Liver segments and biliary tree

Couinaud segments

• Eight segments, each with its own portal branch, artery and bile duct, so each can be resected alone; numbered clockwise from the caudate viewed from the front.
• Vertical planes = hepatic veins: the right hepatic vein splits the right lobe into an anterior sector (V, VIII) and a posterior sector (VI, VII); the middle hepatic vein lies in Cantlie line (gallbladder fossa to IVC), the true division into right and left lobes; the falciform ligament (umbilical fissure) separates segment IV (medial) from II and III (lateral).
• Horizontal plane = portal vein bifurcation: above it II, IVa, VII, VIII; below it III, IVb, V, VI.
• Segment I (caudate): separated from segment II by the fissure for the ligamentum venosum; receives both left and right portal branches and drains directly into the IVC. It is spared and enlarged in Budd-Chiari syndrome (central enhancement, patchy periphery, non-opacified hepatic veins) and in cirrhosis.
• Gallbladder cancer invading the liver bed involves IVb and V.

Liver: normal imaging

• Blood supply: about 75% portal vein, 25% hepatic artery. Normal liver enhances most in the portal phase; HCC is fed by the artery, so it is bright in the arterial phase and darker than liver in the portal phase (washout).
• Portal veins on ultrasound have bright walls (fibrous sheath) and run towards the porta; hepatic veins have barely visible walls and converge on the IVC.
• Echogenicity: renal cortex is less echogenic than liver, liver about equal to or slightly less than spleen, pancreas brighter than liver in adults.
• Riedel lobe: tongue-like downward extension of the right lobe.
• Iron on MRI (dark on T2): transfusion overload darkens liver and spleen (reticuloendothelial); genetic haemochromatosis darkens liver and pancreas, sparing the spleen.
• Percutaneous biopsy: right 8th–9th space, midaxillary line, in expiration.

Biliary tree

• Common bile duct normal internal diameter up to 6 mm (larger in old age and after cholecystectomy).
• Calot (hepatocystic) triangle: cystic duct, common hepatic duct, inferior surface of liver; contains the cystic artery (usually from the right hepatic) and cystic lymph node. Rouvière sulcus marks the level of the bile duct: dissect above it. A tortuous right hepatic artery looping into the triangle is the Moynihan hump.
• Supraduodenal bile duct is supplied by axial arteries at 3 and 9 o'clock, so stripping or tension causes ischaemic stricture.
• Commonest duct variant: right posterior duct joining the left hepatic duct (danger in donor surgery).
• MRCP is heavily T2-weighted: static fluid (bile, pancreatic juice) is bright, liver and flowing blood dark.

Pancreas

• Head in the duodenal C-loop; uncinate process hooks behind the SMV, in front of the aorta and IVC; neck lies in front of the SMV–portal confluence; body arches over L1–L2; tail reaches the splenic hilum in the splenorenal ligament (tail injury at splenectomy gives an amylase-rich leak).
• Splenic vein runs along the back of body and tail; tortuous splenic artery along the upper border. Behind the head: IVC, bile duct, renal veins.
• Development: ventral bud forms the uncinate and lower head; dorsal bud forms the rest. Pancreas divisum: dorsal duct drains most of the gland through the minor papilla. Annular pancreas: ring around D2. Islets are most numerous in the tail.
• Imaging: adenocarcinoma is hypodense in the pancreatic phase (dense desmoplastic stroma, few vessels); chronic pancreatitis shows stippled calcification at L1–L2; acute pancreatitis fluid lies in the anterior pararenal space; an encapsulated collection with solid debris after 4 weeks is walled-off necrosis.

Spleen

• Lies along the left 9th–11th ribs (injured with their fractures); a medial notch distinguishes it from a kidney.
• Ligaments: gastrosplenic (short gastric and left gastroepiploic vessels), splenorenal (splenic vessels, pancreatic tail), phrenicocolic (supports the lower pole). Lax ligaments allow a wandering spleen that can twist.
• Arterial phase shows striped (arciform) enhancement from unequal flow through the red pulp; uniform by the portal phase.
• Accessory spleen is commonest at the hilum (mesodermal origin); splenosis is scattered implants after trauma. Infarct is wedge-shaped with its base on the capsule (end arteries). Subcapsular haematoma: crescent that flattens the splenic surface. Gamna-Gandy bodies in portal hypertension: dark foci that bloom on gradient-echo (haemosiderin).

Remember Hepatic veins split the liver vertically, the portal vein splits it horizontally.

Exam trap The caudate lobe drains straight into the IVC, which is why it survives and enlarges in Budd-Chiari syndrome.

Exam trap Striped splenic enhancement in the arterial phase is normal flow, not injury.