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Small intestine

The small intestine is the longest part of the gastrointestinal tract, measuring approximately 5–7 meters in adults. It extends from the pylorus of the stomach to the ileocecal junction, where it empties into the large intestine. It is divided into three main parts:

  • Duodenum (25 cm): C-shaped, fixed to retroperitoneum, receives bile and pancreatic secretions.

  • Jejunum (2.5 m): Proximal two-fifths of intraperitoneal small bowel, characterized by thicker walls, larger diameter, and prominent circular folds (plicae circulares).

  • Ileum (3.5 m): Distal three-fifths, thinner walls, smaller diameter, more mesenteric fat, Peyer’s patches, and fewer circular folds.

The small intestine is the primary site for digestion and nutrient absorption. It has a large mucosal surface area due to villi and microvilli. The duodenum plays a major role in enzymatic digestion, while the jejunum and ileum specialize in absorption of nutrients, electrolytes, and vitamins.

Clinically, it is central to conditions such as Crohn’s disease, celiac disease, small bowel obstruction, ischemia, and neoplasms.

Synonyms

  • Intestinum tenue

  • Small bowel

Function

  • Completes digestion of food via enzymes and bile

  • Absorbs carbohydrates, proteins, fats, vitamins, minerals, and water

  • Provides immune protection through gut-associated lymphoid tissue (GALT), especially in ileum

  • Maintains intestinal motility and coordination with stomach and colon

Nerve Supply

  • Sympathetic: Thoracic splanchnic nerves via the superior mesenteric plexus (jejunum & ileum) and celiac plexus (duodenum)

  • Parasympathetic: Vagus nerve via celiac and superior mesenteric plexuses

  • Provides motor control for peristalsis and secretomotor function

Arterial Supply

  • Duodenum: Celiac trunk via gastroduodenal artery and superior pancreaticoduodenal artery; SMA via inferior pancreaticoduodenal artery

  • Jejunum and Ileum: Jejunal and ileal branches of the superior mesenteric artery forming vascular arcades and vasa recta

Venous Drainage

  • Corresponds to arterial supply, draining into superior mesenteric vein (SMV)

  • SMV joins splenic vein to form the portal vein

  • Important for portal circulation and site of porto-systemic collateral formation in pathology

MRI Appearance

T1-weighted images:

  • Bowel wall shows intermediate signal, lumen content variable (gas low signal, fluid low-to-intermediate, fat hyperintense if present)

  • Useful for mural thickness and mesenteric anatomy

T2-weighted images:

  • Normal lumen fluid is bright hyperintense, bowel wall appears intermediate signal

  • Detects bowel wall edema, inflammation, or obstruction

STIR:

  • Suppresses mesenteric fat, enhancing visibility of bowel wall edema, abscesses, or inflammatory infiltration

  • Useful in Crohn’s disease assessment

T1 Fat-Saturated (Pre-contrast):

  • Bowel wall appears intermediate signal, easily distinguished from suppressed mesenteric fat

  • Highlights wall thickening and mucosal folds

T1 Fat-Saturated Post-Contrast (Gadolinium):

  • Normal wall shows thin, homogeneous enhancement

  • Pathology (Crohn’s disease, ischemia, tumors) shows thickened, irregular, or layered enhancement

  • Active inflammation appears with mural hyperenhancement and stratification

MRI Non-Contrast 3D Imaging (MR Enterography):

  • Provides 3D view of small bowel loops, wall thickness, strictures, and fistulas

  • Excellent for inflammatory bowel disease mapping

CT Appearance

CT Pre-Contrast:

  • Bowel loops visible as soft-tissue tubes with variable intraluminal content (air, fluid, feces)

  • Detects obstruction, calcifications, or gross masses

CT Post-Contrast:

  • Bowel wall enhances uniformly in normal state

  • In pathology, wall thickening, mural stratification, and abnormal enhancement patterns appear

  • Identifies complications such as perforation, abscess, ischemia, or tumors

CT Enterography (CTA protocol):

  • Optimal for small bowel evaluation

  • Uses oral neutral contrast to distend bowel, with IV contrast to show mural enhancement

  • Demonstrates strictures, fistulas, Crohn’s disease activity, ischemia, and neoplasms

  • Provides high-resolution 3D reconstructions for surgical planning

MRI images

Small intestine MRI coronal anatomy image -img-00000-00000

CT images

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CT images

Small intestine CT axial anatomy image -img-00000-00000_00001

CT images

Small intestine CT coronal anatomy image -img-00000-00000