Internal Urethral Orifice: Anatomy, Location, Relations, Function, Clinical Significance, MRI & CT Imaging

Topic

Internal urethral orifice

The internal urethral orifice is the anatomical opening at the junction of the urinary bladder and the urethra. It represents the beginning of the urethral passage and is located at the bladder neck, at the inferior angle of the trigone. This structure is clinically significant because it regulates urinary outflow, is closely associated with the internal urethral sphincter, and is involved in conditions such as bladder outlet obstruction, urethral stricture, and prostate enlargement in males.

Synonyms

Internal urethral opening
Bladder neck orifice
Orificium urethrae internum

Location and Structure

Found at the inferior angle of the trigone of the bladder
Lies at the bladder neck, where the detrusor smooth muscle fibers are organized to form the internal urethral sphincter
In males, it marks the beginning of the prostatic urethra
In females, it opens directly into the short female urethra

Relations

Anteriorly: Pubic symphysis (separated by retropubic fat and connective tissue)
Posteriorly: In males, the bladder trigone leading to the seminal colliculus of the prostatic urethra; in females, the bladder trigone leading to the urethral lumen
Superiorly: Cavity of the bladder
Inferiorly: Prostatic urethra in males, membranous urethra continuation in females

Function

Acts as the outlet of the bladder into the urethra
Works in coordination with the internal urethral sphincter to maintain urinary continence
Prevents retrograde flow of semen into the bladder during ejaculation in males
Facilitates voluntary initiation of micturition with support of the external urethral sphincter

Clinical Significance

Narrowing or obstruction at the internal urethral orifice may cause bladder outlet obstruction
In males, enlarged prostate can compress or distort the orifice leading to urinary symptoms
Site of involvement in urethral stricture disease and congenital anomalies
Landmark during cystoscopy and urological procedures
Associated with conditions such as vesicoureteral reflux, where bladder outlet function is altered

MRI Appearance

T1-weighted images:

Appears as a low-signal opening at the bladder neck
Surrounded by intermediate-signal smooth muscle of the internal sphincter

T2-weighted images:

Urethral lumen appears as high-signal intensity due to urine or fluid
Surrounding sphincter muscle appears as a hypointense ring

STIR (Short Tau Inversion Recovery):

The orifice itself remains hypointense
Pathologic changes (inflammation, edema) appear hyperintense

T1 Fat-Sat Post-Contrast:

Normal bladder neck and orifice show mild, uniform enhancement
Pathology (tumor, inflammation) shows irregular, nodular, or asymmetric enhancement

CT Appearance

Non-Contrast CT:

The orifice is not directly seen; bladder neck appears as a funnel-shaped narrowing
Calculi at the bladder neck may be visualized as hyperdense foci

Post-Contrast CT (CT Urography):

Contrast outlines the bladder lumen and flows through the internal urethral orifice into the urethra
Pathology may appear as narrowing, irregular contour, or filling defect at the orifice

MRI image

CT image