Real-time magnetic resonance imaging (MRI): anatomical changes during physiological voiding in men

To investigate the interactions between the bladder, urethra, pelvic floor and the function of the prostate during 'normal' voiding.

In all, 16 men with no history of urinary incontinence, urgency or obstructive voiding dysfunction were enrolled. We analysed the interaction between the bladder, urethra, pelvic floor and changes in the prostate during the Valsalva manoeuvre and voiding using real-time magnetic resonance imaging (rtMRI). The axis through the external sphincter (AES) to pubo-coccygeal line (PC-line) and the angle between the axis of the os pubis (AOP) and ventral prostate (VP) was measured before and at the end of voiding. Additionally, the angle between the AOP and the VP was measured during the Valsalva manoeuvre. Change of position, or contraction, of the VP was measured.

The mean age of the men was 69.8 years and mean prostate volume 33.1 mL. Before voiding, the mean AES to PC-line was 10.5 mm. At the end of voiding, the mean AES to PC-line was 20 mm. The mean angle between AOP/VP was 31.6° in the storage phase and increased to a mean of 54.5° during voiding. During the Valsalva manoeuvre, the angle between the AOP/VP remained constant. There was a mean vertical contraction of the VP of 48.25 mm before voiding and a declining of the cranio-caudal distance of the VP with a mean of 33.92 mm during voiding.

All the men in our study showed relaxation of the pelvic floor, followed by a descent of the bladder neck. Voiding could not be initiated unless the prostate rotated around the symphysis. The study suggests that both the rotation and a vertical contraction of the prostate precede voiding. The anatomy of physiological voiding or voiding dysfunction can be investigated non-invasively using rtMRI.