Image courtesy: http://www.mayoclinic.com
Breast tissue is composed of fatty (nondense) tissue and connective (dense) tissue.
Radiologists use a grading system to describe the density of breast tissue based on the proportion of fat to connective tissue.
The four levels of density are shown in these mammogram images.
From left to right: level 1 (a very fatty breast), level 2 (fatty tissue makes up more than 50 percent of breast), level 3 (dense tissue makes up more than 50 percent of the breast) and level 4 (a very dense breast with very little fat).
It is easier to interpret lesions in Level 1 & Level 2 breast densities whereas Level 3 & 4 pose a problem in identification of lesions and micro-calcifications.