The term "density" is sometimes used to refer to the degree that a body structure or substance appears dark on an x-ray picture. Different body structures and substances absorb x-rays (a form of energy) at different rates. Blood and bone absorb x-rays at a high rate and appear white on x-ray pictures. When looked at on an x-ray picture, blood and bone are referred to as areas of high density. Air and water absorb x-rays at low rates and appear black on x-ray pictures. When looked at on an x-ray picture, air and water are referred to as areas of low density. Brain tissue is between high density and low density, and is gray in appearance on x-ray pictures.

It should be noted that whereas a relatively new bleed in the brain appears white, blood that has been around in the brain for a long time will appear dark because it does not absorb x-rays as well. Thus, a new bleed will show up as a high density area, whereas an old bleed will show up as a low density area. If the appearance of new blood or bone shows up as a high density area and represents a change from the previous appearance, this is referred to as a "high density change." Because of the neutral density of the brain, it is easy to detect low or high density changes. Injured brain tissue shows up on x-ray pictures as a low density change.

The term "high density" is typically used in CT (Computerized Tomography) scan reports. CT scanning is an advanced imaging technique that uses x-rays and computer technology to produces more clear and detailed pictures than a traditional x-ray. High density is also known as high attenuation. Density comes from the Latin word "densus" meaning "thick."

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