Deciphering Bronchial Patterns
Ellie Nuth, DVM, DACVR
Whether you are relatively new to looking at digital radiographs or not, the amount of “background” pattern in the lungs can often be misleading. The parenchymal interstitium is actually visible on a normal radiograph especially in the caudodorsal lung fields on the lateral projection because of the large size of the lobes and larger amount of superimposed parenchyma. The interstitium appears as the lacy soft tissue opacity between airways and vessels and is more prominent in expiratory images. This opacity, however, should not blur margins of the vessels, and as long as the pulmonary vessels are distinctly visible, you can infer that lung parenchyma is adequately aerated and normal. On top of this background opacity, the walls of the primary bronchi should be visible in a normal patient as opaque double parallel lines leaving the trachea up to the level of the second divisions. Visible bronchial walls should stay thin, linear, and distinct without blurring opacity in the immediate peribronchial parenchyma (Figure 1).
Yes, you WILL see airway walls near the hilus of the lungs in a normal dog, but you should not be able to follow these airways out into the periphery. The hallmark of a bronchial pattern is visualizing airway walls as double parallel lines (side-view, “tram tracks”) or rings (end-on-view, “doughnuts”) in the pulmonary periphery. The best places to look in the pulmonary periphery on the lateral projection are superimposed with and just cranial to the cardiac silhouette, overlying the diaphragm, and just ventral to the vertebral bodies. On the ventrodorsal projection, look lateral to the cardiac silhouette, and overlying the diaphragm caudally. Try to avoid the central hilar portions of the lung fields as these will always look “busy.” The thickening of and excess of visible airway walls in a pathologic state causes the lung fields to overall appear linear and “streaky” (Figure 2). Bronchial patterns are typically diffuse in location. One of the reasons bronchial patterns are often difficult to distinguish is that the abnormal opacity will not be as noticeable per unit area because the interstitium between airways is often adequately aerated and normal if disease is truly limited to airway wall infiltration, as opposed to focal patterns such as aspiration pneumonia or a pulmonary mass. However, as with many things in radiology lung patterns are not always black and white (pun intended), and mixed patterns such as bronchointerstitial may exist with certain disease processes. Caution must be taken to ensure an artefactual change is not to blame (i.e. underexposure or an expiratory image causing the appearance of unstructured interstitial opacity).
Differentials for a bronchial pattern
- Allergic bronchitis – asthma
- Infectious bronchitis
- Paragonimus kellicoti (lung fluke) – dogs and cats, often with cavitary lesions
- Alurostrongylus abstrusus (lungworm) – cats, often with patchy interstitial and alveolar patterns
- Heartworm – often with interstitial pattern, enlarged pulmonary arteries, and right-sided cardiomegaly
- Fungal – Histoplasmosis in cats. Other types often have pulmonary nodules and lymphadenopathy
- Neospora caninum – dogs
- Toxoplasma gondii – cats, often with regions of patchy consolidation
- Other inflammatory bronchitis
- Toxic/inhaled irritant – i.e. smoke inhalation
- Acute lung injury/acute respiratory distress syndrome
- Cushing’s disease, hypercalcemia, hyperparathyroidism – bronchial wall mineralization mimicking a bronchial pattern
- Bronchogenic carcinoma, although often associated with mass lesions
- Pulmonary edema (peribronchial cuffing) – possible manifestation of cats and doberman pinschers in congestive heart failure
What does it mean?
Cellular infiltrate from primary airway disease causing wall thickening is often the cause of increased conspicuity and a bronchial pattern on radiographs, but a bronchial pattern may also result from wall mineralization, luminal exudate, thickened bronchial mucosa or peribronchial cuffing. Depending on the type and stage of the particular disease process, other pulmonary patterns may be concurrently present.
Here is a list of canine/feline differentials to consider – note some of these may be disregarded due to lack of certain endemic infectious diseases here in Washington State, but travel history must be considered. Further characterization of airway disease may be obtained with airway sampling (i.e. bronchoalveolar lavage or transtracheal wash) with cytology and culture, and in some cases thoracic computed tomography is helpful in better evaluating distribution and severity of disease.
Figure 1. Normal canine thorax (a) in which vessel margins and background interstitium are distinctly visible. In the close-up of the hilar region (b), note only the walls of the primary bronchi are visible (white arrows).
Figure 2. Lateral thoracic radiograph of a 1-year-old border collie who presented for coughing, sneezing, oculonasal discharge and fever, diagnosed with infectious tracheobronchitis. Note thickened/fuzzy and prominent airway walls in the pulmonary periphery (white arrows = “tram tracks”, white circles = “doughnuts”).