Liver

The liver is the largest organ in the body and is sheltered by the ribs in the right upper quadrant (RUQ) of the abdomen. It is kept in position by peritoneal ligaments and intraabdominal pressure from the muscles of the abdominal wall. The functions of the liver are multiple: metabolism of substances delivered via its portal circulation; synthesis of substances, including those concerned with blood clotting, storage of vitamin B, and other materials; and detoxification and excretion of various substances.

The liver has a double supply of blood, coming from the hepatic artery and the portal vein. The hepatic artery usually originates from the celiac axis and takes oxygenated blood to the liver. The portal vein is formed by the union of the superior mesenteric and splenic veins. It is located within the liver and serves to return venous blood from the abdominal viscera to the inferior vena cava (IVC). Any interference with blood flow, which may occur with liver disease, results in consequences elsewhere in the abdominal viscera and spleen.

Biliary Tree

A system of ducts acts to drain bile produced in the liver into the duodenum (Fig. 6-2). Bile from the liver’s two main lobes is drained by the right and left hepatic ducts. These unite to form the common hepatic duct, which is joined usually in its midportion by the cystic duct from the gallbladder. Together, the cystic duct and the common hepatic duct form the common bile duct.

The common bile duct descends posterior to the descending duodenum to enter at its posteromedial aspect. Before its entrance into the duodenum, the common bile duct may be joined by the pancreatic duct from the head of the pancreas. The short part of the common bile duct, after joining the pancreatic duct, is known as the hepatopancreatic ampulla or, more commonly, the ampulla of Vater.

The flow of both bile and pancreatic juice into the duodenum is regulated by the hepatopancreatic sphincter, more commonly known as the sphincter of Oddi. The release of bile into the duodenum is triggered by cholecystokinin, a hormone released by the presence of fatty foods in the stomach. The purpose of bile is to emulsify fats so that they may be absorbed.

Gallbladder

The gallbladder, a digestive organ, is a pear-shaped sac located on the undersurface on the right lobe of the liver. Normally, the walls are quite thin, but they often thicken in the presence of inflammation. The sole function of the gallbladder is to store and concentrate bile that has been produced in the liver. The gallbladder receives bile from the liver via the hepatic duct and empties bile into the duodenum to aid the digestion and absorption of fats in the gastrointestinal (GI) system.

Pancreas

The pancreas is an elongated, flat organ that obliquely crosses the left side of the abdomen behind the stomach; it is a powerful digestive organ. Its functions are both exocrine and endocrine. Exocrine function is concerned with production of the three digestive enzymes trypsin, amylase, and lipase. Trypsin assists in the digestion of proteins. Amylase helps to break down large molecules of starch into smaller molecules of maltose, which eventually are broken down into glucose and stored in the liver. Lipase assists in breaking down lipids into fatty acids and glycerol. These are discharged through the pancreatic duct into the duodenum. The endocrine portion of the pancreas consists of multiple clusters of specialized cells, which are termed the islets of Langerhans. The specialized cells are classified as α-cells and β-cells. The function of β-cells is to produce insulin, and α-cells produce glucagon, both of which are discharged directly into the blood from the pancreas. Insulin and glucagon regulate carbohydrate metabolism. Insulin’s role in this function includes making and storing glucose in the liver and in muscle tissue throughout the body and enabling the body to burn the stored glucose. When the blood flowing from the pancreas to the liver through the portal vein is hyperglycemic, insulin is released into the bloodstream via the portal vein. Glucagon acts in a similar manner, but it functions to increase the glucose level in blood. If the blood released into the portal vein is hypoglycemic, glucagon stimulates the liver to break down glycogen into glucose, thus increasing the glucose level in the bloodstream.

Percutaneous transhepatic cholangiography (PTC) is used to visualize the biliary tree and involves insertion of a needle into the biliary tree by puncture directly through the wall of the abdomen. With the use of a flexible, 22-gauge, skinny needle (Chiba), this procedure is safe and fairly easy to perform. The subsequent injection of iodinated contrast medium (Fig. 6-3) is useful in distinguishing medical jaundice, caused by hepatocellular dysfunction, from surgical jaundice, which results from biliary obstruction. Also, the examination is useful for detecting the presence of calculi or a tumor in the distal common bile duct. It has a high success rate in imaging the biliary ductal system, is less expensive than an endoscopic retrograde cholangiopancreatogram, and has a low complication rate of approximately 3.5%. It also may be immediately followed by a therapeutic procedure such as a biliary drainage, stone removal or crushing via contact lithotripsy or laser fragmentation, stent placement, or biopsy. This procedure is preferred in the evaluation of proximal obstructions involving the hepatic duct bifurcation, which is difficult to image with the retrograde approach via an endoscopic retrograde cholangiopancreatogram (see next section).

Endoscopic Retrograde Cholangiopancreatogram.

The endoscopic retrograde cholangiopancreatogram (ERCP), an imaging procedure performed by a gastroenterologist, is a means of visualizing the biliary system and main pancreatic duct, which provides drainage for the pancreatic enzymes into both the digestive tract and the common bile duct. A fiberoptic endoscope is passed through the duodenal C-loop to visualize the hepatopancreatic ampulla (ampulla of Vater). A thin catheter is then directed into the orifice of the common bile duct or pancreatic duct, followed by an injection of contrast medium (Fig. 6-4). In many cases, the ERCP is preferred over the PTC and is often preceded with a sonographic examination or computed tomography (CT) investigation of the pancreas. Although an ERCP is more expensive than PTC, it is often used to visualize nondilated ducts, distal obstructions, bleeding disorders, and the pancreas. The complication rate (2% to 3%) is similar to that of PTC and also offers the ability to perform therapeutic procedures such as sphincterotomy, stone extractions, stent placement, and biliary dilatation. Cytology and biopsy may also be performed.

Operative Cholangiography.

Operative cholangiography is performed during surgery at the time of a cholecystectomy to detect biliary calculi and the need for common bile duct exploration (Fig. 6-5). A needle is placed directly into the cystic duct or common bile duct by the surgeon, and a small volume (6 milliliters [mL]) of iodinated contrast material is injected, followed by radiography. A second injection of 5 mL is made, followed by radiography a second time. The resulting images are reviewed for possible areas of concern before the surgery is completed. It is imperative that no air bubbles be injected into the ductal system with the contrast agent during this procedure because they can mimic stones.

Diagnostic Medical Sonography

Real-time diagnostic medical sonography is now the modality of choice for evaluating the gallbladder (Fig. 6-6) and biliary tree. This procedure is noninvasive, and the gallbladder can be imaged in almost all fasting patients regardless of the body habitus or clinical condition of the patient. When sonography is performed by a skilled sonographer, it has been proven to be almost 100% accurate in detecting gallstones, which are demonstrated as echogenic areas within the echo-free gallbladder. Thickening of the gallbladder wall is also easily identified. Sonography is also an excellent tool for determining the presence of common bile duct obstruction, evaluation of the intrahepatic biliary ductal system, and identification of abscesses. The liver may be evaluated by sonography because of its ideal location in the RUQ and broad contact with the abdominal wall. Hepatic lesions of 1 cm or greater are easily identified, with cystic lesions appearing echo-free and solid masses appearing echogenic, allowing excellent guidance for aspiration and biopsy of these lesions.

Doppler sonography enhances the diagnostic capabilities of sonography to allow for clear analysis of the circulatory dynamics, including portal blood flow and hepatic artery thrombosis following liver transplantation. Doppler sonography can also differentiate between vessels and biliary ducts based on flow characteristics.

Computed Tomography

The role of CT in the hepatobiliary system is similar to its role in the GI tract. It is the accepted modality for following malignancies and assessing masses, particularly of the gallbladder, liver, and pancreas. It is also helpful in evaluating complications of cholecystitis such as perforations and abscess formations. The use of spiral or helical CT ensures that the entire liver is imaged in one breath, eliminating respiratory artifacts and in many cases demonstrating the liver parenchyma and associated structures better than sonography. In addition to the excellent contrast resolution offered by CT, the use of large-bolus intravenous (IV) iodinated contrast media injections during dynamic CT examination has also improved evaluations of the hepatobiliary ductal system and blood flow via three-phase imaging of the liver to capture the arterial and portal venous blood flow (Fig. 6-7). If a biliary obstruction is not visible on sonographic examination, CT is generally used to identify the location and extent of the obstruction because it is not limited by patient size or the presence of bowel gas. Lacerations of the liver and resultant abdominal bleeding are readily detected on CT (Fig. 6-8), as are metastatic lesions within the liver. CT also demonstrates good visualization of pancreatic tumors and pseudocysts. In addition, CT-guided biopsy procedures for the liver (Fig. 6-9), pancreas, and kidney allow for analysis and drainage and offer significant advantages over conventional surgical biopsy and drainage.

Nuclear Medicine Procedures

Single photon emission computed tomography (SPECT) examinations permit excellent detection of hepatobiliary lesions, especially those located deep within the liver parenchyma. SPECT provides a noninvasive method of evaluating hepatic function as well as hepatic and splenic perfusion. Because nuclear medicine imaging provides information regarding physiologic function, combining SPECT and CT can often provide information about both anatomic changes and physiologic function, thus enhancing the ability to diagnose pathologies earlier than using any one modality alone. Labeling of white blood cells (WBCs) with radioactive indium is useful in locating sites of infection for treatment.

Cholescintigraphy performed in nuclear medicine is very useful to confirm cholecystitis and for distinguishing acute cholecystitis from chronic cholecystitis (Fig. 6-10). Radioactive technetium is cleared from blood plasma into bile, demonstrating the physiologic function of the liver, excretion into the biliary ductal system, and visualization of the gallbladder about 1 hour after injection. Delayed visualization or nonvisualization of the gallbladder indicates pathology. In addition, it is a noninvasive method of evaluating biliary drainage, hepatobiliary leaks following trauma or surgery, and segmental obstruction.

Magnetic Resonance Imaging

The role of magnetic resonance imaging (MRI) of the hepatobiliary system has improved greatly as a result of shorter scan times, which allow the acquisition of several images of the abdomen in a single breath. MRI is often used in conjunction with CT to evaluate pathologies and anomalies of the peritoneum, especially the liver and pancreas. MRI may also be used to identify retroperitoneal bleeds following trauma (Fig. 6-11). Contrast-enhanced three-dimensional dynamic scans of the liver imaged at timed intervals help to differentiate certain tumors from hemangiomas.

Magnetic resonance cholangiopancreatography (MRCP) is an imaging procedure that uses magnetic resonance to visualize the gallbladder and biliary system. MRCP is noninvasive and does not require the use of a contrast agent (Fig. 6-12). A heavily T2-weighted sequence is used to suppress the tissues around the biliary system, allowing the gallbladder and bile ducts to appear bright and enabling visualization of stones or other obstructions. MRCP usually accompanies other imaging sequences of the liver, but it takes only about 15 seconds to acquire an image.

The American College of Radiology (ACR) provides a highly researched scheme for determining which of the diagnostic imaging studies is best suited for specific clinical conditions. For patients who present with RUQ pain that may be attributed to the biliary system, sonography of the abdomen is the highest rated imaging examination. CT of the abdomen (with and without contrast), cholescintigraphy, and MRI of the abdomen (with or without contrast) are also recommended and may be appropriate for obtaining the diagnosis. Additionally, if the patient is suspected of having inflammation of the gallbladder (cholecystitis), a percutaneous cholecystotomy may be appropriate for patients in the intensive care unit, who are likely considered nonsurgical candidates.