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Sources of Funding: None

Introduction

Anatomical description of the male urethral sphincter complex (internal lissospincter and external rhabdosphincter) has undergone several revisions as previous conclusions were mostly derived from gross cadaveric dissection of adult male pelvis. These findings have been plagued by distortions of the anatomical structures in cadavers. Our objective was to elucidate the anatomy of the urethral sphincter muscles by resorting to in vivo, non-invasive imaging, using proton-density (PD), diffusion tensor imaging (DTI) and fiber tracking and thereby improve our understanding by avoiding the errors in cadaveric studies.

Methods

Normal healthy five male young subjects, (mean age ~25 yrs) were scanned on a 3T GE MR scanner, using a multi-channel cardiac coil, lying supine, feet-first. After acquisition of a high resolution sagittal scout scan (Fig. A), axial morphological proton-density scans were prescribed and acquired extending from a few slices below the base of the bladder to beyond the entry of the urethra into the penis (~18-22 slices depending on the height of the subject). Three mm thick DTI scans were acquired and analyzed to obtain first the tensor axial images (Fig. B-C) to construct fibers within this urethral complex.

Results

A consistent finding was that of possibly two sphincter like muscles (Fig. D-E), with one proximal near the bladder neck (yellow arrow in Fig. A, B) and the other more distal (Fig. C). In the tensor images, blue color indicates fibers oriented superior- inferior, red left-right while green anterior-posterior directions. Fiber tracking of these tensor images yields fiber structures within the urethral sphincter complex. The proximal sphincter is shown in Fig. D, the distal sphincter in Fig. E. The connecting superior-inferior longitudinal fibers are shown in blue in Fig. F. A consistent finding was the distance between the two sphincters is between 21 and 24 mm (i.e., 7 to 8 slices).

Conclusions

This is perhaps the first DTI and fiber tracking of the male urethral sphincter complex. Our findings support the two sphincter concept to constrict/close the urethral opening. These observations will form the basis of normal urethral morphology that can be monitored in post-surgery patients and correlated with age /urinary incontinence symptoms.

Funding

None