Introduction
This session will use case studies to review important aspects of imaging disorders of the oesophagus and gastrointestinal tract of small animals. In short, radiography permits an opportunity to visualize the serosal and mucosal surfaces of the stomach and intestines, while diagnostic ultrasound permits visualization of these structures as well as the individual layers of visceral walls. Video fluoroscopy permits appreciation of motility abnormalities which, in some circumstances is preferable to studying motility with ultrasound. Contrast radiography opacifies the visceral lumen and defines the mucosal margins clearly.
The Oesophagus and Cardia
Survey radiography only visualizes the oesophagus if the lumen contains gas or opaque ingesta such as bones or other foreign material. When not dilated and not opacified [positively or negatively] the oesophagus negatively silhouettes with surrounding tissues and is either poorly or not visualized.
Traditionally, the lumen of the oesophagus is visualized following oral administration of a radiopaque contrast medium, commonly barium or iodine. The lumen is also directly visualized using oesophagoscopy.
Functional studies require fluoroscopy, and the current modality of choice is image intensified video fluoroscopy, which is done when the animal is conscious.
Ultrasonography can be used to inspect the cervical oesophagus and the oesophagus at the cardia, but its usefulness is restricted to circumstances when gaseous material isn't present within or around the oesophagus.
The Stomach and Pylorus
Survey radiography permits visualization of the gastric lumen, and if there's adequate abdominal adipose tissue, its serosal margin. The lumen is best visualized when there is intraluminal gas and when it is present it can be moved to the radiographers advantage by postural variation so that the gas can fill the area of interest. For example, gas will rise into the fundus when the animal is in right lateral recumbency, but will move into the pyloric antrum when than animal is rotated so that it is in left lateral recumbency. Any fluid in the stomach will move in the opposite direction.
Negative contrast gastrography takes advantage of the usefulness of gas within the stomach, and is a relatively simple procedure in sedated or anaesthetized animals. Positive contrast gastrography, using barium or iodine, permit evaluation of the gastric lumen, the gastric mucosal margin and over time can be used to assess gastric emptying.
Gastroscopy permits visualization of the gastric contents and the gastric mucosa. Flexible fiberoptic gastroscopes also facilitate mucosal biopsy.
Video fluoroscopy permits examination of gastric motility and gastric emptying. It is particularly useful where there are abnormalities causing pyloric outflow obstruction but the study of gastric contractions can also assist location of abnormalities in the stomach wall such as ulcers or infiltrative masses.
Ultrasonography is extremely useful to evaluate the gastric wall and the pylorus. Wall layering is clearly visible and gastric motility can be observed. At the pylorus the gastroduodenal junction can be seen, and the layering and thickness of the pylorus measured. Ultrasonography of the stomach is limited in the presence of gastric intraluminal gas, but is its usefulness is enhanced when there is fluid or ingesta in the stomach. Ultrasonography assists collection of material for cytology or histology, as instruments used to collect tissue can be visualized and guided into the target area. In this respect ultrasonography is superior to endoscopy, which is restricted to collection of material from the mucosal layer.
The Small Intestines and Ileo-caeco-colic Junction
Survey radiography permits visualization of the intestinal lumen when there is gas or ingesta present. In animals with a moderate volume of omental adipose tissue the intestinal serosal surfaces can also be visualized. Indeed, it is the loss of serosal visibility that is used to assess whether or not an animal may have ascites. The intestines are poorly visualized when fat or gas is absent and in juveniles, whose fat composition makes it similar in opacity to soft tissue.
Positive contrast intestinal studies generally use barium, but iodine can be substituted when intestinal perforation is suspected. Positive contrast media permit evaluation of the lumen, so their usefulness is generally confined to searching for intraluminal or intramural intestinal obstruction. Barium contrast permits visualization of the mucosal interface, so mucosal irregularities or thickening may be detected in this way.
Video fluoroscopy can provide useful information about intestinal motility, but is not commonly used for this purpose.
Endoscopic examination of the small intestine requires an experienced operator and is usually confined to the descending duodenum.
Ultrasonography permits examination of the entire small intestinal volume. It provides exquisite detail of intestinal layering and is capable of detecting small changes in intestinal thickness. As well as infiltrative diseases, intraluminal obstructions such as foreign bodies or intussusceptions can be visualized. Gas, the enemy of ultrasound, sometimes prevents complete examination of the small intestines but in these circumstances survey radiography is the perfect foil, as gas is the radiographer's friend.
The ileocaecal junction can be identified by its characteristic anatomy.
Ultrasonography assists tissue collection by fine aspiration or true cut biopsy. It also permits evaluation of adjacent structures such as the pancreas, the common bile duct and lymph nodes. In many institutions abdominal radiography has been replaced by ultrasonography as the primary imaging modality of abdominal disorders.
The Large Intestine
Survey radiography permits evaluation of the large bowel and is enhanced when there is gas or opaque material in the large intestinal lumen. The caecum has a characteristic appearance and reliable location. The large intestine can usually be distinguished from the small intestines by its size, shape and location.
Contrast radiography of the large bowel isn't commonly practiced, largely because it's a messy procedure and requires prior bowel preparation and general anaesthesia. You can use either negative [air] or positive contrast media, but the volume required can be large as it must approximate the potential space within the colon and caecum.
Colonoscopy has largely replaced barium enemas, as is the procedure of choice when large bowel disease is suspected.
Ultrasonography can be used to assess the large intestine, but the presence of gas within the large intestinal lumen [a common finding] can limit the usefulness of ultrasound. When the bowel is evacuated [of solids as well as gas] ultrasound permits visualization of wall layering and the mucosal margins.
Rectum and Anus
Visual inspection can be helpful.
Ultrasonography of the anus, the anal saccules and the rectal canal can be achieved with transducer placement on the perineum.