Page 270 - Veterinary diagnostic imaging birds exotic pets wildlife

Chapter 23

Rats

III INJURIES rotates on its vertical axis, further altering the appear-
ance of the cardiac silhouette. Under the described

Most injuries to rats and other pet rodents are the conditions, it can be very difficult to distinguish lung
result of being accidentally stepped on. Crush injuries consolidation from collapse, in the normally vague
occur occasionally and can be quite serious, especially cranioventral region of the thorax.

if the chest or diaphragm is involved. Attacks by other There are other interpretive difficulties as well. For
pets, particularly cats and small dogs, can be fatal. example, in the VD view the width of the cranial medi-
astinum can be up to four times greater than the cranial
mediastinum of a cat. Also, in the VD view, the right
III THORACIC DISEASE half of the heart often appears overly large and conical,
appearing to lie abnormally close to the right chest
wall, especially if there is right-sided obliquity.
Cardiopulmonary Disease and Related
Interpretive Diffi culty When the gastrointestinal tract is distended with
ingesta, the diaphragm is forced forward, and typi-
We probably do more cardiopulmonary examinations cally assumes a near-vertical position as seen previ-
on pet rats than any other rodent, and these are studies ously in Figure 23-2. This creates the illusion of an
that I often fi nd difficult to interpret. In the ventrodor- enlarged heart because of the less visible background

sal (VD) view, I sometimes have diffi culty deciding if lung; this phenomenon is also known as an increased
the heart is enlarged or simply projected in a nonstan- cardiac-thoracic ratio.
dard manner because of obliquity, making it seem
abnormal (Figure 23-1). In the lateral view, I often fi nd Mediastinal Disease
it hard or impossible to clearly see the cranial border
of the heart, making it impossible to accurately measure Rats can develop mediastinal disease, which most
heart length (Figure 23-2). Apparently, this is common, often takes the form of a cranial mediastinal mass. The
as evidenced by other authors publishing normal majority of these are malignant tumors, and some of
lateral radiographs of rats with color overlays mark- these tumors may become as large as the heart, making

ing the otherwise obscure cranial and caudal heart it difficult to distinguish between the two. The visual
borders. impression of two heartlike objects in the thorax has
Another interpretive problem with thoracic radiol- been termed the double heart sign (Figure 23-4). In such
ogy in rats (and most other small mammals) relates to situations, the heart can usually be distinguished from
the gas anesthesia often used for restraint. Specifi cally, the contiguous mass by following the trachea in the
the lowermost lung in an anesthetized animal gradu- dorsoventral projection, which should terminate at the
ally collapses, while its contralateral lung expands, a level of the heart base.
process termed postural atelectasis. The resulting infl a-
tionary imbalance causes the heart to shift toward the Abdominal Disease
partially collapsed half of the lung, which is termed
mediastinal shift, or more precisely, a cardiac shift (Figure Abdominal Distention. Rats, like other small pet
23-3). rodents, regularly develop abdominal distention,
In the process of being displaced from the center of which is not a disease but may indicate the presence
the thorax to either the right or left side, the heart also of one. Most instances of abdominal distention are
266

2/11/2008 11:11:12 AM
ch023-A02527.indd 266 2/11/2008 11:11:12 AM
ch023-A02527.indd 266