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Introduction to Urethra
Bryan R. Foster, MD; Paula J. Woodward, MD
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Anatomy

  • Female Urethra

    • The female urethra is a short, tubular muscle ~ 4 cm in length coursing from the bladder neck to the external urethral meatus between the clitoris and vagina. It is lined by transitional epithelium (urothelium) proximally and squamous epithelium distally. Deep to the epithelium is a vascular network surrounded by 2 layers of smooth muscle (internal sphincter) and an outer layer of striated muscle (external sphincter). The urethra passes through the pelvic diaphragm, which is formed by the levator ani and coccygeus muscles and complex ligaments and fascia. Due to its short course, traumatic injury isolated to the female urethra is extremely rare.
    • Male Urethra

      • The male urethra is ~ 18-20 cm in length and has 2 major divisions: The anterior and posterior urethra, each of which can be further subdivided into 2 parts. From proximal to distal, the posterior urethra is composed of the prostatic and membranous portions, and the anterior urethra is composed of the bulbous and penile portions.
      • The prostatic urethra begins at the bladder neck and extends to the apex of the prostate gland and contains the verumontanum, a ridge of tissue on the posterior wall where the prostatic and ejaculatory ducts drain. The membranous urethra traverses the urogenital diaphragm, and, being the shortest and most fixed portion of the urethra, is prone to injury. The bulbous urethra extends from the inferior border of the urogenital diaphragm to the penoscrotal junction within the corpus spongiosum. The bulbous urethra is commonly injured with a straddle mechanism. The penile urethra is distal to the penoscrotal junction and widens into the fossa navicularis at the distal glans.
      • Cowper glands are located within the urogenital diaphragm, but their ducts course through the corpus spongiosum ~ 2 cm to enter the bulbous urethra. Multiple small mucosal glands (glands of Littré) line the mucosa of the anterior urethra. These structures may be opacified on fluoroscopic imaging and should not be confused with injury.

      Imaging Techniques and Indications

      • Retrograde Urethrogram

        • Retrograde urethrogram (RUG) is the study of choice for evaluating the male urethra and is most often used in the setting of acute trauma with blood at the meatus. It is also the study of choice for evaluating for strictures or fistula.
        • Proper positioning is very important when performing RUG. The pelvis should be imaged in an oblique position and penis placed under tension to optimally visualize the full length of the urethra. Often, the membranous and prostatic urethra are difficult to opacify due to contraction of the external urethral sphincter. In these cases, careful cystoscopy may be performed to pass a catheter, or voiding cystourethrogram (VCUG) may be done through a suprapubic tube to fully evaluate the posterior urethra.
        • Urethral strictures are commonly imaged with RUG for diagnosis and as follow-up after therapy. Various etiologies include iatrogenic trauma, straddle injuries, inflammatory disease, and infections.
        • Voiding Cystourethrogram

          • The urethra can also be evaluated with VCUG. This is most often used in children but can have a role in adults for some indications.
          • MR

            • T2WI provides the best images of the urethra and is particularly helpful for demonstrating female urethral diverticula. T2WI and T1WI C+ FS are key sequences for evaluating local invasion of urethral carcinoma.
            • CT

              • Female urethral diverticula can be seen on CT and should not be confused for a mass or abscess. Urethral bulking agents can mimic a diverticulum or stone/calcification at CT depending on the type of agent used.
              • US

                • US can image the female urethra with either transvaginal or translabial approach. US also shows the male anterior urethra well.

                Selected References

                1. Wongwaisayawan S et al: Imaging spectrum of traumatic urinary bladder and urethral injuries. Abdom Radiol (NY). 46(2):681-91, 2021
                2. Childs DD et al: Multimodality imaging of the male urethra: trauma, infection, neoplasm, and common surgical repairs. Abdom Radiol (NY). 44(12):3935-49, 2019
                3. Sekhar A et al: Imaging of the female urethra. Abdom Radiol (NY). 44(12):3950-61, 2019
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                Anatomy

                • Female Urethra

                  • The female urethra is a short, tubular muscle ~ 4 cm in length coursing from the bladder neck to the external urethral meatus between the clitoris and vagina. It is lined by transitional epithelium (urothelium) proximally and squamous epithelium distally. Deep to the epithelium is a vascular network surrounded by 2 layers of smooth muscle (internal sphincter) and an outer layer of striated muscle (external sphincter). The urethra passes through the pelvic diaphragm, which is formed by the levator ani and coccygeus muscles and complex ligaments and fascia. Due to its short course, traumatic injury isolated to the female urethra is extremely rare.
                  • Male Urethra

                    • The male urethra is ~ 18-20 cm in length and has 2 major divisions: The anterior and posterior urethra, each of which can be further subdivided into 2 parts. From proximal to distal, the posterior urethra is composed of the prostatic and membranous portions, and the anterior urethra is composed of the bulbous and penile portions.
                    • The prostatic urethra begins at the bladder neck and extends to the apex of the prostate gland and contains the verumontanum, a ridge of tissue on the posterior wall where the prostatic and ejaculatory ducts drain. The membranous urethra traverses the urogenital diaphragm, and, being the shortest and most fixed portion of the urethra, is prone to injury. The bulbous urethra extends from the inferior border of the urogenital diaphragm to the penoscrotal junction within the corpus spongiosum. The bulbous urethra is commonly injured with a straddle mechanism. The penile urethra is distal to the penoscrotal junction and widens into the fossa navicularis at the distal glans.
                    • Cowper glands are located within the urogenital diaphragm, but their ducts course through the corpus spongiosum ~ 2 cm to enter the bulbous urethra. Multiple small mucosal glands (glands of Littré) line the mucosa of the anterior urethra. These structures may be opacified on fluoroscopic imaging and should not be confused with injury.

                    Imaging Techniques and Indications

                    • Retrograde Urethrogram

                      • Retrograde urethrogram (RUG) is the study of choice for evaluating the male urethra and is most often used in the setting of acute trauma with blood at the meatus. It is also the study of choice for evaluating for strictures or fistula.
                      • Proper positioning is very important when performing RUG. The pelvis should be imaged in an oblique position and penis placed under tension to optimally visualize the full length of the urethra. Often, the membranous and prostatic urethra are difficult to opacify due to contraction of the external urethral sphincter. In these cases, careful cystoscopy may be performed to pass a catheter, or voiding cystourethrogram (VCUG) may be done through a suprapubic tube to fully evaluate the posterior urethra.
                      • Urethral strictures are commonly imaged with RUG for diagnosis and as follow-up after therapy. Various etiologies include iatrogenic trauma, straddle injuries, inflammatory disease, and infections.
                      • Voiding Cystourethrogram

                        • The urethra can also be evaluated with VCUG. This is most often used in children but can have a role in adults for some indications.
                        • MR

                          • T2WI provides the best images of the urethra and is particularly helpful for demonstrating female urethral diverticula. T2WI and T1WI C+ FS are key sequences for evaluating local invasion of urethral carcinoma.
                          • CT

                            • Female urethral diverticula can be seen on CT and should not be confused for a mass or abscess. Urethral bulking agents can mimic a diverticulum or stone/calcification at CT depending on the type of agent used.
                            • US

                              • US can image the female urethra with either transvaginal or translabial approach. US also shows the male anterior urethra well.

                              Selected References

                              1. Wongwaisayawan S et al: Imaging spectrum of traumatic urinary bladder and urethral injuries. Abdom Radiol (NY). 46(2):681-91, 2021
                              2. Childs DD et al: Multimodality imaging of the male urethra: trauma, infection, neoplasm, and common surgical repairs. Abdom Radiol (NY). 44(12):3935-49, 2019
                              3. Sekhar A et al: Imaging of the female urethra. Abdom Radiol (NY). 44(12):3950-61, 2019