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Ileum

The ileum is the final and longest part of the small intestine, extending approximately 2–4 meters from the jejunum at the jejunoileal junction to the ileocecal junction, where it opens into the cecum. It lies predominantly in the right lower quadrant and pelvic cavity, suspended by the mesentery.

The ileum differs from the jejunum in being narrower, thinner-walled, and paler. Its mucosa has fewer and shorter circular folds (plicae circulares) and contains abundant Peyer’s patches (aggregated lymphoid nodules) along its antimesenteric border, reflecting its important immunological role.

The ileum functions in the absorption of bile salts, vitamin B12, and other nutrients not absorbed proximally. It is also an important site for immune surveillance of intestinal contents.

Clinically, the ileum is relevant in Crohn’s disease, Meckel’s diverticulum, intestinal obstruction, ischemia, tumors, and malabsorption syndromes. The terminal ileum is a frequent site of disease evaluation in imaging and endoscopy.

Synonyms

  • Distal small intestine

  • Terminal ileum (for distal segment)

  • Pars ileum intestini tenuis

Function

  • Absorbs bile salts, vitamin B12, and products of fat digestion

  • Completes absorption of electrolytes and water

  • Provides immunological defense via Peyer’s patches

  • Transfers digested contents into cecum for further processing

Nerve Supply

  • Sympathetic: Superior mesenteric plexus (derived from lesser splanchnic nerves, T10–T12)

  • Parasympathetic: Vagus nerve (posterior vagal trunk)

  • Regulates peristalsis, secretion, and vascular tone

Arterial Supply

  • Multiple branches from the superior mesenteric artery (SMA) via ileal arteries in the mesentery

  • Terminal ileum also supplied by the ileocolic artery

Venous Drainage

  • Corresponds to arteries, draining into the superior mesenteric vein (SMV)

  • SMV contributes to formation of the portal vein

MRI Appearance

T1-weighted images:

  • Ileal wall: intermediate signal intensity, lumen varies depending on content

  • Mesenteric fat appears hyperintense, aiding wall delineation

T2-weighted images:

  • Normal wall: low to intermediate signal

  • Lumen: bright when fluid-filled

  • Hyperintense thickening indicates edema or inflammation (e.g., Crohn’s disease)

STIR:

  • Suppresses fat, accentuating wall thickening, edema, or inflammatory changes

  • Highlights active disease in Crohn’s ileitis

T1 Fat-Saturated (Pre-contrast):

  • Ileal wall shows intermediate signal, distinguished from suppressed mesenteric fat

T1 Fat-Saturated Post-Contrast (Gadolinium):

  • Normal wall: thin, uniform enhancement

  • Abnormal wall: thickened, stratified, or heterogeneous enhancement in inflammation, neoplasia, or ischemia

  • Active Crohn’s disease shows layered mural hyperenhancement with surrounding fat stranding

MRI Non-Contrast 3D Imaging:

  • Provides 3D overview of bowel loops and mesenteric vessels

  • Useful in preoperative mapping, fistula evaluation, and bowel motility assessment

CT Appearance

CT Pre-Contrast:

  • Ileum seen as a soft-tissue tubular loop with variable luminal content (air, fluid, fecal matter)

  • Calcifications may suggest chronic disease or foreign body

CT Post-Contrast:

  • Wall enhances thin and uniform in normal state

  • Wall thickening, stratified enhancement, and engorged vasa recta (“comb sign”) indicate active Crohn’s disease

  • Detects tumors, ischemia, obstruction, and Meckel’s diverticulum

MRI images

Ileum MRI coronal anatomy image -img-00000-00000

CT images

Ileum ct axial anatomy image -img-00000-00000

CT images

Ileum, ct coronal anatomy image -img-00000-00000