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Topic

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Rectum

The rectum is the terminal portion of the large intestine, extending from the sigmoid colon to the anal canal. It measures approximately 12–15 cm in adults and lies within the pelvis, following the concavity of the sacrum. Its primary function is to store feces before defecation, but it also plays a key role in continence and coordination of bowel emptying. The rectum is of great clinical significance due to its involvement in colorectal cancer, inflammatory bowel disease, and anorectal malformations.

Synonyms

  • Terminal large intestine

  • Pelvic rectum

  • Lower bowel segment

Location and Course

  • Begins at the rectosigmoid junction at the level of the third sacral vertebra (S3)

  • Follows the curve of the sacrum and coccyx within the pelvis

  • Ends at the anorectal junction at the level of the pelvic floor (where it continues as the anal canal)

  • Lies retroperitoneally in its upper two-thirds and subperitoneally in its lower third

Relations

  • Anteriorly:

    • In males: urinary bladder, seminal vesicles, vas deferens, prostate

    • In females: vagina, uterus, and pouch of Douglas (rectouterine pouch)

  • Posteriorly: Sacrum, coccyx, piriformis, and coccygeus muscles

  • Laterally: Levator ani muscles and ischiorectal fossae

Arterial Supply

  • Superior rectal artery (continuation of inferior mesenteric artery)

  • Middle rectal arteries (branches of internal iliac artery)

  • Inferior rectal arteries (branches of internal pudendal artery)

Venous Drainage

  • Superior rectal vein → inferior mesenteric vein → portal system

  • Middle and inferior rectal veins → internal iliac vein → systemic circulation

  • This dual drainage forms the porto-systemic anastomosis, clinically relevant in portal hypertension (hemorrhoids)

Function

  • Acts as a reservoir for feces before defecation

  • Coordinates bowel emptying with internal and external anal sphincters

  • Maintains continence with the help of the puborectalis muscle (anorectal angle)

  • Assists in water and electrolyte absorption in a limited capacity

Clinical Significance

  • Common site for colorectal carcinoma

  • Involved in ulcerative colitis and Crohn’s disease

  • Hemorrhoids occur due to venous congestion at the anorectal junction

  • Surgical importance in rectal resections, pelvic exenteration, and stapled anastomosis

  • Important in imaging for cancer staging, fistula mapping, and inflammatory disease evaluation

MRI Appearance

T1-weighted images:

  • Rectal wall: intermediate signal intensity

  • Perirectal fat: bright

  • Tumors: intermediate-to-low signal lesions

  • Rectal air: very low (signal void, black)

T2-weighted images:

  • Rectal wall: layered appearance (mucosa/submucosa bright, muscularis dark)

  • Perirectal fat: bright

  • Tumors: intermediate-to-high signal disrupting layers

  • Rectal air: very low (signal void, black)

STIR (Short Tau Inversion Recovery):

  • Rectal wall: low-to-intermediate baseline signal

  • Edema/inflammation: bright

  • Fistulas/abscess: bright

  • Rectal air: complete signal void (black)

T1 Fat-Sat Post-Contrast:

  • Rectal wall: mild uniform enhancement

  • Tumors: heterogeneous enhancement

  • Abscesses: rim enhancement, non-enhancing center

  • Rectal air: signal void, no enhancement (black)

CT Appearance

Non-Contrast CT:

  • Rectal lumen density varies with stool/fluid

  • Wall thickening: suggests inflammation, infection, or tumor

  • Rectal air: very low attenuation

Post-Contrast CT:

  • Normal rectal wall enhances mildly and uniformly

  • Tumors enhance irregularly or heterogeneously

  • Abscess: rim enhancement with central low attenuation

MRI image

Rectum MRI axial anatomy image1

MRI image

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MRI image

Rectum MRI sagittal anatomy image 1

MRI image

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CT image

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CT image

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