What changes on a thoracic radiograph are age acceptable? (Proceedings)

Interpretation of radiographic findings must take patient age and breed into account. Both cats and dogs have typical or age acceptable juvenile and geriatric findings that should not be assumed pathologic. The following is a partial list of age and breed acceptable thoracic findings.

Juvenile

Both puppies and kittens frequently have a visible thymus, prominent hearts and mild hepatomegaly. The thymic shadow results in a soft tissue opacity in the cranial mediastinum sometimes obscuring the cranial cardiac margin. The prominent heart can raise concern for congenital cardiac disease however alerting the owner that a puppy or kitten can have a prominent heart normally can assuage their fears.

Geriatric

Obesity

Obese dogs and cats have a few typical findings. Fat deposition in the mediastinum typically results in widening to the cranial mediastinum, relative cardiomegaly and pseudo pleural effusion. The cranial mediastinal widening due to fat deposition can be differentiated from a mass typically by looking at the extrathoracic tissues on the V/D projection and the mediastinum on the lateral view. The extrathoracic tissues can be used as a gauge of obesity. Differentiating relative cardiomegaly and pseudo pleural effusion due to fat deposition is fairly straightforward. Because fat and soft tissue have different opacities the soft tissue margin will be visible within fat. This is not true when another soft tissue opacity, such as pleural effusion, is present. Pleural effusion adjacent to the heart will obscure the cardiac margin. True cardiomegaly will not have the visible cardiac margin that is seen with pericardial fat.

Weight loss

With weight loss in cats comes the appearance of hyperinflation or relative hyperinflation. Relative hyperinflation can be distinguished from true hyperinflation by evaluating the degree of soft tissues, or lack thereof, over the lumbar spinous processes. Without realizing the patient was thin one could easily misinterpret relative hyperinflation for the real thing and suspect that the cat has obstructive bronchial disease. This appearance is not recognized in dogs.

The wandering heart

People and cats share two things. 1) Spontaneous hyperthyroidism and 2) elongation of the aorta with age. The aortic elongation results in older cat hearts having a different location/ position. The elongating aorta will commonly have a serpentine path in the caudodorsal thorax and result in the heart having a more horizontal rather than vertical position. On the V/D projection the aorta will result in a soft tissue bulge in the left cranial thorax. This bulge is commonly misinterpreted as a tumor.

Chronic airway changes

Both older cats and dogs sustain chronic airway damage from living in urban settings. These changes manifest on radiographs as bronchointerstitial patterns. The trick in determining if these changes are significant or not is just that. Is the patient coughing? Showing any signs? And how old is the patient. If a 13 year old dog has the lungs of a 13 yr old and is not coughing then these are age acceptable changes. However if a 6 year old cat has the lungs of a 12 year old cat then those are significant changes.

Breed specific changes

Bullldog

Hypoplastic trachea. Cranial mediastinal widening due to fat deposition. This is the most common breed to be affected by an upper airway obstruction.

Bulldog, Boston, Pug

Hemivertebrae

Deep Chest Dogs

Pseudopneumothorax. This occurs because the dependent lung collapses due to atelectasis and the heart falls away the sternum. Relying on the heart "lifting" from the sternum can result in the misdiagnosis of a pneumothorax in deep chested dogs.

Lab heart

Labs and other athletic breeds have a tendency to have rounded hearts and prominent right hearts. This is most commonly due to an athletic change and not due to right sided enlargement or a right atrial mass. However be careful since labs also get tricuspid dysplasia which will cause right heart enlargement.

Small dogs

Relative cardiomegaly. In smaller dogs the normal heart takes up a bigger percentage of the thorax resulting in relative cardiomegaly. This can be difficult to differentiate a normal small dog heart from cardiomegaly particularly when a murmur is present. Separation of the trachea from the spine as you evaluate cranial to caudal is a good indication that cardiomegaly is not present.

This is a partial list of age and breed specific changes. There are others out there.