Diagnostic Imaging of the Gastrointestinal Tract

Chapter 26 Diagnostic Imaging of the Gastrointestinal Tract


Kari L. Anderson and Daniel A. Feeney

Technical Considerations

There are several reviews on specific radiographic techniques published elsewhere.16 Several decisions need to be made before considering any imaging procedure, particularly those that are invasive or more costly. Specifically, what is the aim of the radiographic study? Is it to investigate organ geometry (i.e., size, shape, location, surface character, number or degree of organ involvement), organ opacity (i.e., gas, fluid, soft-tissue parenchyma, mineral, or metal), organ function (i.e., motility, breakdown, secretion, absorption, or excretion), organ leakage, organ obstruction, organ wall anatomy, organ wall thickness or pliability, etiology of dilation (segmental or diffuse), gas patterns, or effect of a diseased adjacent organ? Once the intent of the imaging procedure is established, it should then be determined whether survey radiographs alone are adequate to make the diagnosis or whether additional imaging procedures will be necessary. Horizontal beam or contrast procedures could be a necessary part of the additional medical workup. If a contrast procedure is indicated, the type of contrast medium should also be considered. Considerations for a contrast radiographic procedure should include the goal of the study (e.g., assessment of organ distention, motility, leakage, or foreign bodies), potential for body cavity or pulmonary contamination, options for contrast medium administration (i.e., oral, gastric, nasoesophageal, or nasogastric intubation), potential for air embolism from negative contrast medium, and the best choice for the contrast agent. Micropulverized barium sulfate suspensions are utilized in the assessment of motility and mucosal morphology, but can cause inflammation if the suspensions gain entry into the lungs or body cavities. Ionic iodinated oral preparations are associated with rapid propulsion, but are less useful for evaluating mucosal morphology, and side effects because of airway aspiration may be significant. Nonionic iodinated media are useful for the assessment of motility but not mucosal morphology, and fewer side effects have been reported if the media gains entry into lungs or body cavities. Another consideration is whether the positive contrast medium should be used as constituted or administered with food. In general, contrast agents should be administered without food for evaluation of mucosal morphology, detection of leakage, and timing of organ transit (e.g., stomach and small bowel) unless the goal is to assess the solid phase of gastrointestinal transit. Once these questions have been answered, other considerations related to the patient may be addressed.

Preprocedural Considerations

Patients with alimentary disease can be nauseous, weak, or dehydrated as well as having complications of alimentary tract disease, including aspiration pneumonia, sepsis, electrolyte and acid–base abnormalities, and abdominal distention. A balance between what is optimal for the patient and what is optimal for the imaging procedure should take these complications into account. Dyspnea and acute pain, for example, could be significant factors to consider in delaying the decision to perform an imaging procedure. Patient attitude and preparation are equally important patient parameters to take into account. Fractious patients may harm staff or themselves and must be adequately restrained or sedated. The authors prefer light phenothiazine sedation if sedation is needed.7,8 Generally, no specific preparation is necessary for survey radiographs. In fact, unnecessary preparations or patient alterations may interfere with image quality. For example, sedation may lead to slight accumulation of air in the esophagus that may be confused with megaesophagus. No specific preparation is required for an esophagram, but both the upper gastrointestinal (GI) series and the barium enema require cleansing enemas and/or cathartics, and, unless imaging is considered an emergency, food should be withheld for at least 18 to 24 hours.

Consideration should also be given to personnel safety. Appropriate radiation safety precautions must be taken for any radiographic procedure, including use of personnel shielding devices (e.g., aprons, gloves, eye shields, and/or thyroid shields). Appropriately placed personal body monitors must be used. Radiographic procedures should be planned such that manual restraint of the patient is minimized. Potential radiation exposure, including scatter and primary beam radiation, should also be minimized.

Positioning and Views


Upper Gastrointestinal Series


Alimentary disease should be classified as luminal, mural, extramural, or transmural (e.g., perforation [Fig. 26-3]). The intent of this classification is to define the general likelihood of various differential diagnoses. An understanding of basic principles of pathology makes this less complicated. For example, extramural lesions affect alimentary organ position and dimensions (e.g., compression of the colorectal junction by an enlarged prostate gland), but do not originate in the alimentary organ itself. Intraluminal lesions, on the other hand, are generally completely surrounded by the administered contrast medium. Finally, mural lesions, such as tumors or strictures, are generally only partially surrounded by the administered contrast medium. When considering mural lesions, wall thickness measurements play an important role. The normal wall thickness is 2 to 4 mm for the esophagus, 4 to 5 mm for the stomach (interrugal), and 3 to 4 mm for the small bowel and the colon.1,36 Any deviation from these measurements is usually a result of local or regional infiltrates causing wall thickening that can range from diffuse infiltrative bowel disease to focal/multifocal mural infiltrates such as tumors and strictures. Annular lesions are a unique subset of mural lesions and resemble either a napkin ring or an apple core because of their circumferential distribution and concentric luminal narrowing. Although possibly seen with strictures, this pattern is most commonly seen with intestinal neoplasia.

Considerations for Alimentary Tract Imaging

The use of a few common sense rules can greatly simplify the imaging of the alimentary tract. Survey radiographs are crucial for the diagnostic process as they allow assessment of the GI tract without being affected by procedure-related stress and aerophagia, peristaltic effects of drugs or tube passage, and the soothing (barium) or irritating (ionic iodine) effects of contrast agents. It is also important to have a perspective on the relative roles of imaging procedures and endoscopic procedures. In general, endoscopic procedures are superior to imaging procedures for mucosal characterization, although this may change as imaging-based virtual endoscopic procedures are further investigated and refined. By comparison, radiographic procedures, particularly positive contrast procedures, are superior for characterization of intramural, transmural, and extramural diseases.

Of particular value is an assessment of the fluid and gas volumes along the gastrointestinal tract. First, any reproducible distention of the esophagus is abnormal. As a general rule, approximately 30% to 60% of the small bowel in dogs contains air, which is spread evenly throughout the bowel and relates in some reasonable proportion to the gas and fluid in the stomach. By comparison, most of the feline alimentary tract is devoid of gas, except in aged cats. This is a consequence of both anatomy and physiology (Fig. 26-4). The gastrointestinal tract is one continuous tubular lumen and there should be an orderly flow of ingesta from proximal to distal. There should be no disproportionate accumulation of fluid or solute or lack of any of these contents along the course of the GI tract. If such changes are identified on survey radiographs, this is an indication for further imaging investigation. The alimentary organs can be affected by regional and systemic influences as well as the local effects of the ingesta. The fluid/gas balance coupled with the organ size or diameter and distribution of a perceived abnormality can provide clinically useful insights into the pathophysiology of the disease. For example, a focal or multifocal distribution is associated with foreign bodies, tumors, strictures, and small incarcerations. In comparison, a diffuse distribution is associated with widespread intramural disease (e.g., infiltrative bowel disease), as well as widely differing luminal osmolarities related to bacterial proliferation, digestion, or absorption. A diffuse distribution can also result from disease involving other components of the digestive system, especially the liver (e.g., portal hypertension causing fluid buildup in the small bowel). Regional effects can be the result of intraluminal, intramural, or serosal irritation or inflammation (e.g., caused by peritonitis, pancreatitis, or a migrating foreign body).

The size of an alimentary organ as well as its contents can provide an insight concerning the underlying abnormality. Organ size is governed by several factors, including peristaltic capacity (i.e., neuromuscular disease, ischemia, inflammation, wall trauma, or altered electrolyte status), downstream or outflow resistance (i.e., sphincter dysfunction, strictures, or any obstruction), the digestibility and bulk of intraluminal contents (e.g., foreign material or engorgement), wall pliability (e.g., fibrosis or cellular infiltrates), wall thickness (e.g., tumor), upstream delivery of materials, and the volume and viscosity of ingested or administered material (e.g., aerophagia, psychogenic polydipsia, or polyphagia). Impedance to flow is characterized by buildup of fluid, ingesta, and air proximal to the impedance, and a lack of these contents distal to the impedance. Partial obstructions are characterized by buildup of fluid and ingesta with only small amounts of gas. Near complete obstruction is characterized by a mixture of gas and fluid, but with little accumulation of ingesta. Dysmotility may mimic any of the previously mentioned conditions.

Gas volume and its distribution can be key components in the interpretation of alimentary tract disorders. In omnivores, most alimentary gas derives from swallowed air. From low-grade impedance of gastrointestinal flow to complete obstruction, the likelihood of intraluminal buildup is air, fluid, digestible solids, and nondigestible solids (e.g., small bone fragments or small foreign objects). Unless an obstruction is nearly complete, only limited amounts of air will accumulate proximal to it because air can move past a partial obstruction more readily than fluid, food, or mobile fragments suspended in the partially digested chyme. The relationship of relative air versus fluid in a tubular organ segment and the influence of gravity are relevant variables to the assessment of gut wall thickness using survey radiography. As illustrated in Figure 26-5, the appearance of the wall of a hollow alimentary organ is based on the optical perception of the serosal and mucosal surfaces with the distance between them presumed to represent the wall thickness on routine vertical-beam radiographs. Overestimation occurs when the lumen is more than half full, which is common on alimentary survey radiography and must be factored into every attempt to assess wall thickness and may prompt a decision to perform additional imaging, including contrast or ultrasonographic procedures.

For interpretation of any survey radiograph, the well-accepted “Roentgen sign” approach (e.g., size, shape, location, surface characteristics, opacity, and function) provides useful basic information. Several questions should be considered to avoid over- or underinterpretation of these findings. Radiographs should be reviewed for any unexpected or disproportionate regions of organ distention, and whether any distention is caused by fluid, gas, solids, or some combination of all three phases. Radiographs should also be reviewed for signs suggestive of mechanical disruption or dysmotility. Gas-to-fluid ratios should be determined and whether luminal gas is randomly distributed. Another question that needs to be addressed is whether the distribution of fluid, gas, and ingesta is diffuse or segmental. Areas of segmental distribution, regional peristaltic abnormalities, and wall rigidity should be identified. The mixture of fluid and gas in the bowel should be assessed for evidence of obstruction and to differentiate it from ischemia, the latter of which is mostly associated with intraluminal air. Radiographic review should also include identification of unexplained localized intraluminal opacities or persistent material. Serosal contrast should especially be evaluated in areas of gut abnormalities. Masses should be identified and assessed for visceral displacements, invasion, or origination from the gut. Another very important question to pose is whether there is any free air or fluid in the peritoneal or retroperitoneal cavity. The etiology of the alimentary problem may not be obvious on survey radiographs alone, and if that is the case, the radiographic interpretation may still help focus selection of more appropriate imaging techniques for further clarification or definitive diagnosis.

There are some useful clinical clues to the underlying etiology of an alimentary tract disease.1,36 The buildup of a grainy opacity can indicate a chronic partial obstruction in either the stomach or the small intestine. A “thumbprinted” bowel, which describes a segment of the bowel that looks similar to a tubular piece of clay into which a thumb has been pressed, is suggestive of segmental or diffuse intramural infiltration because of neoplasia or pyogranulomatous infiltration. The thumb-printing effect comes from loss of wall pliability and variability in wall thickness. Another finding that can be observed is the so-called onion-skin or coil-spring appearance caused by contrast medium spreading between contiguous intestinal loops involved in an intussusception. The so-called washboard effect can be caused by regional mesenteric ischemia. The appearance of mural or wall layering may also be noted during radiography, which can be a result of infiltration of the wall with gas or minerals. Layering of luminal content may also be seen on horizontal-beam views, as different types of bowel content occupy different “layers” relative to gravity. Finally, the appearance of tubular plication, which appears as an eccentric accumulation of air, is caused by intestinal bunching occasioned by ingestion of linear foreign bodies (e.g., cloth, string).

The authors advocate two approaches to interpreting radiographs. The first is the gamut approach9 in which the radiographic signs are listed and the pathophysiologic or pathoanatomic causes are considered. The findings are then ranked in descending order of likelihood based on risk modifiers including clinical history, age, gender, breed, and species. Another helpful approach is the use of the so-called DAMNIT system, which adopts a pathogenesis approach to diagnosing radiographic findings. The disease processes considered in the DAMNIT system are degenerative, drug-induced, and developmental (D); anatomic, allergic, autoimmune, and anomalous (A); metabolic and mechanical (M); nutritional and neoplastic (N); infectious (bacterial, fungal, viral, parasitic), inflammatory, and immune-mediated (I); and, toxic, traumatic, and teratogenic (T). Judicious use of the DAMNIT approach limits the likelihood of a diagnosis being missed because it was not included in the gamut approach.

Specific Considerations for Different Organs of the Gastrointestinal Tract

Oral Cavity and Pharynx

Anatomic considerations

Coordinated prehension, mastication, bolus formation, and swallowing are all necessary to facilitate coordinated aborad propulsion of food and water into the esophagus and beyond. Any morphologic or functional abnormality can be detrimental to the process of ingestion (see Chapter 54). The anatomic structures important for ingestion include the tongue, teeth, supporting bones of the maxilla and mandible, oropharynx, hyoid apparatus, and larynx. The tongue is important for complex movements that allow for prehension, mastication, and swallowing. The teeth and the supporting bones of the maxilla and the mandible are needed for mastication, and the oropharynx, pharynx, and larynx are needed for propulsion, swallowing, and protecting the airway during swallowing, respectively. The hyoid apparatus acts as a suspensory mechanism for tongue and larynx during swallowing. Positioning is crucial for radiographs of the aforementioned structures to avoid erroneous interpretation. A lateral view provides the most information, but orthogonal views should always be obtained.10 The size and shape of the air-filled oropharynx varies with the position of the tongue, but the rectangular air-filled pharynx should maintain its size and shape independent of tongue position and phase of respiration (Fig. 26-6).

Abnormal Radiographic Signs

Close inspection of lateral and DV or VD views will elucidate some of the diseases of the pharynx, but most will require further investigation by visual inspection, special procedures (e.g., contrast studies), and alternate imaging studies (e.g., fluoroscopy, ultrasound, computed tomography [CT], or magnetic resonance imaging [MRI]). It is important to closely evaluate the surrounding soft tissues for air, which can be caused by penetrating foreign bodies, lacerations, or abscessation. Poor pharyngeal tone, pharyngeal collapse, mural infiltrates, edema, and mural or extrinsic masses may lead to an abnormal size and shape of the pharynx (Fig. 26-7). Extrinsic masses often lead to displacement of the pharynx, larynx, and/or the trachea and are also often associated with accompanying compressions of these structures (Fig. 26-7C). The hyoid apparatus may become displaced or may contain fractures in association with dysfunction (Fig. 26-7A and B).

A contrast procedure utilizing a nonionic iodinated contrast agent can be helpful when evaluating the pharyngeal region for draining tracts. It is important to determine the extent of tissue involvement, especially as it is not uncommon for foreign bodies to penetrate the oropharynx or pharynx. Interpretation of the study involves determining the direction and extent of the tract, communication of the tract with the oropharynx, pharynx, esophagus, or other regional structures, and the cause of the draining tract (Fig. 26-8).


Pharyngeal function is best observed fluoroscopically using liquid barium and barium-coated material (dynamic fluoroscopy).11 Sometimes an impression regarding pharyngeal function can be gained using static survey radiographs obtained during inspiration and expiration. Survey radiographic findings suggesting pharyngeal dysfunction include caudal migration of the hyoid apparatus during inspiration, hyperinflation of the pharynx during expiration, collapse of the pharynx during inspiration (see Fig. 26-7A), and persistent gaseous distention of the proximal esophagus. These findings should be further evaluated with the use of pharyngeal fluoroscopy. Diseases associated with pharyngeal dysfunction include myasthenia gravis and other muscular disorders; cricopharyngeal achalasia, which is characterized by cricopharyngeal hypertension and inability of the sphincter to relax; and cricopharyngeal dyssynchrony, which is characterized by an incoordination of pharyngeal contraction and cranial esophageal sphincter relaxation.11,12

Pharyngeal Diseases

Disease processes resulting in abnormal size and shape of the pharynx will be governed by multiple factors. Downstream resistance leading to hyperinflation can be caused by laryngeal paralysis, laryngeal neoplasia, laryngeal pyogranulomatous inflammation, and tracheal obstruction as may occur due to a foreign body. Abnormal pharyngeal tone may be caused by neuromuscular disorders such as myasthenia gravis. Upstream resistance may be a result of stenotic nares, severe obstructive rhinitis, nasal neoplasia, or an inflammatory polyp. Pharyngeal collapse or mural lesions may be caused by redundant pharyngeal tissue, pharyngeal edema, inflammation, polyp, granuloma, or neoplasia. Extrapharyngeal lesions, such as retropharyngeal lymphadenopathy, neoplasia, abscess, granuloma, or foreign body, may also lead to pharyngeal compression.

The location and shape of the larynx may be governed by similar and additional factors. As with pharyngeal disease, downstream resistance may be a result of laryngeal paralysis, laryngeal neoplasia, laryngeal pyogranulomatous inflammation, or various causes of tracheal obstruction, including foreign bodies. Extralaryngeal lesions, such as retropharyngeal lymphadenopathy, neoplasia, abscess, granuloma, foreign body, or failure of the supporting structures such as hyoid luxation or hyoid fracture can all lead to radiographic changes of the larynx. An additional important consideration is that of regional opacity. Radiographic signs noted may include extrapharyngeal or laryngeal radiolucency caused by regional emphysema from pharyngeal or laryngeal perforation. Regional mineralization due to neoplasia, chronic hematoma, abscess, pyogranulomatous inflammation, or foreign body may also be observed. Penetrating pharyngeal foreign bodies can be potentially serious events in the dog and cat.13 Metal foreign bodies are usually easily identified, but less-dense objects may be easily missed. The most frequent radiographic abnormality identified in patients with a pharyngeal foreign body is subcutaneous gas accumulation or gas in fascial planes or gas accumulating subcutaneously.13 In some cases, a foreign body may be misinterpreted as a soft-tissue mass caused by regional inflammation or edema.10


Abnormal Radiographic Signs

The normal esophagus is not visible on survey radiographs and needs to be rendered visible by contrast agents. The esophagus originates dorsal to the larynx and the proximal cervical trachea, courses gradually to the left of the trachea at the thoracic inlet to again become dorsal to the trachea in the cranial mediastinum, and track caudally dorsal to the heart to the esophageal hiatus of the diaphragm (Fig. 26-10). When rendered visible by positive contrast media, the location, morphology, and distribution of esophageal dysfunction can be identified (Figs. 26-11 and 26-12).6,14 In addition, when surface-coating agents such as barium are being used, the mucosal surface can be assessed. In normal patients the esophageal mucosal surface should be smooth and regular. The most common cause of ventrally displaced trachea is esophageal dilation.

Esophageal Diseases

As with other parts of the digestive system, a systematic approach should be used to arrive at a list of differential diagnoses based on radiographic findings. The peristaltic capacity of the esophagus can be altered by idiopathic megaesophagus, dysautonomia, esophagitis, overt electrolyte imbalance, heavy metal toxicity, congenital or acquired diverticula, and thyroid status (see Chapter 55). Downstream resistance can be the result of a stricture, tumor, anomalous constrictions such as vascular ring anomalies, or any intraluminal obstruction, such as a foreign body. Wall thickness and pliability can also be altered because of fibrosis, inflammatory reactions, or neoplastic infiltrates. The upstream delivery of ingested materials is governed by a primary esophageal peristaltic wave that takes less than 10 seconds to move from the pharynx to the stomach. Luminal distention triggers a secondary peristaltic wave that originates at the site of the distention and proceeds toward the stomach in less than 5 to 10 seconds. Esophageal abnormalities should be further classified as intraluminal, intramural, transmural, or extramural. Examples of intraluminal abnormalities are foreign material or retained ingesta because of ineffective peristalsis or increased downstream resistance. Intramural abnormalities can be a consequence of strictures or tumors. Transmural abnormalities can be caused by penetrating objects or by bronchoesophageal fistulae. Extramural abnormalities can be caused by compression, distortion, or displacement by a regional organ or mass. Finally, the esophageal mucosa should be evaluated. Mucosal ulceration can be caused by erosion from gastric or gastrointestinal reflux, trauma from lodged or migrating foreign bodies, tumors that disrupt the mucosal surface, or the ingestion of corrosive chemicals such as sodium hydroxide. Esophageal leakage because of traumatic or ulcerative lesions can lead to septic mediastinitis and should be carefully considered. In the authors’ experience, pneumomediastinum is a rare finding with esophageal penetrating lesions. The material that leaks into the mediastinum is usually of insufficient volume to noticeably distend the mediastinum. This makes the positive contrast esophagram all the more important in the assessment of a suspected esophageal perforation. This is usually initially performed with an iodine-based contrast agent, as these agents are isotonic and do not cause irritation if there is a tract involving the airway or lung. If the initial study is negative, it is followed by a barium study, which is more useful for detecting and localizing small leaks. Any barium contamination of the mediastinum should be surgically irrigated as soon as is practicable, but definitely in less than 12 hours.


Anatomic Considerations

The canine and feline stomach usually contains modest amounts of fluid, but generally not enough to cause the appearance of distention on any radiographic view or recumbency. If the stomach begins to appear rounded rather than elongated in the transverse plane in a fasted animal, it should probably be considered to be abnormal. The cardiac region is more or less anchored behind the left diaphragmatic crus as a result of the distal esophagus passing through the esophageal hiatus. The stomach position is influenced by the liver, and to a lesser degree by the spleen, regional masses, the morphologic and functional integrity of the diaphragm, as well as the degree of luminal distention. In general, the long axis (i.e., fundus to pylorus) of the stomach is parallel to the last few ribs in lateral recumbency and perpendicular to the spine in sternal or dorsal recumbency. The cardiofundic region of the stomach is in the left craniodorsal abdomen behind the left diaphragmatic crus and the gastroesophageal junction is located on the right medial aspect of the cardia. The pylorus is in the right cranioventral abdomen behind the liver near the right lateral abdominal wall in the dog, and just to the right of the midline in the cat. Cats usually have little gastric gas, whereas dogs often have equal amounts of fluid and gas in the stomach. The position of the fluid and gas as a function of gravity and recumbency can influence the appearance of the stomach, giving the illusion of a thickened wall (see Fig. 26-5) or a ball-shaped foreign body, which can be a result of fluid accumulating in the pylorus in right lateral recumbency. Depending on whether the patient is fed or unfed, the stomach may contain varying amounts of food. Unfortunately, many ingested foreign materials cannot be distinguished from food except by recheck radiographs after 18 to 24 hours. Digestible solid food will exit the stomach by 14 to 18 hours after eating, unless there is an outflow problem (see Chapter 26). In contrast, nondigestible foreign material will persist for many hours. Gaseous distention of the stomach has many causes including aerophagia and atony, which can be pathologic, but can also be caused by sedative or anesthetic drug effects. By comparison, gastric distention caused by an outflow obstruction is usually associated with a mixture of fluid and gas and is often accompanied by the retention of grainy material. This tends to accumulate proximal to the outlet obstruction and may gravitate within the lumen depending on patient recumbency. These opacities may be an indication for a foreign body or they may simply indicate the buildup of materials normally dispersed in food (e.g., bone fragments).

Functional Considerations

The stomach has many functions, including storage, digestion, propulsion of solids and liquids, and secretion of gastric acid, pepsins, and mucus (see Chapters 1 and 56). A disease process involving any one of these functions can result in gastrointestinal signs ranging from anorexia to vomiting and can affect the size of the stomach as well as the makeup of the luminal contents. Because of pyloric resistance, fluid may remain in the stomach for up to 2 to 3 hours after ingestion, whereas solid food requiring greater breakdown can remain in the stomach for up to 18 hours. From a practical standpoint, the least invasive and most cost-effective radiographic procedure to determine whether gastric outflow is decreased is to repeat survey radiographs the following day or to obtain an alternate radiographic view. However retention of material in the stomach can be secondary because of reflex inhibition of gastric emptying by various intestinal (see Chapter 1) and/or peritoneal conditions. However retention can also be pathologic as a result of pyloric stenosis, foreign body, or tumor. Considerations of the various causes of gastric retention may help to facilitate the decision to use endoscopy, upper GI, laparoscopy, or exploratory laparotomy to further investigate the problem. To further characterize stomach wall conditions, including those of the pyloric sphincter, contrast gastric radiographic studies should be considered. The most practical and useful is a barium-based upper GI study.

Abnormal Radiographic Signs

The normal and abnormal stomach are both visible on survey radiographs. Consequently, it is important to differentiate transient physiologic conditions, such as postprandial gastric distention from disorders that may cause alterations of gastric size, shape, and position. The localization of gas and fluid in the stomach depend on the position of the patient during radiography. Before arriving at a definitive diagnosis based on one radiographic view, additional views may provide helpful clues and limit the likelihood of premature or inaccurate diagnosis. Identification of discrete cardiofundic, gastroesophageal, and pyloric regions is clinically important and is usually accomplished by shifting the gas–fluid interface with changes in patient positioning (Fig. 26-13). Once an assessment of physiologic versus pathologic distention has been made, an assessment of gastric contents and presence or absence of segmental mural variations can be carried out. Other important parameters such as mucosal integrity, which may be altered because of ulceration, luminal status that may indicate the presence of a foreign body (Fig. 26-14), and wall thickness, which may be altered as a consequence of hypertrophy or tumor (Fig. 26-15), are best assessed by use of contrast studies.1519 Similarly, the position of the gastroesophageal junction, and integrity of the gastric wall can be best assessed by contrast studies. Finally, the initiation and continuity of the peristaltic wave (normally there are two to five peristaltic waves per minute) are best assessed by gastric contrast studies. The choice between barium and an iodinated contrast medium should be made based on the goals of the study, such as mucosal coating, assessment of peristaltic capacity and pyloric function, or the identification of a leak.

Gastric Diseases

The overall size and position of the stomach should be evaluated, particularly as it relates to regional changes, such as a diaphragmatic hernia or microhepatica. The position of each of the gastric compartments (i.e., cardia, fundus, corpus, antrum, and pylorus) should be assessed at the same time. The size of the stomach or any of its compartments is governed by multiple factors including peristaltic capacity, dysautonomia, inflammation, chemical corrosion, overt electrolyte imbalance, and smooth muscle disorders. Downstream resistance caused by pyloric spasm, stricture, or tumor, for example, can affect gastric size and position. Intraluminal obstruction and alterations in gastric wall pliability have predictable effects on gastric size, position, and function. In case of a gastric outflow problem, reflex inhibition of gastric emptying may be contributing to the overall pathology. As with other components of the gastrointestinal tract, a useful classification of gastric abnormalities is to consider them as intraluminal, intramural, transmural, or extramural. Gastric mucosal abnormalities, including ulceration, can be caused by acid or peptic erosion, trauma from stationary or migrating foreign bodies, surface disruption caused by tumors, and ingestion of corrosive substances. Although the assessment of gastric erosions is possible by radiographic and ultrasonographic techniques, it is best assessed by endoscopy. Gastric perforation should always be ruled out as leakage from traumatic, neoplastic, and ulcerative lesions may lead to septic peritonitis. In the authors’ experience, horizontal-beam radiography is a clinically useful tool for the initial assessment of free peritoneal air and suspected gastric perforation. If free peritoneal air is found and cannot be attributed to a recent surgical intervention, the next step should be a positive-contrast gastrogram. That said, the identification of unexplained free intraperitoneal air is always sufficient justification for an exploratory laparotomy. If a contrast procedure is deemed necessary to further investigate potential gastric leakage, this is usually performed initially using iodine-based contrast agents and, if negative, followed by barium, which is more useful for detecting and localizing small leaks. Any barium contamination of the peritoneum or retroperitoneum should be surgically irrigated as soon as is practicable, but definitely in less than 12 hours.

Small Intestine

Anatomic Considerations

The amount of gas normally contained in the small intestine differs between dogs and cats. In dogs, typically 30% to 60% of the small bowel volume is gas unless the space is occupied by ingesta. In contrast, the normal adult cat usually has almost no gas in the small bowel unless ingesta are moving through it. In aged cats, there may be some stress-induced excess small intestinal gas, which must be taken into account during interpretation. Excessive, but evenly distributed, gas in the small bowel has been reported in association with mesenteric volvulus. The small intestine is usually evenly distributed along the ventral abdominal wall caudal to the stomach and liver and cranial to the urinary bladder. On VD views of obese cats, the left perirenal fat may give the illusion of right displacement of the small intestine. Both species have some degree of diffuse mucosal surface irregularity as can be identified using positive contrast materials. These irregularities in normal patients are a result of the normal mucosal villi.20,21 In both species small intestinal loops should appear homogeneous throughout. That said, active peristalsis will influence the perceived external diameter of small bowel loops and some variation of bowel loop diameter may be observed. The best approach to assess effects of peristalsis is to repeat survey radiographic views. This assures that patterns are not reproducible and that changes of loop content do occur.

To facilitate assessment of small intestinal size, the authors suggest a subjective comparison of the diameter of the small intestinal lumen to the size of a midlumbar vertebral endplate. Canine and feline small bowel diameter is usually less than the DV diameter of a vertebral endplate. Further assessment and/or observation by repeat radiographic views should be considered if the small bowel diameter exceeds the transverse diameter of a vertebral endplate. If the small bowel diameter approaches or exceeds the length of a vertebral body, pathologic dilation is likely present. Small bowel dilation can be a result of obstruction, inflammation (e.g., luminal, mural, or serosal), trauma, irritation (e.g., by a migrating foreign body), or ischemia (incarceration or thromboembolism). Ingesta have a grainy appearance and, if present, should be evenly distributed throughout the length of the small bowel. Well-organized contents, particularly when seen in only one section of small bowel, should be noted. Such organized contents are a common indicator of a soft-tissue opacity foreign body. Reproducibly dilated small bowel segments can be associated with foreign bodies, annular lesions (i.e., tumor or stricture), intestinal incarceration, and infiltrative bowel disease–induced bowel wall rigidity. In addition, gas in the bowel lumen should be centrally distributed. Peripherally distributed gas may indicate a diverticulum or plication because of a linear foreign body. Finally, gas in the gut wall, which is termed pneumatosis intestinalis, can be associated with ulceration, ischemic necrosis, or bacterial translocation.

Functional Considerations

The small intestine has many functions, including propulsion and segmentation, secretion, digestion, and absorption. These functions are achieved through the interaction of peristalsis, small intestinal microbiota, and the nutritional or volume effects of the chyme. A problem with any one of these can result in signs of gastrointestinal disease, ranging from anorexia to vomiting and diarrhea, and can affect the size of the small bowel as well as its luminal contents. Fluids normally move through the bowel in 2 to 3 hours, whereas solid food is slower and quite dependent upon the rate of gastric emptying. As with gastric studies, the least-invasive and most cost-effective special radiographic procedure may be repeated or alternate radiographic views may be obtained within 24 hours. Retention of material in small bowel segments can be a result of inadequate short-segment peristalsis; downstream intraluminal mechanical impedance to fluid, gas, or ingesta; or local disruptions in bowel homeostasis as a consequence of mucosal, mural, or perimural inflammation. Other intestinal pathologies, like ischemia, fibrosis, neoplasia, malabsorption, maldigestion, and bacterial overgrowth, can contribute to chyme retention. The decision to commit to small bowel endoscopy requires consideration of disease type and distribution as well as potential morbidity associated with anesthesia. Other imaging procedures, particularly upper GI contrast studies and abdominal ultrasound, may provide useful, cost-effective information with less patient stress.

Abnormal Radiographic Signs

In most patients, the normal or abnormal small bowel is readily visible on survey radiographs. The key to interpretation of survey radiographs is to be sure to differentiate transient physiologic changes such as postprandial luminal distention from pathologic changes in small bowel size, shape, and position. The localization of gas and fluid in the small bowel depends on the position of the patient during radiography. Several survey radiographic views may help to differentiate pathologic from physiologic findings. As discussed previously, the type and distribution of luminal contents may provide clues to the duration and severity of a disease process. In general, large-diameter intestinal loops are associated with greater duration, and high gas-to-fluid ratios are associated with greater severity. It should be kept in mind that bowel dilation has four specific causes: obstruction, inflammation, trauma or irritation, and ischemia. Large quantities of gas involving many or all small bowel segments may be a result of ischemia or severe serosal inflammation (Fig. 26-16), and not necessarily distal small bowel obstruction. There is also a continuum of bowel luminal compromise ranging from partial to complete obstruction. The recognition of these differences is paramount to the diagnostic process. Partial obstructions are usually segmental (i.e., two distinct populations of bowel can be recognized) and are characterized by greater fluid accumulation (than gas) whereas a complete or high-grade obstruction is characterized by equal amounts of gas and fluid or a clear predominance of gas. Partial obstructions can be due to mural lesions, such as tumors, strictures, or other intraluminal abnormalities like foreign bodies. The relative likelihood of these vary with age, behavior, and medical history (Figs. 26-17 and 26-18). As a general rule, conditions associated with large gas accumulations in the small intestine tend to be surgical diseases. Those conditions with proportionally more fluid than gas are more of a diagnostic and prognostic dilemma.2225 Assessment of small bowel mucosal changes (e.g., ulcerations), luminal abnormalities (e.g., foreign bodies), wall thickness, peristaltic capacity and continuity, as well as wall integrity is best achieved by positive contrast studies. However the choice of barium versus an iodinated contrast medium should be made based on the goal of the study, including mucosal coating, assessment of peristaltic capacity, or the identification of a leak. The role of abdominal ultrasonography in relation to contrast radiography has not been systematically studied and needs to be determined in large prospective studies. Concerns persist regarding the predictability of the sonographic diagnosis in the hands of inexperienced individuals and problems associated with user-dependent techniques such as ultrasound.2325

Small Bowel Diseases

Differentiation of small intestinal abnormalities into intraluminal, intramural, transmural, or extramural lesions is useful not only for narrowing the list of differential diagnoses, but also for determining what diagnostic or therapeutic procedures may be useful in the further management of the patient.

Small intestinal mucosal ulceration is uncommon, except when associated with neoplasia. Smoothly marginated rectangular outpouchings of the duodenal lumen are occasionally seen in normal dogs, and have been termed pseudoulcers (Fig. 26-19). Although identification of small bowel surface erosion is possible by radiographic and ultrasonographic techniques, these lesions are best assessed by endoscopic techniques bearing in mind that only the descending duodenum and the ileum are accessible using this technique. Moreover, visual assessment of the mucosal brush-border to diagnose small intestinal disease or to differentiate between different disease processes has not been shown to be clinically useful.21 As with gastric pathology, the unexplained presence of free peritoneal air is usually associated with alimentary tract perforation. If there is any question about the presence or absence of free air, horizontal-beam radiography is a very useful technique (Fig. 26-20). The assessment of free peritoneal air as discussed for gastric disease is equally applicable for diseases of the small bowel.

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Jul 10, 2016 | Posted by in INTERNAL MEDICINE | Comments Off on Diagnostic Imaging of the Gastrointestinal Tract
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