Acceptance and constancy testing in projection radiography with film foil systems

The relation of dose and optical density in a conventional X-ray unit working with Screen Film Systems is subject to inherent fluctuations. For conventional equipment, the fluctuations of the components in conformity to the manufacturer instructions are calculated in such a fashion that the X-ray images of the patients fulfil the requirements of the Radiological Guidelines. The tolerance requirements of a mammogram can only be maintained if the operator runs the subsystem of film and processing such that the processing of films with single emulsion number remains over a longer period of time as close as possible to the manufacturer specifications for the type of film. The testing strategy used thus far in acceptance tests is the random sampling. In conventional X-ray units, and within the frame of constancy tests, this leads to an insufficiently accurate conclusion in case of exceeding tolerances of test-phantom images: of 99 images, 9 give a wrong result (rate of false negative+false positive). In mammography units, the introduction of stricter tolerances (ΔD=±0.15; desirable according to the 'European Protocol for the Quality Control of the Physical and Technical Aspects of mammography Screening'), would further decrease this ratio: 37 of 99 images would give a wrong result. This forces to a different strategy of testing when the reference values are fixed for the image of a test phantom and for the constancy testing of film processing. (orig.)