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

Cardiac Silhouette

The appearance of the cardiac silhouette is breed and body confirmation 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. Sometimes the cardiac silhouette is larger due to presence of pericardiac fat. 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. Other possibility of the present of bronchial mineralization is due to 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 the bronchial mineralization as it most likely represent the active process of bronchial disease.

Pleural Cavity

Presence of pleural fissure lines is an important radiographic finding of pleural effusion. However this is often over interpreted in thoracic imaging. Fibrosis of the pleural and fat in the mediastinum can mimic the presence of a small amount of pleural effusion. 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. Over inflation lungs sometimes may mimic pneumothorax.

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 presence of abundant mediastinal fat. This is sometimes misinterpreted as a mediastinal mass.

Esophagus

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

Trachea

Collapsing trachea is one of the most difficult to diagnose conditions as this is a dynamic condition. Normally, fluoroscopy is the recommended imaging modality for detection of collapsing trachea. Sometimes, 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 indicates of diskospondylitis. Spondylosis deformans is a common incidental finding. Vertebral malformation such as transitional vertebra, blocked vertebrae and hemivertebra normally do not cause any neurological sign. Lysis of any vertebrae is considered as important and most likely are due to neoplastic invasion.

Sternebrae

Pectus excavatum normally does not cause any clinical sign unless it severely compresses the heart and leads to clinical complication. 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.