Radiographic opacity refers to the actual penetrative ability of x-rays to pass through an object and reach the film. Radiographic opacity of a part is determined by its thickness and its atomic weight. Therefore, atomic weight and thickness are closely related.
Image formation is dependent on the phenomenon of differential absorption. When x-rays penetrate tissue, they are not homogeneously absorbed; some tissues absorb x-rays more efficiently than others. If x-ray absorption were uniform, the resulting radiographic image would be grey or white. Some of the x-rays are absorbed by the tissues such as bone. Other x-rays pass through the tissues and produce the diagnostic image on the film. Other x-rays pass into the tissues and are deflected or scattered in the tissues and may exit onto the film. These are unrepresentative of the tissue through which they have passed. These are scattered x-rays and cause distortion of the final image. These scattered rays also contribute to the radiation hazard of the procedure by virtue of their unpredictable exit from the animal.
At this point we must differentiate
between different types of subject tissue density.
The term tissue density is used to describe the degree to which a patient
or object absorbs incident x-rays. In the accompanying radiograph
the bone tissue is denser than the adjacent soft tissue, but the differences
in image tone should be described in terms of radiolucency and radiopacity.
For example, in this radiograph, the soft tissues are more radiolucent
than the bones.
Air and fat absorb relatively less radiation and are consequently radiolucent, so their images on the film will appear black and pale grey repectively. Bone and metal absorb much more X-radiation and are radiopaque, so their images are white. Most soft tissues in the body are composed mainly of water and appear as shades of grey. The radiopacity of most fluids (blood, urine, transudates, exudates, bile and cerebrospinal fluid) and non-mineralised non-adipose tissues (muscle, cartilage, tendons, ligaments, fascia and parenchymatous organs) is the same. Lead and other metals have high physical density and effective atomic number, which renders them extremely radiopaque.