The use of model observers for image quality assessment in digital mammography is currently being considered. Model observers assign decision variables to signal present and signal absent images which, if they are independent, can be used as a measure of performance. In this study, the impact of different dependencies at pixel level between the signal present and signal absent images were studied for the detection of 0.25 mm and 2.5 mm diameter disk-shaped objects. Clinical images were acquired on an Amulet Innovality (FujiFilm, Tokio, Japan) mammography unit and modified multiple times to appear as acquired at 75% of the original dose level and to simulate different noise realizations. From these modified images, regions of interest (ROIs), with and without an embedded signal were obtained. Subsequently, detection experiments were created for which the images with and without embedded signals had: 1) exactly the same background structures, 2) the same background structures but different quantum noise realizations, and 3) completely different background structures. The ROIs were evaluated using a channelized Hotelling observer (CHO) with a dense difference of Gaussian channel set. It was found that if the background structures within the ROIs with and without signal are dependent, the CHO decision variables also show strong dependencies. However, the performance measurement of the CHO yielded values that were not affected by the dependency in pixel values. This finding is important for future developments of phantom-based image quality analysis in mammography using model observers when using a single or a limited number of anthropomorphic phantoms.