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21

Normal Gastrointestinal Anatomy
Judith Hudson

Department of Clinical Sciences, College of Veterinary Medicine, Auburn, AL, USA

21.1 Normal Radiographic comma‐shaped, and rarely contains gas. The colon has a
Anatomy variable size and is usually either filled with fecaloid mate-
rial or gas or is empty. Smooth nongranular contents can
The esophagus is fluid opaque and blends in with other indicate the presence of fluid in cases of diarrhea. In an
fluid‐opaque structures in the cervical region and mediasti- evaluation of colonic diameter in cats, comparison was
num (Figure 21.1). It occasionally can be seen as a vague made between 50 normal cats and cats with megacolon or
opacity in the caudodorsal thorax, particularly if it is constipation. The ratio of maximal diameter of a normal
distended at the time of radiography. Gas in the lumen may feline colon to L5 was determined to be less than 1.28 [2].
reveal its presence in other parts of the thorax and the neck
but the gas is normally transient. 21.2 Contrast Radiography
On a lateral projection of the abdomen (Figure 21.2), the
normal gastric axis is 30° caudal to a line perpendicular to
the spine. Most cats have a large falciform fat pad elevating 21.2.1 Commonly Used Contrast Media
the liver and stomach. On the ventrodorsal or dorsoventral Contrast radiography or ultrasonography is frequently nec-
projection, the feline stomach is J‐shaped with the pylorus essary for complete evaluation. Positive contrast agents
superimposed on or to the right of the spine. The stomach include barium sulfate and iodinated contrast [3]. Barium
contains a variable amount of gas and ingesta. In some sulfate is less expensive than iodinated contrast but its
cats, a radiolucent band can be seen in the gastric wall, main positive attribute is its ability to coat the mucosa. It is
which appeared to correlate with a hypoattenuating layer not absorbed or diluted, isotonic, and stays in suspension.
in four of 15 cats on computed tomography (CT). The CT It can only be given orally. Not uncommonly, clinical signs
finding and the finding of fat in this area of one cadaver may improve subsequent to a barium series, possibly due to
suggest that the radiolucent band represents fat in the an antiprostaglandin effect. Iodinated contrast agents can
submucosa [1]. be divided into two categories: ionic and nonionic. Ionic
Other portions of the gastrointestinal tract (GIT) can be contrast agents are hyperosmolar because each molecule
identified because of intraluminal gas or their shape. dissociates into positive and negative ions. Nonionic water‐
Normal small intestinal loops are seen as longitudinal soluble iodinated contrast does not have this property.
undulating fluid opacities. Gas is generally present in the Some products are manufactured specifically for GI appli-
stomach and small intestine but the diameter of the small cations but iodinated contrast intended for intravenous use
intestine should not exceed twice the height of the central can be diluted and given orally.
part of a lumbar vertebra or about 12 mm from serosa to Although the most common GI contrast medium is
serosa. In transverse images, the intestine may resemble a barium, it is contraindicated if a ruptured or lacerated
donut if there is a small amount of gas in the lumen. If a esophagus or GIT is suspected because barium is harmful to
radiopaque foreign body is seen, always check for evidence the mediastinum or peritoneum. A fulminating granuloma-
of perforation or obstruction. Unlike the large pigtail‐ tous inflammatory response can result from the presence of
shaped cecum of the dog, the cecum of a cat is small, barium in the peritoneal cavity. Barium can also create an

Feline Diagnostic Imaging, First Edition. Edited by Merrilee Holland and Judith Hudson.
© 2020 John Wiley & Sons, Inc. Published 2020 by John Wiley & Sons, Inc.