The importance of anatomical noise in mammography

Normal tissue structure in a radiological image, called anatomical noise, can prevent radiologists from seeing the pathology they are looking for. The goal of this paper is to show the importance of this noise component compared to the system one (quantum mottle, screen structure and film noises) in the particular case of mammography. Two normal mammographic images are digitized in order to provide an anatomical structure. On this background, an object simulating a microcalcification (a sphere) or a tumor (a spherical lens) is filtrated by the modulation transfer function of the imaging system representative of a mammographic unit and superimposed. A two alternative forced choice experiment is effected on these synthesized images in which the amount of object contrast and system noise is varied. The images are presented on a high resolution screen to five observers used for this kind of experiment. The experimental results are then compared to the signal to noise ratios given by the non-prewhitening matched filter model in which the human visual transfer function is taken into account. When comparing the experimental results with the model in which the anatomical noise is considered as a noise or as a signal, it is observed that the experimental results are in-between, This means that the anatomical background has a component that can be considered as a noise and another that can be recognized as signal. At the present time, there is no observer model who takes the anatomical noise effect into account. The use of a homogeneous test object without anatomical structure to qualify the system performance or to optimize the radiological procedure then appears questionable.