Bones show up well on X-rays, but soft breast tissue does not.
The term "uncompressed breast tissue" refers to a region along the periphery of the breast that escapes the gentle squeezing action that is part of every mammogram. Because the uncompressed region consists mostly of subcutaneous fat, radiologists often call it the zone of uncompressed fatty tissue.
Background
A mammogram is simply an X-ray picture of the breast. The principle behind X-ray imaging is that structures in your body block X-rays to differing degrees based on how dense they are. Dense structures like bone block a lot of X-rays and show up on the X-ray image as white. Less substantial structures show up as black or gray.
Breast Compression
Since none of the soft tissues in the breast block X-rays very well, getting a good picture can be difficult. A further complication is that the breast varies in thickness. The thicker central portion blocks more X-rays simply because it contains more tissue. As a result, tumors in that region might not be visible on the X-ray film. Compressing the breast between two panels during a mammogram momentarily spreads the tissues out to give a better picture and make it easier to spot any abnormalities.
Mammogram Landmarks
When examining a mammogram, radiologists use a number of visual cues to spot potential problems. A common strategy involves visually segmenting the mammogram into regions depending on how various types of breast tissue show up on X-ray film.
One of these landmarks consists of the uncompressed fatty tissue at the periphery of the breast near the skin. Another region, known simply as "fatty tissue," includes breast fat in the more central areas that experience compression. This area of breast fat lies adjacent to the uncompressed zone and surrounds denser inner structures like ligaments and milk ducts. Radiologists refer to that denser central region on the mammogram as the fibroglandular disk.
Cancer Risk
According to Dr. Daniel Kopans, author of the book, "Breast Imaging," and professor of radiology at Harvard Medical School, the subcutaneous fat layer near the surface of the breast is not the most likely area for breast cancer to develop. Instead, according to Kopans, more than 70 percent of breast cancers develop within the zone that lies within 1 cm of the outer edge of the fibroglandular disk.
However, radiologists must carefully examine that the zone of uncompressed fat for tumors. Although breast compression increases image quality, some imperfections remain. In particular, the outer periphery of the breast often shows up poorly on a mammogram. Even with compression, the X-ray contrast needed to image properly the denser inner structures does not give good detail for the thinner regions. As a result, tumors occurring at the edge of the breast can also be hard to see.
Contrast Correction
Expert radiologists can adjust mammogram contrast by hand on a case-by-case basis to "see through" the dense inner structures without losing detail at the outer edge. Computer-aided mammography also can enhance detail in this region by applying peripheral equalization, a type of digital image processing technique.
Peripheral equalization for uncompressed fatty tissue also helps computerized algorithms identify the breast-skin boundary line. Accurately locating mammography landmarks such as this makes it easier to find differences between right and left breast images that could signal tumors or other problems.
Tags: fatty tissue, block X-rays, inner structures, periphery breast, uncompressed fatty, uncompressed fatty tissue
