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.

Gastrointestinal Tract Anatomy

🧩 Key Structures

🎞️ Imaging Planes & Modalities

📏 Landmarks & Normal Values

🔀 Anatomical Variants

🧠 Pathology Correlation

💡 Pitfalls & Mimics