to the amount of increase appear to be subjective. The current study investigated objective approaches to optimising the SID and exposure factors for full length femur radiography.Multiple pelvis and femur exposures were made on a cadaver. Dose Area Product was recorded for each exposure, and the image from each was assessed by four independent observers. Image quality was scored using reliable anatomical criteria. The mean image quality scores were considered in the context of the relevant DAP dose to permit comment on an optimal exposure factor combination.Results demonstrated that a source image distance (SID) of 140 cm will permit visualisation of an average length femur, but a SID of 150 cm is recommended as standard to ensure visualisation of the full femur of taller patients. The optimal femur exposure is derived from that of a pelvis using this simple algorithm:1.Remove the grid and increase SID to 140 cm;2.Maintain the kVp;3.Reduce the mAs by the grid factor of 4;4.Then double the mAs to account the 1.5× increase in SID.The study highlights the greater value of objective and evidence based approaches to radiography over a more subjective approach, and reinforces the ongoing need for research in all aspects and areas of imaging.