Endometriosis is a spectrum of diseases which consist of three main subtypes: superficial peritoneal endometriosis (not seen in MRI), ovarian endometriosis (endometriomas, most common form of endometriosis) and deep infiltrative endometriosis (most of the endometriotic lesions visible on MRI). Fig.1

The most frequent site of ectopic endometrial implantation is the ovary, showing typical characteristics in magnetic resonance imaging (MRI) summarize in figure 2. Endometriotic cysts are associated with a risk of malignant transformation (the two prevalent histological subtypes are endometrioid adenocarcinoma and clear cell carcinoma. The most sensitive MRI finding to diagnose malignant endometriosis is the emergence of enhanced mural nodules within the ovarian endometriotic cyst. Fig.2

Magnetic resonance imaging (MRI) plays an increasingly important role in the diagnosis and mapping of endometriosis making an exhaustive roadmap essential in preoperative decisions.

However, many endometriosis findings, especially DIE lesions, are challenging to assess, particularly for young or non-expert radiologists. Moreover, there is significant heterogeneity in the description of endometriosis findings and the sites of involvement.

There is significant variability in the literature regarding the MRI protocols, but in our institution we use the MRI protocol summarized in Fig 3.

To systematically describe imaging findings in the female pelvis, radiologists initially divided it into three primary compartments: anterior, middle and posterior. In 2023, the ENDOVALIRM group introduced a more detailed MRI assessment lexicon by dividing the pelvis into nine compartments using vertical lines that intersect the three primary compartments. Fig.4.

This new lexicon includes lateral endometriosis (which increase the surgical complexity and perioperative morbidity) and establishes three levels depending on how many compartments are affected: mild disease with two compartments or less; moderate dis- ease with three or four compartments; and severe disease with five compartments or more. In addition, the degree of involvement of this disease evaluated with this lexicon was correlated with operating time, hospital stay and postoperative complications . Thus, this classification allows better patient stratification.

When preparing the report, it must be done in a standardized way, for example, starting by reviewing the three central compartments, followed by the three lateral compartments (first right side and then left side).

• The CENTRAL COMPARTMENTS

The anterocentral compartment contains the bladder and the proximal round ligament.

Bladder involvement is considered when there is a nodule or mass involving the muscularis layer. It is important to note the three different locations within the bladder wall: anterior dome, vesicouterine pouch and bladder base; the detrusor or ureteral infiltration next to de the bladder. When endometriosis is restricted to the dome, simple partial cystectomy can be performed. In contrast, with bladder base involvement the risk of ureteral resection-reimplantation is high.

The proximal round ligament involvement is considered when there is a fibrotic nodular thickening (generally > 1cm) +/- hemorrhagic cystic area. Fig.5.

The mediocentral compartment contains the torus uterinum and uterosacral ligaments (USL), the posterior vaginal fornix, the rectovaginal septum (RVS) and external adenomyosis. Fig.6

Torus uterinum and proximal USL involvement is suspected when there is a fibrotic thickening (> 5 mm) or retraction (spiculated morphology “spider shape”) with or without hemorrhagic implants. Fig 7,8

Posterior vaginal fornix and rectovaginal septum involvement has to be suspected when it is observed a nodule or thickening of the posterior vaginal wall or extended between the vagina and the rectum, below the peritoneal pouch of Douglas reflexión (It is important to describe the length, transverse axis, thickness, circumfered by degree in vaginal fornix involvement and distance to the anal verge in rectovaginal septum involvement). Fig 9

External adenomyosis corresponds to a nodular fibrotic extrinsic infiltration of the myometrium with ill-defined borders, separated from the junctional zone, which remained intact.

The posterocentral compartment contains the rectum and de rectosigmoid junction. They are involved when a thickening or mass of the rectosigmoid wall is seen. It is important to note the location (recto-sigmoid junction, upper/middle/lower rectum). The mushroom cap sign is one of the important signs of deep recto-sigmoid endometriosis seen on T2 weighted MRI sequence. It indicates the submucosal involvement in the rectosigmoid colon. The hypertrophic muscularis propria appears as heterogeneous low signal intensity surrounded by the high signal intensity of mucosa and submucosa. Fig.10

• The LATERAL COMPARTMENTS

The anterolateral compartment contains the distal round ligament. It is critical to report an endometriosis involvement of the round ligament as well as its location as the surgical procedure can be modified according to the proximal or distal involvement location. In the case of distal involvement, the proximity to the epigastric and external iliac vessels can increase the surgery complexity while involvement at the superficial orifice of the inguinal canal is considered in the extra pelvic compartment and may require a perineal approach. Fig 11

The mediolateral compartment contains the parametrium, ureter, uterine artery, and pelvic wall. Fig.12,13

The posterolateral compartment includes the distal USL, sacrorectalgenital septum (SRGS), and pelvic wall.

• The EXTRAPELVIC LOCATIONS

The appendix, terminal ileum, cecum, colon, abdominal wall and inguinal regions can be involvedby endometriosis. The endometriosis in this locations appears as a nodule or nodular fibrotic infiltration with or without haemorrhagic cyst area Fig.14