|Factors affecting the appearance|
What causes a structure to appear black, white, or gray on a plain radiograph? How much of the x-ray beam is absorbed by the structure, and how much passes through determines this. The density of the object being imaged influences how much of the x-ray beam will be absorbed.
Dense structures absorb (attenuate) more of the x-ray beam than less dense structures. Thus, less of the beam passes through to hit the cassette. The result of this is that dense structures appear white; these structures are termed radioopaque.
On the right is a radiograph of a pair of scissors. Scissors are comprised of metal, which is very dense, and so they appear white on the x-ray. Other dense structures include calcium, barium and iodine, all of which appear white on radiographs. Barium and iodine will be discussed in later sections of this module.
Structures which are not very dense, such as air, absorb very little of the x-ray beam. Most of the beam passes through the air and strikes the cassette. The result is that these structures appear black on x-rays; these structures are termed radiolucent. Note that on the x-ray image on the right the air surrounding the scissors appears black.
The variable densities of structures in the body result in the four basic radiographic densities:
a. Air, which appears black
Fat has a low density, but its density is slightly greater than air. Thus fat will appear nearly black on a radiograph but slightly less black than air. Muscles, organs, and soft tissues are shades of gray, ranging somewhere between white and black depending on the structure’s density. These shades of gray are referred to as water density.