The axial location of structural regions in the urethra: a magnetic resonance study in nulliparous women*1

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Abstract
To define and quantify the appearance and location of distinct regions of the bladder neck and urethra by using axial magnetic resonance images from healthy, continent, nulliparous women. Seventy-eight asymptomatic, healthy, nulliparous women (mean age 29.2 ± 5.4 years) volunteered for this study. All women were proven continent on urodynamic examination. Axial proton density magnetic resonance images of the pelvic floor were analyzed at 5-mm intervals. A geometric origin was established at the internal urethral meatus. The presence or absence of each of six structural regions—the bladder base, bladder neck, striated urethral sphincter, compressor urethrae and urethrovaginal sphincter, perineal membrane, and distal urethra—was then noted in each more distal image. The proportion of women in whom a structural region was seen at each 5-mm interval was recorded. The striated urogenital sphincter was observed at 5–25 mm distal to the bladder base. It was observed 10 and 15 mm below the bladder base in 91% (95% confidence interval [CI] 85%, 98%) and 99% (95% CI 97%, 100%), respectively, of all women. The perineal membrane, marking the distal end of the muscular urethra, was located 20–35 mm distal to the bladder base. The length of the muscular region of the urethra ranged from 20 to 35 mm (mean 24 mm, 95% CI 24, 25 mm). Magnetic resonance images allow the normal appearance and location of urethral and bladder neck structures to be quantified in healthy, continent, nulliparous women.