| Breast composition is expressed as the proportion of fibroglandular parenchyma in relation to the fatty breast tissue. The BI-RADS lexicon describes breast tissue composition under the following four breast density categories: 
 
ACR category a: The breasts are almost entirely fatty.ACR category b: There are scattered areas of fibroglandular density.ACR category c: The breasts are heterogeneously dense, which may obscure small masses.ACR category d: The breasts are extremely dense, which lowers the sensitivity of mammography.
 
 
 
 Normal mammogram, ACR category a (the breasts are almost entirely fatty)
 
 
 
 
 Normal mammogram, ACR category b (there are scattered areas of fibroglandular density)
 
 
 
 
 Normal mammogram, ACR category c (the breasts are heterogeneously dense, which may obscure small masses) 
 
 
 
 
 Normal mammogram, ACR category d (the breasts are extremely dense, which lowers the sensitivity of mammography)
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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