Thoracic radiology: lung patterns made easy (Proceedings)

The lecture is a review of thoracic radiographic interpretation in small animals. The outline of this lecture is as follows:

     • Review of normal. Breed variations (like the hellish bulldog!) and changes occurring as dogs and cats age will be stressed. Some comparisons between film-screen radiography and digital radiography will also be discussed, primarily with regards to the superior contrast resolution of digital radiography.

     •Review of lung patterns. Examples of interstitial, alveolar, bronchial, and vascular lung patterns will be illustrated. The clinical significance of these patterns will be discussed.

     •Application of lung patterns to common clinical presentations will be discussed.

     •A final note on the benefits of CT to thoracic imaging will be illustrated by comparing radiographs to CT in specific cases.

1) Normal Anatomy: Anatomy is power! There are times when a variation in normal anatomy can steer the clinical diagnosis in the wrong direction. Here are some examples.

          • BREEDS:

a) Bulldogs: Always a difficult thoracic radiograph to interpret, even for those with experience. The cranial mediastinum of a bulldog is often wider than other breeds. The right cranial lung lobe does not extend cranially and medially to outline the cranial aspect of the cardiac silhouette. Additionally, bulldogs tend to have a 'normal' (or at least clinically insignificant) diffuse bronchial pattern. Mid- to caudal thoracic vertebral column anomalies are very common, especially in French Bulldogs. Be prepared to accept this breed's odd conformation!

b) Basset Hounds: The VD projection of a Bassett Hound (and some Dachshunds) outlines prominent costochondral junctions at the lateral pleural surface of the lungs. This prominence makes the pleural surface undulate, giving the impression of pleural effusion. This is normal and not pleural effusion! The lateral view will confirm this.

c) Collies, shelties, and other deep-chested dogs: Shelties and collies (in particular) can have prominent small (1-2 mm) mineral foci in the cranial lungs that represent incidental pulmonary osseous metaplasia (osteoma, osteomata, or heterotopic bone are synonyms). Other breeds will have this as well and this should not misinterpret this as pulmonary metastasis. Soft tissue pulmonary metastasis (nodules) must be greater than 4-5 mm to be visible radiographically. When a 1-3 mm nodule is visible in the lung it is either mineralized (as in the case of osseous metaplasia) or it represents an end-on blood vessel. Deep-chested dogs in general tend to have an upright cardiac silhouette. This gives the heart a perfectly ovoid shape on the VD projection. This is normal. Skin folds can increase the opacity of the lungs, particularly on the VD projection.

d) Sharpei dogs: Deal with it! Skin folds.

          • AGE-RELATED CHANGES:

Another variation of normal occurs as the dog or cat ages. A classic example of this is the "lazy heart" appearance of the feline heart. The geriatric cat's heart will have increased sternal contact on the lateral view. This also causes the arch of the aorta to abruptly arc in a ventral to dorsal direction. On the VD projection, this end-on blood vessel (the aorta) will look like a rather large nodule to the left of midline, just cranial to the heart. This is not a lung nodule or mediastinal mass. Also, the descending aorta tends to undulate on the lateral view. Claims as to whether this appearance of the heart and aorta is related to systemic hypertension in cats are unproven. In dogs, a bronchial pattern, or more commonly a mineralization of the larger airways, can be identified as the dog ages. Mineralization/bronchial patterns can be a variant of normal in cats also, but this is not necessarily age-related.

          • MISCELLANEOUS VARIATIONS:

Some examples regarding the differences in right versus left lateral, VD/DV, and oblique projections will be illustrated. Inspiratory and expiratory phases of respiration will be compared.

2) Lung Patterns: The ultimate goal of this lecture is to convey how lung patterns are unnecessarily complicated and generally useless! The different patterns are just fancy words used to describe a fairly simple concept.

a) Interstitial and alveolar lung patterns: These are just pathologist's words that are used to describe a radiologist's simple observation. An interstitial lung pattern is an opaque (white) lung that PARTIALLY obscures the margins of the pulmonary blood vessels. An alveolar lung pattern is an opaque lung that COMPLETELY obscures the margins of the pulmonary blood vessels. The only distinction these patterns make with regards to clinically relevant information is the SEVERITY of the disease. The patterns alone tell us nothing about WHAT disease we are dealing with.

b) Bronchial Patterns: This pattern comes closest to helping shed light on WHAT disease the pet is suffering from. The bronchial pattern is an increased opacity of the lung that often partially obscures the margins of blood vessels (hence the pussy-foot term, "broncho-intersitial pattern"). What makes it different from an unstructured interstitial lung pattern is that the opacity follows the lower airways. Some people describe "donuts" or "tram lines" to describe this. I will be describing the nooks and crannies of an English muffin.

c) Vascular Patterns: The apparent increased opacity we see in the lungs when pulmonary blood vessels are too large defines a vascular pattern.

d) Structured Interstitial Patterns: Some texts make a distinction between unstructured partial obscuring of the pulmonary vessel margins (unstructured interstitial lung pattern) and pulmonary nodules (structured interstitial lung pattern). How about we just call nodules NODULES?!!!

The defining sign that helps us determine why the dog/cat can't breath is the DISTRIBUTION (or location) of the lung pattern rather than the lung pattern itself. Examples of this will be shown.

— Cranioventral distributions tend to be associated with pneumonias.

— Caudodorsal distributions tend to be associated with cardiogenic and non-cardiogenic pulmonary edemas.

— Cats do not follow this rule (do cats follow any rules?). However, some tips can help steer you away or towards cardiogenic vs infectious etiologies.

3) Clinical Applications: Variations in the radiographic appearance of pneumonia and cardiogenic edema will be presented. An important finding that can differentiate diseases is the presence of pleural effusion in addition to the lung pattern.

     • *** Congestive heart failure is always a potential differential diagnosis in a cat with pleural effusion.

     • *** Heart failure and pneumonia are uncommon differentials in a dog with pleural effusion.

     • These rules help to prioritize the differential list while you wait to see how the animal responds to Lasix or antibiotics.

4) Computed Tomography (CT) Solves World Hunger: Actually, CT does not solve world hunger but it can help shed light on some non-specific findings of a thoracic radiograph. CT also can further evaluate the extent of certain diseases, like pulmonary metastasis. CT does this because of two basic advantages over standard thoracic radiography:

a) CT has superior contrast resolution: Only 5 opacities can be differentiated with radiography, but THOUSANDS of shades of gray can be differentiated with CT.

b) CT removes superimposed structures: CT makes the third dimension of the patient really thin, removing certain structures that would obscure anatomy on a two-dimensional shadow of a radiograph. Examples of this will be shown.