Imaging the GI Tract
Norman Ackerman, DVM, DAC
Veterinary Radiology Consultng
Huntsville, Al, USA
Of the methods of GI imaging, radiography remains the most efficient. When the medical problem is obscure, other means do not yield a diagnosis or an animal resists palpation abdominal radiography is indicated. Contrast studies are valuable but are time consuming and expensive. Ultrasound can eliminate the need for GI contrast studies and is quicker and less expensive. The decision to use it is often based on the ultrasonographer's experience and training. Nuclear imaging techniques are valuable for functional studies. MRI and CT have great potential but their cost and non-availability make them less useful. Endoscopy is valuable for evaluating the stomach, proximal duodenum and colon and provides biopsy material.
The Stomach can be evaluated on radiographs. Note the gastric content and evaluate it relative to the patient's history. Although granular material in the stomach could represent food, in anorectic or vomiting patients it is more likely foreign material. Using right and left lateral, VD and DV views, the entire stomach can be evaluated. Gastric wall and rugal fold thickness depends on gastric distension and the content amount and type. If the wall appears abnormal on survey radiographs, air or contrast must be administered to confirm the abnormality. Normal gastric fold thickness ranges from 1 to 8 mm in dogs ranging in weight from 2 to 50 kg.Shape alteration with or without wall thickening is more significant than gastric size.
The Small Intestines are visible because of fat present within the peritoneal cavity and the luminal air. Non-digestible material such as bone may be present. Small intestines are curvilinear when seen longitudinally and round in cross section. Except for the descending duodenum, small intestine location is variable. The diameter varies but should be no wider than a lumbar vertebral body height. Small intestinal wall thickness cannot be evaluated accurately without contrast.
The Large Intestine (colon) begins at the cecum. Its position is variable, usually in the midabdomen ventral to L3 or L4 and slightly right of the midline. The dog's cecum has a spiral or comma shape and frequently contains gas. The cat's cecum is small, not spiraled and located ventral to L2 or L3. The ascending colon is short. The transverse colon is caudal to the stomach. On the lateral view it may appear as a round fluid, gas or granular structure. The colon's position varies. It may be on the right side, may take an irregular course through the abdomen and may appear tortuous. The colon diameter is usually no more than two to three times that of the small bowel. It is difficult to recognize colon lesions without the aid of special procedures.
Additional Radiographs are valuable. If a lesion can be identified on two consecutive lateral views, on both VD and DV views or in the same view obtained after some time has elapsed additional studies are unnecessary.
Selective Abdominal Compression utilizes a radiolucent wooden spoon or paddle to isolate the intestines. With the animal in lateral or dorsal recumbency the area of interest is palpated, the spoon holds the intestine in place and the radiograph is exposed. Since the abdomen is compressed the radiographic technique is reduced. The x-ray beam must be collimated to the area of interest. This technique can be used to isolate an intestinal mass obscured by overlying gas or ingesta.
Ultrasound. Even when GI disease is suspected the entire abdomen should be scanned. Hepatic, gall bladder and pancreatic disease can produce GI signs and are easily detected. Lymph nodes can be detected and is evidence of systemic disease. Ultrasound can be used as guidance for biopsy or aspirate of masses.
The Stomach's appearance varies with content and distension. Peristalsis can be observed. Scan the stomach in longitudinal and transverse planes. Wall thickness varies from 3 to 5 mm. Five layers may be identified: hyperechoic serosa, hypoechoic muscularis, hyperechoic submucosa, hypoechoic mucosa and the hyperechoic lumen or mucosal surface. Luminal gas produces reverberation artifacts. The rugal folds may be identified if the stomach is relatively empty but disappear when it is distended. Fluid and gas may be seen swirling around in the stomach as a result of peristalsis. It is not necessary to fill the stomach with fluid for complete evaluation. Although intraluminal gas usually restricts evaluation to the wall closest to the transducer patient rotation and transducer movement permit evaluation of the entire stomach. Ultrasound has been used for evaluating gastric emptying but is inaccurate and has not replaced other methods.
Small intestines always contain gas bubbles. The hyperechoic lumen and serosa can be identified. The mucosa, submucosa and muscularis appear as a single hypoechoic structure. Wall thickness (normally 2-4mm), luminal content and bowel shape should be examined. Although ultrasound is very sensitive for evaluating intestinal wall thickening this abnormality is non-specific and identification of a normal wall does not rule out disease. Intestines can be followed for only a short distance. Peristalsis should be observed. The descending duodenum can be traced from the pylorus, has a relatively consistent position along the right lateral abdominal wall and has a thicker mucosa (up to 4.5mm). The ileum can be traced to the ileocecocolic junction; other specific portions cannot be distinguished.
The colon size, position and absence of peristalsis make it recognizable. Gas and fecal material obscure wall details. When fluid filled, mucosal folds may be visible resulting in a wrinkled appearance. The wall width is 2-3 mm but appears thinner when the colon is distended. In a transverse plane the colon appears as a semicircle since gas shadows the far wall. In the longitudinal plane it appears as an echoic line. The portions of the colon can be recognized by their position.
Radiographic Contrast Procedures are performed for GI tract evaluation. Methods vary; interpretation principles are the same. GI Contrast rarely results in a specific diagnosis and most often identifies a lesion that should be biopsied. The GI series can be normal despite the presence of disease. Itis always preceded by non-contrast radiographs that should be repeated even if obtained a day before. The most common mistakes are improper patient preparation, too little contrast and too few radiographs. Withhold all food for at least 12 hours and use laxatives and/or enemas to empty the GI tract. Sedate dogs with intravenous acetylpromazine (0.1 to 3.0 mg); cats with intravenous (10mg) or intramuscular (12mg/lb) ketamine, acepromazine/ketamine (0.1mg/lb; 6mg/lb) or ketamine/valium (6mg/lb; 0.2mg/lb). Administer barium suspension (25 to 40 percent weight/volume) by gastric intubation (5-8 ml/lb). Ionic and non-ionic water-soluble iodinated contrast can be used. The advantage of rapid transit is outweighed by poor contrast and detail. Ionic agents coat the mucosa poorly and may irritate the mucosa causing vomiting. Ionic agents draw fluids into the intestine resulting in decreasing contrast density and dehydration. In bowel rupture, barium will identify minimal leakage and can be flushed from the abdomen at surgery. A stomach tube is superior to oral instillation because animals may struggle and inhale barium or may swallow large amounts of air. Although each individual is consistent gastric emptying times vary greatly among normal dogs and most figures are estimates. Barium reaches the dog's colon in 180±90 minutes and the cat's in 30 to 60 minutes. In dogs, radiographs should be taken immediately after barium administration and at 15 and 30 minutes, 1, 2 and 3 hours, and periodically thereafter; in cats, barium passes through the GI tract more rapidly, take radiographs at 0, 10, 20 and 30 minutes, 1 hour, and hourly thereafter until the barium reaches the colon and the stomach is empty. Four views (VD, DV, and left and right recumbent laterals) should be taken immediately and at least a VD and lateral view should be taken at each time thereafter. The VD view minimizes superimposition of structures and allows for evaluation of the majority of the stomach. Gastric contractions alter the gastric shape. An open pylorus is rarely seen because compared to gastric mixing, pyloric opening is infrequent and of short duration. Gastric emptying time varies; a marked delay in contrast exit from the stomach implies outflow obstruction. Gastric emptying is prolonged in nervous patients; if left in a quiet location for a few minutes normal peristalsis usually resumes. Mixing food with barium to evaluate gastric emptying serves no useful purpose. The barium will separate from the food and transit the intestines while the food remains in the stomach. Solid particles in the stomach may be mistaken for foreign material. Emptying times for barium food mixture ranges from 5 to 14 hours. Normally the contrast passes through in an organized fashion. Loops are smooth walled and uniformly distended with a few constrictions representing normal peristalsis. A brush border may be present. Symmetrical oval contractions termed a string of beads have been described in the duodenum of the normal cat. In the duodenum of the dog one or more crater like lesions result from the presence of gut associated lymphoid tissue (Peyers patches). The paramount principle in evaluating a GI series is lesion repeatability. A study with a limited number of radiographs may be nondiagnostic. An abnormality on one film that does not remain constant in appearance on at least that same view throughout the series should be regarded as spurious and the study considered normal. The GI series, a poor evaluator of function, is a more accurate evaluator of gross anatomic change. The colon should not be evaluated with a GI series.
Barium Impregnated Polyethylene Spheres (BIPS) have been recommended for evaluation of GI transport. Small spheres are designed to evaluate gastric emptying; larger spheres are designed to accumulate at an obstruction or stricture. Inconsistent results and not outlining the GI mucosa limit the technique.
Colon diseases may be evaluated by endoscopy or by pneumocolon or barium enema. The fiberoptic endoscope is usually more accurate than most radiographic special procedures. The major exception occurs if luminal narrowing or stricture prevents passage of an endoscope.
Pneumocolon is performed by inserting a syringe tip through the rectal sphincter and injecting 1 to 3 ml of air per kg of body weight. Lateral and VD views are then exposed. More air may be added as needed. Intraluminal, intramural and extraluminal lesions can be detected. The cranial extent of a mass or stricture that prevents endoscope passage can be demonstrated. The colon can be distinguished from distended small intestines.
Barium Enema/Double Contrast Enema. If the lesion is not accessible to colonoscopy and a pneumocolon will not provide sufficient information, a contrast enema is indicated. Colon preparation using multiple enemas or laxatives is essential. The patient must be anesthetized. A 15 to 20 percent weight per volume barium suspension is instilled at a dose of 22.2 ml/kg via a cuffed enema tube. The barium is drained and the colon insufflated with an equal volume of air to create a double contrast enema. This is rarely used since colonoscopy is relatively easy, permits directly visualizing lesions, determining its extent and obtaining biopsy or cytology.
Nuclear Medicine is most often used for physiologic evaluation. It has mostly been used for research into the drug and diet effects on gastric emptying and can be useful for detection of GI bleeding. Equipment expense and availability are major limitations.
Magnetic Resonance and Computed Tomography have great potential for evaluation of the GI tract however their expense and availability limit their use. Technologic improvements have decreased blurring of the images due to motion.
Endoscopy is an important method of GI imaging. Equipment expense is reasonable. Anesthesia or tranquilization is usually required. It is limited to the stomach and proximal duodenum and to the colon. An important advantage is being able to obtain biopsy or cytology. Operator experience is critical.
Selection of the imaging modality is based on availability, expense and personal experience. In general the abdominal radiograph should be the first choice. Additional and repeat radiographs are helpful usually followed by ultrasound and GI contrast studies if ultrasound is unavailable. Endoscopy is also appropriate at this time depending upon the lesion's site. The role of computed tomography and magnetic resonance is yet to be determined. Nuclear medicine has a limited value.
Norman Ackerman, DVM, DACVR