Mammogram films showing the four levels of breast density

Breast tissue is composed of milk glands, milk ducts and supportive tissue (dense breast tissue) and fatty tissue (nondense breast tissue). Radiologists use mammogram images to grade breast tissue based on the proportion of dense to nondense tissue. According to the BI-RADS reporting system, the levels are (from left to right) almost entirely fatty, scattered areas of fibroglandular density, heterogeneously dense and extremely dense.