Small Animal Radiology and Ultrasound: A Diagnostic Atlas and Text

Chapter Three The Abd omen 253
compensate for the decrease in abdominal thickness caused by the compression. A 50%
reduction in mAs is the norm. This technique will result in a radiograph with the structure
of interest relatively isolated from other organs that might have been creating confusing shadows.
Another method of isolating relatively immobile abdominal organs radiographically is
to position the animal so that gravity pulls the relatively freely moveable organs away from
the area of interest. In facilities that have a table that tilts, this is done by securing the patient
to one end of the table and then tilting it up 20 to 30 degrees from horizontal.
A view that may be useful in cases in which free peritoneal air is suspected is the left lateral
decubitus. For this view, the animal is positioned in left lateral recumbency, and a radiograph
is taken with a horizontally directed beam centered on the cranial abdomen.
Normally, in this view, there will be liver lobe or fat seen adjacent to the diaphragm. If free
air is present in the abdominal cavity, it will be trapped above the liver immediately below
the diaphragm and right lateral abdominal wall.
Ultrasonographic examinations may be performed with the animal in dorsal or lateral
recumbency. The choice usually is determined by the experience and training of the ultrasonographer.
Some patients are more comfortable in lateral recumbency and therefore that
position can be used. For animals that resist both lateral and dorsal recumbency, the examination
may be accomplished with the animal standing. Moving the animal and repeating the scan
is helpful to document a change or lack of change in position of a lesion. This can help to distinguish
bladder tumors, which remain fixed in location, from blood clots, which are usually
moveable. Elevating the head- or tail-end of the table may be useful for increasing access to the
liver in the former case, and to the bladder and prostate in the latter instance. Rectal palpation
may be used to displace the prostate out of the pelvic canal, making it more easily examined.
T E C H N I C A L E VA LUAT I O N O F T H E R A D I O G R A P H
Abdominal radiographs first must be evaluated for technical adequacy (positioning and
exposure) before attempting to interpret the images for pathologic change. The radiographs
should be exposed at the end of expiration to minimize the crowding and bunching of structures.
Expiration can be evaluated by the position of the diaphragm on the lateral view. In full
expiration, the diaphragmatic crura should cross the spinal column at approximately the
eleventh thoracic vertebra. The lateral view is assessed for rotation by determining if the lateral
processes of the lumbar vertebrae and the ilia of the pelvis superimpose. The ventrodorsal
view is evaluated by checking whether the dorsal spinous process bisects the vertebral
body and whether the lateral vertebral processes are identical in appearance. Occasionally,
when taking the ventrodorsal view, a dog will twist or curl its body to one side. If this occurs,
a repeat radiograph may be needed to eliminate this positioning artifact. Good radiographic
technique will yield a radiograph that is neither too light nor too dark but one that readily
differentiates the various densities. In some deep-chested dogs it may be necessary to use different
radiographic techniques for the wider cranial as opposed to the narrower caudal
abdomen. Presuming that these criteria are met, the radiographs may be evaluated. Both ventrodorsal
and lateral views should be displayed simultaneously side by side so that any suspected
change can be correlated on both projections.
R A D I O G R A P H I C D E TA I L
One can recognize various abdominal structures because of the differences between their
radiographic densities. Whenever two structures of the same density touch, their margins will
be obscured, and whenever two structures of different densities touch, a margin will be visible.
The greater the difference in density (e.g., air against soft tissue versus fat against soft tissue)
the more distinct the margin will be. The major contribution to abdominal detail is the difference
in density between the retroperitoneal, mesenteric, and omental fat, and the soft tissue– or
fluid-dense viscera. Poor abdominal detail (a lack of contrast) may occur for several reasons. A
cachectic animal will have poor abdominal detail, both peritoneal and retroperitoneal, and minimal
soft tissue dorsal to the spine. The ventral midline will be displaced dorsal to its normal
position, giving a “tucked-up” appearance. Juvenile animals usually exhibit poor abdominal
detail, presumably because of a lack of abdominal fat or the presence of a small amount of peritoneal
fluid (Fig. 3-5). 4 The presence of fluid within the peritoneal cavity will obscure visceral
detail and, depending on the volume of fluid, will produce abdominal distention (Fig. 3-6). 5