Read the German translation: Interpretation der Lungenmuster beim Hund und bei der Katze

Introduction

Thoracic radiographs are routinely used in dogs and cats with respiratory disease, but their interpretation remains challenging. The reasons why the pulmonary parenchyma is difficult to evaluate is the fact that many different diseases can have a similar appearance, and there is a large degree of overlap of radiographic manifestation of diseases. The concept of pulmonary patterns is based on the assumption that different diseases affect different anatomical structures within the lung parenchyma. However, the model of pulmonary patterns is not a perfect one, as many diseases involve several and varying components of the lungs, and disease in transition can move from one component to the other. Nevertheless, the pulmonary pattern model, if used appropriately, is a valuable diagnostic tool. In the following the different pulmonary patterns, their radiographic appearance and significance, but also an alternative approach to interpretation of the pulmonary parenchyma in dogs and cats is described.

Interpretation of Pulmonary Disease

Interpretation of thoracic radiographs is often too focused on indentifying a specific pulmonary pattern rather than taking all findings into account. Since the approach to pulmonary disease using a strictly pattern-based approach has its limitations, several other factors have to be considered.^1,2 First of all, thoracic radiographs should always be assessed for technical quality, especially in patients with suspected pulmonary disease. Underexposure, expiration and patient-related artifacts such as obesity will result in an increased opacity of the thoracic cavity which may be misinterpreted as an interstitial pulmonary pattern. Then, a decision has to be made if the pulmonary parenchyma is too opaque or too radiolucent. If the lungs are too radiopaque, and artifacts and extrapulmonary disease have been ruled out as a cause, the type of opacification using pulmonary patterns, and most importantly the distribution (focal, lobar, multifocal or diffuse) and the location (cranioventral, caudodorsal etc.) should be noted. Often, a mixture of pulmonary patterns is present, and in those cases it is most efficacious to determine the predominant pattern as it will best define the source of the problem. There are 4 pulmonary patterns described.

Pulmonary Patterns

Alveolar Pulmonary Pattern

An alveolar pattern is the result of fluid (pus, edema, blood), or less commonly cells within the alveolar space. A total collapse of the alveoli (atelectasis) leads to a similar appearance. Radiographic signs include border effacement with other soft tissue structures such as the pulmonary vessels, cardiac silhouette or diaphragm. Smaller, ill-defined patches of alveolar pattern can coalesce into a more homogeneous soft tissue opacity where the only remaining gas is retained in the larger bronchi, seen as "air-bronchograms". Air-bronchograms may not be visible early on in the disease process, their absence does not rule out alveolar disease. If the disease process does not spread to an adjacent lung lobe, the border of the affected lobe is sharply demarcated ("lobar sign"). Distribution of the changes allows determining the most likely cause for the alveolar infiltrate:

Table 1. Selected differential diagnoses for alveolar pulmonary pattern based on distribution.^3-6

Bronchial Pulmonary Pattern

A bronchial pattern is an abnormal lung opacity caused by peribronchial cellular, fluid and fibrotic infiltration, or bronchial mucosal and submucosal thickening (chronic bronchitis). Radiographic signs of a bronchial pulmonary pattern are: Excessive number of opaque rings and lines, best recognized in the periphery of the lungs where normal bronchial walls should not be visualized anymore. End-on projection of thickened bronchi results in rings or "donuts", whereas long axis projection of a thickened bronchus results in two parallel lines "tram lines". The overall pulmonary opacity is not markedly increased as the alveoli are still air-filled, in fact, hyperinflation of the lungs can result from lower airway obstruction.⁷ Chronic bronchial disease can lead to bronchial dilation and loss of normal tapering of the bronchi (bronchiectasis). It is important to recognize the presence of bronchiectasis as those patients are more prone to chronic respiratory infections, due to impaired mucociliary clearing function. Bronchiectasis is usually irreversible.⁸

Table 2. Selected differential diagnoses for bronchial pulmonary patterns.

Interstitial Pulmonary Pattern

The interstitial space consists of alveolar and interlobular septa as well as microscopic vessels. In order to subdivide pathology of the interstitial space, interstitial pulmonary pattern are further divided into unstructured and nodular interstitial pattern. An unstructured interstitial pattern is caused by a collection of fluid, cells or fibrin within the connective tissue framework of the lung, between the alveoli and around vessels and airways. This results in a generalized increase in pulmonary opacity with a hazy appearance, the outline of the pulmonary vessels and airway walls becomes unsharp. Summation of multiple linear opacities can lead to a net-like or reticular pattern. The lung volume is usually unchanged, but may be smaller with pulmonary fibrosis.⁹ Alveolar disease in transition (early or resolving) with partially fluid-filled or partially collapsed alveoli results in the same appearance as alveolar septal thickening, therefore the same principles are applied when it comes to pattern distribution.

Table 3. Selected differential diagnoses for diffuse and focal interstitial pulmonary patterns.

Aggregations of cells in the supportive lung tissues result in a nodular pattern. Pulmonary nodules have to reach a considerable size to be recognized on a radiograph (7-9mm).¹⁰ The areas where nodules can best be detected are the periphery of the lungs and the portion of the lung superimposed with the cardiac silhouette and the diaphragm where superimposition of other pulmonary structures is limited. Two lateral views, or ideally 3 thoracic views, are extremely important, as a partial collapse of pulmonary parenchyma in dependent portions of the thorax will obscure pulmonary nodules. Care should be taken not to confuse pulmonary nodules with osseous metaplasia, end-on pulmonary vessels and extrathoracic structures such as skin nodules or nipples (use nipple markers or barium on extrathoracic nodules to clarify).

Table 4. Selected differential diagnoses for solitary vs. multiple pulmonary nodules.¹¹

Vascular Pulmonary Pattern

Vascular pattern is not a "pulmonary pattern" strictly speaking, as it does not reflect pulmonary but rather cardiovascular pathology. It is produced by a change in size, shape or number of pulmonary vessels. The blood volume circulating in the pulmonary vasculature has an impact on the overall pulmonary opacity. Overcirculation of the pulmonary vasculature will increase the opacity of the lungs due to the increased blood content, whereas hypoperfusion of the lungs will lead to increased radiolucency of the lungs.

Alternative Approach to Pulmonary Patterns

On a daily basis we have to deal with patients with pulmonary abnormalities that don't fit into one of the classical pulmonary patterns. Rather than agonizing over how to name the radiographic appearance of the lungs, the observer should rather focus on the next step in the diagnostic workup. A definitive diagnosis is rarely made on radiographs and follow up diagnostic tests are required. And radiographs are most helpful in deciding which one to choose. The most important decision is whether the airways (alveoli or bronchi) are affected by the disease process. If this is the case, airway sampling such as trans-tracheal aspiration or bronchoalveolar lavage may be very helpful. If the disease process mainly affects the interstitial space, airway sampling is likely not rewarding and more invasive procedures such as fine needle aspiration or biopsy of the lung tissue should be considered if clinically indicated.¹² Needless to say, to decide which disease process is most likely, history, clinical signs, bloodwork and other radiographic findings have to be integrated with the radiographic changes in the pulmonary parenchyma, and further diagnostic workup will depend on the combination of all these findings.

References

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10. Nemanic, et al. J Vet Int Med 2006; 20: 508-515.

11. Suter, et al. Vet Radiol Ultrasound 1974; 15: 3-24.

12. Berry, et al. 2007 Textbook of Veterinary Diagnostic Radiology 5^th ed. 471-480.