Donald E. Thrall, DVM, PhD, DACVR (Radiology, Radiation Oncology)
Radiographic manifestations of lung disease are not specific. As a result, the cause of respiratory disease can rarely be identified definitively based on radiographic changes. To narrow the list of possibilities, the pulmonary pattern recognition system is taught widely. In the pulmonary pattern recognition system, radiographic lung abnormalities are categorized into either an alveolar, bronchial or interstitial pattern. Once the radiographic abnormality is characterized, the list of considerations for that particular pattern can be formulated. This list is usually shorter than if the lung pattern was not categorized. There is nothing inherently wrong with this approach, other than many veterinarians and students find it confusing and few are able to reliably categorize radiographic lung abnormalities accurately.
Is the pattern recognition system the best one? Common problems are that: 1) whether the radiographic is normal or abnormal is often overlooked; the focus is on defining an abnormal pattern, 2) evaluators get bogged down and frustrated when a pattern can not be categorized accurately, and 3) in actual fact, few lung diseases involve only one compartment.
A simpler approach is needed. The first question to answer is whether the radiograph is normal. To do this, the things that affect the radiographic appearance of the thorax must be understood; this is true regardless of whether the pulmonary pattern recognition system is applied. Once the lungs are determined to be abnormal, the next question is whether the airways are involved. If airway involvement can be substantiated, airway sampling, such as with a transtracheal aspirate or a bronchoalveolar lavage, becomes a potentially valuable intervention to determine the cause of the pulmonary problem. If the airways are not involved, airway sampling may not be informative, and in some instances it could be contraindicated. It is important to remember that a definitive diagnosis of the cause of a radiographic lung abnormality cannot be made regardless of the radiographic interpretive system used. A key point becomes whether the airspaces are involved. Also, the distribution of the lesions can still be used to help formulate the list of ruleouts.
With regard to the question of "normal or abnormal", abnormal generally means an increase in pulmonary opacity rather than a decrease in pulmonary opacity. Only increases in pulmonary opacity are considered here. Unfortunately, there are various technical and patient factors that create an increase in lung opacity that are not pathologic. The most common are radiographic technique, decreased lung aeration and habitus.
Underexposure and overexposure will both render thoracic radiographs non-diagnostic. Even small errors in exposure can be problematic. If using an analog (film-screen) system, a high level of diligence is needed to acquire diagnostic thoracic radiographs consistently. In the digital world, there is considerable exposure latitude and it is less problematic to acquire consistently good thoracic radiographs.
Poor aeration is a big problem. Lung aeration is always reduced when the patient is in lateral recumbency and a diagnosis of lung disease must not be made solely from lateral views; disease suspected in lateral views must be substantiated in the DV or VD view. Sedation or anesthesia will also lead to poor ventilation and should be avoided for thoracic radiography if possible.
Habitus is influential. The volume of overlying tissue will affect lung opacity directly. Some level of increased lung opacity is to be expected in obese or athletic patients and this will complicate assessment of the background lung opacity.
The radiographic signs of an airway pattern include: 1) air bronchogram formation, 2) intense lung disease that is not due to a mass, and 3) the presence of rings and trams. Clinically, many patients with airway disease will have a cough and/or wheeze but this is not always true.
An air bronchogram is a throwback to the days of pulmonary pattern recognition, but it is one part that cannot be discarded. An air bronchogram is visualization of a radiolucent (black) branching structure that traverses a region of relatively homogeneously increased lung opacity. It is created by air in the bronchus being contrasted by increased opacity created by pathologic accumulation of cells or fluid in the alveoli. Air bronchogram formation and visualization is more common in dogs than in cats, and many cats with advanced alveolar airspace disease will not have visible air bronchograms in thoracic radiographs.
Seeing radiolucent lung between normal pulmonary artery-pulmonary vein pairs is often misinterpreted as an air bronchogram. The key to making the distinction between a real air bronchogram and this "fake out" is that the radiolucent region between vessels does not traverse a wide region of intense lung opacity; it is simply bordered by two vessels.
Airspace disease and masses are the two most radiographically intense forms of lung disease. These can be distinguished by the lack of distinct margination to the opacity; a distinct margin is a typical feature of a mass but not alveolar disease. An indistinctly marginated region of intense lung opacity will usually be due to fluid or cells in the airspaces.
"Rings and Trams" are signs of bronchial (airway) disease. Rings are circular opacities with radiolucent centers and represent thick bronchial walls or peribronchial infiltrate. Trams are parallel non-tapering opaque lines with a more radiolucent region between them. Again, the tram lines represent either thick bronchial walls or peribronchial infiltrate. It is important to recognize that some ring shadows can be seen in nearly every thoracic radiograph, so the key is the detection of an excessive number of ring shadows. Trams are rarely seen in normal radiographs. The ring and tram pattern is not very opaque per unit area of lung.
Evaluators will often spend considerable energy trying to determine whether an airway pattern, such as the ring and tram pattern, has coexisting interstitial disease...the so-called "bronchointerstitial" pattern in the world of pulmonary pattern recognition. It doesn't matter! Regardless of interstitial involvement, identification of airway involvement opens the door for airway sampling, which is likely to provide important information regarding the etiology of the lung disease.
In summary, a modified approach to thoracic radiographic interpretation involves the following:
1. Is the radiograph normal? Consider radiographic technique, ventilation and habitus.
2. Start out by assessing the airways
a. Are there air bronchograms?
b. Is there any evidence of intense lung consolidation without a sharp margin, as might occur with intense alveolar disease?
c. Is there a ring and tram pattern?
3. If abnormal airways are detected, consider signalment and history and determine whether airway sampling can be conducted safely and is indicated.