On completion of this chapter, you should be able to:
Describe the normal anatomy and relational landmarks of the pancreas
Name the exocrine and endocrine functions of the pancreas
Describe the laboratory tests used to detect pancreatic disease
Describe the sonographic technique and patterns of the normal pancreas
Define the clinical signs and symptoms of pancreatic disease
Name the congenital anomalies of the pancreas
List the sonographic findings and differential diagnoses of the following diseases: pancreatitis, pancreatic cyst, and pancreatic tumor
The pancreas continues to be a technical challenge for the sonographer because this gland is located in the retroperitoneal cavity posterior to the stomach, duodenum, and proximal jejunum of the small bowel. In addition, the transverse colon may obstruct visualization of the pancreas as it runs horizontally across the abdominal cavity.
Other noninvasive procedures were unsuccessful in visualization of the pancreas before the development of computed tomography (CT), magnetic resonance imaging (MRI), and ultrasound. Plain film of the abdomen may lead to a diagnosis of pancreatitis if calcification is visible in the pancreatic area, but calcification does not occur in all cases. Localized ileus, dilated loops of bowel without peristalsis (“paralyzed gut”) caused by gas and fluid accumulation near the area of inflammation, may be shown on the plain radiograph in patients with pancreatitis. The upper gastrointestinal test series provides indirect information about the pancreas when the widened duodenal loops are visualized.
CT and MRI have become the primary modalities to image the patient with pancreatic disease because of their improved resolution of the retroperitoneal structures. However, the normal pancreas can be visualized in the majority of gas-free patients with sonography by using the neighboring organs and vascular landmarks to aid in localization. The gland appears sonographically isoechoic to more hyperechoic than the hepatic parenchyma. Variations in patient positioning or ingestion of water to fill the stomach (that serves as a window to image the pancreas) are used routinely in many laboratories to further aid in visualizing the entire gland. In addition, clinicians performing the endoscopic retrograde cholangiopancreatography (ERCP) examination of the pancreatic duct are incorporating endoscopic ultrasound as an aid in visualizing the detailed anatomy of the pancreatic area.
Sonography is readily accessible and less expensive than the other imaging modalities. The primary task of the sonographer is to distinguish the normal gland from an abnormal process, to image the ductal system, and to separate inflammation of the gland from malignancies. Sonography may also aid in percutaneous fine-needle aspiration when a lesion is found.
Anatomy of the pancreas
The pancreas lies anterior to the first and second lumbar bodies located deep in the epigastrium and left hypochondrium, behind the lesser omental sac ( Figure 12-1 ). The major posterior vascular landmarks of the pancreas are the aorta and inferior vena cava. The pancreas most commonly extends in a horizontal oblique lie extending from the second portion of the duodenum to the splenic hilum. Other variations in the lie of the pancreas include transverse, horseshoe, sigmoid, L -shaped, and inverted V. When this occurs, it may be more difficult to obtain a single image of the pancreatic gland, as the tail will be in a different plane than the body and the head.
It may be surprising that the majority of the pancreas lies within the retroperitoneal cavity, with the exception of a small portion of the head that is surrounded by peritoneum. Posterior to the pancreas are the connective prevertebral tissues, the portal-splenic confluence, the superior mesenteric vessels, the aorta, the inferior vena cava, and the lower border of the diaphragm. The stomach, duodenum, and transverse colon form the superior and lateral borders of the pancreas, which makes visualization of the pancreas by ultrasound difficult (air and gas interference).
The pancreas is divided into the following four areas: head, neck, body, and tail ( Figure 12-2 ). Each area is discussed as it relates to its surrounding anatomy. The reader is referred to the multiple cross-sectional drawings ( Figures 12-3 and 12-4 ) to gain a relational understanding of the adjacent anatomy to the pancreas.
The head of the pancreas is the most inferior portion of the gland. It lies anterior to the inferior vena cava, to the right of the portal-splenic confluence, inferior to the main portal vein and caudate lobe of the liver, and medial to the duodenum as it “lies in the lap” of the C-loop of the duodenum (see Figure 12-2 ). The splenic vein forms the posterior medial border of the pancreas, where it is joined by the superior mesenteric vein to form the main portal vein, thus forming the portal-splenic confluence ( Figure 12-5 ). The superior mesenteric vein crosses anterior to the uncinate process of the head of the gland and posterior to the neck and body of the pancreas. The uncinate process is the small, curved tip at the end of the head of the pancreas. It lies anterior to the inferior vena cava and posterior to the superior mesenteric vein. As Figure 12-6 shows, the common bile duct passes posterior to the first part of the duodenum and courses through a groove posterior to the pancreatic head, whereas the gastroduodenal artery forms the anterolateral border.
The neck of the pancreas is located between the pancreatic head and body, and often it is included as “part of the body” of the gland. It is found directly anterior to the portal-splenic confluence/superior mesenteric vein (see Figure 12-5 ).
The body of the pancreas is the largest section of the pancreas. It lies anterior to the aorta and celiac axis, left renal vein, adrenal gland, and kidney. The tortuous splenic artery is the superior border of the gland (see Figure 12-5 ). The anterior border is the posterior wall of the antrum of the stomach. The neck of the pancreas forms the right lateral border.
The tail of the pancreas is more difficult to image because it lies anterior to the left kidney and posterior to the left colic flexure and transverse colon. The tail begins to the left of the lateral border of the aorta and extends toward the splenic hilum (see Figure 12-5 ). The splenic vein is the posterior border of the body and tail. The splenic artery forms the superior border of the tail, whereas the stomach is the anterior border.
Two ducts are seen within the pancreas, the duct of Wirsung and the duct of Santorini. To aid in the transport of pancreatic fluid, the ducts have smooth muscle surrounding them. The duct of Wirsung is a primary duct extending the entire length of the gland (see Figure 12-6 ). It receives tributaries from lobules at right angles and enters the medial second part of the duodenum with the common bile duct at the ampulla of Vater (guarded by the sphincter of Oddi). The duct of Santorini is a secondary duct that drains the upper anterior head. It enters the duodenum at the minor papilla about 2 cm proximal to the ampulla of Vater. The duct of Wirsung is easier to visualize on ultrasound as it courses through the midline of the body of the gland. It appears as an echogenic line or lucency bordered by two echogenic lines. The duct should measure less than 2 mm, with tapering as it reaches the tail. Color Doppler imaging may help distinguish the dilated pancreatic duct from the vascular structures (splenic vein and artery) in the area.
Common bile duct.
The common bile duct runs inferiorly in the free edge of the lesser omentum to the level of the duodenum. Then it travels posterior to the first portion of the duodenum and the head of the pancreas to the right of the main pancreatic duct. The common bile duct opens into the duodenum after forming a common trunk with the pancreatic duct.
Size of the pancreas
The normal length of the pancreas (head to tail) is about 15 cm, with the range extending between 12 and 18 cm. The head is the thickest part of the gland, measuring 2 to 3 cm in its anterior-to-posterior dimension. The neck measures 1.5 to 2.5 cm, the body measures 2 to 2.5 cm, and the tail measures 1 to 2 cm. The sonographer should evaluate the total size, contour, and texture of the gland to determine enlargement. The gland appears larger or thicker in children than in adults and decreases in size with advancing age.
The blood supply for the pancreas is the splenic artery and pancreaticoduodenal arteries ( Figure 12-7 ). The anterior and inferior pancreaticoduodenal arteries supply the head and part of the duodenum. The splenic artery supplies the body and tail of the pancreas through four smaller branches: (1) suprapancreatic (rises from the celiac axis/splenic artery), (2) pancreatic, (3) prepancreatic (before leaving the pancreas), and (4) prehilar (before leaving the spleen) and hepatic artery (gastroduodenal artery). The dorsal pancreatic artery rises from the suprapancreatic section, the pancreatica magna artery rises from the pancreatic section, and the caudal pancreatic artery rises from the prepancreatic or prehilar section. Venous drainage is through tributaries of the splenic and superior mesenteric veins.
Vascular and ductal landmarks to the pancreas
Celiac axis and branches.
The celiac axis originates from the anterior abdominal aorta and serves as the superior border of the pancreas. It gives rise to three branches: the left gastric, common hepatic, and splenic arteries (see Figure 12-7 ).
The splenic artery follows a tortuous course along the superior border of the pancreatic body and tail as it crosses horizontally toward the splenic hilum.
Common hepatic artery.
The common hepatic artery rises from the celiac axis and courses along the superior margin of the first portion of the duodenum to divide into the proper hepatic artery and gastroduodenal artery, usually when it crosses anterior to the portal vein. The common hepatic artery forms the right superior border of the body and head of the gland and gives rise to the gastroduodenal artery. In some patients the right hepatic artery rises from the superior mesenteric artery and courses posterior to the medial portion of the splenic vein.
The gastroduodenal artery is seen along the anterolateral border of the pancreas as it travels a short distance along the anterior aspect of the pancreatic head just to the right of the neck before it divides into the superior pancreaticoduodenal branches; they join with the inferior pancreaticoduodenal branches, which rise from the superior mesenteric artery (see Figure 12-7 ).
Superior mesenteric artery.
The superior mesenteric artery rises from the aorta inferior to the celiac axis and posterior to the lower portion of the pancreatic body and courses anterior to the third portion of the duodenum to enter the small bowel mesentery (see Figure 12-7 ).
Portal vein and tributaries.
The main portal vein is formed posterior to the neck of the pancreas by the junction of the superior mesenteric vein and splenic vein (see Figure 12-5 ). The splenic vein runs from the splenic hilum along the posterior aspect of the pancreas. The superior mesenteric vein runs posterior to the neck of the pancreas and anterior to the uncinate process, which forms the small, curved tip of the pancreatic head.
Common bile duct.
The common bile duct crosses the anterior aspect of the portal vein to the right of the proper hepatic artery. The portal vein is anterior to the inferior vena cava. The duct passes along the anterior border of the portal vein and travels posterior to the first portion of the duodenum to course inferior and somewhat posterior in the parenchyma of the head of the pancreas (see Figure 12-6 ). It joins the pancreatic duct close to the ampulla of Vater.
Congenital abnormalities of the pancreas are uncommon. The following abnormalities are presented: agenesis, pancreas divisum, ectopic pancreatic tissue, and annular pancreas.
Agenesis of the body and tail, with hypertrophy of the pancreatic head, is a congenital defect.
This rare condition is caused by the lack of fusion of the dorsal and ventral pancreatic buds. The drainage of the dorsal pancreas is through the minor papilla, with the ventral part draining through the major papilla. On sonography, this diagnosis is challenging. A persistent dorsal pancreatic duct in the head may be identified, but communication with the ventral duct is difficult to ascertain with sonography ( Figure 12-8 , A ).
Ectopic pancreatic tissue.
Ectopic pancreatic tissue is the most common pancreatic anomaly, usually in the form of intramural nodules. The ectopic tissue may be found in various places in the gastrointestinal tract. Frequent sites are the stomach, duodenum, small bowel, and large bowel. On palpation these lesions may seem polypoid, and they characteristically have a central dimple. They consist of elements of the pancreas, usually the acinar and ductal structures and less frequently the islets of Langerhans. They are generally small (0.5 to 2 cm), and acute pancreatitis or tumor may occur within these elements.
Annular pancreas is a rare anomaly in which the head of the pancreas surrounds the second portion of the duodenum ( Figure 12-8 , B and C ). It is more common in males than in females, and all grades (from an overlapping of the posterior duodenal wall to a complete ring) may be found. It may be associated with complete or partial atresia of the duodenum and is susceptible to any of the diseases of the pancreas.
Physiology and laboratory data of the pancreas
The pancreas is both a digestive ( exocrine ) and hormonal ( endocrine ) gland. The primary exocrine function is to produce pancreatic juice, which enters the duodenum together with bile. The exocrine secretions of the pancreas and those of the liver, which are delivered into the duodenum through duct systems, are essential for normal intestinal digestion and absorption of food. Pancreatic secretion is under the control of the vagus nerve and two hormonal agents, secretin and pancreozymin, that are released when food enters the duodenum. The endocrine function controls the secretion of glucagons and insulin into the blood. Failure of the pancreas to furnish sufficient insulin leads to diabetes mellitus.
Exocrine function is performed by acini cells of the pancreas, which can produce up to 2 L of pancreatic juice per day. These cells are arranged in saclike clusters (acini) connected by small intercalated ducts to larger excretory ducts. The excretory ducts converge into one or two main ducts, which deliver the exocrine secretion of the pancreas into the duodenum. The enzymes of the pancreatic juice that aid in digestion include lipase, which digests fats; amylase, which digests carbohydrates; carboxypeptidase, trypsin, and chymotrypsinogen, which digest proteins; and nucleases, which digest nucleic acids ( Table 12-1 ).
|Enzymes of Pancreatic Juice||Digestive Action|
|Trypsin, chymotrypsinogen, carboxypeptidase||Proteins|
Pancreatic juice is the most versatile and active of the digestive secretions. Its enzymes are capable of nearly completing the digestion of food in the absence of all other digestive secretions. Because the digestive enzymes that are secreted into the lumen of the small intestine require an almost neutral pH for best activity, the acidity of the contents entering the duodenum must be reduced. Thus the pancreatic juice contains a relatively high concentration of sodium bicarbonate, and this alkaline salt is largely responsible for the neutralization of gastric acid.
The nervous secretion of pancreatic juice is thick and rich in enzymes and proteins. The chemical secretion, resulting from pancreozymin activity, also is thick, watery, and rich in enzymes. Pancreatic juice is alkaline and becomes more so with increasing rates of secretion. This is because of a simultaneous increase in bicarbonates and decrease in chloride concentration.
The proteolytic enzyme trypsin may hydrolyze protein molecules to polypeptides. Chymotrypsinogen is activated by trypsin. Amylase causes hydrolysis of starch with the production of maltose, which is further hydrolyzed to glucose. Lipase is capable of hydrolyzing some fats to monoglycerides and some to glycerol and fatty acids. Although lipases are also secreted by the small intestine, what is secreted by the pancreas accounts for 80% of all fat digestion. Thus impaired fat digestion is an important indicator of pancreatic dysfunction.
Partially digested food, or chyme, in the duodenum stimulates the release of hormones that act on pancreatic juice formation. These hormones include gastrin, cholecystokinin, acetylcholine (all digestive enzymes), and secretin (stimulates production of sodium bicarbonate).
The pancreatic juice enters the duodenum through the duct of Wirsung. This duct joins the common bile duct as it drains bile from the liver and both enter the duodenum through the ampulla of Vater. The sphincter of Oddi is a muscle surrounding the ampulla of Vater that relaxes to allow pancreatic juice and bile to empty into the duodenum.
The endocrine function is located in the islets of Langerhans in the pancreas. Specialized cells within the islets are called alpha, beta, and delta cells. The beta cells are most prevalent and produce insulin, a hormone that causes glycogen formation from glucose in the liver. It also enables cells within insulin receptors to take up glucose (to decrease blood sugar). Alpha cells produce glucagon, a hormone that causes the cells to release glucose to meet the energy needs of the body. Glucagon stimulates the liver to convert glycogen to glucose to increase sugar levels. Delta cells are the smallest composition of endocrine tissue and produce somatostatin. This hormone inhibits the production of both insulin and glucagon. All the hormones are released into the bloodstream ( Table 12-2 ).
|Pancreatic Hormone||Cell Type||Action|
|Insulin||Beta||Glucose to glycogen|
|Glucagon||Alpha||Glycogen to glucose|
|Somatostatin||Delta||Alpha and beta inhibitor|
There are specific enzymes of the pancreas that may become altered in pancreatic disease, namely amylase and lipase. Increased glucose levels may indicate abnormalities of the pancreas ( Table 12-3 ).
|Acute pancreatitis||Twice normal|
|Chronic pancreatitis||No change|
|Mumps, ischemic bowel disease, pelvic inflammatory disease|
|Carcinoma of the pancreas|
|Condition||Blood Glucose Level|
|Chronic liver disease|
|Overactive endocrine glands|
|Tumor in islet of Langerhans|
Amylase is a digestive enzyme for carbohydrates. It is secreted by the pancreas, parotid glands, gynecologic system, and bowel. In certain types of pancreatic disease, the digestive enzymes of the pancreas escape into the surrounding tissue, producing necrosis with severe pain and inflammation. Under these circumstances there is an increase in serum amylase. A serum amylase level of twice normal usually indicates acute pancreatitis.
Other conditions that may cause an increase in amylase include chronic pancreatitis, obstruction of the pancreatic duct, perforated peptic ulcer, acute cholecystitis, and alcohol poisoning. Less common conditions include mumps, ischemic bowel disease, and pelvic inflammatory disease.
Urine amylase may be elevated in pancreatitis. Diseases not affecting the pancreas may cause the elevation of serum amylase without elevation of urine amylase.
Lipase is an enzyme that is excreted specifically by the pancreas and that parallels the elevation in amylase levels. The lipase test is performed to assess damage to the pancreas. The pancreas secretes lipase, and small amounts pass into the blood. The lipase level rises in acute pancreatitis and in carcinoma of the pancreas. Both amylase and lipase rise at the same rate, but the elevation in lipase concentration persists for a longer period. Lipase may also be elevated with obstruction of the pancreatic duct, pancreatic carcinoma, and acute cholecystitis.
Glucose controls the blood sugar level in the body. The glucose tolerance test is performed to discover whether there is a disorder of glucose metabolism. An increased blood glucose level is found in severe diabetes, chronic liver disease, and overactivity of several of the endocrine glands. There may be a decreased blood sugar level in tumors of the islets of Langerhans in the pancreas.
Sonographic evaluation of the pancreas
The pancreas is one of the most difficult abdominal organs to image with sonography because it lies posterior to the stomach and sometimes the transverse colon. To help visualize the pancreas, the patient should fast 6 to 8 hours; this decreases the amount of air and fluid in the stomach and colon that may impede visualization. It also promotes dilation of the gallbladder and ducts. If fluid is administered to better visualize the gland, real-time visualization of peristaltic movement of food particles within the duodenum and stomach can be a useful landmark to help outline the head, body, and tail of the pancreas. This will be discussed in more detail later in the chapter.
The pancreas is examined as part of a comprehensive general abdominal study ( Figure 12-9 and Table 12-4 ). Specific indications for pancreatic scanning include abdominal pain, clinically manifested acute or chronic pancreatitis, abnormal laboratory values, cholecystitis, or obstructive jaundice. The examination determines the presence of cystic and solid masses, biliary and ductal dilation, and the presence of extrapancreatic masses and fluid collections.
Patient preparation: nothing by mouth for at least 6 hours; may need to give water to fill the stomach as a window to image the pancreas.
Transducer selection: broadband 2.5 to 5 MHz curvilinear.
Patient position: supine, decubitus, or upright.
Images and observations should include the following:
The head, neck/body, and tail should be well delineated once the celiac axis, superior mesenteric artery and vein, aorta, and inferior vena cava are identified. (Often the lie of the pancreas makes it difficult to image the gland in one plane; the tail may be seen on an image that is more superior than the head of the gland.)
Transverse scans along the region of the splenic vein should be performed to demonstrate the body and tail of the pancreas.
The pancreatic duct may be seen on the transverse scan as it courses through the body of the gland.
The longitudinal view of the pancreatic head lies anterior to the inferior vena cava and inferior to the portal vein.
The superior mesenteric vein may be seen to course anterior to the uncinate process of the head and posterior to the body.
The pancreatic tail may be seen as gentle but firm pressure is applied to the abdomen to displace overlying gas in the antrum of the stomach or transverse colon. The tail may also be seen with the patient in a right decubitus position as the transducer is angled through the spleen and left kidney; the pancreatic tail is anterior to the left kidney.
The presence of dilated pancreatic or biliary ducts should be assessed and their size measured.
The presence of cystic or solid masses should be assessed.
The presence of peripancreatic nodes should be assessed.
The presence of peripancreatic fluid collections (e.g., pseudocysts) should be assessed.
The presence of any pancreatic calcifications detected should be recorded.
|Pancreas (Figures 8-58–8-63)||Trv||Head/IVC/SMV |
Body and tail/SMV/SMA
|Long||Head/portal vein/IVC |
Body and tail/aorta
Normal pancreatic texture
The echogenicity of the pancreas is discussed in terms of how it relates to the liver’s homogeneous soft echo pattern. The normal pancreas has an echo pattern that is slightly more hyperechoic and finer in texture than that of the surrounding retroperitoneum. The echo intensity of the pancreas is usually slightly less than that of surrounding soft tissue and slightly greater than that of the liver.
The parenchymal texture of the pancreas depends on the amount of fat between the lobules and to a lesser extent on the interlobular fibrous tissue. The internal echoes of the pancreas consist of closely spaced elements of the same intensity with uniform distribution throughout the gland. Fat is strongly echogenic, and the extensive fatty infiltrations of the pancreas are difficult to visualize by ultrasound because the pancreas blends in with the surrounding retroperitoneal fat. A lesser degree of fatty infiltration may not render the pancreas invisible but may raise the amplitude of returning pancreatic echoes, resulting in the clinical observation that the pancreas returns stronger echoes than the liver. Fibrous tissue may also account for the portion of increased echogenicity. Box 12-1 lists the sonographic characteristics of the normal pancreas.
Size: Head ≤3 cm; neck ≤2.5 cm; body ≤2.5 cm; tail ≤2 cm
Echogenicity: >liver </>spleen (depends on fatty/fibrous texture)
Surface: Smooth to slightly lobular (islets of Langerhans)
Sonographic scan technique
The patient is usually examined in the supine, oblique, and sometimes upright positions. Sonographic techniques vary according to the patient’s body habitus. For adult patients, use a low-frequency broadband transducer with a midfocal zone; for pediatric patients, use at least a 5- to 7.5-MHz transducer. The curved array transducer allows for a better near field of view than the sector transducer allows. The time gain compensation and overall gain should be adjusted so that the pancreatic tissue has the same echo brightness or slightly greater than the normal liver. The texture of the pancreas will appear coarser than the liver depending on the amount of fibrous/fatty tissue interfaces within the gland. The younger pediatric patients tend to have less echogenicity of the pancreas than the older patients do (i.e., more fatty interfaces in the gland of the older patient). The diabetic patient may be challenging to image through the fatty liver texture; therefore a lower-frequency transducer may be useful. With the patient in deep inspiration, gentle pressure on the abdomen with the transducer allows the sonographer to get as close as possible to the pancreatic tissue to improve visualization.
Box 12-2 summarizes the normal pancreatic landmarks. The sonographer should identify the head, neck, body, and tail in the transverse and longitudinal planes ( Figures 12-10 and 12-11 ). The sonographer should evaluate the shape, contour, lie, and texture of the pancreas (compared with the liver parenchyma). The oblique or upright position of the patient may improve visualization of the pancreas and peripancreatic region. The following surrounding structures should be identified: superior mesenteric artery and vein, portal and splenic veins, aorta and inferior vena cava, common bile duct, gastroduodenal artery, left renal vein, duodenal bulb, posterior wall of the stomach, and pancreatic duct.
Head: Anterior to inferior vena cava, lateral to duodenum; gastroduodenal artery is anterolateral border; common bile duct is posterior medial border. On sagittal plane, portal vein is superior to head of the pancreas.
Uncinate process: Anterior to inferior vena cava, posterior to superior mesenteric vein.
Body: Anterior to superior mesenteric artery and vein, aorta, splenic vein; posterior to stomach, inferior to splenic artery.
Tail: Medial to hilum of spleen; superior to left kidney.
Pancreatic duct: Runs through middle portion of body of pancreas.