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.

Abdomen Anatomy

🧩 Key Structures

🎞️ Imaging Planes & Modalities

📏 Landmarks & Normal Values

🔀 Anatomical Variants

🧠 Pathology Correlation

💡 Pitfalls & Mimics