First and second year — the floor first, then every step
First-line: excellent for dilatation and gallbladder stones, poor at the distal duct where bowel gas sits.
Orient first
Ultrasound answers "are the ducts dilated" better and faster than anything else, and answers "why" much less often.
The PARALLEL CHANNEL sign — a dilated duct alongside a portal vein branch — is the intrahepatic finding; colour Doppler on the vein confirms which is which in seconds.
The distal duct is obscured by duodenal gas in a large proportion of patients. That is a limitation of the study, not a negative finding.
A distended, thin-walled gallbladder in a jaundiced patient supports a distal obstruction; a contracted stone-filled gallbladder supports a stone.
Acquire the study
Fasted at least 6 hours so the gallbladder is distended and the ducts are assessable.
Subcostal and intercostal windows; left lateral decubitus and an erect position to move duodenal gas away from the distal duct.
Colour Doppler available throughout — distinguishing duct from vessel is the commonest ambiguity.
The manoeuvre
Assess intrahepatic ducts for the parallel channel sign, and confirm with colour.
Measure the extrahepatic duct internally at the porta hepatis and state where you measured.
Follow the duct distally as far as gas allows, repositioning the patient to improve the window.
Assess the gallbladder: distended or contracted, calculi, wall thickness, and impaction at the neck.
Assess the pancreatic head, and the pancreatic duct for the double duct sign.
State explicitly when the distal duct was not visualised, and recommend MRCP.
What confirms it
Dilatation ABOVE a transition point with a visible cause at that point.
A stone is a dependent, round filling defect confirmed on more than one plane and sequence; a stricture is a segment of narrowing with wall thickening or a mass.
What licenses you to exclude it
⚠️ Normal-calibre ducts do NOT exclude obstruction early, nor after cholecystectomy, nor in a fibrotic liver where ducts cannot dilate. Correlate with the biochemistry rather than reporting a normal study.
Ultrasound alone frequently cannot exclude a distal cause because bowel gas obscures the distal duct — say so explicitly.
The classic misread
Mistaking a portal vein branch for a dilated duct without using colour.
Reporting "no cause seen" when the distal duct was never visualised.
Reference values
Each value carries the caveat that keeps it from being misused. Normal limits and diagnostic criteria are kept apart on purpose: a disease cut-off read as a normal range is the more dangerous mistake.
Normal limits
Common bile duct · internal diameterup to 6 mm
Widens with age (roughly +1 mm per decade over 60) and after cholecystectomy — state the patient context rather than the bare number.
USG · CT · MRI
Common bile duct · Internal diameter after cholecystectomy / with age
wider than the standard 6 mm limit — commonly accepted up to about 10 mm post-cholecystectomy
Measured internal wall to internal wall at the porta hepatis.
⚠️ THIS IS THE ENTRY THAT PREVENTS A FALSE POSITIVE. Applying the 6 mm limit to a post-cholecystectomy duct over-calls obstruction. Some texts also add roughly 1 mm per decade over 60. State which allowance you applied.
USG · CT · MRI
Intrahepatic bile ducts · Calibre of central intrahepatic ducts
up to 2 mm, and under 40% of the diameter of the adjacent portal vein branch
Peripheral ducts are not normally visualised at all — seeing them is itself the finding. Mild central prominence is common after cholecystectomy and in the elderly.
USG · CT · MRI
Main pancreatic duct · diameter in the head/bodyup to 3 mm
Tapers toward the tail; mild age-related dilatation is recognised.
USG · CT · MRI
See it on real cases
Direct links to Radiopaedia — the reference article and worked cases with their images. Each opens on Radiopaedia.