Bile duct measurements use ultrasound

The internal diameter of the common duct is usually taken as 6 mm or less. It is age-dependent,however, and can be 8 or 9 mm in an elderly person, due to degeneration of the elastic fibre in the duct wall. Ensure this is not early obstruction by thoroughly examining the distal common bile duct or rescanning after a short time interval. The diameter can vary quite considerably, not only between subjects, but along an individual duct. The greatest measurement should be recorded, in longitudinal section. Never measure the duct in a transverse section (for example at the head of pancreas); it is invariably an oblique plane through the duct, which will overestimate the diameter. Intrahepatically, the duct diameter decreases. The right and left hepatic ducts are just visible, but more peripheral branches are usually too small to see. Patients with a cholecystectomy who have had previous duct dilatation frequently also have a persistently dilated, but non-obstructed, duct (Fig. A). Be suspicious of a diameter of 10 mm or more as this is associated with obstruction due to formation of stones in the duct. relative to the portal branches is highly variable. Don’t assume that a channel anterior to the PV branch is always a biliary duct—if in doubt, use colour Doppler to distinguish the bile duct from the portal vein or hepatic artery. The proximal bile duct is best seen either with the patient supine, using an intercostal approach from the right, or turning the patient oblique, right side raised. This projects the duct over the portal vein, which is used as an anatomic marker. Scanning the distal duct usually requires more effort. Right oblique or decubitus positions are useful. Gentle pressure to ease the duodenal gas away from the duct can also be successful. Sometimes, filling the stomach with water (which also helps to display the pancreas) and allowing it to trickle through the duodenum does the trick. Try also identifying the duct in the pancreatic head (Fig. B) and then tracing it retrogradely towards the liver. Asking the patient to take deep breaths is occasionally successful, but may make matters worse by filling the stomach with air. It is definitely worth persevering with your technique, particularly in jaundiced patients.