A device that consists of preserved human or animal tissue, or a two or three-dimensional (3-D) tissue-equivalent model designed to mimic the functional, physical, or a combination of these characteristics of normal or diseased human organs. It is used in diagnostic nuclear medicine and incorporates radioactive sealed sources or refillable cavities to which radioactivity can be added in order to simulate anticipated in vivo distributions of radiopharmaceuticals used in nuclear medicine imaging procedures. It is a quality assurance (QA) device permitting both subjective and quantitative evaluation of the overall function of photon detector systems, e.g., gamma camera, SPECT or PET.