Avoiding the biggest mistakes in thoracic radiography (Proceedings)
Mistakes are commonly made in radiographic interpretation of the thorax. Errors are due to the various radiopacities normally present, non-uniform thoracic shape, functional changes occurring during the respiratory cycle and occasionally an incomplete knowledge base. Soft tissues, vessels, fat and bone that make up the thorax are surrounded by or juxtaposed to a large amount of air, producing more subject contrast than is seen elsewhere in the body. This is useful because soft tissue nodules, fluid accumulation, lymphadenopathy and vascular lesions are visualized due to contrast with surrounding air. The pulmonary space is the only area of the body where blood vessels are well-seen on survey radiographs.
Non-uniform shape causes variation in the thickness of the thorax. Thus, an optimal radiographic exposure cannot be selected for the entire thorax-some areas will be relatively under- or over- exposed 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) exposure. Good quality digital radiographic systems are also helpful in this regard because they spread the contrast over a wider range of tissues than with film-screen (analog) radiography.
The respiratory cycle affects the appearance of a thoracic radiograph. An obvious problem associated with the respiratory cycle is motion, thus short exposure times are recommended (typically less than 1/60 sec). Even with short exposures, panting or excessive fractiousness will limit radiographic quality necessitating use of sedatives or tranquilizers. With sight hounds and cats, breath-holding may occur during exposure, causing over inflation and a false impression of airway obstruction. At peak inspiration, the airways and vasculature are widely separated by end air spaces, producing a "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. As a result a radiograph made during expiration may give the false impression that an interstitial or vascular pattern is present. Radiographs made during expiration also limit the conspicuity of small nodules and pulmonary infiltrates.The thoracic radiograph is a two-dimensional image of a three dimensional structure and is subject to the effects of summation, i.e., superimposition of overlying anatomic structures. For instance, on a lateral radiographic projection, the shadows cast by blood vessels of the left lung are superimposed on those of the right lung. Likewise, a skin nodule on the lateral body wall would be superimposed on the lung field giving the false appearance of a pulmonary nodule. Understanding the pitfalls of summation is particularly important with an air-filled lung containing a branching network of water opaque tissues such as blood vessels, bronchi and bronchioles. Using at least two 90 degree orthogonal projections can help overcome the effect of summation. Other issues related to the two dimensional nature radiographs include magnification, distortion and loss of depth perception.
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 reducing aeration and subject contrast. As a result, soft tissue changes such as nodules, alveolar opacities in the dependent lung will not be seen. For instance, in cats an alveolar lesion in the right middle lung lobe is poorly visualized on the right lateral projection, but is well seen on a left lateral projection. Occasionally both ventrodorsal and dorsoventral projections are needed. This applies to lesions involving the dorsal lung field, especially in large dogs. On a ventrodorsal projection, the dorsum of the lung is dependent and underinflated (compressed) and limits the conspicuity a soft tissue lesion, while a dorsoventral projection will have a greater likelihood of revealing the lesion. General anesthesia exaggerates the above gravity dependent effects and should be avoided when making thoracic radiographs.