Determination of correct AEC function with computed radiography cassettes

It has long been recognized that the assessment of Automatic Exposure Control (AEC) function is an important criterion in the assessment of any general radiographic x-ray installation. Of the tests commonly performed on an AEC, only Beam Quality Dependence relates intimately with the optical density produced on the resultant radiograph. In this test the AEC function is ascertained as being satisfactory or otherwise, on the basis of whether the optical density on the radiograph is controlled within well defined limits. With computed radiography (CR), where the conventional screen-film combination is replaced by a storage phosphor screen, the process by which the digital image is presented on film restricts the resultant optical densities to an extremely tight range, independent of the air kerma incident upon the CR phosphor plate. The implication of this is that the measurement of optical density can not be used as a direct measure of AEC function, and a test such as Beam Quality Dependence cannot be performed in the conventional manner. To overcome this, the relationship between incident air kerma and a computer generated index supplied with the CR system was investigated for two commercially available systems, one supplied by Fuji Medical Systems and the other by Kodak. The relationship between the index and incident air kerma could then be used as a substitute for optical density in monitoring AEC performance with respect to beam quality. It has been determined that an inverse linear relationship exists between the incident air kerma and the Sensitivity Number for the Fuji CR system and that a logarithmic relationship exists between the Exposure Index and incident air kerma for the Kodak CR system. In order to quantify these relationships further, the dependence of Sensitivity Number and Exposure Index on beam quality and incident kVp have been investigated. Using the results from the above investigations, acceptable limits for both Sensitivity Number and Exposure Index, can be established to test for correct AEC function, with respect to Beam Quality Dependence.