The Model 156 Mammographic Accreditation Phantom simulates the x-ray attenuation of a 4.2cm slab of compressed human breast composed of 50% adipose tissue and 50% glandular tissue. Target objects in the phantom are of known size, shape, and density. These represent the different structures or malignancies found when imaging the breast. Image quality and system sensitivity is evaluated using these targets and following ACR/MQSA guidelines. The 156 is also an excellent tool to help identify artifacts due to grids, filters, film-processing (for film-screen systems), as well as those produced by digital mammography systems. Routine use of the Model 156 Mammographic Accreditation Phantom helps detect many imaging changes that degrade image quality before they affect clinical results. The phantom consists of a precision wax insert containing the test objects. The insert is protected by an external acrylic shell. The combination of these materials provides the simulation of a 4.2cm thick 50%/50% breast. A reference contact radiograph of the insert is included. This provides information on the location and orientation of the test objects in the individual phantom. An acrylic test disk provides the test step needed to measure density differences.