Intraurethral Ultrasound Correlated with Urethral Histology

Objective: To examine the anatomic identity of sonographically visible sphincteric structures of the female urethra.

Methods: The urethra, urinary bladder, and vagina were removed from 11 fresh female cadavers and placed in a water bath. Intraurethral ultrasound was performed with a 360 degrees-rotating 7.5-MHz ultrasound probe. Afterward, the specimens were fixed and cross sections were made transverse to the urethral axis at 5-mm intervals. Corresponding ultrasonograms and histologic images were matched and depicted simultaneously side by side. The anatomic identity of sonographically visible structures was determined by histologic examination and thickness of the longitudinal smooth urethral sphincter measured.

Results: Structures visible sonographically were the striated and smooth urethral sphincter muscle layers, vagina, and blood vessels with diameters exceeding 0.2 mm. The longitudinal smooth muscle layer appeared as a well-defined internal hypoechoic ring. The outer circular smooth muscle layers and the striated muscle layers were a more irregular and hyperechoic zone. The circular smooth muscle layers and the striated sphincter muscle layers could not always be differentiated easily. With formalin fixation, tissue shrinkage resulted in a smaller thickness of the longitudinal smooth muscle measured on the histologic specimen.

Conclusion: With intraurethral ultrasound, the longitudinal smooth muscle layer appears as a well-defined and measurable hypoechoic ring. The region of the circular smooth muscle and the striated muscle emerges as a hyperechoic and less definable outer zone.