Congenital and Developmental Pancreatic Anomalies: Ultrasound, Computed Tomography, and Magnetic Resonance Imaging Features

Congenital and Developmental Pancreatic Anomalies: Ultrasound, Computed Tomography, and Magnetic Resonance Imaging Features

Congenital and Developmental Pancreatic Anomalies: Ultrasound, Computed Tomography, and Magnetic Resonance Imaging Features Els L.F. Nijs* and Michael...

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Congenital and Developmental Pancreatic Anomalies: Ultrasound, Computed Tomography, and Magnetic Resonance Imaging Features Els L.F. Nijs* and Michael J. Callahan† The purpose of this article is to provide an overview of the most common anatomic, congenital, and developmental anomalies of the pancreas. Familiarity with the imaging appearances of these anomalies as well as the range of normal appearance is important to differentiate them from other conditions, especially since some of these variants are commonly encountered. Basic knowledge of the normal embryology is of utmost importance to be able to recognize these disorders. Semin Ultrasound CT MRI 28:395-401 © 2007 Elsevier Inc. All rights reserved.

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he pancreas develops from dorsal and ventral buds that arise from the endodermal lining of the duodenum. First the dorsal anlage arises in the embryo and it comes directly from the dorsal side of the duodenum before 28 days. The ventral anlage is located close to the bile duct and is visible between 30 and 35 days of embryologic development. At day 37, the ventral pancreas rotates in a counter-clockwise direction, posterior to the duodenum, and fuses with the dorsal pancreas posteriorly and inferiorly. The ventral anlage forms the uncinate process and part of the pancreas head, whereas the larger dorsal anlage forms the remainder of the organ.1 Following this fusion, the ductal systems anastomose, a complicated process with a wide spectrum of possible outcomes. The duct of the ventral anlage usually persists, together with the distal portion of the dorsal anlage, to form the main pancreatic duct (of Wirsung), which drains at the papilla maior. The proximal portion of the dorsal duct may either disappear or persist as the accessory pancreatic duct (of Santorini),2 which drains at the papilla minor. During the embryologic development, the pancreas becomes located in the anterior pararenal space of the retroperitoneum.

*From the Department of Radiology, University Hospital Gasthuisberg, Leuven, Belgium. †Department of Radiology, Boston Children’s Hospital, Boston, MA. Address reprint requests to: Els L.F. Nijs, Department of Radiology, University Hospital Gasthuisberg, Herestraat 49, 3000 Leuven, Belgium. E-mail: [email protected].

0887-2171/07/$-see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1053/j.sult.2007.08.001

Normal Pancreas Normal Pancreatic and Pancreatic Duct Measurements In the first year of life the pancreas grows significantly, with an obvious slower growth in childhood and adolescence.2,3 The gland is proportionally larger in children than in adults: the overall ratio of gland size to patient body size shrinks with age4,5 (Table 1). The size and configuration of the normal pancreas vary considerably according to body habitus and size of nearby organs.4 In children, the pancreatic head is more prominent than the body and tail, and should not be mistaken for a pathologic process.2,3,5 This relative prominence of the pancreatic head is typically less evident in adults. The mean diameter of the pancreatic duct in normally developed children is 1.65 ⫾ 0.45 mm.8 There is a tendency to a small increase in size of the pancreatic duct diameter as adult patients are getting older (Table 2).

Normal Imaging Findings Ultrasound The echogenicity of the pancreatic gland shows a wide range from hypoechoic to hyperechoic, depending on the patient’s age.10 Computed Tomography On computed tomographic (CT) examination, the normal pancreas is homogeneous and has somewhat variable softtissue attenuation. The gland can be isodense or slightly hypodense relative to the liver.4,11 Fatty replacement is common 395

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396 Table 1 Measurements of the Pancreas in Correlation with Age* Age

Head

Body

Tail

Ultrasound (Mean ⴞ SD, cm) <1 month 1.0 ⴞ 0.4 0.6 ⴞ 0.2 1 month–1 year 1.5 ⴞ 0.5 0.8 ⴞ 0.3 1–5 years 1.7 ⴞ 0.3 1.0 ⴞ 0.2 5–10 years 1.6 ⴞ 0.4 1.0 ⴞ 0.3 10–19 years 2.0 ⴞ 0.5 1.1 ⴞ 0.3

1.0 ⴞ 0.4 1.2 ⴞ 0.4 1.8 ⴞ 0.4 1.8 ⴞ 0.4 2.0 ⴞ 0.4

CT (Mean ⴞ SD, mm) 28.6 ⴞ 3.8 19.1 ⴞ 2.1 26.0 ⴞ 3.4 18.2 ⴞ 2.4 25.2 ⴞ 3.6 17.8 ⴞ 2.2 24.0 ⴞ 3.6 16.0 ⴞ 2.0 23.4 ⴞ 3.5 15.8 ⴞ 2.4 21.2 ⴞ 4.3 14.4 ⴞ 2.7

18.0 ⴞ 1.6 16.5 ⴞ 1.8 15.8 ⴞ 1.7 15.1 ⴞ 1.9 14.7 ⴞ 1.8 13.0 ⴞ 2.1

20–30 31–40 41–50 51–60 61–70 71–80

years years years years years years

*Maximum anteroposterior dimensions of pancreas according to age, based on ultrasound6 and computed tomography (CT).7

in older people, but unusual in children and suggests an underlying pancreatic disorder.2 The surface of the pancreas is commonly smooth and otherwise slightly lobulated.

Figure 1 T1-weighted axial image demonstrates a normal hyperintense aspect of the pancreas. The contour is smooth.

Magnetic Resonance Imaging When evaluated on high field strength (1.5 T) magnets, the pancreas should be relatively isointense to the liver on T1and T2-weighted images (Fig. 1). The normal signal of the pancreas may vary slightly, depending on the age of the patient.

ical variant of the pancreatic duct configuration with a persistent duct of Santorini that forms a reversed-S shape and connects with a side branch of the duct of Wirsung (Fig. 2). There may be a higher incidence of pancreatitis.13 Duplications of the main pancreatic duct are not infrequently seen. Cystic dilatations of terminal portion of the duct of Santorini and duct of Wirsung are called Santorinicele and Wirsungocele (Fig. 3) respectively.14

Congenital Anomalies

Developmental Anomalies of the Pancreas

Anatomic Variants Many variations of the pancreatic duct occur, but in 60% of patients a bifid configuration is seen with an equal size of the duct of Wirsung and duct of Santorini. In 30%, the duct of Santorini will be only rudimentary, whereas in 1% the duct of Santorini will be dominant.12 Ansa pancreatica is an anatom-

Pancreas Divisum Pancreas divisum is the most common anatomic variant of the pancreas.2 Its estimated incidence varies from approximately 5 to 10% of the population;4 1.3-6.7% based on endoscopic retrograde cholangiopancreatography (ERCP) and 4-14% based on autopsy series.2 It is a variant of ductal fusion

Table 2 Pancreatic Duct Diameter8,9 Age (Years)

Pancreatic Duct Diameter, Mean ⴞ SD, mm (Range)

1–3 4–6 7–9 10–12 13–15 16–18

According to Age, Based on US6 1.13 ⴞ 0.15 (0.9–1.3) 1.35 ⴞ 0.15 (1.0–1.5) 1.67 ⴞ 0.17 (1.3–1.9) 1.78 ⴞ 0.17 (1.5–2.2) 1.92 ⴞ 0.18 (1.6–2.4) 2.05 ⴞ 0.15 (1.8–2.4)

18–29 30–39 40–49 50–59 60–69 70–81

According to Age, Based on CT7 1.5 ⴞ 0.5 1.6 ⴞ 0.5 1.9 ⴞ 0.3 2.0 ⴞ 0.5 2.1 ⴞ 0.4 2.0 ⴞ 0.5

US, ultrasound; CT, computed tomography.

Figure 2 Magnetic resonance cholangiopancreatography image shows the typical inverted-S shape of the duct of Santorini in ansa pancreatica.

Pancreatic anomalies

Figure 3 Magnetic resonance cholangiopancreatography image demonstrates the dilated terminal portion of the duct of Wirsung, known as Wirsungocele.

with an absent or incomplete fusion between the dorsal and ventral anlage1,2 with the persistence of a double system: the majority of the gland drains into the minor papilla via the duct of Santorini, and the head and uncinate process alone drain through the major papilla via the duct of Wirsung (Fig. 4). This may or may not result in a functional stenosis, on its turn resulting in pancreatitis.15 Separate ventral and dorsal anlagen may be seen with thin-section CT,1 but are not often identified with sonography. However, the modalities of choice are magnetic resonance cholangiopancreatography (MRCP) and ERCP.

Figure 4 Magnetic resonance cholangiopancreatography image shows nonfusion of the ventral and dorsal duct of the pancreas as seen in pancreas divisum.

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Figure 5 Contrast-enhanced axial computed tomography image demonstrating normal pancreatic tissue encircling the duodenum. Within the encircling pancreas, an accessory pancreatic duct can be seen.

Annular Pancreas Although the exact etiology of annular pancreas is unknown, several mechanisms have been proposed. The first hypothesis includes hypertrophy of both dorsal and ventral anlage resulting in a complete ring (hence the name “annular” pancreas). The second theory suggests an abnormal adherence of the ventral duct to the duodenum before rotation.1 The imaging findings depend on the age of presentation of the patient. Roughly one-half of all patients present in the neonatal period with duodenal obstruction, which may be incomplete or complete. The latter results in the “double bubble” on fetal ultrasound (prenatally) or an abdominal radiograph (postnatally). On ultrasound, the fluid-filled dilated descending duodenum may also be seen encircled by pancreatic tissue in infants.16 The differential diagnosis of duodenal obstruction in neonates should include duodenal atresia (most common cause of double bubble), midgut volvulus, and annular pancreas. Annular pancreas is associated with other congenital anomalies in up to 70% of infants,1,4 which includes duodenal stenosis or atresia (40%), Down’s syndrome (16%), tracheoesophageal fistula (9%), and congenital heart defects (7%).17 Other patients will present during adulthood with nonspecific symptoms or some will be asymptomatic during their entire life and annular pancreas will be discovered incidentally on CT or magnetic resonance (MR). In this patient group no associated anomalies are seen. There is an increased prevalence of acute and chronic pancreatitis. On CT, pancreatic tissue may be seen surrounding the descending duodenum and this must be differentiated from thickening of the duodenal wall4 (Fig. 5). MRCP and ERCP can demonstrate the ductal anatomy and are the investigations of choice. Agenesis/Hypoplasia of the Dorsal Pancreatic Anlage Agenesis of the dorsal pancreatic anlage,2,6,18,19 also known as congenitally short pancreas, may be seen as a solitary finding

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recommended. The long-term prognosis varies with the potential development of intra-abdominal tumors, most of which are malignant.26 The short-term prognosis is related to the severity of neonatal hypoglycemia, and macroglossia, which affects swallowing and may cause respiratory problems.

Pancreatic Cysts

Figure 6 Contrast-enhanced axial computed tomography image shows hypoplasia of the dorsal pancreatic anlage.

or in association with polysplenia syndrome. This entity can be recognized as a short, rounded pancreatic head adjacent to the duodenum and absence of the pancreatic neck, body, and tail. Patients with this anomaly have an increased risk of diabetes mellitus, since most of the islet cells are located in the distal pancreas.2 In cases of hypoplasia of the dorsal pancreatic anlage, the size of the pancreatic body varies (Fig. 6). The minor papilla is present as well as a remnant of the duct of Santorini.20 Ectopic Pancreatic Tissue Ectopic pancreatic tissue is histologically normally organized, well-developed, accessory pancreatic tissue that lacks anatomic and vascular continuity with the main body of the pancreas.21 The incidence of this anomaly varies widely (⬍1% to ⬎13%) depending on the series reported.21,22 Approximately 70% are found in the stomach (antrum), duodenum, and jejunum. Other locations include the ileum, liver, spleen, biliary tract, mesentery, or umbilicus.2 The anomaly is usually located submucosally (73%), but can also be located in the muscularis mucosae or on the serosal surface.1 Ectopic pancreatic rests are often asymptomatic2,21 and are discovered incidentally on gastroscopy. Specific clinical manifestations of ectopic pancreatic tissue may be due to either the mass effect or underlying pathology. The former causes bowel intussusception or obstruction at various sites. The latter can be seen in the form of acute pancreatitis (Fig. 7), cystic degeneration,23 or malignant degeneration of the exocrine or endocrine ectopic pancreas.21,24

True Single Congenital Cyst True single congenital cysts of the pancreas are extremely rare.28 They are most often found in females and typically present as an asymptomatic palpable mass. Other symptoms may include epigastric pain, jaundice, and vomiting28 related to compression of surrounding visceral structures. These cysts are most commonly located in the tail and body of the pancreas28 and are typically unilocular cysts with thin-walled cavities ranging in size from microscopic up to 5 cm in diameter. Communication between the cyst and the pancreatic ductal system is rare.27 These cysts are usually anechoic on ultrasound and are low attenuation cystic structures on CT studies with no wall enhancement.28 Associated congenital anomalies include renal tubular ectasia, polydactyly, anorectal malformations, polycystic kidneys, and asphyxiating thoracic dystrophy.28 Polycystic Systemic Disorder Multiple congenital cysts of the pancreas can be present as part of a polycystic systemic disorder, including von HippelLindau disease and autosomal dominant polycystic disease. Von Hippel-Lindau disease. Von Hippel-Lindau disease (VHL) is an autosomal-dominant disorder with variable penetrance, characterized by central nervous system (CNS) lesions, including retinal angiomas and CNS hemangioblastomas.2,29-31 Pancreatic abnormalities in VHL include cysts,

Beckwith-Wiedemann Syndrome (BWS) The classic triad of omphalocele, macroglossia, and gigantism characterizes the Beckwith-Wiedemann syndrome (BWS).25,26 Variable degrees of visceromegaly can affect the kidneys, liver, pancreas, and adrenal glands. Hemihypertrophy may also be present. Patients with BWS are at a greater risk for development of abdominal malignant tumors,25 such as Wilms tumor and pancreatoblastoma.27 Therefore, routine survey abdominal ultrasounds starting at a young age are

Figure 7 Contrast-enhanced coronal computed tomography image shows a soft-tissue mass at the greater curvature of the stomach in the antrum, consistent with acute pancreatitis in ectopic pancreatic tissue.

Pancreatic anomalies

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Figure 10 Axial non-contrast-enhanced computed tomography image shows the fatty infiltration of the pancreas, atrophy of the gland, and multiple calcifications in this young adult with cystic fibrosis. Figure 8 Contrast-enhanced coronal computed tomography image demonstrates innumerable small and larger cysts virtually replacing the pancreas in this patient with von Hippel-Lindau disease.

tumors.30,31

serous cyst which are adenoma, and islet cell Also reported, but less often seen, are pancreatic carcinoma and adenocarcinoma of the ampulla of Vater.30 Pancreatic cysts are reported in up to 30% of patients on imaging studies,30,32 but can be found in more than 70% in patients with VHL at autopsy.30 There is a wide spectrum of the number of cysts, ranging from a single cyst to numerous cysts, virtually replacing the pancreas31 (Fig. 8). Peripheral calcifications may also be present.32 These cysts may be the first indication of disease during routine screening and may precede any other manifestation of von Hippel-Lindau disease by several years.31 Other abdominal manifestations of VHL include cysts of the kidneys, liver, omentum, mesentery, spleen, ad-

Figure 9 Axial ultrasound image demonstrating the hyperechoic appearance of the pancreatic gland in this patient with cystic fibrosis.

renal glands and epididymis, renal cell carcinoma, and renal adenomas. Autosomal dominant polycystic disease. Autosomal dominant polycystic disease is a hereditary disorder with 100% penetrance but variable expressivity. Renal cysts are the predominant feature, but cysts may also be found in the liver, spleen, adrenal glands, and pancreas. Pancreatic cysts are present in about 10% of the patients.27 There are typically fewer cysts in the pancreas than in the kidneys and liver.32

Congenital Anomalies Causing Systemic Disease and Pancreatic Insufficiency Cystic Fibrosis Cystic fibrosis (CF) is the most common life-threatening recessive genetic trait in the white population, and the most frequent cause of exocrine pancreatic insufficiency in children.33 There is a wide spectrum of severity of clinical findings and morphologic changes in the pancreas.34 Affected patients have pancreatic exocrine dysfunction, which is caused by obstruction of the small ductules by mucoid secretions, which results in acinar ectasia and glandular atrophy. Pancreatic parenchyma is replaced by fibrosis and fat, demonstrated sonographically by diffusely increased echogenicity of the gland (Fig. 9), and diffuse low attenuation on unenhanced CT. Other manifestations include diffuse calcifications (Fig. 10), single or multiple macroscopic cysts, which have been referred to as pancreatic cystosis.35,36 Cystosis is thought to be related to ductal inspissation of tenacious secretions, ultimately resulting in an inflammatory response, obstruction, and ductal ectasia.36 Shwachman-Diamond Syndrome Shwachman-Diamond syndrome (SDS) is the second most common cause of exocrine pancreatic insufficiency in children.37-39 SDS is a rare disease, characterized by exocrine pancreatic insufficiency with varying degrees of hemato-

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9.

10. 11.

12.

13.

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Figure 11 Axial opposed-phase image demonstrating significant drop of signal in the pancreatic gland in this patient with pancreatic lipomatosis due to diabetes mellitus type II.

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logical (intermittent neutropenia) and skeletal abnormalities,2,3,37-39 including metaphyseal dysplasia.3 Clinical characteristics are failure to thrive, diarrhea, and short stature.39 A normal sweat test excludes cystic fibrosis.3,39 Exocrine pancreatic insufficiency is the most prominent feature and may vary from mild to very severe.39 Although a majority of the glandular tissue is replaced by fat, the islet cells of Langerhans3,39,40 and much of the ductal architecture remain preserved.3,39 It is usually seen as diffuse fatty infiltration of the pancreas on imaging.

17.

Other Causes of Pancreatic Lipomatosis Other causes of pancreatic lipomatosis include Johanson– Blizzard syndrome (congenital aplasia of the nasal alae, deafness, hypothyroidism, dwarfism, and malabsorption),37,38 chronic pancreatitis, steroid therapy, Cushing’s syndrome,2 obesity,3 hemochromatosis,39 obstruction of the main pancreatic duct,38 diabetes,41 (Fig. 11), and aging. The differential diagnosis between these entities is mainly based on the combination of clinical and biochemical findings.2

22.

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