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Topic

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Anal canal

The anal canal is the terminal portion of the gastrointestinal tract, extending from the rectal ampulla to the anal verge. It measures approximately 3–4 cm in length and is surrounded by specialized sphincter muscles that control defecation. The canal is anatomically divided into an upper mucosal part lined by columnar epithelium, a middle transitional zone, and a lower cutaneous part lined by stratified squamous epithelium. Its location and close relation to the pelvic floor, sphincter complex, and perineum make it vital for continence and defecatory function.

Synonyms

  • Terminal anal segment

  • Distal rectal canal

  • Lower gastrointestinal outlet

Relations

  • Anteriorly: Perineal body (in females, vagina; in males, urethra and bulb of penis)

  • Posteriorly: Anococcygeal ligament and tip of coccyx

  • Laterally: Ischioanal fossae and levator ani muscles

Nerve Supply

  • Upper part (above pectinate line): Autonomic innervation via the inferior hypogastric plexus (visceral sensory)

  • Lower part (below pectinate line): Somatic innervation by the inferior rectal nerves, branches of the pudendal nerve

Arterial Supply

  • Above pectinate line: Superior rectal artery (branch of inferior mesenteric artery)

  • Below pectinate line: Inferior rectal artery (branch of internal pudendal artery)

  • Middle rectal artery provides additional supply

Venous Drainage

  • Above pectinate line: Superior rectal vein draining into the inferior mesenteric vein → portal system

  • Below pectinate line: Inferior rectal vein draining into the internal pudendal vein → systemic system

  • This dual drainage forms the basis of porto-systemic anastomoses, clinically relevant in hemorrhoids

Function

  • Passage of feces from rectum to exterior

  • Maintains fecal continence through coordinated action of internal and external anal sphincters

  • Sensory discrimination between gas, liquid, and solid stool

Clinical Significance

  • Site of common pathology: hemorrhoids, fissures, abscesses, fistulas, anal carcinoma

  • Portal hypertension may present with dilated veins in the canal

  • Critical area in proctologic surgery and pelvic floor imaging

MRI Appearance

T1-weighted images:

  • Anal canal walls show low-to-intermediate signal intensity

  • Fat planes of ischioanal fossae appear bright, delineating sphincters

  • Internal sphincter is low signal intensity, external sphincter is intermediate

T2-weighted images:

  • Anal canal lumen may show bright signal if fluid or mucosa is present

  • Internal sphincter appears as a circumferential low signal band

  • External sphincter and puborectalis appear intermediate signal

STIR (Short Tau Inversion Recovery):

  • Normal anal sphincter complex appears low-to-intermediate

  • Abscesses, fistulas, or inflammation appear as bright hyperintense tracts or cavities

T1 Fat-Sat Post-Contrast:

  • Normal sphincters enhance mildly and uniformly

  • Pathological conditions (fistulas, abscesses, tumors) show marked or rim enhancement

CT Appearance

Non-Contrast CT:

  • Anal canal walls appear as soft tissue density

  • Surrounding ischioanal fat provides natural contrast

  • Masses or calcifications may be detected

Post-Contrast CT:

  • Canal walls enhance mildly and uniformly in normal state

  • Abscesses appear as central low density with rim enhancement

  • Tumors or inflammation show irregular enhancement and wall thickening

MRI image

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MRI image

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MRI image

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CT image

Anal canal ct axial anatomy image

CT image

Anal canal ct sagittal anatomy image