A computer generated phantom has been produced to compare observer performance in detection tasks using various display systems. Simulated lesions of various size and contrast can be generated over a uniform or sloping grey level background. Many such lesions can be generated for a single phantom, and in each case, for example, for the set of all regions within the phantom, two such lesions are generated, one at the center of the region and a second at one of four orientations at a fixed distance from the central lesion. The observer performance test is therefore, for the ensemble of lesions and regions, to indicate which orientation occurs for each region. This phantom is based on that proposed in mammography. In order to evaluate the use of this phantom to assess image quality in general, an observer performance study was performed to assess image quality after data compression using 2 different techniques, capable of compressing the data by ratios of up to 50:1. Preliminary results indicate that this type of phantom is relatively efficient in assessing image quality and that a relatively low number of images, and therefore limited amount of time, is required of the observers. This phantom is part of a larger European project (ISCAMI) where a series of physical phantoms for testing PACS networks are being developed.