Small Animal Radiology and Ultrasound: A Diagnostic Atlas and Text

414 Small Animal Radiolo g y and Ultrasono graphy
fragments. 433 Although plant awns are uncommon, they will produce an irregularly shaped
filling defect with distinct margins within the contrast puddle.
Position of the Urinary Bladder. Occasionally, the location of the bladder may not be apparent
on the survey radiograph. This may be due to displacement of the bladder into a hernia, a
very small bladder, or confusion of the bladder with other caudal abdominal masses. 703-707 A
cystogram may be helpful in these cases to clearly identify the location of the urinary bladder.
A pneumocystogram usually is sufficient for this purpose. If the bladder is displaced into a hernia,
the catheter should be passed carefully to avoid traumatizing the urethra. Retroflexion of
the bladder can be recognized from the position of the urethra. The urethra will fold back
upon itself and the vertex of the urinary bladder will be caudal to the bladder neck. Positioning
of the bladder neck within the pelvic canal can be demonstrated during a contrast cystogram.
If the bladder does not displace cranially as it is distended, the possibility of a pelvic bladder
should be considered. The significance of this is controversial, although in some female dogs
it has been associated with incontinence. 686-688 Incontinence is found frequently in patients
with no morphological abnormalities identifiable by survey or contrast radiography. 708-713
Ultrasonography of the Abnormal Bladder. Ultrasonography works best when the bladder
is distended with urine, is within the abdominal rather than the pelvic cavity, and is not
obscured by intestinal contents. It is ideally suited for evaluating the bladder wall and
lumen. 47,48,714-723
The bladder can be differentiated easily from other caudal abdominal masses such as
the prostate, a retained testicle, a lymph node, or the uterus. On the other hand, it may be
difficult to distinguish between the urinary bladder and a paraprostatic cyst or other urogenital
structures. 704-707 A careful ultrasonographic examination, however, usually identifies
the cyst as a distinct structure separate from the urinary bladder that can be traced to
the region of the prostate gland. A uterus filled with fluid may also mimic a urinary bladder;
however, identifying both uterine horns, multiple fluid-filled loops, or recognizing the
tubular shape of the uterus will help discriminate among these structures. If necessary, a
contrast cystogram can be used to confirm sonographic suspicions about the relationship
of caudal abdominal cavities to the urinary bladder.
The shape of the bladder is influenced by transducer pressure and by regional
structures beneath or against it as well as from distortion of normal bladder wall
structures. 528,529,692,717 Consequently, a careful examination of the bladder from all angles
is important before a diagnosis of an abnormally shaped bladder is made. True bladder
shape alteration usually is accompanied by bladder wall abnormalities, such as thickening
or irregularity, so the diagnosis is less difficult.
Cystitis, bladder wall hemorrhage, and tumors produce similar bladder wall abnormalities
(Fig. 3-211). 724 Cystitis may involve the entire bladder circumference and may produce
a thickened bladder wall with a submucosal hypoechoic zone (Figs. 3-212 to 3-214).
More often, cystitis produces a focal bladder wall thickening at the cranioventral aspect of
the bladder. A urachal diverticulum may be identified during the ultrasonographic examination
as a focal defect in the bladder wall. The wall usually is thickened and irregular and
the anechoic urine helps to define the defect. Most urachal diverticula are small, and
detecting them using ultrasonography and distinguishing them from cystitis are difficult.
The fibrotic remnant of the closed urachus has been identified as a heteroechoic structure
extending cranially from the bladder wall. This probably represented an abnormal urachus
that was not patent at the time of the examination.
Usually, tumors can be detected easily as heteroechoic structures protruding into the
bladder lumen (Figs. 3-215 and 3-216). The attachment of the mass to the bladder wall is
often abrupt, and thickening of the bladder wall at the site of attachment usually can be
recognized. 718 Differentiating between an attached blood clot and a bladder tumor may be
impossible. 717
A focal area of bladder wall thickening may be observed at the trigone of the bladder. This
is the ureteral papillae and should not be mistaken for a focal mass or polyp. Careful examination
using high-frequency transducers permits identification of the ureter within the bladder
wall or allows detection of the jet of urine that enters the bladder from the ureter (see
Figs. 3-22 to 3-24).