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Gastrointestinal

Gastrointestinal Tract Anatomy

The gastrointestinal tract is a continuous muscular tube from the esophagus to the anal canal, suspended and compartmentalized by the peritoneum and mesenteries that direct the spread of inflammation and tumor. Cross-sectional imaging with multiphase CT, MR enterography, and fluoroscopy evaluates wall thickness, enhancement, luminal caliber, and adjacent fat. A working knowledge of segmental anatomy and mesenteric attachments is essential for localizing pathology and predicting disease spread.

Key Structures

  • Esophagus: cervical, thoracic, and abdominal segments; normal wall <3 mm distended; gastroesophageal junction at the hiatus
  • Stomach: fundus, body, antrum, and pylorus with rugal folds; wall thickness varies with distention (up to 5-7 mm in the antrum)
  • Duodenum (four parts) framing the pancreatic head; the C-loop and ligament of Treitz mark the duodenojejunal junction
  • Small bowel: jejunum (left upper, feathery valvulae conniventes) versus ileum (right lower, fewer folds); normal wall <3 mm
  • Colon: haustrated ascending, transverse, descending, and sigmoid segments; cecum with the appendix arising at the confluence of the taeniae
  • Rectum and anal canal: mesorectal fascia and levator ani complex; internal and external anal sphincters
  • Peritoneal reflections and mesentery: transverse mesocolon, small bowel mesentery, greater/lesser omentum defining supra- and inframesocolic compartments

Imaging Planes & Modalities

  • Contrast-enhanced multidetector CT with neutral or positive oral contrast is first-line for obstruction, perforation, ischemia, and staging; multiplanar reconstructions trace bowel loops
  • CT/MR enterography with neutral luminal distension optimally assesses small-bowel Crohn disease, wall enhancement, and mesenteric changes
  • Fluoroscopy (barium swallow, upper GI, small-bowel follow-through, contrast enema) evaluates mucosa, motility, strictures, and leaks
  • Rectal MRI (high-resolution T2) stages rectal cancer relative to the mesorectal fascia and sphincter complex
  • Ultrasound assesses appendicitis, intussusception, and bowel-wall thickening, especially in children
  • CT colonography and PET/CT are used for polyp screening and oncologic staging respectively

Landmarks & Normal Values

  • Ligament of Treitz (duodenojejunal flexure) is the anatomic boundary between upper and lower GI bleeding and orients the small-bowel mesentery
  • Normal small-bowel wall <3 mm and caliber <2.5-3 cm; small bowel >3 cm (or colon >6 cm, cecum >9 cm) suggests obstruction/dilatation
  • Appendix normally <6 mm outer diameter, compressible, without periappendiceal fat stranding
  • Ileocecal valve and the confluence of taeniae coli localize the appendiceal base
  • Mesorectal fascia is the circumferential resection margin reference for rectal cancer staging

Anatomical Variants

  • Intestinal malrotation with an abnormal SMA/SMV relationship and risk of midgut volvulus
  • Situs and rotational anomalies; mobile cecum and redundant sigmoid predisposing to volvulus
  • Meckel diverticulum—an antimesenteric ileal remnant of the omphalomesenteric duct
  • Duodenal diverticula (often periampullary) and paraduodenal fossae associated with internal hernias
  • Retrorectal/presacral space anatomy and variant hiatal anatomy (sliding hiatal hernia)
  • Annular pancreas and duplication cysts affecting duodenal caliber

Pathology Correlation

  • Bowel obstruction: transition point with proximal dilatation and distal collapse; closed-loop and volvulus show whirl sign and mesenteric swirl
  • Ischemia: reduced or absent mural enhancement, pneumatosis, portal venous gas, and mesenteric vessel occlusion
  • Inflammatory disease: mural hyperenhancement, stratification (target sign), comb sign, and fibrofatty proliferation in Crohn disease
  • Perforation: extraluminal gas and fluid tracking along mesenteric and peritoneal planes to dependent recesses
  • Appendicitis: dilated non-compressible appendix, wall enhancement, periappendiceal stranding, and appendicolith
  • Tumor staging follows mesenteric and nodal drainage; rectal cancer relationship to the mesorectal fascia dictates neoadjuvant therapy

Pitfalls & Mimics

  • Collapsed or under-distended bowel mimics wall thickening or a mass—assess adequately distended loops
  • The normal SMA/SMV relationship must be checked to avoid missing malrotation with volvulus
  • Physiologic pneumatosis and stercoral gas can be mistaken for ischemic pneumatosis—correlate with enhancement and clinical picture
  • Do not confuse fecal material or an ingested foreign body with an appendicolith or intraluminal mass
  • Misidentifying the transition point or overlooking a closed-loop obstruction delays surgical intervention
  • Ascites, adjacent inflammation, and adjacent organ pathology can obscure the appendix and bowel margins

References

  • Gore RM, Levine MS. Textbook of Gastrointestinal Radiology. Elsevier
  • Federle MP, et al. Diagnostic Imaging: Abdomen. Elsevier
  • Meyers MA. Dynamic Radiology of the Abdomen: Normal and Pathologic Anatomy. Springer
  • Standring S. Gray's Anatomy: The Anatomical Basis of Clinical Practice. Elsevier

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