Penetration is the degree to which X-rays have passed through the body. Assessment of penetration is traditionally a standard part of assuring chest X-ray quality. With modern digital viewing systems, over or under penetrated images are rarely a problem. Image data can be 'windowed' to optimise visibility of anatomical structures. This is often performed by radiographers after they have acquired the image or can be performed using the windowing tool in the viewing system.
A well penetrated chest X-ray is one where the vertebrae are just visible behind the heart. Although X-rays are still occasionally over or under exposed, a discussion of penetration now best serves as a reminder to check behind the heart. The left hemidiaphragm should be visible to the edge of the spine. Loss of the hemidiaphragm contour or of the paravertebral tissue lines may be due to lung or mediastinal pathology.