Practical imaging of the gastrointestinal tract (Proceedings)

Much can be learned about the gastrointestinal tact (GIT) by careful evaluation of survey abdominal radiographs. Survey radiographs of the abdomen in cases of GIT disease should always include the stomach, liver, and diaphragm. Stomach position, size, shape and contents are evaluated. The small intestine is assessed for position and distribution within the abdominal cavity, diameter, and luminal contents.

Contrast studies of the GIT (barium upper gastrointestinal examination-UGI) still have a solid place in veterinary medicine and when used appropriately can yield diagnostic information. Unfortunately, information gained may be limited due to poor patient selection, preparation, or inappropriate radiographic technique. A complete UGI is time consuming for the technician and veterinarian as well as costly to the client. In addition to collaborating or adding to the survey radiographic findings, contrast examination of the GIT allows determination of gastric emptying and small intestinal transit time, bowel wall thickness, mucosal pattern, length of intestine affected (focal, regional, diffuse disease), radiolucent foreign bodies, mass lesions, abnormalities in peristalsis, and various types of obstructions. The primary contraindication for an UGI is when a definitive diagnosis can be made on survey films (e.g., a mechanical obstruction), or if free abdominal air is diagnosed, indicative of a ruptured viscus. Ultrasound also has a place in diagnosis of GIT disease, yet there are limitation, usually a result of GIT gas that hide foreign bodies or portions of pathologic bowel.

There are four keys to a successful UGI. In noncritical or elective circumstances, the patient should be fasted for 12-24 hours and given an enema. This will ensure that the stomach is empty and that colonic contents will not obscure evaluation of the small intestine or indent the stomach. Second, the type and concentration of barium sulfate is important. Micropulverized barium sulfate suspensions should be used, not powdered barium (barium sulfate, U.S.P.-often purchased because it is less expensive). This is because powdered barium will often flocculate or the barium column will become fragmented as it passes through the intestinal tract. If it does, it may indicate altered GIT physiology. Diluted barium is preferred (1:2 to 1:4 barium:water) as this allows you to "see through" the barium column in the SI. Small filling defects or foreign bodies will not be masked. The third consideration is proper filling of the stomach with barium. A partially full stomach may not have the neurophysiologic stimulus to empty in a timely manner. Also, intermittent emptying can occur, resulting in a fragmented or discontinuous small intestinal barium column. If you do not fully distend the stomach, you may have taken away one important aspect of the UGI examination: transit times. A dose to remember is 5 ml/lb. Last, the timing sequence and patient position during the UGI are important. Immediate films should be obtained, ideally both right and left lateral views as well as VD and DV films. This allows full assessment of the stomach. The next set of films should be taken in 15-30 minutes. Right lateral and VD films are standard. Radiographs taken at hourly intervals should suffice, the end point of the study determined when the stomach is empty (or sooner if a definitive diagnosis is made prior to this, such as an obstruction). If the study is terminated prior to complete gastric emptying, sooner or later you will overlook a small gastric foreign body (e.g., a hairball).

Esophagus

Regurgitation is probably the most common sign of esophageal disease. Difficult or abnormal swallowing, gagging, systemic neuromuscular disease, failure to grow or maintain body weight, and respiratory disease are others. Aspiration pneumonia, tracheitis and nasal discharge may overshadow primary esophageal disease.

The normal esophagus is not visible radiographically unless it contains gas or contains abnormal soft tissue or radiopaque material. Enlargement of the esophagus may displace surrounding structures, most noticeably the trachea (ventrally, right or left). Although gas within the esophagus is often indicative of pathology, it can be present normally as a result of aerophagia, general anesthesia or deep sedation. Gas commonly collects caudal to the cranial esophageal sphincter (cricopharyngeus muscle), within the cranial thoracic esophagus or heart base region. Anesthesia may produce generalized "megaesophagus" and must be differentiated from pathologic megaesophagus.

Generalized megaesophagus is seen radiographically as a gas filled, dilated structure. On lateral radiographs, the dorsal wall of the esophagus is detected in the cranial thorax adjacent to the longus colli muscles and ventrally as it drapes over the trachea, which is displaced ventrally. The heart base may be displaced ventrally as well, with increased distance between the carina and the thoracic vertebrae. Caudally, two thin soft tissue lines are noted overlying the dorsal lung field, converging to the diaphragm. These very important radiographic signs may be overlooked if the radiograph is overexposed. On the dorsoventral (DV) or ventrodorsal (VD) thoracic radiograph, the dilated cranial thoracic esophagus is detected as a widened cranial mediastinum. This may be radiolucent if gas filled or more of a soft tissue mass effect if fluid distended. At the level of the heart base, the descending aorta and azygous vein indent the lateral margins of the esophagus causing an hourglass constriction. The thin soft tissue walls of the esophagus are usually apparent in caudally, converging to the diaphragm.

The site of focal esophageal disease may aid in the diagnosis. For example, vascular ring anomalies will produce a focal esophageal dilatation cranial to the base of the heart. A redundant esophagus, commonly seen in brachycephalic breeds will produce a mass effect at the cranial thoracic inlet and cranial thorax. Caudal esophageal disease may indicate a hiatal hernia, gastroesophageal intussusception, and is a frequent site of esophageal neoplasia (e.g., leiomyoma and leiomyosarcoma). Critical evaluation of the entire thoracic radiograph is important, with particular attention to the ventral lung field for evidence of aspiration pneumonia.

In many instances, an esophagram is necessary to diagnose esophageal pathology. High-density liquid barium is generally used (undiluted to 50% w/w). Occasionally, a barium-coated meal can be used to identify early strictures, or used if there are differences in swallowing solids or liquids. Oral aqueous iodine (e.g. Gastrografin®) may be used if there is strong suspicion of an esophageal tear. Care must be used with these agents, however, as they are very hypertonic and aspiration may lead to severe pulmonary edema (liquid barium is safer in this regard). If the esophagram is ordered as part of a complete UGI examination, it may be preferred to perform this procedure after the UGI, as barium in the stomach from the esophagram may interfere with interpretation of the UGI. Aerophagia may cause the stomach to be distended with air.

Stomach

If the animal has been fasted, is anorexic, or has been vomiting, the stomach should be empty. The presence of gastric contents may indicate lodged foreign material, outflow obstruction, and functional disorders. A greatly distended stomach may signal an outflow obstruction, simple dilatation, or even torsion.

Evaluation of GIT wall thickness on survey radiographs is treacherous at best. Fluid within the lumen will silhouette with the wall, mimicking thickening (since fluid is the same radiographic density as the soft tissue of the wall). Radiographically, the only reliable way to evaluate GIT wall thickness is by the presence of positive contrast (or in certain instances negative contrast).

Stomach position can be more accurately determined when barium has been added. The stomach wall is thin, up to 5 mm thick when fully distended. Rugal fold appearance varies, depending on the degree of distension. They are more tortuous in the nondistended stomach, and more uniform and parallel to the gastric curvatures with increasing distension. If overly distended, rugae may not be visible at all. The rugae are smaller in the pyloric antrum. A rule of thumb is that the height of the rugae should approximate the interspace between them in the dog. Cats have fewer and smaller rugal folds.

The stomach should be assessed for filling defects and focal or diffuse wall thickening. The mucosal surface should be smooth, not ragged or inflamed looking. Careful attention should be paid to the pyloric outflow tract. It is nice to have at least one view demonstrating a wide, patent outflow of the pylorus. The contour of the pylorus should be critically evaluated for indirect evidence of outflow obstruction ("beak" sign).

Gastric emptying may be evaluated when the stomach is fully distended with liquid barium. Emptying should begin within 15 minutes in the dog and cat, and is usually present on the initial contrast radiographs. In the dog, the stomach should completely empty in 1-4 hours. In the cat, complete emptying is usually evident by 2 hours, often within 30 minutes. If there is food in the stomach, emptying may be delayed (canine gastric emptying of food is 8-10 hours, shorter in the cat). If the stomach is not fully distended, emptying may be delayed, leading to a false positive diagnosis of an outflow obstruction. Other factors which may delay gastric emptying include some medications (atropine, xylazine, morphine) and emotional stress (anxiety, fear, rage, pain).

Peristaltic contraction may be seen on radiographs as an indentation of the wall of the stomach. Indentation of the stomach wall is differentiated from peristalsis by observing for differences in the appearance of the wall/shape of the stomach on serial films. Ultrasound or fluoroscopy is necessary to observe the stomach in real-time. The stomach contracts 4-5 times per minute. Contractions are more vigorous in the antrum, less in the body and fundus.

Stomach Pathology

Gastric dilatation, malpositioning, and torsion can be diagnosed on survey radiographs. Malposition refers to displacement of the pylorus relative to the fundus and body, without distension and usually without clinical signs. This is often an incidental finding, but the client should be advised of the potential for dilatation and/or torsion. Torsion is readily identified by a "folded" appearance of the stomach, a soft issue band that compartmentalizes it. On ultrasound examination, a dilated stomach may dominate the field of view and may limit visualization of the liver. The stomach wall may appear to be thinner than normal and rugal folds will be absent. The gastric fluid may be anechoic but is often heterogeneous and echogenic. It may be sonolucent, allowing visualization of the far field gastric wall as well potential foreign material within the lumen. A gas dilated stomach is immediately recognized by a large highly echogenic interface in the near field, with acoustic shadowing and/or reverberation artifact. Only the near field wall can be evaluated in this instance and luminal contents and the remainder of the gastric wall cannot be assessed.

A greatly distended, fluid-filled stomach should signal the clinician that an outflow disorder is likely and prompt a search for an etiology through an UGI, double contrast gastrogram or ultrasound. Gastric dysfunction, either primary or secondary, may be suspected if gastric contractions are reduced in number or intensity. Gastric motility may be reduced or absent in cases of chronic outflow obstructions as the stomach tires, however. Therefore, observation of reduced or absent gastric peristalsis does not necessarily exclude chronic foreign body ingestion or pyloric stenosis as the primary cause of gastric dilatation. The gastric fluid should be carefully evaluated for the presence of a discrete foreign body that can be detected in some cases. The pyloric outflow tract should be evaluated for evidence of wall thickening, and peristalsis with propagation of gastric contents through the pyloric canal into the duodenum should be assessed. Focal as well as diffuse gastric diseases are often associated with dilation of the stomach.

Radiolucent foreign bodies and mass lesions of any etiology will be recognized by the presence of filling defects in the barium pool. Mass lesions may arise from the wall of the stomach or be seen as alterations in the barium shape in the pyloric region. Using ultrasound, foreign bodies may be identified by intense acoustic shadowing (rubber balls, plastic toys, rocks). Other foreign bodies may be diagnosed by a characteristic appearance, such a cob of corn. There are obviously a plethora of potential gastric foreign bodies, and of course material imaged in the stomach may be normal ingesta. An important question to ask is how long has it been since the last meal? Normal dogs and cats will completely empty their stomachs in 8-10 hours. Suspicious findings may be reevaluated later in the day or following day, or followed up with other diagnostic procedures

Pyloric stenosis causing a gastric outflow obstruction in most instances causes gastric dilatation, as described above. Vigorous contractions may be observed without propagation of gastric contents through the pylorus and into the duodenum, although as mentioned, gastric atony can occur over time as the stomach tires. Chronic hypertrophic pyloric gastropathy has been described as concentric hypoechoic thickening of the pylorus. Congenital pyloric stenosis may look similar. In one case of congenital pyloric stenosis the wall was thickened and relatively hyperechoic with poor visualization of wall layers. Focal mass lesions of the pyloric outflow tract or irregular wall thickening may represent neoplasia or inflammatory lesions. Mass lesions, either malignant or benign (e.g., benign polyps) may be seen projecting into the pyloric lumen. The ultrasound findings discussed above may be confirmed with a barium or iodinated contrast material upper gastrointestinal examination. In many instances, the diagnosis is clearly evident when information from both imaging modalities is considered. Endoscopy is an effective diagnostic tool to further evaluate the gastric outflow tract.

Diffuse thickening of the stomach is often indicative of nonmalignant disease such as parvovirus infection, lymphocytic/plasmacytic or eosinophilic infiltrate, uremic induced gastritis or gastritis for other causes such as dietary indiscretion. Lymphosarcoma and mast cell disease are neoplastic diseases to consider when the stomach wall is diffusely thickened. Minimal thickening usually preserves the normal layered appearance of the stomach wall on an ultrasound examination. Severe thickening usually obliterates these layers. Rugal folds also become thickened. Mineralization of the gastric mucosa that may occur with chronic renal disease has been described sonographically as a hyperechoic line at the mucosal-luminal interface, usually without acoustic shadowing. Lymphosarcoma is the most common diffuse neoplastic disease of the stomach. The gastric wall becomes uniformly hypoechoic and thickened with loss of normal layers. Gastric lymphosarcoma can also be a focal disease. Regional lymphadenopathy can be indicative of the severity of gastric disease; more severe lymphadenopathy is usually associated with neoplasia. Differentiation between these various diseases requires integration of the signalment, clinical signs, lab data, radiology interpretation, etc. Ultimately, a fine needle aspirate or biopsy may be necessary to make a final diagnosis.

In most instances, focal gastric lesions represent a neoplastic process. Mass lesions of the stomach are identified as focal areas of gastric wall thickening. These lesions usually obliterate the normal layered ultrasonic anatomy of the stomach wall and can become quite large. Their appearance may be homogeneous or very complex. Gastric dilation is often present. Gastric carcinomas, lymphosarcoma, leiomyoma and leiomyosarcoma are examples of focal gastric neoplasia. Lesions can be readily biopsied with ultrasonic guidance. As mentioned previously, misdiagnosis of a gastric mass lesion or diffuse thickening can occur when the stomach is assessed when empty.

Gastric ulcers are difficult to detect reliably with an UGI, but a double contrast gastrogram may be more rewarding. Evaluation of the wall of the stomach for extensions of barium and adjacent wall thickening are the classic signs. Plication of rugal folds, converging to a focal accumulation of barium is diagnostic (spoke wheel sign). Ultrasound has been used to diagnose gastric ulcers. A hyperechoic area representing gas accumulation within a focal area of gastric wall is seen. There is usually a noticeable depression in the mucosal surface. Gastric fluid distension is often present and regional reduction in gastric wall peristalsis may be observed. The area surrounding the gastric ulcer should be studied for evidence of focal perigastric fluid accumulation and hyperechoic mesentery, evidence of ulcer perforation and focal peritonitis. Ultrasound may be used to assess gastric ulcer healing, with reduction of wall thickness and eventual reestablishment of normal wall layering. Care must be taken to differentiate trapped air bubbles within gastric rugae from true gastric ulcers.

Small Intestine

Much can be learned about the small intestine (SI) from survey abdominal radiographs if a few basic guidelines are kept in mind. First, intestinal contents should be assessed. The typical canine small intestine contains a mixture of both fluid and gas. A totally fluid-filled or gas-filled SI in dogs is usually a sign of disease. The cat SI contains predominately fluid. Granular, calcific material is not normally present in the SI of either the dog or cat. Its presence is highly suggestive of some form of obstructive process, usually a long-standing partial obstruction.

Bowel diameter is another criteria to be assessed on survey abdominal radiographs. A good rule-of-thumb is that SI diameter should not exceed the height of the body of the lumbar vertebrae in dogs, or around 12 mm in cats. Bowel diameter exceeding these criteria suggests some form of pathology. If the SI is diffusely enlarged, this is a fairly easy task. More of a challenge is detection of focal SI dilatation on the survey radiographs. The diameter of the intestinal loops can be reliably evaluated during the UGI. Dilated loops of bowel indicate ileus, either functional or mechanical. "Stacked" loops of gas distended bowel often indicate an obstructive process.

An attempt should be made to locate and visualize the cecum and entire colon (ascending, transverse and descending). It is important to differentiate small intestine from large intestine because colonic diameter will usually exceed that of the small bowel, and the contents of the colon will be granular and sometimes calcific. This is normal for the colon, abnormal for the small intestine.

The barium column should be continuous as it leaves the stomach and enters the small intestine. Fragmentation of the barium column within the SI may indicate delayed or intermittent gastric outflow, abnormal peristalsis, or altered SI physiology (e.g., altered pH, excessive mucus, etc.). Barium often reaches the colon in the dog in 2 hours, as soon as 30 minutes in the cat. Barium should be cleared from the small intestine of the dog by 5 hours, 3 hours in the cat.

The intestinal walls should be evaluated for thickness. Wall thickness in the dog and cat is typically 2-4 mm in the jejunum and ileum, slightly thicker in the duodenum. The mucosal surface is assessed for irregularity or filling defects. The gastrointestinal tract as seen sonographically as alternating hyperechoic and hypoechoic layers, whether viewed in long-axis or in cross-section. Optimally, five discrete layers of the GI tract can be seen, corresponding to the luminal/ mucosal interface, mucosa, submucosa, muscular layer, and the serosa.

The constant movement of the intestinal tract due to segmental and peristaltic contractions can produce some unusual appearances of the contrast medium column. To avoid mistaking a contraction or peristaltic wave for a pathologic lesion, the area of suspicion should be seen on additional radiographs (i.e., the more times you see it, the more likely it is to be a true lesion). Peristaltic activity varies when small bowel dilation is present, from complete absence to hypermotility. Hypermotility is probably more common with acute mechanical obstructions from foreign material and infectious or dietary induced enteritis than with chronic partial obstructions. Two distinctive normal patterns of the proximal SI need to be recognized radiographically. One is the "string of pearls" appearance of the cat intestine, which is simply peristalsis. The pattern is often mistaken for a linear foreign body. The second pattern is the "pseudoulcer", smooth depressions along the antimesenteric border of the canine duodenum that represent filling defects from islands of lymph tissue.

Small Intestine Pathology

Abnormally dilated bowel is easily diagnosed on the UGI examination. Differentiation of SI from the cecum and colon is also possible. Once you have detected dilated SI, assessment of focal, regional or diffuse disease is necessary. Greatly dilated, diffuse SI disease is often inflammatory or infectious (e.g., parvovirus), but one must also consider a terminal SI or colonic obstructive process. If you recognize dilated bowel, the next step is to look for evidence of a radiolucent filling defect within the intestinal lumen. This indicates not only the site of obstruction (with dilated bowel proximal to the site) but may give the clinician a clue as to the etiology of the blockage. Common examples include various foreign bodies (balls, fruit pits, plastic, cloth, leather, wood), tumors, and intussusceptions.

Ultrasound can be used to detect dilated bowel, but caution must be used because scale must be kept in mind. Many times the bowel appears to be enlarged because the image is magnified; when measured, diameter may be only a centimeter, certainly not dilated! Mechanical obstruction results from the presence of foreign material, mass lesions, strictures or intussusceptions. Differentiating obstructive from non-obstructive small bowel dilatation is possible when dilated and normal intestinal segments are both seen or when the site or cause of obstruction is identified. Depending on the location of an obstructive lesion, the dilation may be segmental (proximal obstruction), or involve the entire small bowel if the obstruction is at the level of the terminal small intestine or ileocecocolic region. In proximal small intestinal obstructions, both dilated (proximal to obstruction) and normal (distal to obstruction) segments of intestine will be present. With mechanical obstruction, the degree of bowel distention is dependent on whether or not the obstruction is partial or complete and on the duration of the obstruction. When segmental small intestinal distension is recognized, an attempt should be made to define the site of obstruction and differentiate a foreign body from a mass lesion or intussusception.

Assessment of the intestinal mucosa is essential on an UGI examination. Rough, ragged, irregular filling of the mucosa indicates some form of pathology. Diffuse disease is usually indicative of enteritis (infectious or inflammatory), whereas focal or regional disease is more likely neoplastic in origin. The radiographic appearance may be mild or very severe.

The presence of barium within the SI allows for reliable assessment of bowel wall thickness. While subtle thickening may go undetected radiographically, once the intestinal wall reaches a thickness of 5 mm or more, the clinician can have confidence that pathology is present. Diffuse thickening of the small intestine is seen in cases of lymphocytic/plasmacytic enteritis, corona or parvovirus, dietary indiscretion, acute hemorrhagic gastroenteritis (HGE), and intestinal lymphosarcoma. Careful observation of various intestinal segments may reveal that some areas are slightly thicker than others. It must be noted that mild lymphocytic/plasmacytic enteritis may not be detectable sonographically, yet be confirmed on histologically. The normal layered appearance of the bowel wall will usually be preserved and motility will be normal. However, in many cases of infiltrative bowel disease, the muscularis layer becomes thickened, causing disproportion of bowel layering.

Intestinal lymphosarcoma is usually presents with more severe, advanced bowel pathology than that described above. The bowel wall is usually quite thick (5-25 mm), hypoechoic, and layers are lost. Transmural circumferential thickening (symmetric or asymmetric) is the most common form of intestinal lymphosarcoma in dogs and cats. Other forms of intestinal lymphosarcoma include transmural-bulky, in which larger, complex lesions are seen; a transmural-nodular form, characterized by nodular lesions within the wall and metastasis to regional lymph nodes; the transmural-segmental form, in which only a segment of bowel is affected; and a mucosal infiltrative pattern, with subtle thickening and mottled echogenicity of the mucosa and preservation of intestinal layers. The mucosa infiltrative pattern of lymphosarcoma is difficult to identify in some instances, and if seen, may mimic non-neoplastic enteric disease. Segmental small bowel thickening occurs most commonly with intestinal lymphosarcoma, adenocarcinoma, or undifferentiated sarcomas. Duodenitis as a sequel to pancreatitis, hypergastrinemia, or portosystemic shunts (duodenal ulceration) is a common form of segmental enteritis. The bowel may have a corrugated appearance. In some regions infectious mycoses can produce segmental or diffuse bowel wall thickening. Segmental intestinal thickening may also be present proximal to and at the site of an intestinal obstruction (inflammation and/or muscular hyperplasia).

Primary intestinal tumors (other than lymphoma) are often imaged as mass lesions by the time the patient is presented for clinical signs of bowel neoplasia. Common intestinal masses include leiomyosarcoma and carcinoma. One notable exception is feline adenocarcinoma of the ileocecocolic region, which may cause clinical signs while quite small, and palpate as a small mass lesion that can be detected sonographically. Palpable mass lesions may be imaged while the sonographer or an assistant holds the lesion for positive identification. Intestinal mass lesions can usually be readily identified if clinical signs are present. The mass may be quite variable in appearance. Focal, concentric thickening of the bowel may be present, or the thickening may be eccentric in location, the latter a common finding with leiomyosarcoma. Larger lesions are usually complex, with mixed echogenicity. While it is not difficult to identify large mass lesions, it may be more of a challenge to associate the mass with the bowel. Key points are the presence of gas within the mass and dilatation of the obstructed bowel proximal to the lesion. An attempt to image the dilated proximal bowel as it enters the mass and/or normal bowel exiting the mass is critical. Metastasis to regional lymph nodes and occasionally to the liver or other organs can occur.

Intussusceptions have a characteristic ultrasound appearance. Cross-sectional views of an intussusception show a multilayered, concentric, target-like lesion due to the multiple walls and wall layers that comprise the mass. On a sagittal image, multiple layers of bowel wall are seen "stacked" on one another. Hyperechoic mesentery is often incorporated into the intussuscipien as it accompanies the intussusceptum. In some instances the concentric or layered appearance is distorted and not as easily recognized because of inflammation and edema. Distended bowel proximal to the obstruction will be present.

The position of the intestinal tract is easily determined with the presence of barium. The normal contour and shape of the small intestinal wall is gently rounded or curved. Therefore, abnormal patterns are seen as segments of bowel with persistent straight or flat walls or those, which are excessively coiled or plicated. Plication ("ribbon candy" sign) is present with linear foreign bodies. Linear foreign bodies usually cause partial obstruction and may lacerate the bowel causing peritonitis. Bizarre-shaped SI gas pockets may signal a linear foreign body. Bunching of the SI centrally can occur. Be wary of "normal bunching" of the SI in obese cats.

Linear foreign bodies may be diagnosed with ultrasonography by recognizing the characteristic plicated appearance of the small bowel, but radiography is a more reliable test. The foreign body may be identified as an echogenic luminal structure. Foreign material may be noted in the stomach (e.g., cellophane wrap from ham or roast, with the thick twine descending into the small bowel). Peritonitis from bowel wall leakage in long standing cases is suggested if free peritoneal fluid is detected, the mesentery is hyperechoic with poor sonographic detail, and lymphadenopathy is present.