Topics

Topic

design image
Membranous urethra

The membranous urethra is the shortest and narrowest segment of the male urethra, located between the prostatic urethra and the spongy (penile) urethra. It is surrounded by the external urethral sphincter and passes through the urogenital diaphragm. Despite its short length, it is clinically important due to its vulnerability to traumatic injury, iatrogenic damage during catheterization, and its role in urinary continence.

Synonyms

  • Intermediate urethra

  • Urogenital diaphragm urethra

  • Pelvic urethral segment

Location and Boundaries

  • Proximal boundary: Extends from the apex of the prostate at the end of the prostatic urethra

  • Distal boundary: Continues into the bulb of the penis as the spongy (penile) urethra

  • Course: Runs downward and forward through the urogenital diaphragm, enclosed by the external urethral sphincter

Relations

  • Anteriorly: Perineal membrane and root of the penis

  • Posteriorly: Bulbourethral (Cowper’s) glands

  • Superiorly: Apex of the prostate

  • Inferiorly: Bulb of the penis and beginning of spongy urethra

  • Surrounding structure: External urethral sphincter encircles it, providing voluntary control

Function

  • Conducts urine from the prostatic urethra to the penile urethra

  • Serves as the main site of voluntary urinary continence due to the external urethral sphincter

  • Provides a passage for semen during ejaculation

  • Forms a vulnerable site for trauma and stricture formation

Clinical Significance

  • Trauma: Common site of injury in pelvic fractures (posterior urethral injuries)

  • Strictures: Frequent location of urethral narrowing due to trauma, infection, or instrumentation

  • Surgical relevance: Critical to preserve during prostatectomy to maintain continence

  • Imaging importance: Evaluated in urethrograms, MRI, and CT for strictures, trauma, and tumor spread

MRI Appearance

T1-weighted images:

  • Urethral lumen without fluid: Appears as a thin dark line (low signal)

  • With urine: Lumen shows low signal intensity

  • Fat in surrounding perineum: Bright (high signal intensity)

T2-weighted images:

  • Urethral lumen without fluid: Appears dark

  • With urine: Lumen appears bright due to fluid signal

  • Surrounding fat: Bright signal intensity

  • Fibrosis or stricture: Low-signal thickening around lumen

STIR (Short Tau Inversion Recovery):

  • Normal urethral wall: Dark signal

  • Fluid within lumen: Bright hyperintensity

  • Fat: Suppressed and dark

  • Inflammation or edema: Bright hyperintensity in periurethral tissue

T1 Fat-Sat Post-Contrast:

  • Urethral wall enhances mildly and uniformly

  • Pathology (stricture, tumor, inflammation) shows focal or diffuse enhancement

  • Fluid in lumen: No enhancement

  • Surrounding fat: Suppressed and dark

CT Appearance

Non-Contrast CT:

  • Urethra itself poorly visualized unless contrast or fluid is present

  • Fat around urethra: Low density, outlining the lumen

  • Calcifications or foreign bodies: High density in urethral course

Post-Contrast CT:

  • With contrast instilled (CT urethrography): Urethral lumen clearly visible, filled with contrast

  • Strictures: Narrowing or abrupt cut-off of contrast column

  • Extravasation in trauma: Contrast leaks into periurethral tissues

  • Normal urethral wall enhances faintly; surrounding fat remains low density

MRI image

Membranous urethra MRI axial anatomy image-img-00000-00000

MRI image

Membranous urethra MRI sagittal anatomy image-img-00000-00000