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Common Errors of Biliary Ultrasound
More and more practitioners are using bedside ultrasound to obtain valuable data that can be used to enhance and expedite patient care. It is important to recognize the limitations of bedside ultrasonography and to stay current with the recent literature. Update your skills regularly and continue to challenge yourself by reading, scanning, and attending advanced courses.
1. Use a 5-1 MHz phased array or curvilinear transducer. Probes with smaller footprints are easier to maneuver in between the ribs.
2. The gallbladder can be visualized using three main views: the X-7 approach, the subcostal sweep, and the lateral approach.
3. For the X-7 approach, look for the gallbladder 7 cm lateral to the xiphoid process through the intercostal space.
4. With the subcostal sweep, start with the probe in a longitudinal fashion just lateral to the xiphoid process, with the indicator pointing towards the patient’s head. Aim the beams towards the patient’s right shoulder and sweep the probe laterally just underneath the costal margin until you visualize the gallbladder.
5. The lateral approach utilizes the liver as an acoustic window. Place your probe in a longitudinal fashion, along the anterior axillary line, with the indicator pointed towards the patient’s head. Glide anteriorly along the intercostal space, scanning through the liver, until you visualize the gallbladder lumen.
6. Assess whether or not the patient has a normal, dilated gallbladder (between 3-4 cm in the transverse diameter), evidence of cholelithiasis or sludge, gallbladder wall thickening > 3 mm, pericholecystic fluid, common bile duct dilatation > 6 mm, or a sonographic Murphy’s sign. Remember that contracted gallbladders (<3 cm in diameter) can appear to have thickened walls.
7. Remember that the right kidney lies adjacent to the liver. Large renal cysts and prominent hydroureters can be mistaken for the gallbladder if you are not careful. Always scan through the entire gallbladder and follow it’s neck until it joins the cystic duct and eventually the common bile duct to ensure you are visualizing the correct structure.
8. Often times, while scanning through the liver, loops of bowel will be visualized adjacent to the liver’s inferior edge. Stool will appear hyperechoic with prominent acoustic shadowing farfield on the screen. Do not mistake one of these loops of bowel for the gallbladder with an intraluminal stone.
9. Likewise, it is common to overlook a gallbladder filled with a large stone because its sonographic appearance is very similar to a stool-filled loop of bowel. Don’t be fooled by the W.E.S. sign (Wall-Echo-Shadow sign) signifying the presence of a large, and likely symptomatic, gallstone.
10. When in doubt, obtain multiple views from different angles, and reposition the patient as needed.
11. Remember that practice makes perfect. With bedside ultrasound there is no substitute for experience. The more ultrasounds you do, the better you will be able to differentiate abnormal from normal, even when you may not be sure exactly what the abnormality is. Stay tuned for future articles on how to avoid common ultrasound mistakes.