Theoretical analysis of comparative patient skin dose and exposure technique approaches in planar radiography

A theoretical analysis of patient skin dose changes with manipulation of technique factors (mAs and kV) was carried out in order to obtain a description of the relationship between patient dose and the exposure technique approach employed. The standard exposure technique approaches in variable part thickness radiography of “fixed kVp” and “variable kVp” and the “15% rule” in constant part thickness radiography were analysed in terms of the changes in patient skin dose. Analytical expressions describing the variation of dose change ratios have been developed in respect of each approach and also in comparison between the two standard approaches. The resultant model is limited to essentially qualitative predictions due to its inherent limitation of neglecting secondary radiation at the entrance site of the beam, but nevertheless indicates some non intuitive benefits in utilisation of mAs (only) changes in compensating for the thinner part and for lower image densities, as well as indicating that most gain is to be had (in terms of lower skin dose), where only modest part thickness compensation is required. The model also describes the influence of the chosen voltage in a “reference technique” (upon which further compensatory changes in exposure factors are based) on the patient skin dose, when the variable kV approach is utilised.