Small Animal Radiology and Ultrasound: A Diagnostic Atlas and Text

270 Small Animal Radiolo g y and Ultrasono graphy
the rectum. The transverse colon in both animals is located immediately caudal to the stomach
and, when seen on the lateral view in an end-on projection, may appear as a round tissue density
if it is filled with fluid, a round radiolucency if it is filled with gas, or a granular mixture of
fluid and gas if it contains fecal material. On the ventrodorsal radiograph, the descending colon
is usually on the left of the midline, extending caudally along the lateral abdominal wall. On the
lateral radiograph, the colon usually is located in the middle one-third of the abdomen. The
position of the colon varies depending on its content and the amount of retroperitoneal fat that
is present. It may be on the right side in some normal animals and may take an irregular course
through the abdomen when distended with air or filled with fecal material. In some normal
dogs the colon appears tortuous, particularly at the region of the junction between the transverse
and descending colons. The distal colon may be deviated to the right or the left by a distended
urinary bladder, particularly on the ventrodorsal view. The normal colon diameter is
usually no more than 2 to 3 times that of the small bowel. The colon may be completely empty
or filled with fluid and appear as a tissue-dense tube, completely gas filled and appear as a radiolucent
tube, completely distended with fecal material and appear as a tube ranging in density
from tissue to bone, or a combination of the above. Because of this high degree of variability, it
is often difficult to recognize colonic lesions without the aid of special procedures.
The colon can be identified during an ultrasonographic examination; however, most of
its structure is obscured by the highly echogenic nature of its content. 12,18 Gas and fecal
material are so highly echogenic that the details of the colon wall often are obliterated. The
size and position of the colon and the absence of peristalsis enable the ultrasonographer to
recognize it and distinguish it from small intestines. When the colon is empty the mucosal
folds may be visible, resulting in a wrinkled appearance of the wall of the colon. Although
the normal colon wall is approximately 2 to 3 mm in width, it may appear thinner when
the colon is distended. In the transverse plane, the colon usually appears as a semicircle
because the gas within the lumen shadows the far wall. In the longitudinal plane, the colon
appears as an echogenic line. If the colon is fluid filled, the far wall will be evident and the
tubular nature of the bowel can be recognized. The three portions of the colon can be recognized
because of their position within the abdomen.
Urinary Bladder. The urinary bladder, a teardrop-shaped tissue-dense organ, normally is
seen immediately cranial to the pelvic brim (or prostate gland in the male) and varies in
size depending on the amount of urine it contains. The cranial border of the normal urinary
bladder rarely extends cranial to the umbilicus. The degree of urinary bladder distention
usually reflects that the animal has not had the opportunity to urinate, and it is
hazardous to evaluate bladder function based solely on bladder size. When the bladder size
is correlated with the patient’s history (i.e., a markedly distended bladder in an animal that
has just urinated would suggest bladder atony or urinary obstruction), some functional
information can be gleaned from the radiograph.
The urinary bladder is ovoid with a cranial blunt vertex, a rounded middle portion or
body, and a neck that leads to the urethra. In the female dog and in both male and female
cats, the bladder normally has a long, gradually tapering neck. In the male dog, the bladder
neck is shorter, tapers more abruptly, and is closer to the pelvic brim. All of the bladder is
covered by peritoneum except the bladder neck and proximal urethra, which are retroperitoneal.
This is important when evaluating patients for bladder trauma, because rupture of
the bladder caudal to the neck will result in retroperitoneal fluid, while rupture of other
portions of the bladder will produce peritoneal fluid. The bladder should be uniformly tissue
dense; however, air bubbles may be observed in the bladder of a normal dog or cat that
has been catheterized or less often in one that has had a cystocentesis.
Ultrasonography is ideally suited for examination of the urinary bladder. 47 Even a small
bladder not detected by abdominal palpation or radiographs can be identified using ultrasonography.
The anechoic urine contrasts well with the echogenic bladder wall (Fig. 3-22).
Compression of the abdominal wall by transducer pressure frequently distorts the bladder’s
shape. Ultrasonographic artifacts, such as slice thickness and reverberation, can create the
appearance of cellular material within the bladder. The bladder wall can be measured. The
thickness of the bladder wall varies with bladder distention, ranging from approximately 1.5
mm when moderately distended to 3.0 mm when minimally distended. 48 A focal thickening