Radiography is the use of x-rays to produce an image. X-rays are produced by an x-ray tube, pass through the patient and finally hit an image detector plate in the case of digital radiography or the radiographic film within the screen in the case of convention film-screen techniques. As the x-ray beam passes through the patient, the beam becomes attenuated (reduced) as it interacts with tissue. Different tissues result in different amounts of attenuation based on atomic number and density.

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The five radiographic opacities are air, fat, soft tissue/fluid, mineral and metal. Contrast between structures relies on adjacent structures having different opacities. This is called 'subject contrast'. What this means is that two adjacent soft tissue structures, or a soft tissue organ surrounded by fluid, cannot be differentiated on radiography. However if two soft tissue organs are separated by fat or air, then the different opacities mean that the structures will be seen distinctly. 



Size is an important consideration. Mutiple radiographs may be needed to cover an entire area and because a radiograph essentially compresses a 3D animal into a 2D image, minimally orthogonal views (at 90 degrees) if not multiple views are needed for full evaluation of a region. 



In marine mammals consideration of subject contrast is also very important. Radiographs are very useful for the thorax because the lungs are air filled, the ribs are bone and the overlying soft tissues are soft tissue opacity. So detail within the lungs can be seen. However most marine mammals do not have internal abdominal fat due to laying fat in the superficial blubber layer. This results in poor abdominal detail under normal conditions, although evaluation of the gastrointestinal tract can be useful if there is gas present, or if a metallic or bone opacity foreign body has been ingested. Pathologies that result in alteration of tissues from soft tissue to mineral opacity, for example some urinary calculi, can also be seen. But in general abdominal radiography is limited in pinnipeds and cetaceans.