NORMAL ANATOMY

The normal female urethra is a tubular structure that measures approximately 4 cm in length. 

On axial and sagittal T2-weighted and axial fat-suppressed gadolinium-enhanced T1-weighted images, it has a typical bull's eye or concentric ringed appearance.

The lesions are named urethral if they have direct communication with the urethra or periurethral when they arise from other structures and there is no clear relationship with the urethra.

The key anatomic landmark that helps distinguish the different entities is the perineal membrane, which is positioned at approximately the level of the pubic symphysis.

Fig 1: Axial (a) and sagittal (b) T2-weighted MR image shows the normal anatomical references of the urethra with the surrounding pelvic structures. A - Anal canal. B - Bladder. P - Pubic symphysis. R - Rectum. U - Urethra. Ut - Uterus. V - Vagina.

URETHRAL DIVERTICULUM

These lesions typically arise in the posterolateral wall of the mid urethra, at the level of the pubic symphysis.

Epidemiology: They are present in 0,6-6% of women, mostly between the 3rd and 5th decade of life.

Etiology: Remains largely unknown. It is believed that they originate from repeated obstruction and infection of periurethral glands, with abscess formation and fistulation with the urethral lumen, eventually becoming epithelialized.

MRI Findings: They normally appear adjacent or surrounding the urethra, in a horseshoe configuration when of large dimensions. The key feature that enables the diagnosis is the detection of a direct communication between the lesion and the urethra, however, this is not always encountered.

Malignant degeneration can occur, but it is rare and normally in the form of adenocarcinoma. Apart from the other mentioned complications, stone formation is also to be aware of.

Fig 2: Axial (a) and sagittal (b) T2-weighted MR image shows a multiloculated hyperintense lesion completely surrounding the midurethra (arrowheads), at the level of the pubic symphysis. These findings are suggestive of a circumferential urethral diverticulum.

Fig 3: (a) Sagittal T2-weighted MR image shows a large T2-hyperintense multiloculated lesion (arrowheads) surrounding the midurethra and starting at the level of the pubic symphysis, showing slight wall thickening and some internal heterogeneity probably due to infectious process. (b) Axial T2-weighted MR image shows a communication between the urethra and the diverticular formation (arrow) and a fluid-debris level (*) is also present. These findings are consistent with a large near-circumferential urethral diverticulum, measuring approximately 4 cm in diameter.

SKENE GLAND CYST

Also known as paraurethral glands, their main role is to lubricate the distal urethra,  with their ducts draining into the lumen.

Epidemiology: Skene gland cysts are rare in any age group.

Etiology: Secondary to previous trauma, inflammation or infection, that results in duct obstruction.

MRI Findings: They typically appear inferior to the pubic symphysis, anterior to the vagina and lateral to the external urethral orifice.

Usually, Skene gland cysts are not associated with significant symptoms, but urinary tract infections or urethral obstruction can occur. If asymptomatic, they may not require treatment.

Fig 4: Axial T2-weighted fat-suppressed (a) and coronal T2-weighted (b) MR image shows a small hyperintense median cystic lesion adjacent to the external urethral meatus (arrowheads), in relation to a Skene duct cyst.

BARTHOLIN GLAND CYST

Also known as the great vestibular glands, they are located in the posterior aspect of the vaginal introitus.

Epidemiology: It is believed that up to 2% of women will develop a Bartholin cyst or abscess in their lifetime.

Etiology: They arise due to ductal obstruction (in case of infection, trauma or inflammation), with accumulation of mucinous glandular secretions and subsequent superinfection and abscess formation.

MRI Findings: Their typical location is inferiorly or at the level of the pubic symphysis, adjacent to the vaginal introitus, most frequently in its posterior aspect.

If asymptomatic, they may not require treatment.

Fig 5: Coronal (a) and axial (b) T2-weighted MR image shows a T2-hyperintense cystic lesion located in the left aspect of the vulva (arrowheads). This was an incidental finding in a patient with an established diagnosis of colon carcinoma and a probable metastatic tumour to the ovary. These images are suggestive of a Bartholin gland cyst.

Fig 6: Axial (a) and sagittal (b) T2-weighted MR image shows a hyperintense cystic lesion located in the right aspect of the vulva, below the level of the pubic symphysis, adjacent to the vaginal introitus (arrowheads). These images are suggestive of a Bartholin gland cyst.

GARTNER DUCT CYST

Epidemiology: This is the most common benign cystic lesion of the vagina, present in up to 1% of women.

Etiology: These lesions are lined by non-mucinous cuboidal or columnar epithelial secreting cells and they originate from remnants of the mullerian duct, hence are frequently associated with other congenital urogenital abnormalities. 

MRI Findings: The typical location is the upper antero-lateral wall of the vagina, superiorly in relation to the level of the pubic symphysis.

Overall complications are rare because of their general small size (< 2 cm). However, when large, they can have a compressive effect on the urethra and in such cases, surgical intervention may be necessary.

Fig 7: Sagittal (a) and axial (b) T2-weighted MR image shows a large cystic right paramedian lesion, anterior to the vaginal wall and above the level of the pubic symphysis, in relation to a Gartner duct cyst.