Pancreas

Scintigraphy of the biliary system using 9 9 m T c HIDA This accumulates in the liver parenchyma and is excreted by the biliary system, outlining the intrahepatic right and left ducts, the extrahepatic ducts, the gallbladder and the cystic duct. Radioactive material excreted into the duodenum is also seen on the scan.

THE PANCREAS (Figs 5.36-5.42) The pancreas is situated on the posterior abdominal wall at approximately L1 level. It is described as having a head, neck, body and tail. It is retroperitoneal w i th the exception of the tail, which lies in the splenorenal ligament. It is over 15 cm long and lies transversely and slightly obliquely, w i th the tail higher than the head.

The head of the pancreas lies in the curve of the duodenum, w i th the pylorus and the duodenal cap overlapping it slightly on its upper surface. The uncinate process projects posteriorly and to the left from its lower part to lie posterior to the superior mesenteric vessels. The remainder of the pancreatic head lies anterior to the vessels of the posterior abdominal wall, that is, the vena cava and renal veins, the aorta and its coeliac and superior mesenteric branches. The common bile duct passes posterior to the head of the pancreas in a groove or tunnel towards its termination in the second part of the duodenum.

The neck of the pancreas extends from the upper part of the anterior portion of the head. It lies anterior to the union of the splenic vein and the superior mesenteric vein to form the portal vein.

The body of the pancreas curves over the vertebrae and great vessels to reach the left paravertebral gutter. The splenic vein passes posterior to the body, where it receives the inferior mesenteric vein. The splenic artery runs along the upper surface of the pancreas in a sinuous course that is intermittently above and behind the pancreas. The body lies anterior to the left kidney and adrenal.

The tail of the pancreas is related to the splenic hilum. Here it lies in the splenorenal ligament.

The lesser sac is anterior to the pancreas and anterior to this lies the stomach and part of the lesser omentum.

The architecture of the pancreas is finely lobulated.

The pancreatic ducts (see Figs 5.38-5.40) The pancreatic duct begins in the tail by the union of ductules and passes transversely towards the head, closer to the anterior than the posterior surface of the gland. It receives smaller ducts along its length at right-angles and increases in size as it approaches the head. At the neck the duct turns inferiorly, somewhat posteriorly and to the right, and joins the bile duct to form a terminal, common dilated portion called the ampulla (of Vater) before entering the duodenum at the papilla.

An accessory duct (of Santorini) arises in the lower part of the head, which it drains, and then passes upwards anterior to the main duct, to which it is connected by a communicating duct, and drains to the duodenum about 2 cm proximal to the papilla. This duct is occasionally absent.

Variations in pancreatic ductal anatomy are frequent and are shown in Figure 5.40.

Fig. 5.37 Pancreas: (a) anterior relations; (b) and (c) posterior relations.

Fig. 5.38 Pancreatic ducts.

The development of the pancreas (Fig. 5.39) The pancreas arises from the junction of the primitive foregut and midgut as a larger dorsal division and two smaller ventral buds. The ventral buds arise in common w i th the biliary duct. The left ventral bud atrophies and the right ventral bud swings posteriorly to unite w i th the inferior aspect of the dorsal division, trapping the superior mesenteric vessels between divisions. The duct of the smaller ventral portion becomes the main duct and the proximal part of the duct of the larger dorsal division becomes the accessory duct.

Variations in pancreatic anatomy

These are as follows: • Annular pancreas: this is pancreatic tissue surrounding the duodenum and occurs when part of the ventral bud fails to atrophy; • Pancreas divisum: failure of fusion of the dorsal and ventral moieties results in the anterosuperior part of the head and the body and tail draining via the accessory papilla, while the posteroinferior part of the head drains to the ampulla;

Agenesis of the dorsal pancreatic moiety: this results in a pancreas w i th a head but no body or tail and is very rare; • Left-sided pancreas is an effect of age w i th laxity of the suspensory fascia of an otherwise normal gland.

It can be precipitated by a large pancreatic mass (usually benign) which causes the gland to flip over on itself; • Accessory nodules (pancreatic rests) of pancreatic tissue may occur in the wall of the stomach, the duodenum, the small intestine or within a Meckel's diverticulum. The most common site is in the wall of the duodenum closest to the pancreas and close to the opening of the pancreatic duct.

Arterial supply of the pancreas (Fig. 5.41) The pancreas is supplied by branches of the coeliac and superior mesenteric arteries. The coeliac supplies branches via its hepatic and splenic arteries. The gastroduodenal artery, arising from the hepatic artery, divides into the right gastroepiploic artery and the superior pancreaticoduodenal artery. This latter artery divides early into an anterior branch that lies in the groove between the pancreas and the duodenum and a posterior branch that passes posterior to the head of the pancreas.

The splenic artery passes along the upper surface of the pancreas and supplies many small branches to it. One of these is sometimes larger than the others and is called the pancreatica magna artery. The dorsal pancreatic artery arises close to the origin of the splenic artery (or separately from the coeliac artery or from the superior mesenteric artery) and passes vertically downwards behind the pancreas. It divides at right-angles into a left branch, which passes towards the tail of the pancreas as the transverse

Fig. 5.39 Development of the pancreas: (a) appearance of two ventral and one dorsal pancreatic buds; (b) development of one ventral and one dorsal bud; (c) fusion of the ventral and dorsal buds.

Fig. 5.40 Variation in anatomy of the pancreatic ducts: (a) atrophic accessory duct persists as tiny accessory duct in 60%; (b) accessory (upper) duct atrophies completely - no connection with duodenum (20%); (c) major and minor ducts open separately and do not communicate (10%); (d) both ducts persist, communicate and open separately (10%).

Fig. 5.41 Arterial supply of pancreas.

pancreatic artery, and a right branch, which passes between the neck and the uncinate process to anastomose w i th arteries on the anterior surface of the gland.

The inferior pancreaticoduodenal artery arises from the right side of the superior mesenteric artery. It divides early into anterior and posterior branches that anastomose w i th those of the superior pancreaticoduodenal artery.

Venous drainage of the pancreas

The neck, body and tail of the pancreas drain to the splenic vein and the head drains to the superior mesenteric and portal veins.

Lymphatic drainage

Lymphatic drainage is to nodes along the course of the supplying arteries to preaortic coeliac nodes.

Radiological features of the pancreas

Plain films of the abdomen The pancreas is not visible unless calcified. If calcification is distributed throughout the gland it is seen as a transverse structure at L1 level, w i th a larger head on the right side and a body and tail extending to the left and upwards.

When the pancreas is inflamed it may cause ileus formation in the nearby duodenum and proximal jejunum, which is visible on plain films. It may also cause fluid collection in the lesser sac (pseudocyst formation), which causes displacement of the stomach gas anteriorly, which is visible on a lateral f i lm of the abdomen.

Hypotonic duodenography This was a technique for assessment of the pancreas, by visualization of its effects on the paralysed inflated duodenal loop. It has now been replaced by other imaging techniques.

Ultrasound of the pancreas (Fig. 5.42) This is possible when it is not obscured by overlying stomach and transverse colonic gas. The entire gland is seen well in only 60% of studies. It is identified on transverse sections anterior to the splenic vein and has a characteristic sonographic architecture (Fig. 5.42). The coeliac artery division into hepatic and splenic arteries at right-angles can easily be identified on ultrasound sections just above the pancreas. The hepatic artery and bile duct are seen anterior to the portal vein, also cephalad to the pancreatic head. The antrum of the stomach may be identified anteriorly, w i th the pylorus and duodenum above and curving around the right side of the head. In the upper part of the head, two round anechoic structures can normally be identified - a cross-sectional view of the gastroduodenal artery anteriorly and the common bile duct posteriorly. These demarcate the right side of the head of the pancreas from the duodenum. The splenic vein has a characteristic 'tadpole' appearance at the junction of body and neck which helps to identify the gland on transverse imaging in the midline. Just posterior to this, the left renal vein is usually seen crossing the aorta to drain into the IVC (Fig. 5.42) The uncinate process is identified posterior to the superior mesenteric vessels. The neck of the gland is identified in transverse and sagittal imaging anterior to the confluence of portal and splenic veins. The body runs anterior to the splenic vein across the midline. The tail is viewed by angling the transducer superolaterally from midline towards the splenic hilum, or by oblique views through the spleen.

The pancreatic duct is seen within the gland closer to its anterior surface. It is seen in over 80% of cases. It is best seen in the central portion of the body where it is perpendicular to the plane of imaging. It measures approximately 1.5 mm in the tail, 2 mm in the body and 3 mm in the head. On high-resolution images in slim subjects it can be

Fig. 5.42 Ultrasound of upper abdomen: transverse image through head and body of pancreas.

1. Skin surface 2. Left lobe of liver 3. Head of pancreas 4. Neck of pancreas 5. Body of pancreas 6. Splenic vein 7. Confluence of splenic and superior mesenteric veins to form portal vein

8. Aorta 9. Superior mesenteric artery 10. Fat around superior mesenteric artery 11. Left renal vein 12. Inferior vena cava (collapsed) 13. Right renal vein 14. Vertebral body

identified running inferiorly from the neck in the head to the duodenum.

The echotexture of the pancreas is normally homogeneous and iso- or slightly hyperechoic w i th respect to liver. With ageing and in obesity it may be hyperechoic due to the presence of fat. When hyperechoic, it can be difficult to distinguish from surrounding fat.

Computed tomography (see Figs 5.2 and 5.3) As a result of its oblique position, the pancreas must be studied on sequential CT slices. The tail is visible at the splenic hilum on the highest slices and the uncinate process is the lowest part. Normal thickness of the head is 2 cm, the neck 0.5-1 cm and the body and tail 1-2 cm. The height of the head is very variable and may measure up to 8 cm. The body and tail may measure 3-4 cm in height.

The normal pancreatic duct is visible in most cases. The common bile duct and gastroduodenal artery are visible in. the pancreatic head. The formation of the portal vein is seen behind the neck and the mesenteric vessels are seen to pass anterior to the uncinate process. A replaced right hepatic artery may be seen arising from the right of the proximal SMA, running towards the liver between the portal vein and the IVC.