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Trigone of urinary bladder

The trigone of the urinary bladder is a smooth triangular region located at the base of the bladder. Unlike the rest of the bladder mucosa, which is loosely folded, the trigone is smooth and tightly bound to the underlying muscular layer, making it less distensible. Its anatomic position between the ureteric orifices and internal urethral orifice makes it a key region for urinary continence, vesicoureteral reflux prevention, and clinical evaluation in urology and radiology.

Synonyms

  • Vesical trigone

  • Bladder trigone

  • Base of bladder trigone

Location and Boundaries

  • Superior angles: Right and left ureteric orifices (where ureters enter bladder)

  • Inferior angle: Internal urethral orifice (beginning of urethra)

  • Boundaries: Form a smooth triangular area on the posterior bladder wall

  • Base: Continuous with posterior bladder wall

  • Apex: At the internal urethral orifice

Relations

  • Anteriorly: Pubic symphysis (separated by retropubic space of Retzius)

  • Posteriorly: Seminal vesicles and rectum in males; vagina and cervix in females

  • Superiorly: Bladder body and dome

  • Inferiorly: Prostatic urethra in males; urethra in females

Function

  • Acts as a funnel directing urine toward the internal urethral orifice

  • Maintains functional alignment between ureteric and urethral orifices to prevent reflux

  • Contains a high density of stretch receptors important for the micturition reflex

  • Remains smooth during bladder filling, unlike other bladder regions which form rugae

Clinical Significance

  • Frequent site of infection (trigonitis) in chronic cystitis

  • Involved in vesicoureteral reflux due to incompetence of ureterovesical junctions

  • Affected in obstructive uropathy, bladder outlet obstruction, or neurogenic bladder

  • Target region in cystoscopy for evaluating ureteric orifices and bladder outlet

  • Tumors (urothelial carcinoma) often involve trigone due to proximity to ureteric orifices

MRI Appearance

T1-weighted images:

  • No urine: Trigone appears as a thin low-signal intensity region within bladder base

  • Urine present: Urine in bladder lumen appears very low signal (dark); trigone remains low signal

  • Fat: Seen as bright signal surrounding bladder, especially in perivesical fat

T2-weighted images:

  • No urine: Trigone is low signal relative to bladder wall, visible as smooth dark band at base

  • Urine present: Urine is bright signal; trigone remains dark

  • Fat: Intermediate to bright signal outlining the bladder externally

STIR (Short Tau Inversion Recovery):

  • No urine: Trigone remains dark

  • Urine present: Urine is bright hyperintense

  • Fat: Suppressed and appears dark

  • Inflammation or pathology: Appears as focal bright hyperintensity

T1 Fat-Sat Post-Contrast:

  • Normal trigone: Enhances mildly and uniformly

  • Infection/inflammation: Focal or diffuse enhancement of trigone mucosa

  • Tumor: Irregular, nodular, or heterogeneous enhancement at bladder base

CT Appearance

Non-Contrast CT:

  • No urine: Trigone not well distinguished from bladder wall

  • Urine present: Urine appears low attenuation; trigone appears as soft tissue density region at bladder base

  • Fat: Seen as low attenuation perivesical tissue surrounding bladder

Post-Contrast CT:

  • Normal trigone: Enhances mildly and uniformly with rest of bladder wall

  • Infection: Diffuse thickening and prominent enhancement of trigone mucosa

  • Tumor: Irregular mass or nodular enhancement in trigonal region

  • Obstruction: Dilated ureters visible entering trigone if vesicoureteral reflux or obstruction present

MRI image

Trigone of urinary bladder mri sag anatomy image-img-00000-00000

MRI image

Trigone of urinary bladder MRI axial anatomy image-img-00000-00000

CT image

Trigone of urinary bladder ct axial anatomy image-img-00000-00000

CT image

Trigone of urinary bladder ct sag anatomy image-img-00000-00000

MRI image

Trigone of bladder MRI