Small Animal Radiology and Ultrasound: A Diagnostic Atlas and Text

Chapter Three The Abd omen 357
Anesthesia or sedation may be used to help in positioning and manipulating the patient
for radiography. Animals that are tranquilized or anesthetized may still retch or vomit following
contrast administration. Although not proven, there appears to be a reduced incidence
of post–contrast administration vomiting if tranquilizers are used.
Immediately after contrast injection, ventrodorsal and lateral radiographs should be
taken. Lateral and ventrodorsal radiographs should be taken 5, 20, and 40 minutes after
injection. The sequence and timing of the radiographs are determined by the information
provided by the urogram. If the position of the kidneys are the only information desired, a
single radiograph taken immediately following contrast administration may be sufficient. If
the position of the ureters relative to the urinary bladder is of interest, then several radiographs
may be required. Some authors recommend the routine use of abdominal compression
to block the ureters, interfere with ureteral peristalsis, and distend the renal
collecting system. 139 We do not recommend abdominal compression because of the variability
it introduces into the study and the lack of any proven efficacy to facilitate diagnosis.
In patients whose renal function is too poor to yield an adequate urographic study, a second
or third intravenous injection of contrast material can be administered. 414 At our current
state of knowledge, we don’t recommend total dosage (by one or more injections) exceeding
1760 mg iodine per kg of body weight (800 mg/lb of body weight). 421,427 Although there
is some degree of correlation between serum urea nitrogen level and the quality of the EU,
azotemia should not be considered as an absolute predictor of a poor-quality EU. 429
Excretion of the contrast material will affect the urinalysis, resulting in an increase in
urine specific gravity, false-positive urine protein determinations, alteration in cellular
morphology, and the presence of unusual-appearing crystals. 430,431 Growth of some bacterial
species is inhibited by the presence of contrast within the urine. 432 Because of these factors,
and because a higher contrast dosage may be required if the urine specific gravity is
low, a urinalysis and urine culture should be obtained before administration of the contrast
material.
The contrast study is complete when the questions raised by the history, physical examination,
or initial radiographs are answered. This may require a single radiograph or multiple
radiographs obtained over a long period of time. However, because the patterns of
nephrographic opacity have diagnostic utility and they may even signal the onset of contrast
medium–induced hypotension, the regular sequence described above is suggested as
a general operating procedure. Combining the EU with a pneumocystogram may facilitate
the diagnosis of ectopic ureters, but the use of this combination is a matter of personal
preference. Although not documented in animals, contrast material may increase the echo
intensity of the kidneys, and the diuresis that results from contrast administration may
cause ureteral dilation that could be mistaken for mild hydronephrosis during an ultrasonographic
examination. For that reason, the ultrasonographic examination usually precedes
the contrast study or should be performed the next day.
C Y S T O G R A P H Y
Examination of the bladder may be accomplished by several special procedures such as the
positive-contrast cystogram, pneumocystogram, or double-contrast cystogram. 433-438 For
these procedures, it is important to ensure that the colon and small bowel are not filled
with fecal material. Food deprivation and the administration of laxatives at least 12 hours
prior to the study are advised. It is usually necessary to perform one or more enemas prior
to the study to evacuate the distal colon. Allow sufficient time for the colon to empty,
because a fluid-filled colon may distort the shape of the bladder.
Complications resulting from cystography are rare. Trauma to the bladder or urethra
may result from catheterization—infection may be introduced into the bladder if the
catheterization is not performed carefully. Rupture of the urinary bladder should not occur
if the bladder is inflated slowly and inflation is stopped when the distended bladder can be
palpated or if the animal shows evidence of pain or discomfort during inflation. On rare
occasion the urinary catheter may become kinked and unable to be removed. 439
If ultrasonographic examination of the bladder is expected, this should precede cystography.
Air within the bladder will interfere with ultrasonographic examination, and
bubbles within the contrast may also degrade the ultrasonographic image.