SPECTRAL ESTIMATION AND CONTRAST CALCULATION IN THE DESIGN OF CONTRAST-DETAIL TEST OBJECTS FOR RADIOTHERAPY PORTAL IMAGING

The calculation of contrast in the design of quantitative contrast-det ail test objects is considered. A spectral model based on the Schiff c ross section is used as a starting spectrum. It is suggested that cont rast should be determined to within +/-5%, if this uncertainty is to b e small compared with uncertainties in threshold contrast estimations. It is shown that the Schiff model gives spectra which can be used to calculate contrast within this range of accuracy. For low values of ef fective accelerating potential (< 6 MV) or when this parameter is know n inexactly, further refinement of the spectrum based on comparison of calculated and measured attenuation data is advisable.