Background and Indications for Examination
Biliary tract disease exists as a spectrum that ranges from asymptomatic gallstones to biliary colic, cholecystitis, choledocholithiasis, and cholangitis. The incidence of gallstones is approximately 10%–20% and is dependent on several factors such as age, gender, fertility, race, ethnicity, and associated comorbidities. Only 1%–3% of individuals with gallstones report being symptomatic. Biliary colic occurs when a gallstone temporarily obstructs either the common bile duct (CBD) or cystic duct. Usually, biliary colic is self-limited and treated with analgesia and elective cholecystectomy. Cholecystitis results from prolonged obstruction of the cystic duct and causes inflammation of the gallbladder (GB), necessitating more urgent surgical removal. Complications of cholecystitis can lead to infection, empyema, gangrene, necrosis, perforation, and sepsis. Cholecystitis is usually caused by an obstructing gallstone, but acalculous blockage does occasionally occur.
Choledocholithiasis is a result of prolonged obstruction of the CBD. This disease can also occur post cholecystectomy when there are retained gallstones after surgery. Cholangitis is an ascending infection of the biliary tract. It can be due to prolonged obstruction from a gallstone and a resultant bacterial infection of the bile. It is a rare complication of cholecystitis, usually occurring in the elderly or in patients with associated comorbidities. Cholangitis has a high morbidity and mortality that increases with a delay in diagnosis.
Ultrasound is a rapid, noninvasive, well-tolerated, and sensitive modality in diagnosing both biliary colic and cholecystitis. Therefore, it is the first imaging modality of choice for suspected biliary disease in both the critical care and outpatient setting. The use of bedside ultrasound is important in order to rapidly identify this disease and prevent complications that result from diagnostic delays. The sensitivity of ultrasound has been reported to be as high as 95%. The four sonographic signs of cholecystitis (stones, sonographic Murphy’s sign, wall thickening, and pericholecystic fluid) are highly specific when all four are present, although this is uncommon and any of the four signs may be nonspecific, particularly in isolation. The combination of a positive sonographic Murphy’s sign and gallstones has been shown to have a positive predictive value for cholecystitis as high as 96%. Hepatobiliary iminodiacetic acid (HIDA) scans (nuclear medicine scans) are used when ultrasound results are equivocal and the diagnosis is still suspected. HIDA scans have a reported sensitivity and specificity of roughly 95%.
Ultrasound is not highly sensitive for diagnosing choledocholithiasis. Although it may not be the best modality for diagnosing choledocholithiasis, the combination of a dilated common bile duct on ultrasound with lab abnormalities helps narrow the diagnosis. Endoscopic retrograde cholangiopancreatography (ERCP) and magnetic resonance cholangiopancreatography (MRCP) remain the gold standards for diagnosing this entity. Cholangitis is a clinical diagnosis (Charcot’s triad: fever, right upper quadrant [RUQ] pain, and jaundice) associated with findings of cholecystitis or choledocholithias identified on ultrasound.
In addition to GB pathology, bedside ultrasound can be useful in evaluating for liver abnormalities, such as cysts, masses, abscesses, and hepatomegaly. A pancreatic pseudocyst or mass may also occasionally be diagnosed at the bedside. Finally, bedside ultrasound is ideal in the rapid diagnosis of new onset ascites in a patient who presents with abdominal pain and distension.
- Any patient complaining of RUQ or epigastric pain
- Unexplained right-sided chest, shoulder, flank, or generalized abdominal pain
- The patient who presents with new jaundice
- The patient with abnormal liver or biliary function tests
- The patient who presents with abdominal distension or pain with suspicion of new ascites
- The sickle cell patient with abdominal pain
- Febrile or septic patients with an unidentified source, particularly the elderly
Probe Selection and Technical Considerations
A lower-frequency probe should be used for all hepatobiliary imaging. This probe provides better penetration, which is especially useful in obese patients who often have a higher incidence of GB disease.
Hepatobiliary scanning should begin at a greater depth in order to not miss pathology in the far field and to adequately image the portal triad (CBD, portal vein [PV], and hepatic artery [HA]). Once the area of interest is identified, the depth can be reduced so that the area of interest takes up approximately three-fourths of the screen.
The focal zone should be adjusted and placed at the depth of the structure being imaged. This improves the lateral resolution of the image. Focal zone may be particularly important when trying to elicit shadowing from suspected gallstones, and should be set at the level of the suspected stone.
The total gain, which increases the intensity or volume of the signal returning to the transducer and brightens the image on the screen, may need to be adjusted in hepatobiliary imaging. Often times, the near field is imaged adequately, but the far field (posterior GB or portal triad) is not visualized entirely. In this situation, time-gain compensation (TGC) can be adjusted in order to increase the far gain of the image. This produces an enhanced brighter signal returning from the far field and a resultant clearer image, but leaves the near field unadjusted. Adjustment of this control may be useful in obese patients with difficult anatomy.
Normal Ultrasound Anatomy
The liver is located in the RUQ of the abdominal cavity just underneath the diaphragm. It is seen to the right of the stomach, superior to the intestines and overlying the GB. It is divided into four sections: the right, left, caudate, and quadrate lobes. The main lobar fissure (MLF) separates the liver into its right and left lobes and can be identified sonographically about 70% of the time, typically seen as a hyperechoic line between the neck of the GB and the portal vessels. The liver has two blood supplies, the hepatic artery (HA), which supplies blood from the aorta, and the PV that returns nutrients from the gastrointestinal tract and blood from the spleen. The normal liver should be less than 12.5–13.0 cm when measured from its superior border with the diaphragm across to the inferior tip. The falciform ligament attaches the liver to the anterior abdominal wall and can sometimes be visualized on ultrasound, especially in the presence of significant ascites.
The bile produced within the liver is contained within the intrahepatic biliary ducts. These intrahepatic ducts empty into the right and left hepatic ducts, which then merge to form the common hepatic duct. The common hepatic duct is part of the extrahepatic system as it exits the liver architecture. The common hepatic duct and the cystic duct from the GB merge to form the CBD, which joins the pancreatic duct to drain into the second part of the duodenum.
The normal liver on ultrasound appears homogenous with a medium echogenicity. The regular appearance of the liver and its large size allow it to be used as an important sonographic window for other structures, such as the right kidney, pancreas, and the heart in a subxiphoid view. The liver will contain regular-appearing anechoic structures within its architecture representing the portal and hepatic blood vessels and bile ducts (Fig. 11-1). These structures can be differentiated on ultrasound based on their different characteristics. Color-flow Doppler can be used to distinguish the vessels from bile ducts.
The PV is formed from the confluence of the splenic and mesenteric veins. The portal vessels can be identified by their hyperechoic (white) border, a result of fatty tissue in the surrounding walls. The PV divides the liver into its superior and inferior segments.
The walls of the hepatic veins will appear thinner than the portal vessels and will not have the bright white hyperechoic border. The right, left, and middle hepatic veins drain the different lobes of the liver and converge as they drain into the inferior vena cava (IVC) (see Fig. 11-1). The IVC runs along the posterior border of the liver and empties into the right atrium of the heart. The hepatic veins divide the lobes of the liver into various segments: the right hepatic vein divides the right lobe into its anterior and posterior areas, the middle hepatic vein divides the liver into its right and left lobes, and the left hepatic vein divides the left lobe into its medial and lateral areas.
The GB lies in the GB fossa located on the undersurface of the liver between the quadrate and right lobes. The GB consists of three portions: the fundus, body, and neck. The neck tapers into the biliary tree and connects to the cystic duct. The common hepatic duct drains the intrahepatic ducts and exits the liver to join the cystic duct and the two form the CBD (Fig. 11-2). The CBD then joins the pancreatic duct at the ampulla of Vater and drains into the second portion of the duodenum.
A normal GB measures about 3 cm wide × 8 cm long when it is fully distended. These measurements may vary somewhat depending on when the patient last ate a meal. A GB measuring over 10 cm in its longest dimension, particularly after a meal, is considered abnormally hydropic. The GB is a fluid-filled cystic organ and will appear as a round or elongated anechoic structure with thin walls on ultrasound. The normal GB wall is usually measured at less than 3 mm.
The landmarks of the GB include the MLF and the undivided right PV. The MLF divides the liver into its right and left lobes and can be identified on ultrasound in approximately 70% of all patients, but may be very short in some. It is visualized as a bright white hyperechoic line that extends from the portal triad up to the neck of the GB. The PV is formed by the union of the superior mesenteric vein and the splenic vein and divides just before entering the liver into the right and left PVs. It is the right portal vein (RPV) that constitutes the second landmark of the GB. In a sagittal image, with the transducer pointing cephalad, the RPV will be seen at one end of the MLF with the CBD and HA sitting just anteriorly to it, while the GB neck will be seen at the other end (Fig. 11-3).
When viewed in a transverse or short axis, the portal triad is often called the “Mickey Mouse sign” with the PV forming the face and the HA and CBD forming the ears (Fig. 11-4). The CBD is measured from inner wall to inner wall, and is typically 2–3 mm. The normal CBD diameter varies based on the age of the patient. The general rule is that the CBD is 1 mm by the age of 10 and increases by 1 mm for every decade after this. The upper limit of normal is considered to be around 7 mm in an older patient.
Ultrasound is usually not the modality of choice for pancreatic imaging. Bowel gas causes significant gas scattering and resultant artifacts and can obscure normal pancreatic anatomy. Oftentimes, the pancreas is identified only when it contains abnormalities, such as pseudocysts, calcifications, or masses.
The vascular landmarks for the pancreas are the splenic and portal veins and the superior mesenteric artery (SMA). The head of the pancreas can be seen anterior to the PV and the body anterior to the splenic vein and SMA. The splenic vein runs posterior to the border of the pancreas in the epigastric region and can be seen draining into the portal vasculature (Fig. 11-5).
Imaging Tips and Protocol
Hepatobiliary imaging should include both longitudinal and transverse images in order to fully interrogate the RUQ. In the longitudinal approach, imaging should begin similar to the FAST scan in a coronal position with the probe in the midaxillary line in between the lower rib spaces pointing toward the patient’s head. This view will allow the sonographer to evaluate the thoracic space and the RUQ for free fluid in addition to the hepatobiliary organs. The evaluation for free fluid is described fully in Chap. 9.
The liver is best imaged in this longitudinal axis from its superior pole down to the inferior tip. The probe should be advanced from the lateral position to the patient’s anterior body surface as the liver echotexture is examined for abnormalities. The probe should be moved caudally until the inferior tip of the liver is fully imaged in order to not miss dependent free fluid that collects in this space. If there is an abnormality in the liver, the sonographer should be able to describe its location based on its relationship to the hepatic and portal veins, as described earlier.
The GB should also be imaged in both its long and short axes. There are two commonly used techniques in obtaining adequate GB images: imaging through the rib spaces or by using a subcostal technique. The most successful technique can depend on a variety of factors, including patient habitus and discomfort and the amount of bowel gas present. Imaging through the rib spaces is more successful in obese patients and in those that have too much bowel gas obscuring the GB. The subcostal technique is ideal in thinner patients without a lot of gas where the probe can be angled up toward the GB without much interference. A combination of these orientations may be necessary in order to adequately image the GB in both long and short axes.
There are certain aspects of hepatobiliary imaging that must be incorporated into every evaluation of the GB in order to have a complete exam. These include imaging the GB in two planes, fanning through the GB from the fundus down to the neck to not miss stones or other pathology, evaluating the anterior wall for thickness, and identifying and measuring the CBD.
The probe can be placed as described earlier in a coronal lateral position and advanced anteriorly until the GB comes into view. The GB is located at the inferior border of the liver and therefore, on a longitudinal view, it will appear on the right side of the screen as a cystic, fluid-filled, anechoic structure. Because the probe is positioned over the ribs in this technique, it is important to make sure that rib shadows are not hiding parts of the GB and possible pathology. If a rib shadow is in the way, the probe can be angled just slightly oblique to bring the GB clearly into view. If the GB is not well visualized laterally, the probe should be slid anteriorly, again avoiding the ribs.
Once an adequate long-axis view is visualized, the probe should be fanned back and forth toward the patient’s right and left sides in order to completely evaluate the lumen from the fundus down to the neck (Fig. 11-6).