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Sigmoid colon

The sigmoid colon is the terminal portion of the large intestine, connecting the descending colon to the rectum. It typically measures 35–45 cm in length and has an S-shaped course, hence the name “sigmoid.” It lies within the left iliac fossa and pelvis and is suspended by the sigmoid mesocolon, which allows a variable degree of mobility.

It plays a vital role in storing fecal matter before defecation and is a frequent site of diseases such as diverticulosis, volvulus, and colorectal cancer. Its anatomical variations and mobility make it clinically important in abdominal and pelvic surgery.

Synonyms

  • Pelvic colon

  • Sigmoid flexure

  • Sigmoid loop

Location and Course

  • Begins at the pelvic brim at the termination of the descending colon

  • Passes through the left iliac fossa into the pelvis

  • Forms an S-shaped loop, with considerable mobility due to the sigmoid mesocolon

  • Terminates at the level of S3 vertebra, where it continues as the rectum

Relations

  • Anteriorly: Small intestine loops (ileum, jejunum), urinary bladder (when full), uterus and adnexa in females

  • Posteriorly: Sacrum, iliac vessels, ureter, and psoas major muscle

  • Superiorly: Descending colon

  • Inferiorly: Rectum

Blood Supply

  • Arterial supply: Primarily from the sigmoid arteries, branches of the inferior mesenteric artery (IMA)

  • Venous drainage: Via the inferior mesenteric vein (IMV), which drains into the portal venous system

Function

  • Acts as a reservoir for feces before defecation

  • Absorbs water and electrolytes from residual intestinal contents

  • Propels stool into the rectum through peristaltic contractions

  • Plays a role in maintaining continence by regulating stool passage

Clinical Significance

  • Sigmoid volvulus: Twisting of the sigmoid loop causing obstruction and ischemia

  • Diverticulosis/diverticulitis: Common in this segment due to high intraluminal pressure

  • Colorectal cancer: Frequent site of adenocarcinoma

  • Surgical relevance: Mobilized in colectomy and colostomy procedures

  • Imaging: Sigmoid colon is routinely evaluated in CT colonography, MRI pelvis, and colonoscopy

MRI Appearance

T1-weighted images:

  • Lumen contents (fluid/stool) appear as variable signal depending on composition

  • Colonic wall shows low-to-intermediate signal intensity

  • Fat within mesocolon is bright

T2-weighted images:

  • Lumen fluid appears bright

  • Colonic wall appears as intermediate-to-low signal

  • Fibrosis, stricture, or tumor appear as intermediate-to-low signal masses

STIR (Short Tau Inversion Recovery):

  • Fluid in the lumen appears bright

  • Colonic wall normally remains dark

  • Inflammation (colitis, diverticulitis) shows bright hyperintensity in wall and mesocolon

T1 Fat-Sat Post-Contrast:

  • Colonic wall enhances homogeneously in normal state

  • Tumors, inflammation, or abscesses show heterogeneous or rim enhancement

  • Pericolonic fat appears dark due to fat suppression, highlighting abnormal enhancement

3D T2 SPACE / CISS:

  • Provides clear delineation of colonic wall and mesocolon

  • Wall shows intermediate signal intensity

  • Surrounding fat and peritoneal reflections provide contrast

  • Useful for detecting subtle strictures, polyps, or small masses

CT Appearance

Non-Contrast CT:

  • Lumen filled with stool: mixed soft tissue and gas density

  • Colonic wall appears as thin soft tissue density

  • Fat planes of sigmoid mesocolon appear as low density

Post-Contrast CT:

  • Colonic wall shows mild uniform enhancement in normal state

  • Tumors: appear as irregular enhancing masses with possible lumen narrowing

  • Diverticulitis: wall thickening with pericolic fat stranding and inflamed diverticula

  • Volvulus: “coffee bean” or “whirl sign” appearance

MRI image

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MRI image

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CT image

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