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.