What is endometriosis and when is MRI ordered?
Endometriosis is a pathological condition in which tissue similar to the endometrium proliferates outside the uterine cavity. It is one of the leading causes of chronic pelvic pain, dysfunctional uterine bleeding, and infertility in women of reproductive age.
Typical sites of involvement include the ovaries, fallopian tubes, uterine ligaments, the pouch of Douglas (pouch of Douglas), rectovaginal septum, bladder walls, and umbilicus. Rare locations include anterior abdominal wall scars after cesarean section and the thorax (causing catamenial pneumothorax).
A definitive diagnosis requires laparoscopy with histological examination, while ultrasound is used for initial imaging. MRI acts as a problem-solving method.
The risk of developing endometriosis is significantly increased in certain Müllerian duct anomalies. For example, in a unicornuate uterus, a rudimentary functioning but non-communicating horn can accumulate menstrual blood (forming a hematometra), which creates favorable conditions for endometrial cells to reflux into the peritoneal cavity.
Endometriotic cysts (endometriomas) on MRI
MRI is the optimal modality for detecting old hemorrhages. Characteristically, ovarian endometriomas demonstrate high signal intensity (hyperintensity) on T1-weighted images and low or variable signal on T2-weighted images due to blood breakdown products. Fat-suppression techniques (Fat suppression) are used for better differentiation.
Key MRI signs:
- T2 shading sign (T2 shading sign). Indicates the presence of old clotted blood. This feature reliably helps differentiate an endometrioma from a simple hemorrhagic ovarian cyst.
- T2 dark spot sign (T2 dark spot sign). Appears as a well-defined hypointense focus within the cystic cavity on T2-weighted images. It has high specificity (93%), but low sensitivity (about 36%).
In addition, due to thick proteinaceous contents within benign endometriotic cysts, diffusion restriction may be observed on diffusion-weighted images.
Deep infiltrative endometriosis and adhesions
Deep implants typically appear on tomograms as irregular formations with poorly defined margins. They are characterized by low signal on T2-weighted images, intermediate signal on T1-weighted images, and minimal contrast enhancement. Small cystic or hemorrhagic foci may be visualized within the implants.
Involvement of adjacent structures (vaginal walls, rectum, uterosacral ligaments) manifests as wall thickening, nodular transformation, and hypointense signal. In severe cases, serosal lesions on the bowel or bladder lead to severe fibrosis.
MRI excellently visualizes the sequelae of endometriosis—adhesions:
- Kissing ovaries sign (kissing ovaries). The ovaries are adhered to each other along the midline posterior to the uterus.
- Posterior displacement of the uterus (retroversion) and angulation (kinking) of bowel loops.
- Hematosalpinx (haematosalpinx). Defined as a tortuous serpiginous structure in the adnexal region with high signal on T1-weighted images.
Malignant transformation of foci
Malignant transformation (malignancy) of endometriomas or pelvic implants occurs in 2.5% of patients. Endometrioid carcinoma (67%) or ovarian clear-cell carcinoma (clear-cell carcinoma, 15%) is most frequently diagnosed. Unlike other types of ovarian cancer, these tumors develop in younger women, are detected at earlier stages, and have a better prognosis.
Risk factors include the onset of postmenopause and the presence of multilocular endometriotic cysts measuring 9 cm or larger.
MRI signs of malignancy:
- Unilateral enlargement of the cystic mass.
- Disappearance of the characteristic T2 shading sign.
- Appearance of a solid mural nodule with avid contrast enhancement (best visualized on subtraction images).
When ovarian endometrioid carcinoma is suspected, the radiologist should purposefully evaluate the uterine endometrium, as in 15–30% of cases the tumor is associated with endometrial cancer or hyperplasia.
To remember the MRI signal of a classic endometrioma, use the "Bright-Dark" rule: it is Bright (hyperintense) on T1 (due to blood) and Dark (with shading) on T2.
Class and exam questions
How to differentiate an endometrioma from a hemorrhagic cyst on MRI?
The main distinguishing feature is the T2 shading sign, indicating the accumulation of old blood. Another specific feature is the T2 dark spot sign within the cavity.
In what cases is an endometriotic cyst suspicious for cancer?
Multilocular cysts larger than 9 cm should raise suspicion, especially in postmenopausal women. Warning MRI signs include unilateral growth, loss of the T2 shading sign, and the appearance of a contrast-enhancing solid mural nodule.
What does the "kissing ovaries" sign mean on MRI?
This is a sign of severe pelvic adhesions. Due to adhesions, both ovaries are displaced posteriorly and touch (adhere to) each other along the midline.
What else to study on this topic
- MRI criteria for diffuse and focal forms of uterine adenomyosis
- Differential diagnosis of adenomyoma and leiomyoma (fibroid) on MRI
- Müllerian duct anomalies: bicornuate uterus and uterine didelphys
- Ultrasound characteristics of deep infiltrative endometriosis
- Imaging of pelvic inflammatory disease (PID) complications ( tubo-ovarian abscess, pyosalpinx)
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