Introduction

The gastrointestinal system is one of the most evaluated systems in small animal veterinary practice. A thorough clinical history and physical examination are critical in attempting to narrow down the area to be evaluated in these patients. The complexity of the anatomy and the varied complex disease processes involved in the patient’s clinical problem can make assessment difficult. Radiography is an excellent survey tool for evaluating the gastrointestinal tract. With the addition of contrast medium, one can evaluate the gastrointestinal tract in a more comprehensive manner.

Diagnostic Imaging Modalities

Radiography is routinely used in evaluating the gastrointestinal tract. Unfortunately, the fluid contents of the gastrointestinal lumen are soft tissue opaque, the same as the soft tissue opaque gastrointestinal wall. To improve our assessment of the gastrointestinal tract, we can use contrast radiography, whereby material of a different opacity is introduced into the gastrointestinal lumen. Gas opaque negative contrast medium (such as room air or carbon dioxide gas) can be introduced via tube into the esophagus, stomach, or colon. Mineral to metal opaque positive contrast medium (such as barium or iodine‐based solutions) can be administered to assess function in real time via fluoroscopy or radiography, as well as the morphology of the lumen of the entire gastrointestinal tract. The risk of aspiration of positive contrast medium must be mentioned, as aspiration of barium may act as a physical barrier to gas exchange (Figure 25.1) whereas iodine may be associated with pneumonitis.

Ultrasonographic evaluation of the gastrointestinal tract is commonly done, replacing abdominal radiography in many institutions. However, the two imaging modalities are complementary and initial survey abdominal radiographs can help in dictating the course of the ultrasound examination. The wall of the esophagus, stomach, small and large intestines is composed of four layers: inner mucosa, submucosa, muscularis, and serosa. The high spatial resolution of ultrasound allows assessment of wall thickness, and the integrity of the wall layers.

Computed tomography (CT), with its speed of image acquisition and ability to image larger patients, is increasingly being employed in the workup of gastrointestinal cases, with and without the administration of intravenous or luminal contrast medium.

Stomach

The stomach is a large tubular organ which extends between the caudal esophageal sphincter and the pyloric sphincter. It is located caudal to the liver and cranial to the colon, pancreas, spleen, and kidneys. The stomach is divided into fundus, body, and pylorus. The pyloric antrum continues to the pyloric sphincter and proximal duodenum.

The stomach is routinely evaluated using radiography and ultrasonography, radiography acting as a screening test for gastric disease and ultrasound yielding further information about mural and intraluminal changes. CT is being used more and more as an adjunct to these modalities in complicated or equivocal cases. In human radiology, magnetic resonance imaging (MRI) is being increasingly used for gastrointestinal imaging but is not routinely used in veterinary medicine.

Ultrasonographically, the gastric wall is composed of four layers, like the rest of the gastrointestinal tract: inner mucosa, submucosa, muscularis and serosal layers. The mucosal surface is corrugated by indentations called rugal folds within the gastric fundus and orad gastric body but not within the pylorus. Due to the indentation of the mucosal rugal folds, the wall thickness must be measured at the thinnest point, which would be at the base of the rugal fold. The normal wall thickness should be 2–5 mm in the dog and 2–4 mm in the cat; however, it must be remembered that stomach filling will affect wall thickness. A limitation of ultrasound is the effect of gas dilation of the stomach leading to inability of ultrasound beam penetration.

Radiographically, normally the empty stomach is soft tissue opaque, and border effaces with the hepatic silhouette. With gas in the stomach, the gastric location can be better determined. In left lateral recumbency, gas moves freely into the gastric pylorus, pyloric antrum, and proximal duodenum. In right lateral recumbency, gas moves into the gastric fundus. In dorsal recumbency, gas moves into the gastric body. Lastly, in sternal recumbency, gas moves into the gastric body and pylorus. As such, each component of the stomach lumen can be evaluated if a moderate amount of gas is present in the gastric lumen.

The gastric axis is a line drawn from the gastric fundus to the gastric pylorus. On lateral view, this line should be within an angle parallel with the 10th ribs, or perpendicular to the long axis of the L2 vertebral body. On ventrodorsal or dorsoventral views, the gastric axis should be perpendicular to the vertebral column in the dog. In the cat, the stomach forms more of a “J‐shape,” with the pylorus located in a more caudal position relative to the fundus. Additionally, the pyloroduodenal junction is located close to midline in a cat and not along the right lateral body wall, as noted in a dog.

Contrast gastrography can be done using negative and/or positive contrast medium. As such, pneumogastrography can be done using an orogastric or nasogastric tube with the introduction of gas into the gastric lumen (2–20 mL/kg; Figure 25.2) [1].

Alternatively, positive contrast medium gastrography, either alone or as part of an upper gastrointestinal study, can be performed whereby in dogs <20 kg, 8–12 mL/kg of 30–60% w/v barium sulfate solution can be administered and in dogs >20 kg, 5–7 mL/kg of 30–60%w/v barium sulfate can be administered either by syringe or via orogastric tube [2]. Liquid barium contrast should progress through the stomach so that there is gastric emptying in the normal dog by 3 hours and in the normal cat by 1 hour [3].

Computed tomography is useful in investigation of the stomach in that superimposition is avoided and the differences between soft tissue‐ and fluid‐attenuating material can be appreciated, especially post intravascular contrast administration with enhancement of the gastric wall.

Gastric Dilation

Gastric dilation can be identified radiographically as a gas‐ or soft tissue opacity‐filled stomach which extends caudal to the costal arch. The gastric fundus, recognized by the rugal folds extending into the gastric lumen, remains in a normal position within the left dorsal abdomen. Gas distension of the stomach can be secondary to aerophagia secondary to anesthesia or sedation. The stomach can be filled with mixed soft tissue and gas opaque material, or well‐defined ovoid‐shaped soft tissue opaque material corresponding to food engorgement, or due to gastric outflow obstruction secondary to pyloric hypertrophy, gastritis, pyloric neoplasia or foreign bodies [4] (Figures 25.3 and 25.4).

Foreign Bodies

Gas‐filled, mineral or metal opaque foreign bodies are easier to identify within the gastric lumen on routine radiography (Figures 25.7 and 25.8). Frequently, patients with gastric foreign bodies are presented with a history of vomiting or regurgitation. As such, clinical signs and radiographic features of aspiration pneumonia must be considered. Gastric foreign bodies which are soft tissue opaque can be difficult to see if surrounded by soft tissue opaque stomach wall and luminal contents. They are easier to identify with gas surrounding the foreign body (Figure 25.9). Positive contrast gastrography can also be useful in highlighting soft tissue opaque foreign bodies (Figure 25.10).

Given that the variable opacities of gastric foreign bodies and the gastric ingesta may be similar, it is frequently difficult to determine the difference between normal ingesta and a clinically significant obstructive foreign body. Screening radiographs may show suspicion of gastric outflow obstruction with/without gastric dilation, however, if the patient has been vomiting secondary to gastric outflow obstruction. In such cases, ultrasound is useful in the assessment of the gastric wall and contents, taking the limitation of gas hindering penetration of the ultrasound beam into account (Figure 25.11).

Gastric Neoplasia

Gastric neoplasia includes gastric carcinoma, lymphoma, and leiomyomas. Gastric adenocarcinoma accounts for up to 80% of gastric neoplasia in the dog. Gastric neoplasia may be associated with intraluminal extension of the mass and can be easily confused with gastric foreign bodies. Pneumogastrography or positive contrast gastrography can identify a soft tissue opaque nodule or mass confluent with the gastric wall.

Gastric ultrasound is useful in characterizing the nature of the gastric wall thickening, to differentiate gastric neoplasia, gastric polyps, and foreign bodies.

In more complex cases and in areas where ultrasound is unable to penetrate to image the stomach due to gastric, intestinal, or even free peritoneal gas, CT can be used to assess the gastric wall (Figures 25.12–25.14).

Gastritis and Ulceration

Gastritis can be caused by ingestion of a chemical or physical irritant, or as a side‐effect of drugs. Radiographically, the stomach often has a normal appearance, in the absence of the identification of foreign bodies. Ultrasonography can be useful to assess gastric wall thickness in cases of gastritis, and in more severe cases aid in the detection of gastric wall ulceration. Gastric ulcers are erosions of the gastric mucosa, whereby gastric contents can penetrate the wall of the stomach. Ultrasonographically, mobile echogenic fluid or gas can be identified in defects within the gastric mucosa (Figure 25.15). Gastric ulcers may also extend through the entire gastric wall, resulting in gastric perforation.