Small Animal Radiology and Ultrasound: A Diagnostic Atlas and Text

28 Small Animal Radiolo g y and Ultrasono graphy
Fig. 2-4 A 2-year-old female Persian
cat with anterior uveitis. A, In the initial
ventrodorsal thoracic radiograph
the cardiac silhouette is shifted into
the left hemithorax. This is due to
malpositioning with rotation of the
sternum to the left. Note that the dorsal
spinous processes are angled to
the right (arrows). B, The ventrodorsal
thoracic radiograph was repeated.
The cardiac silhouette is in its normal
position. The dorsal spinous
processes are centered on the thoracic
vertebrae (arrows). Diagnosis:
Normal thorax.
A
B
num. An apex shift to the right will accentuate or mimic an apparent right heart enlargement;
a shift to the left will minimize the apparent size of the right ventricle. 15
The forelimbs should be pulled cranially to avoid superimposition of their density over
the cranial lung lobes, which will increase cranial lung lobe and mediastinal densities. This
can create or obscure pulmonary or mediastinal lesions (Fig. 2-5).
The best radiographs for thoracic evaluation are those in which the animal is most
comfortable and therefore can be positioned symmetrically. Once right or left lateral, ventrodorsal,
or dorsoventral radiographs are obtained, those positions should be repeated on
all subsequent examinations.
R E S P I R AT O RY P H A S E
Obtaining a radiograph at peak inspiration provides optimal contrast, detail, and visibility
of thoracic structures. Although many animals spontaneously will inspire maximally, some
will not. It may be necessary to interfere with respiration momentarily by obstructing the
animal’s nares and closing its mouth. This stimulates a deep inspiration when the obstruction
is removed.
The increased pulmonary density seen at expiration can mimic the appearance of pulmonary
disease. The pulmonary vessels are shorter, wider, and less sharply defined at expiration
than at inspiration. The cardiac silhouette appears relatively larger at expiration
because it is surrounded by less aerated lung. The caudal vena cava may appear wider and
may be defined less clearly at expiration (Fig. 2-6).
The difference between inspiration and expiration must be recognized due to the artifacts
created by an expiratory radiograph. 16 Compared with the lateral radiograph exposed
at full expiration, the lateral radiograph exposed at full inspiration has:
1. Increased size of the lung lobes cranial to the cardiac silhouette
2. Slight elevation of the cardiac silhouette from the sternum
3. Extension of the pulmonary cupula cranial to the first rib
4. Increased ventral angulation of the trachea
5. A flatter diaphragm
6. Greater separation of the diaphragm from the caudal cardiac margin