Reading thoracic radiographs is a very challenging task as there are many organ structures that need to be evaluated. Two main objectives of thoracic radiology are to find the abnormalities, then formulate reasonable differential diagnoses. There are many normal variations for thoracic organs/structures depending on the age, breed, and body conformation. Additionally, some radiographic changes detected on thoracic images may not have any clinical significance for the patient.

Cardiac Silhouette

The appearance of the cardiac silhouette is breed- and body conformation-dependent. The most common error in reading thoracic images is overestimation of the size of the cardiac silhouette. This is especially true for the barrel-chested dogs when there is a heart murmur detected during physical examination. The cardiac silhouettes of the deep-chested dogs are taller and the barrel-chested dogs are slightly shorter and rounder. Presence of pericardiac fat will occasionally lead to an enlarged cardiac silhouette. Oblique positioning of the VD sometimes will make the cardiac silhouette appear larger on the right and smaller on the left. In older cats, the cardiac silhouette will have a more horizontal position and a bulge may be present at the aortic arch.

Lungs

Many veterinarians remain confused about the interstitial and bronchial patterns of the lungs. The two main factors that can affect the appearance of the interstitial pattern are the age of the patient and the stage of respiration at the time of exposure of the thoracic images. Older animals, especially dogs, tend to have a slight interstitial pattern of the lungs and normally do not cause any clinical signs. Bronchial mineralization is usually an age-related bronchial change. It normally does not cause any clinical signs. Another possibility for the presence of bronchial mineralization is prior bronchial disease. Another component of bronchial pattern commonly seen on thoracic images is bronchial wall thickening or peribronchial cuffing. This change is more significant than bronchial mineralization as it most likely represents the active process of bronchial disease.

Mediastinum

Widening of the cranial mediastinum is one of the radiographic signs of cranial mediastinal lymphadenomegaly. In some cases of obese or brachycephalic dogs, the mediastinum could be very wide due to the presence of abundant mediastinal fat. This is sometimes misinterpreted as a mediastinal mass.

Pleural Cavity

Presence of pleural fissure lines is an important radiographic finding of pleural effusion. However, this is often overinterpreted in thoracic imaging as fibrosis of the pleura and fat in the mediastinum can mimic this. Visualization of the lung margins does not necessarily represent pleural effusion. In many instances, this is seen due to partial inflation of the lung lobes. Overinflation of the lungs sometimes mimics a pneumothorax.

Esophagus

Normally, only a small amount of gas is present in the mid-thoracic esophagus. Gas-filled distended esophagus may be secondary to anesthesia or aerophagia. This should be clinically correlated. Caudal thoracic esophagus may be seen as a soft tissue, or fluid-filled structure in left lateral recumbency.¹ This is not commonly seen in the right lateral recumbency.

Trachea

A collapsing trachea is one of the most difficult to diagnose conditions as it is a dynamic condition. Normally, fluoroscopy is the recommended imaging modality for detection of a collapsing trachea. Sometimes, an incidental finding of narrowing of the caudal cervical trachea may mimic collapsing trachea.

Spinal Vertebrae

Narrowing of the thoracic intervertebral disc space is normally a non-significant finding unless it is accompanied by lysis of the endplates of two adjacent vertebrae indicative of discospondylitis. Spondylosis deformans is a common incidental finding, especially at the mid-thoracic vertebral column. Vertebral malformations such as transitional vertebra, blocked vertebrae, and hemivertebra normally do not cause any neurological signs. Lysis of any vertebra is considered as important and most likely due to neoplastic invasion.

Sternebrae

Pectus excavatum normally does not cause any clinical signs unless it severely compresses the heart and leads to clinical complications.² Degenerative changes and narrowing of the intersternebral spaces usually are not significant clinically. Malformation of sternebrae may accompany other congenital anomaly such as peritoneopericardial diaphragmatic hernia.

References

1.  Avner A, Kirberger RM. Effect of various radiographic projections on the appearance of selected thoracic viscera. J Small Anim Pract. 2005;46:491–498.

2.  Gifford AT, Flanders JA. External splinting for treatment of pectus excavatum in a dog with right ventricular outflow obstruction. J Vet Cardiol. 2010;12:53–57.