Imaging of the Gastrointestinal Tract
Martha Moon, DVM, MS, Dipl.ACVR
The stomach and intestines can be imaged using several different imaging modalities. Each has advantages and disadvantages, and more than one imaging technique may be needed to make a final diagnosis.
Radiology of the stomach
The stomach varies in appearance depending on contents and patient position. On lateral radiographs, the axis of the stomach, from the dorsal fundus down to the ventral pylorus, is perpendicular to the spine, parallel to the ribs, or somewhere in between. The pylorus is located in the cranioventral abdomen, caudal to the liver, and may occasionally be superimposed over the gastric body . In right lateral recumbency, the pylorus may be fluid filled, mimicking a mass or foreign body. On ventrodorsal (VD) views, in the canine, the cardia, fundus, and body of the stomach are typically located to the left of midline, with the pylorus to the right; the stomach may be perpendicular to the spine, or “U” shaped. The feline has more of a “J” shaped stomach, with the pylorus located on the midline. Positioning affects the appearance of the stomach due to different distributions of fluid and gas. In right lateral recumbency, gas is present in the fundus and body, while fluid accumulates in the pylorus. In left lateral recumbency, the opposite occurs, with gas located in the pylorus and fluid in the body and fundus. In dorsal recumbency (VD view), gas accumulates in the pylorus, while fluid fills the fundus and body. Finally, on DV views, gas will be located in the fundus and body, and fluid in the pylorus. Gastric wall thickness is not accurately determined on most survey abdominal radiographs.
Radiology of the small intestines
In the canine, normal small intestinal diameter should not exceed the height of the midbody of L2, or twice the width of a rib. The cat has a more uniform body size, so a diameter of 12 mm is fairly consistent for upper limits of bowel diameter. The duodenum in both species may be slightly larger in diameter than the rest of the small bowel. As in the stomach, wall thickness is not reliably assessed on plain radiographs.
Ileus is a failure of intestinal contents to pass through the bowel lumen, and may be due to lack of peristalsis (paralytic, adynamic, or functional ileus), or due to obstruction (obstructive, mechanical, or dynamic ileus). Paralytic ileus results in normal or mildly dilated gas filled bowel loops, and is usually generalized. Peritonitis, normal post-op abdomen, pain, enteritis, systemic infection, metabolic change, anticholinergic drugs, hypokalemia, and spinal trauma have all been reported to result in paralytic ileus. Occasionallly, focal paralytic ileus can occur with pancreatitis or thrombosis of segmental mesenteric artery. Obstructive ileus results in more dramatically dilated bowel loops, and only those segments orad (proximal) to the obstruction will be affected. A low obstruction will cause more diffuse bowel dilation, however. The obstruction may be due to foreign body, neoplasia, stricture, granuloma, hernia, adhesions, or intussusception. The actual cause of the obstruction, such as foreign body, may not be visualized radiographically, but surgery is indicated if an obstructive pattern is present. In some cases, parvoviral enteritis (a paralytic ileus) will mimic obstructive ileus, with generalized, more dramatic bowel dilation. Mesenteric volvulus may also result in generalized gaseous distension of bowel, resembling obstructive ileus. Chronic obstructions (partial) usually result in fluid distended bowel loops.GI contrast techniques
An upper GI series is occasionally necessary for further diagnosis or evaluation of stomach or small intestine. Because of the time involvement and expense, the GI series should be done properly to get as much information as possible from the study. The choice of oral contrast material depends on the patient and suspected diagnosis. For routine studies where GI perforation is not suspected, barium is the agent of choice. Commercial preparations are preferred over the powdered agent which must be mixed with water. Most barium suspensions come as 60% w/v solutions. The choice of full strength or diluted with water to make a 30%w/v solution is up to the individual. Oral iodine products are available for use in patients in which a bowel perforation is suspected. These iodinated agents are hypertonic, and will draw fluid into the stomach and bowel after oral admiistration, resulting in dilution of contrast, and possible dehydration and electrolyte imbalance in the fragile patient. If aspirated, pulmonary edema may result. However, the iodinated agents are safe if free in the peritoneal space. They also traverse the bowel more rapidly than barium. Nonionic contrast agents (iopamidol, iohexol), typically used for myelograms, are very safe for upper GI series, as they are less hypertonic than the regular ionic iodinated contrast agents. They cause no reaction if leaked into the peritoneal space, and minimal reaction if aspirated. They also give a much better mucosal coating than the regular oral iodinated contrast agents. The disadvantage to the non-ionic contrast agents is their expense, which mostly limits their use to cats and small dogs.
Transit time in the dog varies, but barium contrast material should begin to enter the duodenum within 15 to 30 minutes. Normal small bowel transit time is three to five hours. The stomach should be empty, or have only a minimal coating by the time that most of the contrast is in the colon. Transit time is faster in the cat, and contrast material may reach the colon in 30 minutes. Average transit time is one hour, although in some cats it takes three to four hours to reach the colon. When organic iodide contrast agents are used, transit times are much more rapid. Contrast may reach the colon within an hour in dogs, and 30 minutes in the cat. These agents can be useful in quickly determining the patency of the bowel lumen. Abnormally shortened transit times may indicate an inflammatory condition of the bowel and have been associated with acute enteritis. Delayed transit times associated with normal bowel diameters are consistent with previous administration of anticholinergic drugs or insufficient contrast dose (insufficient volumes to stimulate peristaltic activity). If transit times are delayed in the face of dilated bowel lumen, an obstruction should be suspected.
Each contrast-filled segment should be evaluated for an abnormal mucosal pattern, wall thickness, luminal diameter, and location within the abdomen. To be considered significant, a lesion should be visible on serial radiographs, because changes associated with normal peristalsis can mimic strictures or filling defects. Transient peristaltic contractions are normal and are especially prominent in the feline duodenum, giving rise to the "string of pearls" appearance.
The mucosal surface should be smooth to slightly fimbriated, depending on the degree of small bowel distention. Smudging of the contrast-mucosa interface, along with flocculation of barium indicates mucosal inflammation, edema, and increased mucous production and can be seen in patients with enteritis. This might be accompanied by a very rapid transit time, and increased segmentation of the bowel.
Variations in lumen size and mucosal surface indicate a possible mass lesion in the bowel. Positive contrast should allow evaluation of the location and extent of the mass lesion. Intraluminal masses show up as lucent filling defects in the contrast column and can be pedunculated or sessile. Intramural masses vary from intraluminal protrusions to diffuse thickening and rigidity of the bowel wall. They can cause eccentric or annular narrowing of the bowel lumen, along with partial obstruction. Smooth masses tend to be benign in nature, while those with irregular mucosal surfaces are more likely to be malignant. The "apple core" sign is an irregular, annular thickening indicative of intramural neoplasia. The most common intestinal tumors reported are adenocarcinoma, leiomyosarcoma, leiomyoma, and lymphosarcoma. Lymphosarcoma tends to be multicentric, while other neoplastic processes are more localized. Other differentials for intramural lesions include granuloma, scar tissue and adhesions, abscess, and hematoma.
While neoplastic masses can cause partial or complete intestinal obstruction, a more common cause is an obstructing foreign body. A radiopaque foreign body might be seen on survey radiographs, along with accompanying signs of obstruction such as intestinal dilation. However, in some cases, administration of contrast material is necessary to diagnose the obstruction and to differentiate obstructive ileus, caused by luminal obstruction, from functional ileus caused by impaired intestinal motor fuction. In animals with incomplete obstruction, the barium should outline the foreign body as it passes around the site. With complete obstruction, the contrast column stops abruptly at the point of obstruction. Another radiographic sign of obstruction is dilation of the proximal bowel loops. Dilation greater than 50% of normal luminal diameter is abnormal. Fluid present in the dilated segment might dilute the contrast agent. If the obstruction is located in the duodenum or proximal jejunum, there may be no proximal dilation because the duodenal contents can be relieved by vomiting. It is important to note that if no foreign body is visible radiographically but a classic obstruction pattern is present, a contrast series is not necessary to locate or identify the foreign body. Surgery would be the logical next step.
Linear foreign bodies have a characteristic radiographic pattern of their own and are often diagnosed on survey radiographs. However, if the diagnosis is in question, a positive contrast agent may be given to further evaluate the intestines. There should be a careful search for peritonitis in animals with suspected linear foreign bodies, as barium is contraindicated if intestinal rupture has occured. Characteristic radiographic signs of a linear foreign body include plication of the small intestines, centralization of the bowel, and eccentric, comma-shaped intestinal gas bubbles. If these signs are observed, surgery to remove the foreign body should be performed as soon as possible to prevent rupture or laceration of the intestines.
Ultrasound of the GI tract
Ultrasound of the gastrointestinal tract has become an increasingly popular diagnostic procedure for evaluation of GI obstructions, mass lesions, and infiltrative and inflammatory bowel diseases. Although the stomach and bowel may be incompletely evaluated because of intraluminal gas and resultant shadowing, GI wall thickness and integrity of layers, peristaltic activity, and bowel contents can often be imaged.
Normal GI anatomy
The stomach is located just caudal to the liver. Often, rugal folds can be seen, especially if the stomach is fluid-filled. With high frequency transducers, 5 layers of the stomach may be visualized. From outer to inner, these layers are:
With less resolution, only 3 layers may be seen; hyperechoic outer serosal layer, hypoechoic mucosa, submucosa, and muscularis, and hyperechoic mucosal surface.
Normal gastric wall thickness in the dog is 3-5 mm. In the cat, normal gastric wall thickness is 2-3mm. Four to five contractions/minute should be seen with normal stomach peristaltic activity.
The small intestines are tubular hypoechoic structures with a central echogenicity (mucosal stripe), which represents luminal contents of mucus and gas. The duodenum can be traced from the stomach in the right cranial abdomen. The descending duodenum is usually the most lateral and ventral bowel loop along the right cranial abdominal wall. The rest of the small bowel fills up the remaining abdomen between stomach and bladder. The small intestines have the same 5 bowel wall layers as the stomach, and are best seen with high frequency transducers. If the lumen is gas-filled, prominent shadowing is present, allowing visualization of only the near intestinal wall. If fluid-filled, the lumen is hypoechoic, although the presence of fluid in the lumen is infrequent. Standards for normal duodenal wall thickness are less than or equal to 4.7 and 5.5 mm for dogs < 20 and > 20 kg respectively. Jejunal wall thickness is considered normal if less than or equal to 4.2 and 4.7 mm in dogs < 40 and >40 kg respectively. In cats, duodenal wall thickness is considered normal if 2.7mm or less, and jejunual wall thickness is normal if less than or equal to 2.1mm. About 1-3 peristaltic contractions are noted per minute. The colon can be located caudal to the stomach, and usually is gas filled, with prominent shadowing. The colon has a slightly smaller wall thickness than the small bowel in both the dog and cat.
Abnormal GI ultrasound
Increased thickness of gastric or intestinal wall, and/or loss of normal layered appearance is the most common ultrasound sign of GI disease, and should be carefully evaluated. Small bowel dilation may be an indication of obstruction due to either neoplasia or foreign body. Small intestinal obstruction is characterized by dilated bowel loops proximal to the obstruction. They may be fluid-filled, especially in chronic obstruction. Peristaltic activity is usually increased in the acute obstruction, but may be decreased with chronic obstruction or functional (paralytic) ileus. Foreign bodies occasionally can be visualized, depending on their physical properties. A curvilinear shadowing in the stomach is highly suggestive of a ball foreign body, but could also represent gas or ingesta. Rocks, needles, and some balls attenuate the sound beam, resulting in a reflective near surface and acoustic shadowing. Some balls allow through transmission of the sound beam, and are thus well visualized. Linear foreign bodies may result in visible bowel plication, with the presence of an echogenic linear structure extending through the affected bowel segment. The presence of fluid enhances foreign body visualization. Intussusceptions can usually be diagnosed on ultrasound examination by their characteristic “target”, or “ring” sign. This is caused by multilayered concentric bowel wall layers with a central echogenicity. (in cross section). Again, the bowel proximal to the intussusception is often dilated.
GI neoplasia may result in either a localized mass, or a diffuse wall thickening. Moderate to severe wall thickening, with loss of layers, is the most common appearance. Regional lymphadenopathy may also be present. Inflammatory diseases may cause a less severe wall thickening, typicallly with preservation of wall layers.