All breasts contain ducts, glands, fibrous tissue, and fat. Dense tissue is made of glands and fibrous tissue (“fibroglandular” tissue). Dense tissue blocks x-rays and therefore shows up as white on a mammogram. The muscle behind the breast, lymph nodes, and most other breast masses will also absorb x-rays and appear light gray or white on a mammogram, too. Fatty tissue allows more x-rays to penetrate and therefore shows up as black or dark gray on a mammogram. Each woman’s breasts are different than the next and contain a unique mix of fatty and dense tissue. Some women’s breasts are almost all fat, some have very little fat, and some are in between. Some dense breasts are mostly fibrous tissue and some are mostly glandular tissue. Dense breasts are normal and tend to become less dense with age and menopause. Breast density is not determined by how a breast looks or feels. Density is determined from a mammogram, either by a radiologist or by software.
Figure Diagrams of the normal breast. Left: The normal breast is composed of milk-producing glands at the ends of ducts that lead to the nipple. Varying amounts of fibrous tissue surround the glands. There is layer of fat just beneath the skin. Often a few lymph nodes and the underlying muscle are seen near the underarm (axilla). Right: On a mammogram, fat appears dark gray, glands and fibrous tissue (dense tissue) as well as muscle and lymph nodes appear light gray or white. Masses due to cancer also appear white.