Pulmonary Parenchyma

Radiographic examination of the lungs can: help confirm pulmonary disease; help evaluate severity and chronicity of disease; help to determine aetiology; demonstrate location of disease; show associated changes of lung; and demonstrate progression or regression. The overall opacity of the lung depends upon the ratio between air within the alveoli and bronchi and soft tissue within the vessels and interstitium. Any process that alters this ratio alters the appearance of the lungs. The components of the lung include airways and alveoli, vessels, interstitium (lymphatics, supporting frame-work for vascular and bronchial structures, alveolar septa) and bronchi.

Evaluating thoracic radiographs for pulmonary abnormalities (Figure 1) should include assessment of:

 Radiographic technique:

 Was the radiograph taken at inspiration?

 Was the radiograph under- or overexposed? (Look at the spine: a radiograph exposed properly for assessment of lungs should be slightly underexposed for examining the vertebrae.)

 Was positioning adequate?

 Opacity: is pulmonary opacity normal, decreased or increased?

 Distribution: is the abnormal change in opacity diffuse or localised?

 Location: what is the location of the abnormal opacity? (i.e., what lung lobes or portions of lung lobes?)

 Pattern: what is (are) the predominant pattern(s) of abnormal opacity?

Figure 1. Descriptions of pulmonary lesions.

Alveolar Disease

Alveolar implies involvement of the distal air spaces of the lungs. This results when fluid (blood, oedema or exudate) or cellular material fills the alveoli. It can also occur when the alveoli lose air or collapse (atelectasis). In the latter case the involved lung lobe(s) will have a decrease in size or pulmonary volume. When fluid fills the alveoli, the volume of the lung is normal or near normal. When the lung lobe appears opaque and enlarged, neoplasia should be a primary rule out. An alveolar pattern (Figure 2) is recognised by: soft tissue opacity that causes border effacement (positive silhouette sign, obscuring visualisation of heart, diaphragm, vascular structures); air bronchogram (air-filled bronchi surrounded by homogenous opacity which silhouettes vessels and obscures them); typically fluffy, indistinct margins which tend to coalesce into a lobar margination. Alveolar opacities may overshadow other patterns that are present. Alveolar pattern is very labile and may change appearance in hours. The location and distribution of this pattern may help differentiate the cause of the disease (Figure 3).

Figure 2. Radiographic appearance of alveolar lung disease.

 Increased pulmonary opacity (due to airless alveoli)

 Fluffy, patchy, ill to poorly defined areas of increased opacity (inhomogeneous)

 Coalescence of the fluffy, ill defined areas (with disease progression)

 Eventually may become lobar in distribution

 Air bronchogram

 Air bronchiolograms: when some large groups of alveoli are flooded and others are not. Occurs at the edge of a lesion, while it is mild or early or in regression

 Border effacement (heart, diaphragm, vessels)

 Rapidly changing appearance

Figure 3. Diseases characterized by alveolar lung pattern.

Interstitial Disease

This pattern results when fluid or cellular material infiltrates the interstitium. Two different interstitial radiographic patterns are a generalised increase in pulmonary opacity (unstructured) and nodular (structured) (Figure 4).

Figure 4. Interstitial lung disease.

Bronchial Disease

Diseases that accentuate the faint or invisible normal bronchial appearance create a radiographic 'bronchial pattern'. A bronchial pattern is identified as an opacity of bronchial walls resulting from thickening of the bronchial walls or fluid and/or cellular material in the peribronchial tissue. These diseases may be acute or chronic, reversible or not (Figure 5). Acute diseases may not show radiographic changes. Most of the patterns of bronchial disease are uniformly distributed throughout the lungs. Bronchial opacity appears radiographically as: linear; non-vascular; parallel lines ('tram tracks'); small circular ring shadows ('donuts') (Figure 6).

Other radiographic changes that may occur with bronchial disease include larger sometimes bizarre configurations of ring shadows and irregular parallel opacities with loss of normal peripheral tapering (bronchiectasis), and hyper-inflation of lungs may occur secondarily with air trapping.

Figure 5. Aetiologies of radiographic bronchial changes.

 Mineralised bronchi in old dogs (Cushing's disease in dogs)

 Allergic (feline allergic bronchitis)

 Infection (bacterial, viral, mycotic, parasitic)

 Neoplasia (bronchogenic carcinoma)

 Oedema

Figure 6. Radiographic appearance of bronchial pattern.

 May see no changes with acute disease

 Thickened bronchial walls (thickened walls or peribronchial cellular infiltrate or peribronchial fluid)

 Thickened walls may appear as dense ring shadows or 'donuts', or in other plane as dense parallel lines ('tram lines')

 Hyperinflation due to air trapping (allergic bronchitis in cats--secondary collapse of right middle lung lobe)

Vascular Disease

Vascular patterns result from a change in the normal vascular appearance. This can be either a reduction in size or number of vessels or an increase in size or number (Figure 7). Artery and vein should be about the same size. The width of the vessels on the lateral radiograph in the forth intercostal space should be no larger than the upper third of the third rib and no less than one third the width of the third rib. On the ventrodorsal VD (or dorsoventral, DV) view the caudal pulmonary vessels can be evaluated. The artery is lateral and the vein central to the bronchus. Artery and vein should be about the same size. The width of the vessels should be no larger than the width of the ninth rib as they cross the ninth rib. End-on vessels need to be differentiated from pulmonary nodules.

Figure 7. Causes of vascular patterns.

Decrease in Pulmonary Opacity

This may occur as a focal or generalised radiographic change. A generalised decrease may represent changes in one or more lung lobes. A decrease in pulmonary opacity (increase in pulmonary lucency) occurs usually because of an increase in air content of the lung(s) or decrease in lung perfusion (Figure 8).

Figure 8. Increased pulmonary radiolucency.

Focal Pulmonary Disease

Solitary and cavitary pulmonary lesions (Figure 9) due not fit as well into the previously described classification scheme.

Figure 9. Solitary pulmonary lesions.