The Large Bowel

Imaging Options for Large Bowel Disease

Both survey and contrast radiographic procedures can be used to assess the colon.1–3 Now, however, endoscopy has largely replaced radiographic contrast studies of the colon with the additional advantage of obtaining aspirates and biopsies if needed.⁴

Ultrasound is a sensitive and practical modality that is less time consuming than most radiographic contrast studies of the colon. It also provides information complementary to endoscopic and survey radiographic findings.⁵ Although air and feces in the bowel limit the usefulness of colon sonography, near-field bowel wall thickness and symmetry, mural and extramural bowel masses, regional lymph nodes, and intussusceptions can be assessed. Transabdominal cytologic sampling of colon masses can also be obtained with ultrasound-guided techniques.^6,7

Less commonly used techniques for examining the colon include rectocolonic lymphangiography, mesenteric angiography, and colonic transit scintigraphy. These techniques enable assessment of anatomic or functional abnormalities but require specialized equipment and expertise.8–10 Computed tomography (CT) is an excellent modality to assess the pericolonic and perirectal areas, particularly for the pelvic canal.¹¹

Normal Radiographic Anatomy

The large bowel of the dog and cat is composed of the cecum, colon, rectum, and anal canal (Fig. 45-1). The cecum, a diverticulum of the proximal colon, has different anatomic and radiographic appearances in the dog and the cat (Fig. 45-2).¹ The canine cecum appears semicircular and compartmentalized and normally contains some intraluminal gas. The cecum joins the colon through a cecocolic junction. The intraluminal gas and characteristic shape enable recognition of the cecum in the right midabdomen on most survey radiographs. The feline cecum is usually not visible on survey radiographs. It is a short, conelike diverticulum of the colon with no distinct cecocolic junction and no compartmentalization. The feline cecum rarely contains gas or feces.

Fig. 45-1 Survey lateral **(A)** and ventrodorsal **(B)** radiographs of a normal canine abdomen. The large bowel is divided into the cecum *(1),* ascending colon *(2),* transverse colon *(3),* descending colon *(4),* right colic flexure *(5),* left colic flexure *(6),* rectum *(7),* and anal canal. Note the admixture of gas and feces in the cecum, colon, and rectum.

The colon of the dog and the cat is a thin-walled distensible tube that is divided into ascending, transverse, and descending parts. These divisions are recognized easily on survey radiographs based on shape, size, and location. The distal ileum enters the ascending colon from a medial direction by way of the ileocolic sphincter. This circular sphincter is not visible on survey radiographs, but it can be identified as a filling defect when barium is present in the colon adjacent to the sphincter.

The shape of the colon is similar to that of a question mark or a shepherd’s crook (see Fig. 45-1). The junction between the ascending and transverse colon is the right colic flexure, and the junction between the transverse and descending colon is the left colic flexure. The ascending colon and right colic flexure are to the right of midline. The transverse colon passes from right to left cranial to the root of the mesentery. The left colic flexure and proximal descending colon are to the left of midline. The distal descending colon courses to the midline and enters the pelvic canal to become the rectum. The rectum is the terminal portion of the colon, beginning at the pelvic inlet and ending at the anal canal.

An understanding of the anatomic relation of the large bowel to other viscera is important for the radiographic recognition of diseases of the large bowel and adjacent organs (Fig. 45-3).

Fig. 45-3 Viscera adjacent to the large bowel may cause a change in position of a portion of the colon. This change in position may be indicative of disease or a variant of normal, depending on the cause of the deviation (e.g., enlarged bladder versus enlarged medial iliac lymph nodes). *Arrows*, Usual direction of large bowel position displacement when an organ enlarges (also see Fig. 45-10). (Reprinted from O’Brien TR: *Radiographic diagnosis of abdominal disorders in the dog and cat,* Davis, CA, 1981, Covell Park Veterinary.)

• The ascending colon lies adjacent to the descending duodenum, right lobe of the pancreas, right kidney, mesentery, and small bowel.

• The transverse colon lies adjacent to the greater curvature of the stomach, left lobe of the pancreas, liver, small intestine, and root of the mesentery.

• The proximal descending colon lies in close proximity to the left kidney and ureter, spleen, and small bowel. The right ureter travels directly adjacent to the colon wall in the mesocolon toward the bladder neck.

• The midportion of the descending colon lies adjacent to the small bowel, urinary bladder, and uterus. Because it is less fixed, the midportion of the descending colon has a variety of normal positions in the caudal left abdomen. In some dogs, the descending colon is positioned along or slightly right to the median axis of the body. Such normal variations are caused by various amounts of ingesta within the bowel, intraabdominal fat, and urinary bladder distention (Fig. 45-4). Some dogs appear to have an excess of length of colon. This finding, called redundant colon, is a variant of normal and is not clinically significant.^1,3,3

Fig. 45-4 Ventrodorsal radiograph of a dog with a distended urinary bladder that resulted in displacement of the descending colon to the right *(white arrow).* This is a common malpositioning and is not clinically significant.

• The distal portions of the descending colon and rectum are also closely associated with the urethra, the medial iliac, hypogastric and sacral lymph nodes, the prostate or uterus and vagina, and the pelvic diaphragm.

Radiographic Techniques of Large Bowel Evaluation

Survey Radiography

Because feces and gas produce contrasting radiographic opacities and are usually present in the large bowel, some or all of the large bowel is identifiable on survey radiographs of the abdomen. Normal large bowel content usually has a characteristic pattern of fine and evenly distributed gas bubbles, which is helpful in differentiating the colon from small intestinal loops and abnormal conditions of the large bowel. When present, mineral- or metal-opaque foreign bodies are recognized easily. Neither the wall thickness nor the mucosal pattern of the large bowel can be evaluated from survey radiographs.

When the large bowel is evaluated radiographically, the entire abdomen and pelvic area must be included on orthogonal radiographic views. Rectal examination, vigorous abdominal palpation, aerophagia from restraint and struggling, and enema administration before survey radiography may increase the amount of gas or fluid present within the colon and in other parts of the gastrointestinal tract. Although an abnormality in position, size, or shape of the large bowel may be seen on survey radiographs, it may not be a significant finding.

Compression Radiography

Compression radiography of the abdomen is a simple technique that may help clarify the presence of a lesion. When the abdomen is compressed with a wooden or plastic spoon or paddle, bowel or masses adjacent to the large intestine are displaced or compressed, which enhances radiographic conspicuity (Fig. 45-5). More definitive radiographic evaluation of the large bowel usually requires a contrast study with barium sulfate suspension (barium enema), air (pneumocolon), or a combination of barium sulfate suspension and air (double-contrast study). There is a risk of rupturing masses or hollow viscera with this technique, and it should be used with caution.

Fig. 45-5 A, Lateral radiograph of a cat with a uterine stump pyometra *(black open arrows)* interposed between the descending colon and urinary bladder. B, Survey lateral radiograph of the abdomen during lateral compression of the caudal abdomen with a compression paddle. The mass appears fixed and separate from the descending colon and urinary bladder. C, Survey lateral radiograph of the abdomen after a pneumocolon performed by retrograde introduction of gas. The soft tissue mass is visualized as an extramural mass. Feces were not removed before the contrast study was conducted.

Barium Enema

Barium enema findings in large bowel disease include (1) irregularity of the barium/mucosa interface, (2) spasm of the bowel lumen, (3) partial or complete occlusion of the bowel lumen, (4) outpouching of the bowel wall from a hernia or diverticulum, (5) displacement of bowel, and (6) perforation with peritonitis. Unfortunately, the barium enema findings are usually nonspecific. Although spasm and mucosal irregularity are commonly associated with severe local inflammation, other causes include toxicity, reflex mechanism, and idiopathic factors. Bowel inflammation may occur with generalized or regional areas of bowel wall thickening from edema and small ulcerations. There are frequently no abnormal findings in the acute stage of bowel inflammation.

A barium enema is indicated when (1) narrowing of the lumen prevents passage of an endoscope; (2) limitations of the endoscope prevent examination of all the colon and cecum; and (3) a mural or extramural lesion is suspected, but the mucosa is normal endoscopically.⁴ Survey radiographs should always be made before the contrast study. For a high-quality barium enema, the colon should be cleansed thoroughly. This is best done by withholding food for 24 hours followed by a warm-water enema. The colon should be free of fecal material with a clear effluent visible on the enema performed immediately before the study. Generally, the radiographic technique should be increased by 6 to 8 kVp over the survey technique when barium is used. Although the techniques can vary, barium is administered at room temperature through an inflatable cuffed catheter in the distal rectum.^1,12–14 General anesthesia is almost always necessary. Micropulverized barium suspension is the contrast medium of choice for obtaining a smooth coating of the mucosal surface. The colon should be filled slowly by gravity, preferably with fluoroscopic observation. Because fluoroscopic equipment may not be available, and the volume of barium needed to fill the colon is variable, the contrast medium should be given in small increments until the desired effect is seen radiographically. The approximate barium dosage is 7 to 15 mL per kilogram of body weight. Multiple radiographic views, left lateral, ventrodorsal, right ventral–left dorsal oblique, and left ventral– right dorsal oblique, should be made when the colon is distended with barium and again after evacuation of the barium from the colon. The detection of subtle mucosal lesions may be enhanced by a double-contrast study. In most instances, this is done by removing as much of the barium as possible and then inflating the colon with room air through the catheter.

When distended with barium, the normal colon has a smooth contrast medium/mucosa interface and a uniform diameter. After evacuation of the barium, longitudinal mucosal folds are visible. If air is then infused, a double-contrast study is obtained, which provides the most detailed visualization of the mucosal surface.

A variety of radiographic appearances result from adherence of barium to mucus, clumping and flocculation of barium, and filling defects of feces that are either within the lumen or attached to the wall. The colon of the dog and the cecum and colon of the cat have lymph follicles in the mucosa, which can appear as spicules on a barium enema study or as pinpoint radiopacities when visualized en face with a double-contrast study. These normal follicles must be differentiated from small ulcers.

The large bowel cannot be evaluated properly after oral administration of contrast medium because large bowel luminal distention is inadequate and there will be intraluminal filling defects from ingesta carried aborally with the barium.

Barium enemas are time consuming and must be done meticulously to assess the mucosa, wall, lumen, and adjacent viscera and to avoid artifacts, complications, and technical failures. Partial large bowel contrast studies, which are less thorough, quicker, and easier, may be performed with the introduction of small amounts of air or barium into the rectum using a dose syringe. These studies do not allow visualization of the entire large bowel or of small lesions, such as mucosal irregularities; however, they may enable visualization of large intraluminal lesions and differentiation of the colon from adjacent organs and masses (Fig. 45-5, C).