An assessment of the usefulness of screen-film speed classifications.

Speed classifications of screen-film combinations are frequently quoted in the day-to-day functioning of X-ray departments. These values are utilised for a variety of functions by X-ray personnel. This study aimed to compare a range of commonly used screen-film combinations in order to establish the level of agreement between stated speed classifications and actual speed of systems. Relationships between system speed class and image quality were also investigated. Six commonly used screen-film systems were studied; three had a speed classification of 200 while the remaining systems had a classification of 400. Characteristic curves for each system were produced, from which relative speeds were calculated at four beam energies. Psychophysical tools and visual grading analyses were used to assess image quality. The sensitometric results demonstrated that at all energies the speed class quoted did not predict the actual relative speeds of the film-screen systems. The image quality study demonstrated disagreement with conventional assumptions regarding speed classifications with mean values for 400 speed systems being higher or at least equal to the 200-system scores. There is no evidence from this study to support the ongoing use of current screen-film classifications. Personnel within all X-ray departments should carry out rigorous speed measurements and image quality assessments of all systems within their department before nominal classifications are used.