The thorax is traditionally examined by a compartment approach—6 basic compartments or "spaces" plus the thoracic wall should be considered during the radiographic examination. The compartments include the mediastinum, the pleural space and four pulmonary divisions-bronchial, vascular, interstitial and alveolar. The heart is part of the mediastinal compartment, but often discussed separately. The advantages of the compartment approach over haphazard assessment of the thoracic radiograph are twofold: 1) a systematic evaluation is performed minimizing incomplete assessment and bias. Pulmonary lesions are categorized according to anatomic location. Next, gamuts or lists of diseases that might be considered for a particular anatomic location can be made and rank-ordered in the context of the signalment, history and clinical signs. Although the above paradigm is widely used in human and veterinary medicine, an alternate method of evaluation the pulmonary compartments has been proposed (Nykamp, et al).

The concept of an "interstitial pattern' is abstract because the interstitium contains numerous sub-gross structures, including capillaries, lymphatics and alveolar septae and walls. In addition, structures in other compartments, i.e. e., vessels and bronchioles, traverse the interstitial space. Pathologic interstitial patterns are due to increased opacity in the interstitium, resulting from abnormal cellular or fluid accumulation. Underinflation (expiration) can also cause interstitial pattern in a normal patient because the components of the interstitium are less widely separated by aerated lung. Classification includes structured and un-structured interstitial patterns. Un-structured patterns are typified by "veiling" and "smudging" of vascular structures while structured patterns result in formation of nodules and linear markings. Peribronchial accumulation of cells and fluid are truly in the interstitium and are included in the structured classification and referred to as cuffs or ring shadows. Interstitial patterns do not compromise the alveolar space, thus air is still present within the affected lung, unlike that seen with alveolar lesions.

Technical considerations

Non-uniform shape causes variation in the thickness of the thorax. Therefore, an optimal radiographic exposure cannot be selected for the entire thorax-some areas will be relatively under- or overexposed on the radiographic image. This problem can be minimized with careful selection of film screen combinations and using a technique chart that produces a wide range of opacities, e.g., a long scale of contrast. This is obtained by using a high peak kilovoltage (kVp) low milliampere-second (mAs). Digital radiography virtually eliminates this problem.

The respiratory cycle affects the appearance of a thoracic radiograph. Motion is a common problem, thus short exposure times are recommended (typically less than 1/60 sec). An attempt should always be made to expose thoracic radiographs during inspiration. At peak inspiration, the airways and vasculature are widely separated by end air spaces, producing a relative "black" lung field. The compliant pulmonary vessels, especially the veins and capillary beds, are less distended compared to peak expiration because of increased intrathoracic pressure. The opposite is true on a radiograph made at peak expiration where airways and vessels are not widely separated by air spaces. The pulmonary vessels also contain more blood and are larger. These factors, along with less air being in the lungs, leads to a more opaque lung field and a 'lighter" radiograph with prominent bronchial and vascular structures. A radiograph made during expiration can give misleading information.