A low- to medium-frequency ultrasound transducer will suffice for most typical examinations; however, if other options exist that may be of more benefit, the sonologist must be willing to adapt and use them. This may include the use of a linear transducer for a very anterior gallbladder in a thin patient. The goal is to answer a focused question that enhances the diagnostic workup or therapeutic disposition.
Evaluation of the Gallbladder and Common Bile Duct
Four components are necessary to complete the evaluation of the gallbladder in cases of suspected biliary colic and acute cholecystitis. First, the gallbladder should be viewed in at least two planes to minimize the chance of missing an inconspicuous finding. Second, the gallbladder should be traced from the fundus to the neck carefully examining for small stones impacted in the neck. Third, the anterior gallbladder wall should be measured at its clearest point, which is usually in the mid-portion of the ultrasound beam emanating from the transducer. If focal wall thickening is encountered, several measurements should be made, one of which includes the focally thickened area. Fourth, the common bile duct should be measured and traced as far medially as possible if dilated.
A sagittal approach will initially locate the gallbladder (Figure 8-3B). Placing the probe under the right costal margin at about the mid-clavicular line with the probe marker directed toward the patient's head, the sonologist can sweep the right upper quadrant until an image of the gallbladder is obtained (Figure 8-3C). This will usually be a transverse or oblique cut through the fundus. Having the patient take and hold a deep breath may be very helpful if the gallbladder is not readily identified. Color and power Doppler aid in delineating large vascular structures. The gallbladder should have no Doppler signal and is typically the most anterior cystic structure in the abdomen (Figure 8-3D). Conversely, identifying the main portal vein also helps identify the gallbladder. Typically, the main portal vein connects to the gallbladder via a thick fibrous band known as the main lobar fissure. Occasionally locating the main portal vein and following the main lobar fissure is the only way to locate an obscure gallbladder.
The next step is to obtain a longitudinal profile of the gallbladder by gently sweeping the ultrasound beam, keeping other three-dimensional aspects of the beam's orientation constant. Thus, a three-dimensional mental image of the gallbladder in long axis is built out of multiple two-dimensional images. The gallbladder neck should be interrogated whenever possible. Having the patient take a deep breath will help the sonologist identify the neck leading into the main portal triad. One of the most difficult skills for the novice sonologist to master is the technique of converting transverse or long-axis sections into longitudinal or short-axis cuts, respectively. To change planes, the transducer should be moved slowly, keeping some element of the gallbladder in view. This is the time to adjust the patient's position or perform respiratory maneuvers if the gallbladder is not initially visualized. Short-axis views should be obtained after the longitudinal images (Figure 8-4). These views assist with clarifying whether shadowing is from stones inside the gallbladder or from adjacent bowel gas outside the gallbladder. If imaging remains difficult after patient positioning and deep inspiratory efforts, consider intercostal views (Figure 8-5). These views are helpful when the gallbladder is positioned under the ribs and/or the patient is unable to take a deep breath.
Transverse views of the gallbladder. Initial probe position with patient in lateral decubitus (A) and supine positions (B). Note the deep inspiration used and cephalad angulation of the probe to view under rib margins. Corresponding ultrasound image (C). Shadowing is from adjacent bowel gas outside the gallbladder. (Courtesy of James Mateer, MD)
Intercostal views of the gallbladder. Initial probe position with patient in lateral decubitus (A). Initial imaging plane is aligned parallel to the ribs. Corresponding ultrasound image (B). A small segment of the portal vein is seen below the neck of the gallbladder. (Courtesy of James Mateer, MD)
The gallbladder should be scanned for the presence or absence of gallstones, determination of wall thickness, and for the presence or absence of pericholecystic fluid. An accurate measurement of wall thickness is made on the anterior wall that is perpendicular to the imaging plane (greater than 3 mm being abnormal). If gallstones are detected, the patient should be rolled or elevated into various positions in an attempt to document movement of the gallstones. If a stone or stones do not respond to gravity by rolling “downhill” with body position change, then they may be impacted in the gallbladder neck. Alternatively, the sonologist may be dealing with a calcified polyp or sessile mass. If clinically applicable, the probe should be used to compress the gallbladder in an attempt to elicit a “sonographic Murphy's sign.”
Identification of the portal vein in the longitudinal axis aids in finding the main portal triad (Figure 8-6A). The portal vein will have bright echogenic walls compared with the hepatic veins. It can usually be visualized coursing toward the porta hepatis. The main portal triad is made up of the main portal vein, hepatic artery, and the common bile duct. By rotating the probe 90 degrees (counterclockwise) from a longitudinal orientation, a transverse image of the portal vein with the associated common bile duct (anterior/lateral) and hepatic artery (anterior/medial) may be visualized (Figure 8-6B). This is the classic “Mickey Mouse sign.” It is advantageous to use color or power Doppler to identify vascular structures. The portal vein and hepatic artery will usually give off color Doppler signals while the remaining common bile duct will not (Figure 8-6C,D). Measurement of the common bile duct is made from inner wall to inner wall; normal is in general less than 7 mm thick.31–33 Tissue harmonics may also help delineate the common bile duct. However, tissue harmonics increases the measured thickness of walls and must be used with caution. The site of common bile duct obstruction from stones is often near its termination close to the pancreatic head. This area may be visualized in a thin patient with little interfering gas (Figure 8-7). It will be difficult to visualize this point in the majority of patients.
Portal vein and normal common bile duct. Longitudinal view of the portal vein (A). Transverse view of the portal vein (B). Corresponding color Doppler images (C,D). Patient has a duplicated hepatic artery. IVC = inferior vena cava, PV = portal vein, Ao = aorta, Sp = spine, GB = gallbladder, CBD = common bile duct. (Courtesy of James Mateer, MD)
(a) Short-axis view of the portal vein (PV), with the associated common bile duct (anterior/lateral)[arrow] and hepatic artery (anterior/medial) [arrowhead]. The relative positions of the gallbladder (Gb) and inferior vena cava (IVC) are noted. (b) Transverse view of the upper abdomen. The position of the the common bile duct (arrow) near its termination by the pancreatic head (P) is noted along with the relative positions of the gallbladder (Gb) and aorta (Ao). (Courtesy of James Mateer, MD)
Evaluation of Acute Jaundice and Biliary Obstruction
No gallbladder examination is complete without evaluation of the bile ducts. The initial step of this examination technique is to locate the main portal triad, which can be accomplished in two ways. The first method uses the portal vein as a landmark and is described above. The second method involves tracing more peripheral branches of the portal venous system as they course centrally toward the hilum. Their echogenic walls and their normal enlargement can identify portal venous branches as they course centrally toward the hilum to join the main portal vein in the main portal triad. They are clearly distinguished from the hepatic venous system, with its thin, hypoechoic walls and enlargement as it converges on the inferior vena cava posteriorly (Figure 8-8). Without the presence of biliary tract disease, they are rarely visible within the hepatic parenchyma. Color Doppler helps differentiate between the portal and biliary trees by demonstrating flow within the former. (Figure 8-6). Once the main portal triad has been identified, examination and measurement of the common duct are performed. The duct is usually normal if the transverse diameter, in millimeters, is less than one tenth of the patient's age. However, after cholecystectomy, the common bile duct may normally range up to 1 cm in all age groups.
The most convenient method for intrahepatic duct evaluation involves transverse imaging of the left lobe of the liver. Intrahepatic ducts run in the transverse plane in this location, allowing longitudinal images of the ducts to be obtained by orienting the ultrasound beam axis transversely. Longitudinal imaging of the ducts allows easier detection and evaluation of abnormalities in the intrahepatic system. While it was once thought that any visualization of the intrahepatic ducts was abnormal, this may not be the case with modern equipment and the greatly improved resolution it delivers.
(a) Transverse view of the upper portion of the liver. The hepatic venous system, with its thin, hypoechoic walls and enlargement as it converges on the inferior vena cava (IVC) posteriorly is noted. The right hepatic vein (R) and a portion of the middle hepatic vein (M) are seen in this view. (b) Transverse/oblique view of the upper liver demonstrating the junction of the inferior vena cava (IVC) with the the middle hepatic vein (M) and left hepatic vein (L). (Courtesy of James Mateer, MD)