OVARIAN ENDOMETRIOMA: 

• Sometimes referred to as "chocolate cysts”, they are defined as unilocular or multilocular cystic ovarian lesions with well-defined contours and thick walls. [6]
• They are bilateral in up to 50% of cases.
• On MRI, they show high signal intensity on T1-weighted and T1-weighted fat-saturated images, and low signal intensity on T2-weighted images (“Shading sign”) due to the presence of blood degradation products in different developmental stages. [6]

Fig 1: Figure 1. Ovarian endometrioma seen on transvaginal US (A) and during laparoscopic surgery (B).

Fig 2: Figure 2. Ovarian endometrioma visualised in different studies: A. Cystic lesion with echogenic and homogeneous content in the right adnexal region, observed on transvaginal ultrasound. B and C. Axial pelvic MRI T1W (B) and T1-weighted fat saturation (C) sequences reveal the hyperintense cystic lesion on T1, with no signal loss on fat-saturated sequences indicating the presence of haemorrhagic content, consistent with an endometrioma.

DEEP PELVIC ENDOMETRIOSIS

It is important to classify the deep pelvic implants according to the compartments involved, in order to facilitate systematic reading and standardised radiological reporting. 

Pelvic compartments: [1,2,8]

• Anterior compartment: Prevesical space, bladder, distal ureters, round ligaments, vesicouterine space and vesicovaginal septum.
• Middle compartment: Vagina, uterus, fallopian tubes, ovaries and broad ligaments.
• Posterior compartment: Uterosacral ligaments, torus uterinus, rectovaginal septum, rectouterine and rectocervical spaces, sigmoid colon and the rectum. Is the most commonly affected site in deep infiltrating endometriosis.

Fig 3: Figure 3. Pelvic compartments. Division of the female pelvis into the anterior compartment (AC), middle compartment (MC), and posterior compartment (PC).

Below, we present some of the most characteristic imaging findings that guide us towards the presence of deep pelvic endometriosis, focusing on the pelvic locations that, due to their frequency and mobility, radiologists must be familiar with, correlating these with findings observed during laparoscopy.

Anterior compartment

- Bladder: [1,3]

• The bladder is the most frequently affected organ in the anterior pelvic compartment in endometriosis.
• The posterior wall of the bladder and the vesicouterine pouch are the most frequently affected sites.
• Implants can be confined to the serosal surface or involve the muscular layer.
• In imaging techniques, they are typically observed as focal parietal thickening, or less commonly, as diffuse thickening.
• A careful radiological assessment is crucial, as bladder urothelial involvement is not typically evident on cystoscopy.

Fig 4: Figure 4. Transvaginal US image (A) showing a nodular lesion on the bladder wall, corresponding to an endometriotic implant on the bladder serosa. Cystoscopic view (B) confirms bladder involvement.

Middle compartment

- Ovaries:

• Are the organs most commonly involved of the middle pelvic compartment.
• In addition to visualising ovarian endometriomas, the ovaries may also be affected in this pathology due to the presence of adhesions, which represent an indirect sign of endometriosis.
• Adhesions and fibrotic implants are typically visualised on MRI as intermediate signal on T1 and low signal on T2-weighted images, leading to a posteromedial displacement of both ovaries in a retro-uterine location, where they may come into contact with each other, producing a characteristic sign known as the “Kissing Ovaries sign”, which indicates severe disease. [1,2,8]

Fig 5: Figure 5. Kissing Ovaries. Axial (A) and coronal (B) T2-weighted images: In the uterine torus, a hypointense fibrous plaque with spiculated morphology is identified, suggestive of an endometriotic implant. It causes traction of the anterior wall of the upper rectum as well as the uterus and both ovaries, which are medially displaced to a retrouterine location and in close contact (kissing ovaries). These findings are also be observed in the images obtained during laparoscopy.

Posterior compartment:

- Uterine torus: 

• Another typical site where adhesions are found is at the level of the uterine torus, which may lead to retroflexion of the uterus. [9]
• It is common for adhesions to extend posteriorly and infiltrate the anterior intestinal wall.

Fig 6: Figure 6. Posterior compartment adhesions. Sagittal (A) and axial (B) T2-weighted images demonstrating the uterus with intramural and subserosal fibroids. Thick adhesions between the uterus and sigmoid colon are identified both in MRI images and laparoscopy, causing anterior traction of the intestinal loops.

 - Rectosigmoid colon: 

• The gastrointestinal system is the most common site of extragenital endometriosis, with the rectosigmoid junction being the most frequently affected intestinal region. [5]
• The most specific sign of endometriotic invasion of the sigmoid colon is the “Mushroom Cap sign”. This sign appears as a nodule in the rectouterine space that expands and protrudes towards the anterior intestinal wall, showing hypertrophy and fibrosis of the muscularis propria, which is seen with low signal on T2-weighted MRI, and edema of the mucosa and submucosal layers, which are hyperintense on T2-weighted images. [3,5]

Fig 7: Figure 7. Pelvic MRI. Mushroom Cap sign. Sagittal (A) and axial (B) T2-weighted images demonstrating a nodule located in the rectouterine space, extending towards the anterior intestinal wall. It exhibits hypertrophy and fibrosis of the muscularis propria (low signal) along with edema in the mucosal and submucosal layers (high signal). C) Image obtained during the laparoscopy showing the nodule.

Fig 8: Figure 8. Images obtained during laparoscopic surgery of the same patient as in Figure 7, showing a thick adhesion (blue arrows) causing traction on the left ureter (black arrow), sigmoid colon (yellow arrow), and left ovary (purple arrow). It can be observed that indocyanine green is used for intraoperative localization of the ureter.

Fig 9: Figure 9. Pathology specimen of the same patient as in figures 7 and 8: Transverse section at the level of the mid-rectum showing a pseudonodular thickening of the rectal wall reaching a thickness of 18 mm. The cut surface appears greyish with a fasciculated pattern and well-defined borders, with extension of the affected tissue towards one of the surgical margins.