Optimisation of patient dose for the horizontal beam technique in lateral lumbar spine radiographic examinations.

In the emergency department, patients undergoing lateral lumbar spine radiography examinations are positioned either lying on their side on an X-ray table with the X-ray beam vertical or lying supine on a trolley with the X-ray beam horizontal. The measured dose-area product (DAP) values were found to differ significantly, typically 1.3 Gy cm(2) for those patients examined on the X-ray table and 2.7 Gy cm(2) for those on a trolley. This work investigates the reason for higher DAP values with the horizontal beam technique. The UK's current recommended national diagnostic reference level (NDRL) for the lateral lumbar spine is 2.5 Gy cm(2). The measurements of body diameter on volunteers showed that rotating the patients from their side to their back resulted in an increase in tissue thickness of between 2 cm and 9 cm for the lumbar region. X-ray absorption increases exponentially with increasing tissue thickness. An increase of 5 cm in body diameter for a lateral lumbar spine at 93 kV will increase the DAP by more than two-fold. Mathematical modelling and measurements with polymethyl methacrylate provided data to predict the tube potential increase necessary to reduce the DAP. For the horizontal beam technique, the tube potential was increased from 93 kV to 102 kV and the average DAP reduced to 2.3 Gy cm(2). Radiographic technique should be understood when auditing the dose. Tube potential must be increased to optimise the horizontal beam technique. The 2.5-Gy cm(2) NDRL relates predominantly to the more common vertical beam technique. Separate local diagnostic reference levels for horizontal and vertical beam techniques are recommended.