CAMPI: computerized analysis of mammography phantom images-effects of film nonlinearity and digitizer noise

The purpose of this work was to extend the recently introduced CAMPI method to allow for the nonlinear sensitometric response of film. Sensitometric strips were digitized and used to calibrate the phantom images by converting the pixel value to a linearized intensity-like variable. The corresponding linearized CAMPI measures were found to have simpler (and expected) dependencies on X-ray variables as long as the optical density of the film was not too large. At higher densities (>2.0) the digitizer noise becomes significant, and a correction was necessary. A simple model for the digitizer noise was proposed and tentatively verified. With this work CAMPI can now perform objective and accurate evaluations on analog and direct digital images, compensating for phantom variations, mAs differences, screen-film characteristics (if applicable), and even allowing for digitizer noise limitations