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Gastrointestinal

Abdomen Anatomy

The abdomen contains the solid organs (liver, spleen, pancreas, kidneys, adrenals), the hollow gut, and the mesenteric and retroperitoneal vasculature. Multiphase contrast-enhanced CT and MRI define organ segmentation, vascular supply, and peritoneal spaces. Understanding Couinaud liver segments, pancreaticobiliary anatomy, and retroperitoneal compartments is central to localization and surgical planning.

Key Structures

  • Liver: eight Couinaud segments defined by hepatic veins and portal branches; segment I (caudate) drains directly to the IVC; portal triads and hepatic veins are the key landmarks
  • Biliary tree and gallbladder: intrahepatic ducts, common hepatic and common bile ducts, cystic duct, ampulla of Vater; gallbladder in the interlobar fossa
  • Pancreas: head with uncinate process (surrounding the SMA/SMV), neck, body, tail; main pancreatic duct (Wirsung) joining the CBD at the ampulla
  • Spleen: left upper quadrant organ with early heterogeneous "zebra" arterial enhancement, supplied by the splenic artery
  • Kidneys and adrenals: cortex/medulla with corticomedullary differentiation, collecting system, renal hila; adrenals as inverted-Y/V retroperitoneal glands
  • Gut: stomach, duodenum (four parts), jejunum (left upper, feathery folds), ileum (right lower), colon frame with haustra; appendix off the cecum
  • Vasculature: aorta with celiac, SMA, IMA and renal branches; IVC; portal vein formed by splenic and SMV confluence; mesenteric vessels

Imaging Planes & Modalities

  • Multiphase CT: arterial (~35 s) for hypervascular lesions/arterial map, portal venous (~70 s) for parenchyma and metastases, delayed for washout/urothelium
  • MRI with T1 in/out-of-phase (fat/iron), T2, DWI, and dynamic gadolinium (including hepatobiliary agents) characterizes liver and pancreatic lesions
  • MRCP heavily T2-weighted for the biliary and pancreatic ducts without contrast
  • Ultrasound first-line for gallbladder, biliary dilatation, renal, and free fluid; Doppler for portal/hepatic flow direction
  • CT/MR angiography and venography map mesenteric vessels, portal patency, and variant anatomy
  • Coronal reformats display craniocaudal extent, ureters, and bowel; axial remains the primary review plane

Landmarks & Normal Values

  • Portal vein normally ≤13 mm; common bile duct ≤6-7 mm (allow ~1 mm per decade over 60 and larger post-cholecystectomy)
  • Pancreatic duct normally ≤3 mm in the head, tapering distally
  • Adrenal limbs normally <10 mm thick; a lesion <10 HU on unenhanced CT is a lipid-rich adenoma
  • Spleen craniocaudal length normally <12-13 cm
  • Celiac axis at ~T12 and SMA just below; left renal vein passes between the SMA and aorta
  • Retroperitoneal compartments (anterior/posterior pararenal and perirenal spaces) bounded by Gerota and lateroconal fascia

Anatomical Variants

  • Replaced/accessory right hepatic artery from the SMA and replaced left hepatic artery from the left gastric artery
  • Pancreas divisum (dorsal duct draining via the minor papilla) and annular pancreas
  • Retroaortic or circumaortic left renal vein; duplicated IVC or left-sided IVC
  • Duplicated collecting systems, horseshoe kidney, and crossed fused ectopia
  • Accessory spleen(s)/splenules near the splenic hilum; wandering spleen
  • Riedel lobe and other hepatic contour variants; variant portal vein branching (trifurcation)

Pathology Correlation

  • Couinaud segmentation guides hepatic resection and localizes tumors and metastases relative to vascular pedicles
  • Biliary and pancreatic duct anatomy explains obstruction patterns (double duct sign in pancreatic head cancer)
  • Mesenteric vascular anatomy underpins ischemia, SMA syndrome, and median arcuate ligament compression
  • Adrenal washout characteristics distinguish adenomas from metastases and pheochromocytoma
  • Peritoneal and retroperitoneal spaces predict spread of collections, abscesses, and carcinomatosis
  • Portal venous anatomy and flow direction are central to portal hypertension, thrombosis, and TIPS assessment

Pitfalls & Mimics

  • Transient hepatic attenuation differences and perfusion anomalies mimic lesions on arterial phase
  • Pseudolesions around the falciform/gallbladder fossa from aberrant venous drainage (focal fat sparing or deposition)
  • Collapsed bowel and fluid-filled loops mimic masses; adequate distension and multiplanar review help
  • Adrenal collision tumors and myelolipoma fat can confound the washout algorithm
  • Splenic "zebra" arterial enhancement mistaken for infarct or laceration
  • Peripancreatic fat/duodenal diverticula simulating pancreatic head masses or fluid collections

References

  • Federle et al. Diagnostic Imaging: Abdomen
  • Gore, Levine. Textbook of Gastrointestinal Radiology
  • Haaga et al. CT and MRI of the Whole Body
  • Netter. Atlas of Human Anatomy

⚕️ Educational reference — correlate with clinical context and confirm with a qualified radiologist.