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.
Three radiographic projections are recommended for evaluating the thorax. Either ventrodorsal or a dorsoventral projection
and two lateral projections are made. Both left and right lateral radiographic projections are necessary because the dependent
lung is compressed by overlying tissues, which reduces aeration and masks soft tissue changes such as nodules and alveolar
opacities in the dependent lung. General anesthesia exaggerates the above gravity dependent effects and should be avoided
when making thoracic radiographs.
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.