A parametric method for determining mammographic X-ray tube output and half value layer

In the National Health Service Breast Screening Programme, regular assessment of the mean glandular doses received by a group of women is recognized as an important part of a duality assurance programme. The use of different tube voltages, to improve the beam penetration for thick or dense breasts, and of X-ray units with programmable exposure modes, requires a large number of measurements to ensure that all the values of tube output and half value layer required for the dose calculations are available. In this work, a computer model is used to produce data that allow the: calculation of tube output and half value layer for the range of clinically encountered conditions from measurements routinely obtained during quality assurance tests. The data are given as a series of equations and parameters, enabling the calculations to be easily incorporated into a dose calculation spreadsheet. The results of an experimental verification of the model are presented, showing good agreement between the measured and predicted values of half value layer and tube output for a range of combinations of target, filter and tube voltage.