The CIRS Model 020 BR3D Mammography Phantom is designed to assess detectability of various size lesions within a tissue equivalent, complex, heterogeneous background. This phantom provides more realistic challenges for standard screen and FFDM mammography systems as well as tomosynthesis and breast computed tomography. The phantom consists of a set of 6 slabs made of heterogeneous breast equivalent material that exhibits characteristics of real breast tissue and demonstrates how underlying targets can be obscured by varying glandularity. Each slab contains two tissue equivalent materials mimicking 100% adipose and 100% gland tissues ¡°swirled¡± together in an approximate 50/50 ratio by weight. One of the slabs contains an assortment of micro-calcifications, fibrils and masses. Each semicircular shaped slab measures 100 x 180 x 10 mm. Each slab, with its unique swirl pattern, provides varying backgrounds when arranged in multiple combinations and thicknesses.