Müllerian Duct Anomalies (MDA)

MDAs are congenital abnormalities that arise from the improper development, fusion, or resorption of the embryological structures responsible for forming the female reproductive tract. These anomalies can affect the uterus, cervix, and upper vagina, and their impact on fertility and pregnancy outcomes varies depending on the specific type (1, 2, 4, 6).

MRI has emerged as the gold standard in detecting and classifying MDAs due to its ability to provide detailed images of uterine anatomy, particularly the external fundal contour (3).

Types of MDAs and their MRI findings

• Müllerian agenesis: the uterus, cervix, and upper vagina are completely absent. MRI shows bilateral rudimentary uteri and can detect associated renal anomalies, such as ectopic kidneys or renal agenesis. One of the most recognized forms of this anomaly is Mayer-Rokitansky-Küster-Hauser Syndrome (MRKHS), often presenting with primary amenorrhea in young women (Figure 1).

  

  Fig 1: (A–C) MRKHS with rudimentary uterine horns and vaginal agenesis in an 18-year-old woman. Axial T2-weighted MR image (A) shows fat in the expected location of the lower vagina (arrow). Coronal T2-weighted MR image (B) shows bilateral rudimentary uterine horns (arrows). Axial T2-weighted MR image (C) shows low-signal-intensity fibrous bands (arrows), extending from the rudimentary horns to the midline, which is a common feature in these patients. Coronal T2weighted MR image (D) in a 30-year-old woman with MRKHS shows bilateral pelvic kidneys (arrowheads) and abnormal positioning of the left ovary in the left paracolic gutter (arrow).

• Unicornuate uterus: one Müllerian duct fails to develop, resulting in a single elongated uterine cavity. This may include a rudimentary horn, which can cause cyclic pain or obstruction (Figure 2).

  

  Fig 2: Developmental MDAs. (A) 3D US image (with corresponding illustration, top) of a unicornuate uterus in a 32-year-old woman that was acquired during saline solution infused sonohysterography shows a single fluid-filled uterine cavity (arrow). (B) Axial T2weighted MR image (with corresponding illustration, top) in a 36-year-old woman shows a unicornuate left uterus (solid arrow) and an atretic noncommunicating right uterine horn (dashed arrow).

• Uterus Didelphys: complete failure of fusion results in two separate uteri, each with its endometrium, myometrium, and often a double cervix. Fertility is generally preserved, but there are higher risks of miscarriage and preterm birth (Figure3).

  

  Fig 3: MR images (with corresponding illustrations, top) of MDAs of absent or incomplete fusion. (A) Coronal T2-weighted MR image of a uterus didelphys in a 26-year-old woman shows two separate unicornuate uterine cavities (arrows) and a double cervix (arrowheads). (B) Coronal T2-weighted MR image of a bicornuate uterus in a 31-year-old woman shows external indentation of the uterine fundal contour that is slightly larger than 1 cm, with a septum extending to the level of the internal cervical os (arrowhead).

•  Bicornuate uterus: partial fusion failure leads to two endometrial cavities separated by a shared myometrium. MRI shows a wide-angle divergence (>105°)between the cavities and a prominent external indentation (>1 cm) (Figure 4).

  

  Fig 4: MR images (with corresponding illustrations, top) show classification of normal or arcuate (A), septate (B), and bicornuate (C) uteri on the basis of ASRM 2016 criteria, which apply for both complete septate and partial septate uteri. (A) Axial oblique T2-weighted MR image of a normal or arcuate uterus in a 33-year-old woman shows a depth of internal indentation (solid line) of less than 1 cm (double-headed arrow) and an angle of indentation of greater than 90°. (B) Axial T2-weighted MR image of a septate uterus in a 31-year-old woman shows an internal indentation (solid line) of greater than 1.5 cm (double-headed arrow), an angle of indentation of less than 90°, and a flat external uterine contour (dashed line). (C) Axial oblique T2-weighted MR image of a bicornuate uterus in a 37-year-old transgender man shows external uterine fundal indentation (dashed line) of greater than 1 cm (dashed double-headed arrow).

• Septate uterus: incomplete resorption of the central uterine septum causes a fibrous or muscular partition within the cavity. MRI reveals a deep internal indentation (>1.5 cm) of the endometrium with a normal external contour (< 1 cm) (Figure 5).
  

  Fig 5: MR images (with corresponding illustrations, top) of MDAs related to failure of resorption (A, B) compared with a normal uterus (C). (A) Axial oblique T2-weighted MR image of a septate uterus in a 34-year-old woman shows a convex uterine fundal contour (dashed line), internal indentation of greater than 1.5 cm (double-headed arrow) from the interostial line (solid line) to the apex of the indentation of the endometrial cavity, and a fibrous septum extending to the level of the internal cervical os (arrow). (B) Axial T2-weighted MR image of a partial septate uterus in a 36-year-old woman shows a convex uterine fundal contour (dashed line) and internal indentation of greater than 1.5 cm (double-headed arrow). (C) Axial T2-weighted MR image of a normal or arcuate uterus in a 35-year-old woman shows a convex uterine fundal contour (dashed line) without significant internal indentation (<1 cm).

MRI is particularly valuable in distinguishing between a bicornuate uterus and a septate uterus, which require different management strategies. In a septate uterus, the angle between cavities is <90°, whereas a bicornuate uterus shows a wider angle (5). (Table 1)

Table 1: Key MRI findings and differences between septate and bicornuate uterus.

Fibroids (Leiomyomas)

Fibroids are benign myometrial tumours, affecting up to 70% of women of reproductive age. Although they are a rare cause of infertility, fibroids can interfere with implantation by distorting the uterine cavity or compressing the fallopian tubes. Submucosal fibroids have the greatest impact on fertility (17, 18).

MRI provides superior accuracy in mapping and characterising fibroids compared to TV-US. It can identify the fibroid type (subserosal, intramural, submucosal, or extrauterine) and quantify the degree of submucosal extension. This information is vital for planning surgical interventions such as hysteroscopic resection. Additionally, MRI distinguishes fibroids from adenomyosis, which requires different treatments (14).

Fibroids and their MRI findings

Conventional leiomyomas: homogeneous, low signal intensity on T2-weighted images, and isointense or mildly hypointense on T1-weighted images. Show early homogeneous enhancement post-contrast (15, 16). Based on their localization and following the FIGO classification system (Figures 6 and 7), fibroids are distinguished into:

Fig 6: Schematic overview of leiomyoma locations based on FIGO subclassification system. Stage 0 = intracavitary pedunculated Stage 1 = submucosal with less than 50% of tumor being intramural Stage 2 = submucosal with 50% or more of tumor being intramural, stage 3 = intramural with endometrial contact Stage 4 = 100% intramural Stage 5 = subserosal with 50% or more of tumor being intramural Stage 6 = subserosal with less than 50% of tumor being intramural Stage 7 = subserosal pedunculated

Fig 7: Based on their localization and following the FIGO classification system fibroids are distinguished into:  - Subserosal Leiomyomas (FIGO Stages 5-7): located on the outer surface of the uterus. Well-circumscribed masses projecting outward from the uterine serosa. - Intramural Leiomyomas (FIGO Stages 3-4): located within the myometrium. - Submucosal Leiomyomas (FIGO Stages 0-2): located beneath the endometrium and protrudes into the uterine cavity.

• Subserosal Leiomyomas (FIGO Stages 5-7): located on the outer surface of the uterus. Well-circumscribed masses projecting outward from the uterine serosa. Bridging vessels between the uterus and the mass are a key diagnostic feature. Pedunculated subserosal leiomyomas may show a "claw sign." (Figure 8)
  

  Fig 8: Subserosal Leiomyomas (FIGO Stages 5-7): located on the outer surface of the uterus. Well-circumscribed masses projecting outward from the uterine serosa. Bridging vessels between the uterus and the mass are a key diagnostic feature. Pedunculated subserosal leiomyomas may show a "claw sign."
• Intramural Leiomyomas (FIGO Stages 3-4): located within the myometrium. Low T2 signal intensity relative to the myometrium. Often homogeneous unless degeneration is present. Show early homogeneous enhancement post-contrast. (Figure 9)
  

  Fig 9: Intramural Leiomyomas (FIGO Stages 3-4): located within the myometrium. Low T2 signal intensity relative to the myometrium. Often homogeneous unless degeneration is present. Show early homogeneous enhancement post-contrast.
• Submucosal Leiomyomas (FIGO Stages 0-2): located beneath the endometrium and protrudes into the uterine cavity. Show a T2-hypointense mass in the endometrial cavity. These tumors can appear with a T2-hyperintense rim due to edema or obstructed veins. (Figures 10 and 11)
  

  Fig 10: Submucosal Leiomyomas (FIGO Stages 0-2): located beneath the endometrium and protrudes into the uterine cavity. Show a T2-hypointense mass in the endometrial cavity. These tumors can appear with a T2-hyperintense rim due to edema or obstructed veins.
  

  Fig 11: Submucosal Leiomyomas (FIGO Stages 0-2). On the left: nondegenerated conventional leiomyoma in a 32-year-old woman. (A) Sagittal T2W MR image shows a round T2-hypointense leiomyoma (arrow) of the anterior uterine body (FIGO stage 1). On the right: FIGO stage 0 leiomyoma in a 42-year-old woman who presented with heavy menses. Sagittal T2W MR image shows a hypointense pedunculated conventional leiomyoma (arrow) extending from the lower uterine segment into the upper vagina. The thin stalk (arrowhead) can be seen within the endocervical canal. There is also a FIGO stage 1 submucosal leiomyoma partially imaged in the superior aspect of the endometrium.
• Extrauterine Leiomyomas (FIGO Stage 8): found outside the uterus, such as in the cervix, broad ligament, or parasitic leiomyomas. Typically have imaging characteristics similar to uterine leiomyomas.

Degenerated leiomyomas: leiomyomas can undergo one or several types of degeneration that may alter their imaging appearance (Figure 12).

Fig 12: Leiomyomas can undergo one or several types of degeneration that may alter their imaging appearance. The most common form of leiomyoma degeneration is hyaline degeneration, whereby smooth muscle is replaced by collagen, which is present in up to 60% of leiomyomas. At histopathologic analysis, there is hyalinization of the stromal component with collection of proteinaceous tissue, leading to homogeneous eosinophilic bands. Ghost outlines of typical smooth muscle cells are surrounded by a zone of hyalinized fibrous tissue. At MRI, areas of hyaline degeneration typically enhance less than the myometrium while appearing isointense on T1W images and hypointense on T2W images.These findings are in contrast to those of a typical leiomyoma, which, similar to the myometrium, demonstrates early homogeneous enhancement. Hemorrhagic, cystic, and myxoid degeneration of leiomyomas is also possible and may satisfy additional histopathologic criteria to be subtyped as apoplectic, hydropic, and myxoid leiomyoma, respectively, in the 2020 WHO 2020 classification of smooth muscle tumors. However, these forms of degeneration may also be found histopathologically within a leiomyoma without meeting the criteria for a specific leiomyoma subtype.

Adenomyosis

Adenomyosis occurs when endometrial glands infiltrate the myometrium. It can coexist with endometriosis and may cause infertility by interfering with implantation (8).

MRI findings in adenomyosis:

- Location: diffuse or focal thickening of the junctional zone (JZ) of the uterus.

- T2-weighted images: thickened junctional zone (>12 mm) with poorly defined borders. Hyperintense myometrial cysts are a hallmark feature.

- T1-weighted images: small hyperintense foci representing hemorrhagic foci within ectopic endometrial tissue.

- Post-contrast: shows poor or heterogeneous enhancement of the thickened JZ (Figure 13).

Fig 13: Adenomyosis. - Location: diffuse or focal thickening of the junctional zone (JZ) of the uterus. - T2-weighted images: thickened junctional zone (>12 mm) with poorly defined borders. Hyperintense myometrial cysts are a hallmark feature. - T1-weighted images: small hyperintense foci representing hemorrhagic foci within ectopic endometrial tissue. - Post-contrast: shows poor or heterogeneous enhancement of the thickened JZ

Key differences in MRI between fibroids and adenomyosis 

• Fibroids: well-circumscribed masses with low T2 signal intensity and early homogeneous enhancement.
• Adenomyosis: diffuse thickening of the junctional zone with hyperintense myometrial cysts on T2 and poorly defined borders (11). (Figure 14), (Table 2)
  

  Fig 14: Key differences in MRI between fibroids and adenomyosis.
  

  Table 2: Key differences in MRI between fibroids and adenomyosis.

Endometriosis

Endometriosis is characterised by the presence of endometrial tissue outside the uterus, affecting 30-50% of women with infertility. The condition can lead to chronic pelvic pain, dyspareunia, and dysmenorrhea. Endometriosis impacts fertility by causing peritubal adhesions, blocked fallopian tubes, and impaired implantation. (Figure 15)

Fig 15: Endometriosis. On the left: Sagittal (a) T2-weighted MRI image shows a hypointense focal area of adenomyosis (black arrow) located on the posterior wall of the uterus (U). Left endometrioma (white arrow) and focal area of adenomyosis (black arrow) (b). T1-weighted fat-saturated (FS) image shows the hyperintensity of other multiple foci of endometriosis (black circle) on the posterior surface of the uterus (U), suggestive of subacute blood products (c). T1-weighted axial MRI images (d, c) show the hyperintensity of the endometrioma (white arrow). On the right: Deep infiltrating endometriosis and adenomyosis in a 37-year-old patient. T2-weighted sagittal (a) and axial (b) images show a fibrotic endometriotic plaque (white arrow) infiltrating the posterior wall of the bladder (B) and a thickening of the junctional zone of the uterus (U) due to the presence of adenomyosis (double white arrow). Foci of hyperintensity within the plaque indicate the presence of active endometriosis (black arrows) in a T1-weighted axial MRI image. Another T2-weighted axial image shows thickening of the right round ligament (white arrowhead) and the presence of adhesions (white circle).

MRI in endometriosis

MRI is effective in detecting endometriosis, particularly deep infiltrative endometriosis (DIE). Accurate imaging is critical for surgical planning and improving patient outcomes (12, 13). (Figure 16)

Fig 16: Deep infiltrating endometriosis. On the left: Cul-de-sac obliteration in a 33-year-old woman. Fibrotic plaque obliterating the Douglas pouch, which appears hypointense on a T2 axial image (a). In another T2weighted axial image (b) other fibrotic strands (white circle) are seen between the uterus (U) and the left ureter (black arrow), with an hyperintense focus within the tissue (white arrow; c), which causes an obstruction of the ureter (black arrow), leading to ureteronephrosis (d, e). An hemorrhagic focus is seen also on the surface of the posterior vaginal fornix (white arrowhead, f). In the middle: Tubal endometriosis in a 25-year-old patient. The tube appears dilated (white arrows) and with hyperintense content in the T1-weighted image with fat saturation (a). Fibrotic strands (black arrows) are seen surrounding the tube on the sagittal T2-weighted image (b). On the right: “Mushroom cup” (white circle) due to the presence of deep bowel endometriosis in a 40-year-old woman. A solid hypointense tissue is seen on the rectal wall obliterating the Douglas pouch (white arrows), on T2-weighted MRI images (a, b). The mucosa layer is preserved (black arrow).

Pelvic Inflammatory Disease (PID)

PID refers to an infection that spreads from the vagina or cervix to the upper genital tract, potentially causing infertility through tubal and peritubal damage.

MRI in PID

MRI is reserved for complex or unclear cases and can identify complications such as hydrosalpinx, pyosalpinx, or tubo-ovarian abscesses. (Figure 17)

Fig 17: Tubo-ovarian abscess.

Polycystic Ovary Syndrome (PCOS)

PCOS is a common endocrine disorder characterised by chronic anovulation and hyperandrogenism. Diagnosis is based on the Rotterdam criteria, which require the presence of at least two of the following features:

• Oligo- or anovulation
• Clinical or biochemical signs of hyperandrogenism
• Polycystic ovarian morphology on ultrasound or elevated serum AMH levels

MRI in PCOS

MRI findings in PCOS are nonspecific but can provide supportive evidence. Typically, ovaries in PCOS contain multiple small follicles arranged peripherally around a dense stromal core. (Figure 18)

Fig 18: Polycystic Ovary Syndrome (PCOS) Signal characteristics include: - T2: follicles have high T2 signal while the central stroma is of comparatively low T2 signal. - T1: small uniform follicles are low in signal while the central stroma is of intermediate signal (vs normal myometrium).