The Pancreas

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Chapter 96

The Pancreas

Embryology, Anatomy, and Physiology

The pancreas (from the Greek words pan, meaning “all,” and kreas, meaning “flesh”) arises from two anlagen that develop from the endodermal lining of the duodenum. Before 28 days of gestation, the dorsal part develops from a diverticulum from the dorsal aspect of the duodenum caudal to the hepatic diverticulum. It grows upward and backward into the dorsal mesogastrium to form part of the head and the entire body and tail. The ventral pancreatic bud develops between 30 and 35 days of gestation as a diverticulum from the primitive bile duct that forms part of the head and uncinate process. The ventral pancreas rotates counterclockwise posterior to the duodenum at day 37 of gestation, and the two portions fuse at about the sixth week of embryonic life. The ductal systems fuse, and the duct from the dorsal bud becomes the accessory pancreatic duct (of Santorini); the duct from the ventral bud enlarges to become the main pancreatic duct (of Wirsung), after it fuses with the distal two thirds of the dorsal duct. The opening of the accessory duct is often obliterated.

Developmental deviation from this embryologic pattern can give rise to variants. The usual ductal configuration is most commonly bifid, formed by the ducts of Wirsung and Santorini (60% of cases). Less common configurations include a rudimentary duct of Santorini (30%); a dominant duct of Santorini (1%); and ansa pancreatica, in which the duct of Santorini curves as it courses to the duct of Wirsung. Ductal narrowing can be seen at the site of fusion of the dorsal and ventral ducts. The absence of proximal dilation allows differentiation of this normal variant from a true stricture. Duodenal obstruction, pancreatobiliary maljunction pancreatitis, and biliary cysts occur secondary to developmental variants. Pancreatobiliary maljunction is associated with congenital common bile duct webs.1

The pancreas grows substantially in the first year of life, and growth slows from year 1 through 18. The gland is relatively larger in children than in adults, and the overall ratio of gland size to patient body size decreases with age (Table 96-1). The pancreatic head is more prominent in children compared with the body and tail. The diameter of the pancreatic duct also varies with age (Table 96-2). Enlarged ducts (>1.5 mm at 1 to 6 years, >1.9 mm at 7 to 9 years, and >2.2 mm at 13 to 18 years) are associated with pancreatitis.

Table 96-1

Normal Sonographic and Computed Tomographic Dimensions of the Pancreas

image

Modified from Heuck A, Maubach PA, Reiser M, et al. Age-related morphology of the normal pancreas on computed tomography. Gastrointest Radiol. 1987;12:18-22; and Siegel MJ, Martin KW, Worthington JL. Normal and abnormal pancreas in children: US studies. Radiology. 1987;165:15-18.

Table 96-2

Normal Diameter of the Pancreatic Duct by Ultrasound and Computed Tomography

image

Modified from Siegel MJ, Martin KW, Worthington JL. Normal and abnormal pancreas in children: US studies. Radiology. 1987;165:15-18; Heuck A, Maubach PA, Reiser M, et al: Age-related morphology of the normal pancreas on computed tomography. Gastrointest Radiol. 1987;12:18-22; Chao HC, Lin SJ, Kong MS, et al. Sonographic evaluation of the pancreatic duct in normal children and children with pancreatitis. J Ultrasound Med. 2000;19:757-763; and Glaser J, Hogemann B, Krummenerl T, et al. Sonographic imaging of the pancreatic duct. New diagnostic possibilities using secretin stimulation. Dig Dis Sci. 1987;32:1075-1081.

The pancreas lies transversely in the retroperitoneum. It is divided into the head, body, and tail (Fig. 96-1). The head is to the right of midline, situated within the “C-loop” of the duodenum. At the junction of the inferior and left margins of the pancreatic head is an extension of the gland called the uncinate process. The anterior surface of the pancreatic head is in contact with the transverse colon, gastroduodenal artery, and loops of small intestine. The anterior surface of the uncinate process is in contact with the superior mesenteric artery and vein. The posterior surface of the head is adjacent to the inferior vena cava, common bile duct, renal veins, and the abdominal aorta.

The pancreatic body is in contact with the stomach anteriorly and superiorly. Its posterior portion abuts the abdominal aorta, splenic vein, left kidney, adrenal gland, and origin of the superior mesenteric artery. Loops of jejunum and ileum lie inferiorly. The tail may be more bulbous in children than the head or body and is narrower in adults. The pancreatic tail lies in the phrenicolienal ligament in contact with the gastric surface of the spleen and the splenic flexure of the colon.

Pancreatic function is both exocrine and endocrine. Exocrine functions are directed toward digestion, with secretions exiting through the pancreatic duct into the duodenum. The islets of Langerhans represents endocrine tissue that contains several types of hormone-producing cells (insulin, glucagon, somatostatin, etc.) that help regulate blood glucose levels and digestive function. The B cells produce insulin; A cells, glucagon; G cells, gastrin; D cells, somatostatin; and D1 cells, vasoactive intestinal peptide (VIP) and secretin.

Imaging the Pancreas in Children

The pancreas itself is not seen on plain radiographs, although calcifications in patients with chronic pancreatitis or cystic fibrosis may be identified on abdominal radiographs (Fig. 96-2). In acute pancreatitis, dilated loops of bowel and fluid levels within the upper midabdomen may suggest localized ileus. A pancreatic mass may be sufficiently large to displace adjacent gas-filled portions of the gastrointestinal (GI) tract.2

Ultrasonography (US) is the primary screening tool to evaluate the pediatric pancreas.3 The pancreas is most easily seen if the stomach and duodenum are not distended with gas. Ingestion of water devoid of gas bubbles may improve visualization. The distal body and tail may also be imaged in the prone position using the left kidney as an acoustic window. Age-matched normal dimensions of the pancreas are given in Table 96-1.3 Pancreatic size is best measured at its body, but individual variation is sufficient to warrant caution when determining pancreatic size. Enlargement of the pancreas should be diagnosed when the anteroposterior dimension of the pancreatic body is greater than 1.5 cm.4 The normal duct may be seen as a single- or double-track echogenic line anterior to the junction of the splenic and mesenteric veins (see Fig. 96-1). The pancreas has a spectrum of echogenicity relative to that of the liver, but in most children, the pancreas is hypoechoic or nearly isoechoic with the liver. However, in neonates, particularly in premature infants, the pancreatic gland is more echogenic.5 US is also helpful for image-guided biopsy and in aligning radiotherapy.6

Computed tomography (CT) of the pancreas7 is indicated less frequently than US, but it is valuable in certain conditions, particularly pancreatitis, tumors, and pseudocysts with uncommon features. The pancreas is best visualized during bolus injection of intravenous (IV) contrast material, which readily identifies the adjacent vessels, and with meticulous administration of GI contrast to opacify the adjacent stomach and duodenum. The pancreas is hypodense compared with the liver, both with and without intravenous contrast. The contours of the pancreas are commonly smooth but may be slightly lobulated. Because the pancreas in children is oblique to the axial plane, multiple thin sections may be necessary for optimal visualization; in-plane reconstructions, particularly from axial volumetric data obtained with multidetector equipment, can image the pancreas in its own oblique plane. Imaging of the pancreatic head may be optimized by scanning the patient in the right lateral decubitus position soon after ingestion of GI contrast material. With this technique, the optimally opacified duodenal C-loop outlines the pancreatic head, and opacified proximal jejunal loops outline the remainder of the gland. CT is the best modality for assessing neoplasms, pancreatic trauma, and pancreatitis and its complications and for further evaluation of abnormalities on US. Thin collimation volumetric CT coupled with curved planar reformations produces quality imaging of pancreatic and peripancreatic tissues.8

Magnetic resonance imaging (MRI) of the pancreas9,10 is more difficult in children than in adults because of adjacent gas-filled loops of intestine and motion artifact from peristalsis and respiration.11 Nevertheless, MRI is a powerful tool for imaging pediatric developmental abnormalities. The pancreas normally has signal intensity equal to that of liver on T1- and T2-weighted spin echo images with midfield strength magnets. Pancreatic images produced with high-field strength magnets may have greater signal intensity than those of the liver. To some degree, signal varies with age. Although normal children do not have as much intrapancreatic macroscopic fat as adults, adolescents have more fat in the pancreatic septa than do preadolescent children, and the amount of intrapancreatic fat may be increased in children with cystic fibrosis. The value of MRI is enhanced by the use of breath-holding techniques (generally not possible in younger children), fat suppression, contrast enhancement, and respiratory gating.

Because of its noninvasive nature, magnetic resonance cholangiopancreatography (MRCP)12 may be more useful than endoscopic retrograde cholangiopancreatography (ERCP)13,14 in children (Fig. 96-3). Reported sensitivity, specificity, and accuracy are 87%, 90%, and 89% respectively for stones; 100%, 98%, and 98% for cholangitis; 92%, 97%, and 96% for bile duct tumors; and 89%, 96%, and 95% for periampullary stenosis.15 MRCP is also useful in certain congenital abnormalities, such as pancreas divisum, and after pancreatic trauma to identify duct of Wirsung transections. Although intravenous CT cholangiography is superior to MRCP in delineating postoperative anatomy after choledochal cyst repair, MRCP is highly accurate (84%) in depicting the anastomotic site, intrahepatic biliary tree, and reconstructed bowel, and it clearly demonstrates pancreatobiliary maljunction, residual distal common bile duct, common channel,16 and pancreatic duct.17 Similarly, MRCP accurately depicts the postoperative anatomy and complications after orthotopic liver transplantation.18 A normal MRCP may obviate the need for ERCP or percutaneous transhepatic cholangiography, and abnormalities visualized with MRCP can direct the modality and route for further intervention. An overview of MRCP pitfalls has been provided by Van Hoe and colleagues.19

Secretin stimulation with MRCP further enhances the imaging information obtained, because it gives additional, valuable, functional and anatomic information about the pancreatic duct and pancreatic excretory capacity. Secretin-enhanced MRCP has been described in detail in recent years20–23 and has been found useful for detection and diagnosis of a variety of congenital, inflammatory, and neoplastic pancreatic conditions.24 Secretin causes temporary dilation of pancreatic ducts, principally by increasing pancreatic exocrine secretions, thus it allows better visualization of the ducts during MRCP.

Congenital and Hereditary Pancreatic Abnormalities

Congenital Pancreatic Abnormalities25,26

Pancreas Divisum

Imaging: CT in children with pancreas divisum and pancreatitis demonstrates enlargement of both ducts, in addition to the characteristic findings of pancreatitis. Further enlargement of the ducts can be provoked with secretin stimulation.27,28 Increased thickness of the pancreatic head has also been described.29 Zeman and colleagues30 reported that thin-section CT demonstrated the unfused ducts in 5 of 12 patients (Fig. 96-4), and two distinct pancreatic moieties separated by a fat cleft was seen in 4 patients. Pancreas divisum may be associated with minor papilla adenoma beyond the childhood years.

Congenital Short Pancreas

Overview: Congenital short pancreas, also known as agenesis of the dorsal pancreatic anlage,32 occurs when the portion of the pancreas derived from the dorsal embryonic bud is absent, and only the smaller portion, derived from the ventral anlage, is present. Thus, the pancreatic neck, body, and tail are absent. This anomaly has been described in patients with polysplenia syndrome, or it may be a sporadic finding.

Imaging: Only a globular pancreatic head can be identified on CT (Fig. 96-5). Size is variable, and some patients show an enlarged or prominent pancreatic head, whereas others may show a normal sized or even mildly atrophic and small pancreatic head. The diagnosis of agenesis of the dorsal pancreas is inconclusive without demonstration of the absence of the dorsal pancreatic duct, either with MRCP or ERCP. Patients with this abnormality have an increased risk of developing diabetes mellitus34 because of the paucity of islet cells, most of which are located in the distal pancreas. This condition may also be associated in later life with pancreatic tumors35 such as intraductal papillary mucinous neoplasms.

Ectopic Pancreas

Overview: Ectopic pancreatic tissue36 is an aberrant rest of normal pancreatic tissue remote from the pancreatic body that occurs in 1% to 13% of the population. The vast majority of pancreatic rests (about 70%) are located in the stomach,37 duodenum, and jejunum, but they can occur elsewhere, such as omphaloenteric duct rest.38 An association with Beckwith-Weidemann syndrome has been reported.39

Annular Pancreas

Clinical Presentation: Annular pancreas is frequently diagnosed in infancy because of associated duodenal obstruction. However, in approximately half the cases, the diagnosis is made beyond infancy (Fig. 96-6). The associated abnormalities described above are most common in patients who also have trisomy 21. Annular pancreas has also been described in de Lange syndrome, with heterotaxy, and as a cause of extrahepatic biliary obstruction.41 Pancreatitis that solely affects the annulus of an annular pancreas has been reported in adults.14

Imaging: MRI has advantages over CT in the diagnosis of annular pancreas, because with MRI it is easier to detect and characterize the tissue surrounding the duodenum as pancreatic. Diagnosis by US has also been described.42 ERCP and MRCP are used to investigate ductal anatomy. Coincidence of congenital short and annular pancreas with gallbladder agenesis and splenic malrotation is rare.43

Congenital Pancreatic Cysts

Imaging: Congenital cysts are anechoic by US; they are usually unilocular, located in the pancreatic tail, and range in size from microscopic to 5 cm.47 Rarely, they may communicate with the ductal system. In contrast to single congenital pancreatic cysts, multiple congenital cysts may be associated with a polycystic disorder such as von Hippel–Lindau disease.48 Juxtapancreatic GI duplication cysts occur as abnormalities of the developing foregut and therefore usually have an alimentary tract epithelial lining. Most of these cysts arise from the stomach or duodenum but may rarely be sequestered within the pancreas.49

Hereditary Systemic Conditions with Pancreatic Involvement

Cystic Fibrosis

Imaging: In young patients with cystic fibrosis (CF), US50 shows a normal pancreas or pancreatic enlargement, but chronic obstruction ultimately results in shrinkage of the gland with fatty infiltration and fibrosis. On US, these histopathologic changes are visualized as increased echogenicity of the gland. CT shows a shrunken pancreas with reduced attenuation secondary to fatty infiltration. Fibrosis without fatty infiltration is found infrequently. Unenhanced scans may show pancreatic calcifications, ductal dilation, and pancreatic cysts (Fig. 96-8).51 MRI findings are variable but can accurately depict the changes of fatty infiltration, fibrosis, and atrophy.5254

Cystic transformation of the pancreas, or pancreatic cystosis, in children and young adults with CF has been described.55,56 This is an unusual form of pancreatic involvement with CF, in that the pancreas is replaced by macrocysts that are rarely more than 1 cm in diameter. This can be imaged with US, CT (Fig. 96-9), and MRI. These are true, epithelium-lined cysts that result from the accumulation of inspissated mucus, produced as a result of residual exocrine secretory function in the acinar cells, proximal to ducts obstructed from inflammation.

Shwachman-Diamond Syndrome

von Hippel–Lindau Disease

Beckwith-Wiedemann Syndrome

Hemochromatosis

Overview:

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