Lung Pattern Identification and Thoracic Radiology

Martha Moon, DVM, MS, Dipl.ACVR
Blacksburg, VA

 

Thoracic radiology is an essential component of the diagnostic plan in animals with signs of respiratory disease.  While alternate imaging, such as computed tomography, nuclear scans, and ultrasound are becoming increasingly useful as diagnostic modalities, thoracic radiology remains the gold standard in evaluation of respiratory disease.  Radiographs of the thorax are used to confirm or rule out a tentative diagnosis, determine the nature and extent of thoracic disease, and follow response to therapy. 

PULMONARY PARENCHYMA

Individual lung lobes are not seen separately on normal thoracic radiographs, but it is important to know the general location of each lobe.  Certain lobes are more susceptible to disease, such as the right middle lung lobe.  This lobe is most commonly affected by aspiration pneumonia, torsion, and atelectasis.  The caudal lobes are more likely to develop neoplastic masses, simply because of their larger size.  The right lung is divided into cranial, middle, caudal, and accessory lobes.  The left lung is divided into cranial and caudal lobes.  The left cranial lobe has a deep fissure separating it into cranial and caudal segments.  The caudal segment is similar in size and location to the right middle lobe. 

Pulmonary vessels, and to a much lesser extent, bronchi, and interstitial tissue, contribute to normal background opacity of the lungs on a thoracic radiograph. Air-filled alveoli, also a component of the pulmonary parenchyma, are not seen. 

Pulmonary vessels are the most prominent soft tissue structures within the pulmonary parenchyma. On the lateral view, the pulmonary arteries and veins are best visualized as they extend through the cranial lung lobes.  In the left lateral view, 2 sets of vessels can usually be seen, going to the left and right cranial lobes.  Arteries lie dorsal to the veins, with a radiolucent bronchus running between them.  Arteries and veins are fairly equal in size. The larger pair of vessels should be those in the non-dependent (and therefore magnified) cranial lobe.  Vessels to the right cranial lobe tend to be ventral to those extending into the left cranial lobe.  Using the magnified vessels, (eg, on a left lateral view, the right cranial lobar artery and vein), the artery and vein in the 4th intercostal space should not exceed the minimum diameter of the right 4th rib, just ventral to the spine.  On the DV view, vessels extending into the caudal lung lobes are best visualized, with the artery lying lateral to the vein, separated by the bronchus.  The diameter of the caudal lobar artery and vein should not exceed the diameter of the 9th rib at the intersection of the rib and vessel.    

Bronchial walls are normally visualized only in the hilar area, and resemble ring-like structures when viewed end-on, and parallel thin linear densities when seen longitudinally. The walls become progressively thinner out in the periphery of the lungs, and are normally not seen.  Bronchial wall mineralization, a normal aging change, can enhance radiographic visualization.  The interstitial tissue forms the framework of the lung, surrounding blood vessels, lymphatics, and alveoli.  Individually the interstitial tissues are too small to see radiographically, but the interstitium as a whole makes a minor contribution to normal background opacity.   

PULMONARY PATTERNS

There are a limited number of ways in which the lung can respond to disease processes; responses include either an increase or decrease in radiographic opacity.  Most diseases result in an increase in pulmonary opacity, usually characteristic for the parenchymal component (alveolar, interstitial, bronchial or vascular) involved, falling into one or more of 4 patterns.  Once the pattern is identified, a list of differentials characteristic of that pattern can be determined.  However, several diseases can have the same radiographic pattern, and one disease can have different patterns depending on the progression or stage.  Recently, both human and veterinary radiologists have attempted to simplify the radiographic diagnosis of pulmonary disease.  Rather than attempting to classify pulmonary infiltrate into a particular pattern, a system of utilizing location and distribution of the pulmonary infiltrate is being attempted.  Along with location, severity (opacity of the infiltrate) and size of the lungs (atelectasis or not) is determined.  No matter what system is used, it is necessary to correlate the radiographic signs with other radiographic changes, such as cardiac size, the presence of bony changes, pneumothorax, etc.  One must also correlate the patient’s signalment, history, clinical signs, and physical exam findings.   A lung aspirate or biopsy might be necessary in some cases for a definitive diagnosis.  In this discussion, the newer classification scheme will be incorporated and utilized along with the more classic description of lung patterns.

ALVEOLAR PATTERN

An alveolar pattern occurs when the air in the alveoli is replaced by fluid, or, in the case of atelectasis, not replaced at all.  Radiographic changes associated with the alveolar pattern include an increased, fairly uniform homogeneous fluid density which may vary from faint and fluffy (fewer alveoli involved) to solid and consolidated (more severe involvement).  Hazy, ill defined areas of infiltrate may coalesce.  The margins may be poorly defined unless the infiltrate extends to the edge of the lobe, in which case the margin is sharply defined.  This is called the lobar sign, and is seen when an affected lobe lies adjacent to an unaffected lobe.  It is an indication of more severe pulmonary infiltration.  A silhouette effect can occur with alveolar infiltrate, where the fluid dense lung lobe may obscure the borders of the heart or diaphragm.  The classic sign of an alveolar pattern is the air bronchogram which is formed by an air-filled bronchus extending through a fluid dense lung lobe.  Only the radiolucent lumen of the bronchus is seen, as the bronchial walls are silhouetted out by the fluid dense lung lobe.  Be careful not to mistake a radiolucent bronchus between 2 vessels, or the radiolucent bronchus superimposed over the heart, for air-bronchograms.  While the air bronchogram is a reliable indication of an alveolar pattern, it is not seen in every case (usually seen with more severe infiltrative disease), and the other radiographic changes associated with the alveolar pattern must be used.  The alveolar pattern is also the dominant pattern, and will obscure other patterns such as bronchial or interstitial.  Diseases causing an alveolar infiltrate tend to progress and clear  rapidly compared to other patterns of pulmonary disease. 

Diseases commonly associated with an alveolar infiltrate include pneumonia, pulmonary edema, pulmonary contusion, and atelectasis.  These diseases can often be differentiated by the distribution of the infiltrate, other radiographic changes such as heart enlargement, and history, signalment, and physical exam findings.  Cardiogenic pulmonary edema is most severe in the hilar and dorsocaudal lung regions, and is usually associated with left heart enlargement, as well as the presence of a heart murmur.  The pulmonary veins are frequently enlarged.  Non-cardiogenic edema may be caused by electrocution, smoke inhalation, near-drowning, head trauma or seizure activity, inhalation of toxic substances, or any cause of severe acute hypoxia.  The distribution of infiltrate tends to be more diffuse than is seen with cardiogenic edema, but may be most severe in the caudodorsal region.  Aspiration and bronchopneumonia usually have a ventral distribution due to the effects of gravity when inhaling gastric contents or bacteria. Hematogenous pneumonia tends to be more diffuse due to spread by the bloodstream.  Pulmonary contusions may be very focal, or involve several lung lobes, and should be associated with a history of trauma. Rib fractures, pneumothorax, and pleural effusion may also be seen.  Atelectasis will cause an alveolar pattern, but, unlike other causes of alveolar disease, a loss of volume is usually present, with an associated mediastinal shift.  Causes of atelectasis include prolonged recumbency, or obstsruction of a bronchus by foreign body, tumor, mucous plug, or misplacement of an endotracheal tube.  Other less common causes of an alveolar pattern include pulmonary thromboemboli, lung lobe torsion, and neoplasia such as lymphosarcoma or broncho-alveolar carcinoma.  Both primary and metastatic neoplasia in cats have been reported to cause an alveolar infiltrate.

INTERSTITIAL PATTERN

An interstitial pattern is caused by thickening or cellular infiltration of the interstitium.  This pattern is divided into structured, or nodular interstitial infiltrate, and unstructured, or linear interstitial infiltrate.  The linear pattern is probably the hardest to identify, and the easiest to create artifactually.  Radiographically, an overall increase in hazy pulmonary opacity, which has a linear appearance is present.  The interstitial infiltrate does not tend to coalesce into a more uniform, homogeneous opacity as does an alveolar infiltrate.  Pulmonary vessels may appear hazy or smudged, but are still visible.  Air bronchograms are not seen because, although the amount of air in the alveoli is reduced, it is not totally eliminated as it is with alveolar infiltrate.   

This pattern can by created by a film taken on expiration (or in any patient unable to inspire deeply), or by underexposure. Obesity may contribute to underexposure, as there is more soft tissue/fat and less lung for a given dimension.  True pathology associated with this pattern includes early stages of cardiogenic pulmonary edema (with a hilar distribution), viral pneumonia, pulmonary contusions, and some forms of neoplasia, including lymphosarcoma and carcinoma.  The most common cause of this pattern, however, is probably pulmonary fibrosis associated with old age. 

The structured, or nodular interstitial pattern is somewhat easier to identify, and is associated with a relatively circumscribed, solid nodule or mass.  These can be single or multiple, and of varying sizes.  A miliary pattern implies many very tiny (less than 5mm) nodules.  If multiple nodules are all the same size, this implies a seeding of the lung from a single incident (such as fungal infection).  If multiple nodules are of varying size, continued seeding of the lung over time is probably occurring, and may be more consistent with pulmonary metastasis.  Mass lesions may contain air, and are termed cavitated.  Diseases commonly associated with an interstitial nodular pattern include neoplasia, both primary and metastatic, fungal disease, abscess, granuloma, hematoma (blood filled bulla), the dry form of FIP, toxoplasmosis, lymphosarcoma, and parasitism (Paragonimus).

BRONCHIAL PATTERN

A bronchial pattern occurs when bronchial walls thicken, and become increasingly prominent radiographically.  When viewed end-on, they are referred to as “doughnuts”, and, longitudinally, are seen as prominent paired parallel lines, or “railroad tracks”.  Thickened bronchial walls can be seen in the lung periphery due to their increased prominence.  Inflammation of the bronchi (bronchitis) or bronchoconstriction (asthma) are associated with this pattern, although radiographic signs and clinical signs often are not well correlated.  Significant bronchial disease can be present without any radiographic signs, especially when acute. Increased bronchial wall thickness is due to smooth muscle hypertrophy, cellular infiltration, edema, and increased mucus production. 

VASCULAR PATTERN

A vascular pattern occurs with diseases which increase or decrease lung perfusion, or cause dilation of the arteries or veins.  Both artery and vein become enlarged with pulmonary overperfusion from left to right shunts (ASD, VSD, PDA).  Occasionally both arteries and veins are enlarged in cats with left sided congestive heart failure.  The pulmonary arteries, especially the caudal lobar arteries, typically become enlarged and tortuous in heartworm disease.  Arteries may also be enlarged with pulmonary thromboemboli, and in cats with pulmonary parasites and asthma. The  pulmonary veins often are enlarged with venous congestion and left sided congestive heart failure.  A decrease in size of both the artery and vein may indicate hypovolemia, dehydration, hypoadrenocorticism, or right to left shunt.

MIXED PATTERN

Most pulmonary diseases result in a combination of patterns, although one may predominate.  Cardiogenic pulmonary edema starts out as a hilar linear interstitial pattern, becoming alveolar as the disease worsens.  Pulmonary trauma often results in a mixed pattern, usually alveolar, and interstitial opacities.  Bronchopneumonia may result in mixed alveolar and bronchial patterns. 

In all pulmonary diseases, the radiographic changes must be correlated to history, signalment, bloodwork, and clinical signs to attempt to reach a more definitive diagnosis.