Pancreas Anatomy Overview on Sonography:

Sonographic evaluation of the pancreas provides key diagnostic clues of the normal pancreas versus pancreatitis and its adverse outcomes. Evaluating the pancreas in its entirety is essential, the pancreas from head to tail is about 15 cm in length on average with a variation between 12 to 18 cm (1). The distinction between the different anatomical areas of the pancreas is well delineated. The head of the pancreas is the thickest portion of the pancreas and lies to the right of the superior mesenteric artery (SMA) and superior mesenteric vein (SMV). Posterior to the pancreas, the junction of the SMV and splenic vein creates the portal confluence which is another important anatomical clue for defining pancreatic anatomy. The uncinate process is an extension of the pancreatic head posteriorly which then results in the uncinate process being posterior to the SMV. The uncinate process can be distinguished sonographically from the head of the pancreas by evaluating the pancreatic tissue in the head region which courses posteriorly to the SMV/portal confluence as shown in Figure 1. The neck is the thinnest portion of the pancreas and lies directly anterior to the portal confluence as shown in Figure 1. The body of the pancreas is the main portion of the pancreas and lies to the left of the SMA and SMV and is where the duct of Wirsung is commonly seen sonographically. Last, the tail will be found between the left border of the aorta and the splenic hilum. Ensuring the tail of the pancreas is properly assessed is key as the tail is oftentimes obscured by bowel gas. Beneath this obscured pancreatic tissue can lie a variety of pancreatic pathologies.

Figure 1: The image shows the sonographic evaluation of the pancreatic head, uncinate process and neck as well as the portal confluence in the transverse view. The uncinate process consists of the tissue that is posterior to the portal confluence and the neck is directly anterior to the portal confluence.

Figure 2: Same image as Figure 1 with the addition of pancreas anatomic labels in the transverse view.

Through the body of the pancreas, the duct of Wirsung also known as the main pancreatic duct can be visualized sonographically with sonolucent duct walls within the body of the pancreas. This duct runs from left to right within the pancreas and drains most of the pancreas except for the inferior portion of the head and the uncinate process. Then, the main pancreatic duct typically joins the common bile duct (CBD) at the hepatopancreatic ampulla before draining into the ampulla of Vater. The main pancreatic duct will measure the largest near the head region of the pancreas at 1-3.5 mm in adults <50 and 2-5 mm in adults 70-79 years old. The main pancreatic duct should then taper at the body of the pancreas measuring 2.5-3 mm in adults <50 and 3 mm in 70-79 year olds. It is then most narrow at the tail measuring 2 mm or less in all adults (3). The duct of Santorini also known as the accessory duct drains the remaining portions of the pancreas, inferior portion of the head and the uncinate process and typically communicates with the main pancreatic duct. The accessory duct is not typically visualized on transabdominal sonography.

Visualizing the pancreatic tissue and duct of Wirsung provides the ability to assess for pathology but gas commonly obscures proper visualization. In an attempt to decrease the likelihood of the pancreas being obscured by bowel gas, the patient needs to be NPO for at least 6 hours prior to the exam. The pancreas can be evaluated with the patient in a supine position and the transducer at an oblique angle or with the patient in right lateral decubitus (RLD) with the spleen used as a window to evaluate the pancreatic tail. In some cases, it may be best for the patient to drink water to fill the stomach which creates a sonographic window to the pancreatic tissue (1). This technique can be appreciated in Figures 3, 4, and 5.

Figure 3: Patient drank 20oz of water approximately 10 minutes before the exam and laid in the supine position. This water-filled stomach allowed for better visualization of the pancreatic tail in long.

Figure 4: Same patient and study as Figure 3. This image shows the transverse pancreatic tail free of bowel gas obstruction due to the water-filled stomach.

Figure 5: Same patient and study as Figure 3 and 4. This image again shows the transverse pancreatic tail free of bowel gas obstruction due to the water-filled stomach.

Acute Pancreatitis

With pancreatitis, the pancreatic tissue becomes inflamed due to blockage of the pancreatic bile ducts which can lead to the tissue being digested by its own enzymes. In cases of acute pancreatitis, the recently inflamed acini release enzymes which seep into the surrounding pancreatic tissues. Due to this acute inflammation, the patient will experience severe epigastric pain as well as nausea. The patient will also present with elevation of serum amylase and lipases which is found to be 90-95% specific for the correct diagnosis (8). A case of acute pancreatitis is commonly related to biliary disease, such as impacted gallstones, or commonly alcoholism in the adult United States population (1). One of the main roles of Sonography in the evaluation of acute pancreatitis in adults is to identify gallstones as a potential etiology. In cases of pediatric acute pancreatitis, abdominal trauma is the most common cause.

Acute pancreatitis has a variety of sonographic characteristics. In the very early stages of acute pancreatitis, the pancreas may not appear to be inflamed but if continued inflammation is present, the pancreas will show changes in echogenicity and overall volume. Increased pancreatic volume is found with a marked decrease in echogenicity. Volume increase can be identified sonographically by the body exceeding 2.4 cm in diameter, with marked anterior bowing and surface irregularity (6,7).

As the pancreas becomes more swollen, the head of the pancreas can compress a portion of the IVC (1). Evaluation for such compression in the longitudinal plane can lead to the diagnosis of acute pancreatitis.

Figure 6: This longitudinal image of the pancreatic head demonstrates the anatomical and spatial relation between the pancreatic head and IVC. As stated above, in cases of severe acute pancreatitis, the pancreatic head can swell and cause IVC compression. Cases of adenocarcinoma and/or pseudocyst within the pancreatic head can also result in IVC compression.

Figure 7: Shows the same longitudinal plane of the pancreas head as seen in Figure 6 with the addition of color Doppler to highlight the IVC.

Pancreatic Fluid Collections Classifications: Acute Pancreatitis

In instances of severe acute pancreatitis, peripancreatic fluid collections as well as fluid within the abdominal cavities can occur. Common locations for these fluid collections to collect include the lesser sac, anterior pararenal spaces, mesocolon, and perirenal spaces (1). Classifying pancreatic fluid collections allows for better treatment options for patients with varying forms and stages of pancreatitis. The Revised Atlanta Classification of Acute Pancreatitis provides a universal way for pancreatic fluid collections to be described in dictation. This new classification system allows for distinction between interstitial edematous pancreatitis and necrotizing pancreatitis.

Interstitial edematous pancreatitis (IEP) is one of the subcategories of acute pancreatitis. It is also sometimes referred to as “uncomplicated pancreatitis” which involves acute inflammation of the pancreas which can be potentially life-threatening. Some sonographic signs which signify IEP or uncomplicated pancreatitis include acute peripancreatic fluid in the first 4-weeks of inflammation. This fluid collection will be non-encapsulated. After the 4-week period, the fluid becomes encapsulated resulting in the formation of a pseudocyst (2). Pseudocyst sonographic characteristics include the visualization of an extra-pancreatic fluid collection with a well-defined wall and no solid internal components or areas of necrosis. This results in a typically anechoic appearance with posterior acoustic enhancement and occasional debris as well as calcification of the capsule walls. In approximately 10-20% of the population who have developed acute pancreatitis, a pseudocyst will develop 4 to 6 weeks after onset (1). The distinction between peripancreatic fluid and a pseudocyst is the development of the capsule 4-weeks after the initial bout of acute pancreatitis.

Patients with a history of large pancreatic pseudocysts have a risk of rupture. If a pseudocyst ruptures, pancreatic ascites can occur. As alcoholism is a common cause of chronic pancreatitis, confirming whether alcohol induced cirrhosis is the cause of the sonographically visualized ascites versus a ruptured pseudocyst can be difficult. The pathologic cause of the abdominal fluid can only be confirmed by high levels of amylase and protein content within the abdominal fluid which is associated with a ruptured pancreatic pseudocyst (1).

Another subset of acute pancreatitis is necrotizing pancreatitis. The Revised Atlanta Classification of Acute Pancreatitis second category of pancreatitis is necrotizing pancreatitis. The fluid collections visualized with this category of pancreatitis includes acute necrotic collections which are apparent in the first 4-weeks of inflammation and present as a non-encapsulated heterogenous, non-liquified area within or adjacent to the pancreas. Then, after the first 4-weeks, the heterogeneous, non-liquefied material becomes walled-off, similar to the progression of a pseudocyst. The distinction between acute necrotic collections and walled-off necrosis include the development of a capsule (2).

Chronic Pancreatitis

Chronic pancreatitis occurs with recurrent episodes of acute pancreatitis overtime. This leads to destruction of the pancreatic tissue which results in permanent damage. While patients with acute pancreatitis typically have severe epigastric pain, patients with chronic pancreatitis do not experience such pain. Chronic pancreatitis is most often linked to a history of alcoholism or biliary disease as well as a history of hypercalcemia and hyperlipidemia (1).

Sonographically, chronic pancreatitis typically presents as severely increased echogenicity of the tissue with punctate echogenic foci throughout the tissue. Calcification of the gland occurs in about 20 – 40% of patients (1). Chronic pancreatitis may also present with a dilated pancreatic duct typically best visualized sonographically at the pancreatic body. The overall size of the pancreas will also be decreased with a potentially irregular and enlarged pancreatic duct. A pancreatic pseudocyst can also occur with chronic pancreatitis (1). In the moderate to severe stages, the pancreas endures morphological changes and becomes inhomogeneous and nodular due to focal inflammation (9). Peripancreatic fluid is not typically seen with chronic pancreatitis, whereas a pseudocyst is a common finding.

Pancreatic Pseudocyst

A pancreatic pseudocyst has sonographic characteristics including a localized fluid collection that has no solid material and a well-defined wall. The cyst can also sometimes contain internal debris. As stated above, most pancreatic pseudocysts are caused by a history of pancreatitis but can also be caused by pancreatic trauma or iatrogenic causes such as post-pancreatic surgery. There are no specific symptoms for a pseudocyst and can range from the most frequent symptoms including abdominal pain, nausea, vomiting, and weight loss. Some more obscure symptoms related to this pathology include jaundice, fever, pleural effusion, or potentially sepsis (4).

Pancreatic pseudocysts are the most common type of cystic lesion of the pancreas. Pseudocysts account for two thirds of all cystic lesions documented with the pancreas compared to a cystic neoplasm which accounts for around 10-15% of pancreatic cystic lesions (4). Pseudocysts contain high levels of amylase which is imperative for distinction between other types of pancreatic lesions. Also when evaluating with transabdominal Sonography, color Doppler is imperative in order to rule out the potential for the cystic lesion being a pseudoaneurysm, especially after iatrogenic causes of the lesion such as abdominal and/or post-pancreatic surgery (4). If a cystic lesion is not well-visualized due to overlying bowel gas, if the patient is able to do so, the consumption of water can lead to better visualization and characterization of the lesion.

If a cystic lesion of the pancreas is a confirmed pseudocyst, intervention is not required and may resolve spontaneously. Unless patients are experiencing symptoms, the pseudocyst will only need to be monitored. Some contraindications of spontaneous regression include a large cyst size and an extended amount of time that the pseudocyst has been present (5). If the pseudocyst is large enough, this can have consequences and may require surgical intervention. The two main indications for drainage of the pseudocyst include persistent patient symptoms or the presence of complications such as infection, gastric or biliary obstruction, or bleeding (5). The current best options for treating a pancreatic pseudocyst include a percutaneous catheter drain, surgical drainage or endoscopic drainage with ultrasound guidance (4).

Pancreatic Atrophy

Atrophic changes of the pancreas are considered non-specific and are commonly found in elderly populations. Most commonly, this is found with end-stage chronic pancreatitis. Rarely, this can be found in young patients with a history of chronic illnesses and/or pathology such as cystic fibrosis (6).

Simple Pancreas Cyst

Simple pancreatic cysts are not a common cystic lesion found within the pancreas (13). These simple cysts are also known as retention cysts. The cysts are lined with epithelium and have no communication with the duct of Wirsung or Duct of Santorini. These rare cysts will be anechoic with no septations or wall thickening as well as being spherical in shape. This is a rare pathology and should be differentiated from a pancreatic pseudocyst, which is the most common cystic lesion of the pancreas.

Intraductal Papillary Mucinous Neoplasm

An intraductal papillary mucinous neoplasm (IPMN) are epithelial cystic tumors of mucin-producing cells. Unlike the simple pancreatic cysts, IPMNs communicate with the pancreatic ducts. The most common locations of IPMN within the pancreas can be seen in Figure 8.

IPMN Location within the Pancreas	Breakdown of IPMN Locations
Head	50%
Tail	7%
Uncinate Process	4%
Other locations of the pancreas	40%

Figure 8: Chart shows the likelihood of IPMNs developing at various anatomical locations throughout the pancreas (10).

The types of IPMNs are divided macroscopically into main duct, branch duct, and mixed-type lesions. The main duct sector is reminiscent of chronic pancreatitis and has the highest malignant potential. The type of IPMN, the main duct, has a 60% likelihood of malignancy. With this type of IPMN, the duct of Wirsung will also be dilated at 5 mm or greater (11).

The branch duct type is typically seen in the head and the uncinate process and has a wider range in appearance. This sector of IPMN pathology has a lower likelihood of malignancy at around 5%.

On sonography, an IPMN can be located anywhere throughout the pancreas and may appear as a small, thin-walled pancreatic cysts with a coinciding dilated duct of Wirsung (12). The key for diagnosing IPMN is connecting the cystic lesion with a duct, which can most often only be done using MRI/MRCP. Therefore, all pancreatic cystic lesions should be evaluated with a follow up MRCP if it would change the patients management (14).

Figure 9: This image shows a likely IPMN lesion at the head of the pancreas.

Figure 10: This image is the same patient as Figure 9 in the longitudinal plane.

Figure 11: This image is the same patient as Figure 9 and 10 with the addition of color Doppler. Note the lack of color Doppler within the cystic lesion.

Conclusion

Sonographic delineation of the anatomic segments of the pancreas including the head, uncinate process, body, and tail allows for more optimized evaluation of all pancreatic pathology including morphology and location. Sonography is able to assess for a wide variety of pancreatic pathologies including acute and chronic pancreatitis as well as adverse outcomes from the inflammation. Visualization of the pancreas is not always feasible, but techniques such as filling the stomach with water to create an acoustic window can be extremely beneficial. Clear visualization of the pancreatic tissue allows for the evaluation of pancreatic fluid collections and lesions, echogenicity changes, calcification formation, and more, which allows for a more concise diagnosis for all patients.

The Pancreas: Fluid and Cysts

Chapter 1