Duodenum

of the liver (see Fig. 5.29b). The antrum is more elongated and is seen behind the left lobe of the liver, just to the right of the midline.

In the infant, ultrasound is used in the study of the pylorus in cases of suspected pyloric stenosis. Pyloric muscle thicknesses greater than 4 mm and muscle lengths greater than 18 mm are considered abnormal.

Angiography of the coeliac trunk (Figs 5.12 and 5.13) This is used to image the vessels that supply the stomach. The stomach can be filled w i th gas to eliminate confusing rugal patterns, and to push other bowel loops out of the field of interest.

THE DUODENUM (Figs 5.14 and 5.15) The duodenum extends from the pylorus to the duodenojejunal flexure, where transition to the small bowel proper is marked by the assumption of a mesentery. The first 2.5 cm of duodenum, like the stomach, is attached to the greater and lesser omentum. The remainder of the duodenum is retroperitoneal and, as a result, less mobile than the small intestine. Its anterior surface is covered by peritoneum,

Fig. 5.13 Branches of the coeliac artery.

except where the second part is crossed by the transverse mesocolon and the third part by the superior mesenteric vessels in the root of the mesentery.

The duodenum curves in a C shape around the head of the pancreas. It is described as having four parts: the first (or superior), second (or descending), third (or horizontal) and fourth (or ascending). These measure approximately 2 cm, 8 cm, 8 cm and 4 cm, respectively. The first part is at the level of L1 lumbar vertebra, the second at L2, the third at L3, and the fourth ascends again to L2 level.

The first part, called the duodenal cap or bulb, passes superiorly, to the right and posteriorly from the pylorus. It is overlapped anteriorly by the liver and gallbladder. On barium examinations the latter may indent the cap. The common bile duct, the portal vein and the gastroduodenal artery pass behind the first part of the duodenum and separate it from the inferior vena cava (IVC). Inferiorly it is in contact w i th the pancreatic head.

The second part of the duodenum has an opening halfway down on its posteromedial aspect for the pancreatic and common bile ducts, variously called the duodenal papilla or ampulla of Vater. This is guarded by the sphincter of Oddi. An accessory pancreatic duct (of Santorini), if present, opens 2 cm proximal to this.

This part of the duodenum is crossed by the transverse mesocolon anteriorly. As a result, its upper half is supracolic and has the liver as an anterior relation. Its lower half is infracolic and has loops of jejunum anteriorly. Its posterior relations are the right kidney and adrenal gland and it is in contact w i th the pancreatic head medially.

The t h i rd part of the duodenum curves anteriorly around L3 vertebra and the IVC and aorta. Its posterior relations also include the right psoas muscle, ureter and gonadal vessels of the posterior abdominal wall. Anteriorly it is crossed by the root of the mesentery and the superior mesenteric vessels. The head of the pancreas is in contact w i th its superior border.

The fourth part of the duodenum passes upwards and to the left on the left side of the aorta, on the left psoas muscle and posterior to the stomach. It raises a peritoneal fold called the ligament of Treitz at the origin of the small bowel mesentery. The inferior mesenteric vessels raise another peritoneal fold lateral to the fourth part of the duodenum. The paraduodenal fossa lies between these.

Arterial supply (Fig. 5.15) The first 2.5 cm of the duodenum is supplied by the right gastric and the right gastroepiploic arteries, as is the adjoining stomach.

The superior pancreaticoduodenal artery supplies from beyond this to midway along the second part. This arises from the gastroduodenal branch of the hepatic artery, which passes behind the first part of the duodenum.

The remainder of the duodenum is supplied by the inferior pancreaticoduodenal artery, the first branch of the superior mesenteric artery. At the midpoint of the second part of the duodenum, therefore, there is a transition from supply by the coeliac trunk to supply by the superior mesenteric artery, representing a transition from foregut to midgut.

Venous drainage

The first part of the duodenum drains to the prepyloric vein (of Mayo), which lies on the anterior surface of the pylorus, and thence to the portal vein. The remainder is drained by veins that correspond to the arteries and which drain to the portal and superior mesenteric veins.

Lymphatic drainage

Pancreaticoduodenal nodes drain to pyloric nodes and to coeliac nodes.

Radiological features of the duodenum

Barium studies of the duodenum (Figs 5.9 and 5.16) The duodenum is usually examined radiologically as part of a double-contrast barium-meal examination (see Fig. 5.9).

Because the first part of the duodenum passes posteriorly as well as superiorly, it is foreshortened in AP views. The best air-filled views are obtained w i th the right side raised in a right anterior oblique view.

The duodenal cap may be indented by the normal gallbladder. The cap has thin mucosal folds that are parallel, or parallel in spiral, from base to apex. These folds are effaced by hypotonic agents. Circular valvulae conniventes proper begin in the second part of the duodenum and are constant, despite distension or hypotonic agents.

The ampulla is visualized in two-thirds of normal examinations and an opening of an accessory pancreatic duct in less than one-quarter. The accessory duct opens more anteriorly than the main pancreatic duct. The ampulla is identified w i th the help of distinctive folds - a hooded fold superiorly and a distal longitudinal fold. An oblique fold extending inferolaterally from the ampulla is also occasionally seen.

The third part of the duodenum is indented by the aorta posteriorly and superior mesenteric vessels anteriorly. The position of the ligament of Treitz is at the highest part of the fourth part of the duodenum, and marks the transition to small bowel. It should lie to the left of the first part of the duodenum, and be at the same height or above. An abnormal position indicates malrotation of the bowel, usually w i th the small bowel lying in the right side of the abdomen and the large bowel on the left.

CT and MRI (see Figs 5.10 and 5.11) The junction of the stomach and duodenum is marked by increased thickness of the pyloric muscle posterior to the left lobe of the liver. The gastroduodenal artery may be seen posterior to the first part of the duodenum. The second part of the duodenum is seen between the liver and gallbladder laterally and the pancreatic head medially. The third part of

Fig. 5.16 Barium follow-through study of the small bowel.

1. Body of stomach 2. Fundus of stomach (undistended) 3. Gastric rugal fold 4. Lesser curvature of stomach 5. Greater curvature of stomach G. Antrum 7. Second (descending) part of duodenum 8. Longitudinal duodenal fold: marks position of ampulla 9. Third (horizontal) part of duodenum (a longitudinal impression may be seen here due to the superior mesenteric vessels and spinal column) 10. Fourth (ascending) part of duodenum (the duodenojejunal flexure is obscured by barium in the stomach) 11. Jejunum: normal 'feather-like' pattern of mucosal folds 12. Ileum: relatively featureless mucosal pattern in the collapsed state