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The small intestine

image of The small intestine
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Abstract

The small intestine comprises the duodenum, jejunum and ileum. On a lateral abdominal radiograph, the small intestine occupies most of the mid-ventral abdomen, lying caudal to the stomach and cranial to the bladder. It appears as smoothly curving ‘tubes’ in long axis views and as circular or ring-shaped opacities in cross section. Peristaltic waves cause transient segmental narrowing, which may be recognized on plain radiographs. The duodenum runs cranially and laterally from the pylorus and then turns caudally, forming the cranial duodenal flexure. The flexure is held against the caudal surface of the right liver lobes by the hepatoduodenal ligament. The descending duodenum runs dorsally and caudally along the right abdominal wall, before turning medially at the caudal duodenal flexure. The ascending duodenum runs cranially and to the left of midline, where it becomes the jejunum at the duodenojejunal junction. The duodenum may occasionally be recognized on a plain radiograph from its characteristic position; it is relatively fixed compared with the jejunum and ileum, which have a long mesentery and are readily displaced by changes in adjacent structures. The duodenum is also slightly wider than the jejunum and ileum. The terminal ileum may be recognized in some cases at the ileocaecocolic junction, which lies at approximately the level of the fourth lumbar vertebra on a lateral abdominal view. On a ventrodorsal (VD) view, the caecum lies on the right at the level of the second to fourth lumbar vertebrae. The jejunum and ileum are otherwise radiographically indistinguishable. Normal radiographic anatomy; Contrast radiography; Ultrasonography; Overview of additional imaging modalities; and Intestinal diseases are discussed in this section.

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Figures

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10.1 VD radiograph of the abdomen in a dog following administration of liquid barium, illustrating the position of the duodenum.
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10.3 Horizontal beam lateral radiograph of the abdomen in a dog, following administration of liquid barium. Barium helps to highlight the fluid levels within the stomach and small intestine, which are at a similar level here, making a functional ileus the most likely diagnosis.
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10.4 Summation of intestinal wall and fluid contents can mimic wall thickening on plain radiographs. Thickened intestinal wall. Normal intestinal wall and fluid contents. These may look identical on plain radiographs; contrast studies and ultrasonography provide information on wall thickness. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and are printed with her permission.
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10.6 Lateral radiograph of the abdomen in a dog taken 24 hours following administration of BIPS, showing all the BIPS in the colon (i.e. normal transit). Lateral radiograph of the abdomen in a cat taken 24 hours following BIPS administration, showing all the large BIPS and some small BIPS accumulating in the small intestine, proximal to a partial obstruction. Some of the small BIPS are seen in the descending colon.
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10.7 Lateral radiograph of the abdomen in a dog following liquid barium administration, illustrating the normal duodenal out-pouchings or ‘pseudoulcers’. VD radiograph of the abdomen in a cat following administration of liquid barium, illustrating segmentation of the duodenum and proximal jejunum, sometimes referred to as a ‘string of pearls’ appearance.
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10.8 Ultrasonographic appearance of the normal small intestinal wall. 1 = Lumen (containing mucus); 2 = Mucosa; 3 = Submucosa; 4 = Muscularis.
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10.10 Ultrasonogram showing a longitudinal view of the ileocaecocolic junction. Note the prominent submucosal layer of the ileum and the relatively thin wall of the colon compared with the terminal ileum.
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10.12 Transverse ultrasonogram of a section of small intestine illustrating the appearance of an ‘extended mucosal interface’.
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10.13 Ultrasonograms illustrating different intestinal contents. Gas on the left, seen as an echogenic line with distal shadowing, and mucus on the right, seen as a thin echogenic line. Fluid seen as predominantly anechoic contents. Ingesta seen as moderately echogenic contents, which may contain ‘swirling’ echoes when imaged in real time.
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10.14 Transverse CT image of a normal canine abdomen at the level of the ileocolic junction. (Courtesy of T Schwarz)
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10.15 VD view of a canine abdomen, showing gas-filled loops of small intestine within an inguinal hernia.
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10.16 Lateral view of the thorax and cranial abdomen in a dog, showing gas- and fluid-filled loops of small intestine overlying and cranial to the cardiac silhouette; these loops were contained within a peritoneopericardial diaphragmatic hernia (PPDH). Note the ‘empty’ appearance of the abdomen and the lack of small intestine in the normal position.
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10.17 Lateral radiograph of the abdomen in a dog, showing caudal and ventral displacement of the small intestine by an ill defined dorsal abdominal mass. The mass was confirmed on ultrasound examination to be of renal origin.
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10.18 Close-up view of a VD radiograph of the abdomen in a dog following barium administration. Obvious corrugation of the small intestine is visible. Ultrasonogram of corrugated small intestine. Note the undulating appearance of the hyperechoic luminal and submucosal layers.
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10.19 Ileus. Lateral radiograph of a dog showing generalized gaseous distension of the small intestine. Lateral radiograph of a cat showing distension of the small intestine by a mixture of gas and fluid. Lateral radiograph of a cat showing small intestinal distension; administration of barium has highlighted the areas of fluid distension.
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10.20 Lateral radiograph of a cat showing a moderately distended loop of small intestine filled with particulate mineral material, running vertically across the caudal abdomen. This is a ‘gravel sign’ and consistent with a chronic partial obstruction. Normal faecal boluses can be seen in the descending colon.
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10.22 Lateral radiograph of a dog taken following administration of barium. The stomach and transverse colon are displaced caudally by moderately dilated small intestine. The oesophagus is also dilated. Free gas can be seen between the liver and diaphragm, consistent with intestinal rupture. The surgical diagnosis was small intestinal entrapment in a mesenteric tear.
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10.23 Examples of radiopaque foreign bodies. Button. Jack. Stone. Skewer (with incidental shotgun pellet).
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10.24 Examples of radiolucent foreign bodies . Corn-on-the-cob. Cork. Peach kernels. Nuts. The characteristic pattern of gas seen in/around these objects may aid in their identification . A peach stone within the small intestine: the ‘internal’ kernel is highlighted by an ovoid gas shadow (arrowed).
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10.25 Close-up of a VD radiograph of the abdomen in a dog showing a characteristic striated lucent pattern, resulting from gas trapped within a fabric foreign body (in this case a sock).
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10.26 Lateral abdominal radiograph of a cat following liquid barium administration. Note the duodenal distension and the ill defined filling defect. VD radiograph taken 24 hours after barium administration. Most of the barium is in the colon; however a small amount is retained within the duodenal foreign body (arrowed).
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10.27 VD radiograph of the abdomen in a dog. Contrast material can be seen leaking out of the jejunum, just medial to the descending duodenum.
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10.28 Ultrasonogram showing a shadowing, echogenic linear structure (arrowed) within the lumen of a distended area of small intestine. This appearance is consistent with an intestinal foreign body. Note also the shadowing artefact (arrowhead).
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10.29 Linear foreign bodies. Plain lateral radiograph of the abdomen in a dog showing variably sized and shaped gas bubbles within the small intestine. Lateral radiograph of the abdomen in a cat following contrast medium administration, highlighting the ‘bunched’ appearance of the small intestine.
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10.31 Different patterns of intestinal wall thickening, frequently associated with neoplasia, which may be recognized with ultrasonography or contrast radiography. Annular. Luminal narrowing giving an ‘apple-core’ appearance. Mural. Eccentric narrowing of lumen. Multifocal. Asymmetrical areas of thickening along intestinal wall. Ulceration. Irregular wall thickening, barium may remain adherent to ulcerated areas. Pedunculated mass. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and are printed with her permission.
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10.32 Ultrasonograms of small intestinal masses in three different patients. Longitudinal image showing symmetrical wall thickening with loss of layering and an irregular lumen. Transverse image showing loss of layering and eccentric wall thickening. Longitudinal image showing localized intestinal dilatation associated with an intraluminal mass. In all cases reverberation artefacts from luminal gas help to identify the mass as being of intestinal origin.
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10.34 Radiographic features of intussusception. Plain lateral abdominal radiograph showing a sausage-shaped soft tissue opacity in the mid-abdomen. A gas lucency highlights the crescenteric end of the intussuscipiens in an ileocolic intussusception.
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10.35 Ultrasonograms of an intussusception in longitudinal and transverse section. Note the multilayered appearance.
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10.36 Lateral abdominal radiograph of a dog with intestinal lymphoma, taken following administration of barium. The small intestine has a chewed out, ‘apple-core’ appearance with multiple asymmetrical areas of luminal narrowing.
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10.37 Transverse ultrasonogram showing a thickened loop of intestine with complete loss of layering. This appearance is typical of, but not specific for, intestinal lymphoma.
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10.38 Ultrasonogram showing a loop of small intestine deep to the spleen. Transversely oriented echogenic striations can be seen within the mucosal layer, which are characteristic of lacteal dilatation.

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