Technique charts for EC film: Direct optical measurements to account for the effects of X-ray scatter

Purpose: To develop a method of measuring technique charts for enhanced con trast (EC) film, to demonstrate how X-ray scatter changes the response of E C film, and to generate technique charts for general use. Methods and Mater ials: We have developed a "digital cassette" - consisting of a metal plate/ phosphor screen, a light guide, a photodiode sensor, and an electrometer - that can be used to measure the light generated in the phosphor screen of t he film cassette. In turn, these measurements can be used to generate techn ique charts for EC film. The digital cassette has been used to measure tech nique charts for 4-MV and 6-MV X-ray beams for a variety of different phant om thicknesses, field sizes, and phantom-to-cassette air gaps. Results and Discussion: We have observed that the signals generated in an ionization ch amber located 9.4 cm behind a 30-cm-thick water-equivalent phantom increase by a factor of 1.9 when the field size is increased from 4 x 4 cm(2) to 40 x 40 cm(2) when irradiated by a 6-MV X-ray beam. However, the change in EC film response is a factor of 3.5 under the same conditions. Irradiations t o optimally expose the EC film predicted by the digital cassette differ by up to 82% compared to those predicted by ion chamber measurements. Neverthe less, the technique charts measured using the digital cassette predict the response of the EC film to +/- 0.2 optical density. The overresponse of the EC film is most likely due to low-energy scattered photons, which interact with the high atomic number (Z = 64) phosphor screen of the enhanced contr ast localization cassette. Therefore, simple solutions, such as placing a h igh atomic number material above the enhanced contrast localization cassett e, can reduce this contribution by scattered photons to the signal generate d in the cassettes. Conclusions: We have developed a digital cassette that can make more accurate measurements of the technique charts for EC films. O ur measurements show that under some conditions, X-ray scatter can generate a large fraction of the signals recorded by the EC film. Technique charts have been generated at 4 MV and 6 MV, and these charts should have universa l applicability. (C) 2001 Elsevier Science Inc.