Endometriosis in magnetic resonance imaging: essentials for radiologists and clinicians

Endometriosis is a chronic disease, characterized by the growth and implantation of endometrial tissue outside the uterus, usually in deep pelvic structures, often resulting in inflammation, fibrosis and pain. The ovaries, the uterosacral ligaments, the rectovaginal septum, and, less frequently, other parts outside of the pelvis such as the diaphragm and the sciatic nerve are frequently affected. Timely diagnosis remains a challenge despite the high prevalence due to the heterogeneous clinical presentation. Magnetic resonance imaging (MRI), provides better resolution compared to other imaging modalities and helps identify different forms of the disease, including superficial, deep infiltrative endometriosis (DIE) and endometriomas. This review describes the recommended MRI protocol, descriptions of key imaging findings, the Deep Pelvic Endometriosis Index (dPEI) and the MRI consensus lexicon, and the ENDOVALIRM group's compartment-based approach an MRI-based system that helps classify disease by severity and location for guiding clinical decisions, support preoperative planning and predict surgical outcomes. According the compartment-based approach an MRI-based system, two horizontal lines divide the pel - vis into an anterior, a middle and posterior region. In addition, vertical lines divide the pelvis into a central, a left and a right compartment. This anatomical framework creates 9 compartments: anterolateral, anteroventral, mediolateral, midcentral, posterolateral, posterior-central with additional extrapelvic areas. This review aims to support radiologists and gynecology teams in the accurate detection and classification of endometriosis through standardized MRI interpretation, increasing diagnostic confidence and contributing to better surgical and clinical outcomes.

Endometriosis. Magnetic resonance imaging. Deep pelvic endometriosis index. ENDOVALIRM group. MRI protocol.

Endometriosis is a common chronic inflammatory dis- ease, characterized by abnormal growth of endometrial tissue outside the uterus, which is usually benign but with a heavy health burden. Ectopic endometrial tissue con - sists of normal endometrial stroma and glands that react to the hormonal changes that occur during the menstrual cycle. These periodic hormonal changes are associated with cyclic bleeding, and fibrosis1. This condition occurs in approximately 5-10% of women of childbearing age, with the global burden estimated to be around 176 million women worldwide. According to some studies, the prevalence in some populations is as high as 20% of women2,4. Endometriosis is present in up to 90% of patients presenting with infertility and chronic pelvic pain. Despite this, around 65% of cases are misdiagnosed5,6. There have been major advances in the detection and characterization of different patterns and locations using diagnostic modalities such as magnetic resonance L.M. Olarte-Bermudez et al. Endometriosis in MRI 143 imaging (MRI). However, timely and accurate diagnosis of endometriosis remains a challenge as the disease presents very differently in terms of both symptoms and imaging findings5. This contributes to the fact that it typ - ically takes 10 years between the onset of symptoms and diagnosis, with most women seeing 3 or more doc - tors and healthcare professionals before receiving the correct diagnosis. Typically, the diagnosis is not made until the fourth decade of life, although symptoms usually begin in the early twenties 6. In light of these cases, this review compiles updates for radiologists, trainees, and gynecologists to take a better approach to endometriosis by introducing a standardized scale that allows radiolo - gists to appropriately communicate their findings to gynecologists. NORMAL GYNECOLOGICAL ANATOMY The uterus is a pear-shaped reproductive organ located in the female pelvis between the rectum in the posterior region and the bladder in the anterior region. This organ can be divided into three main segments: the fundus, the body and the cervix. The body and the fundus are mostly composed of the myometrium, the main muscle layer, which is made up of layered and interwoven smooth muscles interspersed with arterioles, nerves and areolar tissue, which ensures their contractile function and better structural integrity. Distally, the uterine body narrows, and opens into the vagina via the cervical canal. The fibers of the outer layer of the body and the fundus are arranged transversely and continue into the surrounding structures, including the fallopian tubes, the round ligament and the ovary ligament7. The uterus is covered on the outside by the serosa, a thin layer derived from the peritoneum. Inside, the uterine cavity is lined by the endometrium, an epithelial stroma containing tubular endometrial glandular tissue, that changes cyclically with the phases of the menstrual cycle8. In approximately 80% of women, the uterus is in anteversion. Above and anteriorly, the peritoneum covers the uterine body and the bladder and forms the vesico- uterine pouch. Posteriorly, the peritoneum covers the fundus, the cervix and the upper part of the vagina, and form the rectouterine pouch, also known as the Douglas pouch. On MRI, the endometrium can be visualized as a hyperintense structure on T2-weighted images and shows homogeneous, low-signal, non–contrast- enhanced T1-weighted imaging 7. The cervix, which acts as a connecting duct between the uterine cavity and the vagina, normally measures approximately 4 cm in length and 3 cm in diameter in non-pregnant women. On high-resolution T2-weighted MRI of the pelvis, the cervix can be differentiated into three zones: a central hyperintense zone formed by the endo- cervical mucosa, a hypointense middle layer, and an outer layer with low to moderate signal intensity, both consisting of fibromuscular stroma9. A unique anatomical structure that requires special attention is the uterine torus, a small transverse thickening in the posterior part of the cervix where the uterosacral ligaments converge10. The broad ligament, a lateral peritoneal fold that surrounds the fallopian tubes, is the most important of the various ligamentous structures that support the uterus. The fallopian tubes consist of 5 anatomically different segments: The intramural portion, the isthmus, the ampulla, the infundibulum, and the fimbriae, which are finger-like projections connected to the ovaries 8. The ovaries are ovoid organs, that are normally have a volume of 6-7 ml in premenopausal women. However, great variability can occur. They are located in the ovarian fossa, are supported by several ovarian ligaments and are connected to the fallopian tubes via the fimbriae. On MRI, the ovarian cortex appears hypointense on T2-weighted images, while the medulla shows intermediate to high signal intensity, reflecting its vascular and stromal composition 7. PATHOGENESIS AND CLINICAL MANIFESTATIONS Despite continuous research, the exact etiology and pathophysiology of endometriosis are still not fully understood, which significantly hinders the develop - ment of definitive curative therapies. Several environ - mental, immunologic, genetic and endocrine risk fac tors have been identified, including short menstrual cycle duration, young age at menarche, low body mass index, nulliparity and congenital obstructive müllerian defects11. Exposure to certain toxins such as bisphenol A and phthalates, alcohol consumption and low physical activ - ity have also been associated with an increased risk of this condition 12. Some studies also suggest a familial predisposition, with twin studies showing a 50% herita - bility and an increased risk (3-15 times) in first-degree relatives with the disease 13–15. Among the proposed mechanisms, the most widely accepted theory for the pathogenesis of endometriosis is the “retrograde menstrual phenomenon”. This theory proposes that fragments of the endometrium migrate through the fallopian tubes into the peritoneum, implant, grow and invade the pelvic structures, leading to inflammation and fibrosis. In addition, it has been suggested that J Mex Fed Radiol iMaging . 2025;4(3):142-153 144 hematogenous and lymphatic dissemination of endome- trial cells may be a mechanism for the occurrence of ectopic lesions in distant organs 16,17. There are various forms of the disease, including superficial peritoneal endometriosis, deep infiltrative endometriosis (DIE) and endometriomas. The clinical manifestations of patients with endometriosis vary depending on the location of the disease. Superficial peritoneal endometriosis is usually characterized by being asymptomatic, while DIE is associated with pelvic pain, dysmenorrhea, dyspareunia, urinary symptoms and infertility 18. Studies have shown that there is no direct correlation between the severity of symptoms and the extent of impairment of the lesion, as some patients with full-blown disease are unaware of their condition, adding to the mystery of the disease 19. It is important to consider the sensory innervation of endometriotic lesions, as deeply infiltrating lesions can affect nerve fibers, resulting in current pain 18. The definitive diagnosis of endometriosis should be made by laparoscopy. Histologic examination of the biopsied tissue should show ectopic endometrial glands and stroma20. However, several international guidelines recommend a more conservative, image-based initial approach. Transvaginal ultrasound (TVUS) is conside - red the imaging technique of choice due to its easy accessibility and low cost. MRI is a very useful tool for diagnosis and preoperative planning as it offers better contrast resolution, and more detailed anatomy. With an overall sensitivity of 91% to 93.5% and a specificity of 86% to 87.5%, it is an excellent tool for diagnosis, mapping and preoperative planning 11. The anatomical forms of presentation such as super - ficial peritoneal endometriosis, DIE and endometriomas show different imaging findings 21. Endometriomas are cystic lesions of the ovary characterized by high T1 signal intensity and intermediate T2 signal intensity with the classic shading appearance, reflecting hemor - rhagic content and cyclic blood breakdown. Several imaging signs have been described in large endome - triomas. One of the most common signs is the “kissing ovary” sign, in which periovarian adhesions pull the ovaries together near the midline 22. Another feature of endometriomas is the double level of fluid– inside the endometrioma, indicating blood in various stages of degradation within the endometrioma 17. Acute endometriosis lesions shows high T1 signal inten- sity but with variable T2 signal due to different bleeding and glandular contents. Chronic or fibrotic endometriosis shows low signal intensity on both T1 and T2, indicating replacement of active tissue with fibrosis and scar tissue. MULTIMODALITY IMAGING FOR ENDOMETRIOSIS Endometriosis requires imaging as a fundamental tool for diagnosis, disease monitoring, and preoperative planning. TVUS is the imaging modality of choice for the evaluation of suspected endometriosis because it is easily accessible, does not require ionizing radiation, has high diagnostic accuracy, and is cost-effective 23. Further advantages are the possibility of real-time assessment and its non-invasive nature 23. The diagnostic performance of TVUS varies depending on the type and location of endometriotic lesions. A 2016 Cochrane review, which included 17 studies reported a sensitivity of 93% and specificity of 96% for the detection of endometriomas in the ovaries, while the sensitivity for DIE, was 79% and specificity 94%. Recent guidelines from the European Society of Human Reproduction and Embryology (ESHRE) recommend that imaging tech - niques, especially TVUS, should replace diagnostic lap- aroscopy as the first line of investigation due to the advantages mentioned above. Accordingly, laparoscopy is now reserved for cases where imaging is inconclusive or when therapeutic intervention is planned. Expert-guided transvaginal ultrasound (ETVUS) is defined as a dynamic real-time ultrasound examination performed by a clinician with expertise in endometriosis. It has been shown to improve detection rates and pro - vide better information for preoperative assessment of the extent of disease24. ETVUS has a sensitivity of 77.5% for deep endometriosis, comparable to a sensitivity of 78.5% reported by Bazot et al.11 in similar cohorts 24. Contrast-enhanced ultrasound (CEUS) has become a complementary method in some selected cases. CEUS provides a better imaging approach to charac - terize endometriotic lesions by using microbubble con - trast agents to assess tissue perfusion and vascular architecture in real time, especially in differentiating solid endometriotic implants from other adnexal or pel - vic masses 25. Endometriotic nodules may have differ - ent enhancement patterns, typically showing moderate to low vascularity due to associated fibrosis and chronic inflammation. CEUS may also be helpful to detect vas - cular involvement in DIE or to guide targeted biopsies of atypical or suspicious lesions 25. MRI ENDOMETRIOSIS PROTOCOL The literature has shown that the MRI protocol for endometriosis should be performed in a superconducting 1.5T (Tesla) system26. An axial and sagittal T2 single-shot L.M. Olarte-Bermudez et al. Endometriosis in MRI 145 fast spin echo (SSFSE) sagittal plane aligned parallel to the longitudinal axis of the uterus. In addition, the exam- ination requires an oblique coronal and oblique axial T2-weighted Fast Recovery Fast Spin Echo (FRFSE) sequence26. T2-weighted sequences without fat suppres- sion are the preferred sequences for detecting pelvic endometriosis, as they are considered the most effective

due to their superior anatomical delineation and sensitivity to fibrotic changes26. In order to maximize the quality of the diagnostic method, preparation for the examination is crucial. We recommend that patients fast for approximately 4-6 hours prior to MRI and undergo bowel preparation to reduce peristaltic artifacts. In addition, the examination should be performed outside the patient’s menstrual cycle, as this can be a confounding factor26. In our insti- tution, the protocol indicates the use of antispasmodic drugs (glucagon or hyoscine) to minimize bowel peri - stalsis. We also ask patients to keep the bladder full to detect endometriosis lesions near the bladder. An endovaginal gel is also inserted to dilate the vaginal canal and improve the view of the adjacent structures. Table 1 shows the MRI protocol at our institution: cor - onal T2-weighted images covering the entire pelvis and extending from the lesions of the kidneys to the pelvic floor; axial, sagittal, and coronal T2-weighted images; axial T1-weighted in-phase and out-of-phase images; coronal T1-weighted fat-saturated images without contrast; and post-contrast T1-weighted fat- saturated images acquired in the axial, coronal, and sagittal planes. Several authors 18,27,28 have proposed improved MRI protocols specifically designed for the detection of DIE. These protocols include T2-weighted images without fat suppression in the axial, coronal, and sagittal planes for a clear view of the round ligaments and pelvic region27,28. The protocols also recommend T1-weighted images with fat-suppression to detect hemorrhagic foci smaller than 1 cm. T2-weighted fat-suppressed images, either in the axial or coronal plane, help to better iden - tify small amounts of free fluid adjacent to lesions 28. Gadolinium contrast agent is generally discouraged as there is currently insufficient evidence of its significant added benefit 18. ENDOMETRIOSIS LOCATIONS BY COMPARTMENTS Endometriosis in the anterolateral compartment: round ligament The round ligament is part of the upper edge of the broad ligament, which is covered by a peritoneal fold. Its main anatomical function is to support the uterus by anchoring it to the mons pubis and labia majora via the inguinal canal. This canal obliterates physiologically between the eighth month of gestation and the first Table 1. MRI 1.5T and 3.0T protocol for the assessment of endometriosis Field strength Sequence FOVa mm Matrix TR ms TE ms Flip angle, degrees Slice thickness, mm 1.5T Cor T2 FSE panoramic 40 × 60 468 × 468 9480 136 140 4 Axial Dual Echo 43 × 1 288 × 160 120 2.2 85 4 Axial DWI 40 × 40 112 × 128 8270 72 - 4 Sag T2FSE spin eco 20 × 20 300 × 224 7.2 120 140 3 Cor T2 FSE 20 × 20 300 × 224 7.2 120 110 3 T1 LAVA 3D Sag 24 × 24 300 × 176 7.5 2.1 12 2.2 3.0T Cor T2 PROPELLER 20 × 20 320 × 320 442 112 111 5 Sag T2 FRSE 20 × 20 320 × 200 442 54.6 110 3 Axial T2 FRSE 20 × 20 320 × 256 5510 112 100 3 DWI 34 × 34 80 × 128 5510 112 - 5 Dual Eco 36 × 36 240 × 140 5510 112 - 5 LAVA 2D Sag 24 × 24 188 × 128 112 112 12 2.30 × 1.03 aVaries according to the width of the patient’s pelvis. MRI: magnetic resonance imaging; T: tesla; TI: time inversion; FOV: field of view; TR: time repetition; TE: time echo; ms: milliseconds; FSE: fast spin echo; FRSE: fast recovery spin echo; DWI: diffusion-weighted imaging; LAVA: liver acquisition with volume acceleration; 3D: three-dimensional; 2D: two-dimensional; PROPELLER: periodically rotated overlapping parallEL lines with enhanced reconstruction; Sag: sagittal; Cor: coronal. J Mex Fed Radiol iMaging . 2025;4(3):142-153 146 postnatal year. This ligament also helps to maintain the anteversion of the uterus throughout the reproductive period. The round ligament is a 10-12 cm long cord-like structure, composed of fibromuscular tissue. It is con - sidered clinically relevant to endometriosis due to its anatomical proximity to the fallopian tubes, vessels, and nerves contained within the broad ligament 29,30. On MRI, it usually appears as a thin, generally smooth, hypointense structure on T1- and T2 weighted images, extending from the uterine horns to the pelvic wall and running anterior to the external iliac vessels, which are highlighted by the surrounding fat. Endometriosis of the round ligament is rare, with an estimated incidence of 0.3% to 0.6% of cases, most of which occur in the extrapelvic region and on the right side. Signs and symptoms may vary depending on the location of the lesion, with pain usually localized to the lower abdomen31. Endometriotic involvement may cause asymmetric thickening, typically larger than 1 cm, with either a smooth or nodular morphology. The segment adjacent to the uterus is most commonly affected. Figure 1 shows an MRI of the pelvis of a 44-year-old woman with pelvic pain associated with the perception of a mass in the right groin. T2-weighted images show a poorly defined heterogeneous lesion in the right inguinal canal that is predominantly hypointense. There are also irreg - ular fibrotic bands with blurred borders suggestive of endometriotic infiltration. Hyperintense foci suggestive of blood content are seen on the T1-weighted fat-sup - pressed sequences without contrast. The MRI findings are consistent with the diagnosis of endometriosis of the right inguinal canal. Endometriosis of the round ligament exhibits variable MRI signal characteristics depending on the composi - tion of the lesion, including stromal tissue, glandular elements, hemorrhage, inflammatory reaction, or fibro - sis. Purely fibrotic lesions appear hypointense on both T1- and T2-weighted images, whereas hemorrhagic foci show hyperintensity on T1-weighted and/or fat-sup - pressed T1-weighted sequences. However, lesions are usually a mixture of both components and visual sur - gical findings such as shortening, deviation or thicken - ing of the round ligaments of the uterus (RLUs) have a high positive predictive value (83.3%) for the diagnosis of endometriosis in these ligaments; these findings may facilitate the decision to excise these ligaments 18. Although the prevalence of endometriosis in RLUs is not as high as in other structures, endometriosis in RLUs may have clinical implications for the persistence of postoperative symptoms, as comprehensive resec - tion of all visible lesions increases the likelihood of improvement in symptoms and quality of life 32,33. Endometriosis in the anteroventral compartment: bladder Bladder endometriosis is the most common form of urinary tract involvement in endometriosis, and accounts for about 70% to 85% of genitourinary cases. However, only 1% of cases present with isolated urinary tract foci33,34. This form of the disease is clinically significant as it is associated with chronic, often debilitating symp - toms such as dysuria, hematuria, increased urinary frequency, and voiding dysfunction. Therefore, timely Figure 1. MRI of the pelvis of a 44-year-old woman with pelvic pain associated with the perception of a mass in the right groin. A: axial view, T2-weighted sequence shows a round, poorly defined predominantly hypointense lesion, and to a lesser extent, multiple small hyperintense regions, giving a heterogeneous appearance (white arrow). B: axial T1-weighted sequence with fat suppression, without contrast, shows hyperintense foci within the endometrial lesion (white arrow), representing a blood component. C: sagittal T2-weighted view shows fibrotic bands, with poorly defined borders (white arrow), involving the right inguinal canal. The MRI findings are consistent with the diagnosis of endometriosis of the right inguinal canal. MRI: magnetic resonance imaging. A B C L.M. Olarte-Bermudez et al. Endometriosis in MRI 147 TVUS, especially in combination with transabdominal views and bladder filling, can also help to detect endome- triomas. This technique may reveal hypoechoic mural nodules or irregularities along the bladder dome or pos - terior wall. In some cases, loss of the normal interface between the posterior bladder wall and the anterior uter- ine surface may serve as an additional clue. Cystoscopy can be used to confirm mucosal involvement and rule out malignancy, although it is of limited use in detecting lesions outside the mucosa. Accurate identification of bladder endometriosis is critical for planning surgical inter- vention, as complete resection or partial cystectomy may be required to improve symptoms, particularly in inva - sive disease that is unresponsive to medical therapy. Endometriosis of the middle central compartment: uterosacral ligament The uterosacral ligaments are paired fibromuscular structures that extend from the posterolateral cervix and upper vagina to the anterior aspect of the sacrum, providing important support to the uterus and main - taining pelvic floor the stability. These ligaments are commonly affected in DIE, which often involves the mid-central compartment of the posterior pelvic cavity. DIE is defined as invasion > 5 mm of the peritoneal surface by endometriotic lesions, most commonly located at the uterosacral ligaments, rectovaginal space, pararectal space and vesico-uterine fold 35. imaging is required for diagnosis, as it may be associa - ted with the risk of renal failure due to urinary tract obstruction. In bladder endometriosis, MRI typically shows locali - zed or diffuse wall thickening with associated signal abnormalities. Findings include nodular thickening with T2-weighted low signal intensity fibrotic stranding; sometimes, hyperintense hemorrhagic foci may be present in T1-weighted sequences. Figure 2 shows an MRI of the pelvis of a 45-year-old woman with hypo - gastric pain and dysmenorrhea, who had undergone a hysterectomy. On coronal T2-weighted images, the lesion extends to the bladder serosa and infiltrates the anterior perivesical fat in the Retzius space. On sagittal T2-weighted images, an irregular, predominantly hypointense lesion is seen at the level of the bladder dome with distention of the vagina due to the use of intravaginal gel. On T1-weighted images without fat suppression, the lesion is hypointense and pseudono - dular, whereas contrast-enhanced T1-weighted images with fat suppression show heterogeneous hyperinten - sity due to contrast uptake, The MRI findings are con - sistent with the diagnosis of DIE involving the bladder. In most cases, lesions are confined to the serosal layer of the bladder, although extension into the muscularis propria may occur in advanced disease. The mucosa in particular is often spared, which may result in cys - toscopy showing normal findings despite significant extramucosal disease. Figure 2. MRI of the pelvis of a 45-year-old woman with hypogastric pain and dysmenorrhea, who had a history of hysterectomy. A: coronal T2-weighted MRI shows a poorly defined, pseudonodular lesion in the anterior bladder wall (white arrow), extending to the bladder serosa and infiltrating the anterior perivesical fat (space of Retzius). B: sagittal T2-weighted MRI shows an irregular, predominantly hypointense lesion (white arrow) involving the bladder dome. The distended vagina is visible due to the presence of intravaginal gel. C: T1-weighted image without fat suppression showing a hypointense, pseudonodular lesion in the anterior bladder wall (white arrow). D: contrast-enhanced T1-weighted image with fat suppression shows a pseudonodular lesion of the bladder wall, that appears hyperintense due to contrast uptake (white arrow). The MRI findings are consistent with the diagnosis of DIE involving the bladder. DIE: deep infiltrative endometriosis; MRI: magnetic resonance imaging. A B C D J Mex Fed Radiol iMaging . 2025;4(3):142-153 148 The most common site of DIE is the uterosacral lig - aments, which are affected in up to 70% of cases 36. The differential diagnosis of ureteral endometriosis includes ureteral invasion by cervical cancer. On MRI, the main findings are asymmetric shortening, thicken - ing, and nodularity involving the ligaments 37. Figure 3 shows an MRI of the pelvis of a 32 -year-old woman with pelvic pain and dysmenorrhea. The axial T2- weighted image shows smooth thickening of the right uterosacral ligament. The MRI findings are consistent with the diagnosis of DIE in the uterosacral ligament. Impairment of the uterosacral ligament typically presents as fibrotic thickening with low signal on T2- weighted images, which may be spiculated, nodular, or smooth. In some cases, focal areas of high signal inten- sity may be present on T1-weighted images, suggesting hemorrhagic components. Although specific diameter thresholds have been proposed for the diagnosis of endometriosis of the uterosacral ligament, these are not yet well validated 37. Endometriosis of the posterolateral compartment: rectovaginal septum Endometriosis of the rectovaginal septum is a serious condition with clinical manifestations such as abdominal pain, colonic obstruction, and, in some cases, renal obstruction. The diagnostic difficulty of multifocal endo - metriosis begins with its similarity to other pathologies38. Deep dyspareunia, dyschezia, and chronic pelvic pain are common symptoms that intensify during menstrua - tion. In some cases, rectovaginal endometriosis may mimic irritable bowel syndrome, inflammatory bowel dis- ease, or pelvic inflammatory disease, leading physi - cians to misdiagnose. A thorough clinical history, with particular attention to cyclical symptoms, is crucial to raise suspicion and prompt appropriate referral for imaging and avoid unnecessary surgery 21,39. A meta-analysis found that the sensitivity and spec - ificity of MRI for the diagnosis of rectovaginal septal endometriosis were 82% and 77%, respectively. On MRI, rectovaginal endometriosis may present as ill- defined lesions, soft tissue thickening, or obliteration of the pouch of Douglas. On MRI these lesions typically show low signal intensity on T1- and T2-weighted images with late contrast enhancement due to the fibrotic component 40,41. Occasionally, punctate hyperin - tense foci may be present on T1-weighted images, indicating subacute hemorrhage or highly viscous fluid. Figure 4 shows an MRI of the pelvis of a 39-year-old woman with chronic pelvic pain, constipation and Figure 3. MRI of the pelvis of a 32 -year-old woman with pelvic pain and dysmenorrhea. The axial T2-weighted image shows smooth thickening of the right uterosacral ligament (white arrow). The MRI findings are consistent with the diagnosis of DIE in the uterosacral ligament. DIE: deep infiltrative endometriosis; MRI: magnetic resonance imaging. dyspareunia. Sagittal T2-weighted images show irregu - lar, hypointense lesions involving the rectovaginal sep - tum, with a distended vagina visible due to the presence of intravaginal gel. The MRI findings are consistent with the diagnosis of DIE involving the rectovaginal septum. Although MRI is superior to other imaging modalities in the overall visualization of the pelvis, its diagnostic accuracy in the detection of superficial peritoneal lesions is limited. In this context, laparoscopy remains the gold standard for the diagnosis of superficial endometriosis and is more accurate than MRI, TVUS or physical examination for this particular subset of lesions 40. Endometriosis of the posterior-central compartment: rectosigmoid colon Deep endometriosis is an entity that poses a challenge for the gynecologist. The rectosigmoid colon is most commonly affected by intestinal endometriosis, which mainly affects the central posterior compartment (65.7%). In the rectum, it usually affects the middle and upper third of the organ 39. On MRI, endometriosis appears as T2 hypointense thickening of the muscle layer, ranging from plaque-like involvement to nodular, mass-like lesions. A classic sign of rectal involve ment is the ‘mushroom cap’ appearance, caused by T2-hypointense fibromuscular hypertrophy of the muscularis, with wrinkling and retrac- tion of the serosa or adventitia resembling the appear - ance of a mushroom. Figure 5 shows an MRI of the pelvis of a 40-year-old woman with pelvic pain and rectal bleed- ing during menstruation. a lesion of the mucosa of the L.M. Olarte-Bermudez et al. Endometriosis in MRI 149 mid-rectum with thickening of the muscularis propria, forming the mushroom cap sign. The axial and sagittal T2-weighted MRI images show the finding of this mush- room cap lesion is a reliable predictor of infiltration of the muscularis. The MRI findings are consistent with the diagnosis of DIE with involvement of the rectosigmoid colon and show the mushroom cap sign. A typical imaging finding for sigmoid involvement is the ‘fortune cookie’ sign, which appears as a T2-weighted mass with low signal intensity, central retraction, and delayed homogeneous enhancement that resembles the appearance of a fortune cookie. T1-hyperintense foci cor- responding to hemorrhagic components can also be observed in the fortune cookie and mushroom cap sign42. Assessment of rectal segments, classified as low ( 10 cm) from the anal verge is essential when assessing the anal margin42. Extrapelvic endometriosis: diaphragmatic and perihepatic Extrapelvic endometriosis is rare and has several local- izations, such as the abdominal, thoracic and hepatic capsule. The most common location of thoracic endome- triosis is the diaphragm, with an incidence of 0.15% to 1.5%, followed by the pleura and lung 41. Diaphragmatic endometriosis can manifest clinically as catamenial or non-catamenial pneumothorax. Initial imaging modalities include chest X-ray and computed tomography (CT); however, MRI provides better imaging, with a reported sensitivity of 78% and specificity of 83% 41,43. A catamenial pneumothorax is defined as a pneumo - thorax occurring three days before or after menstrua - tion. It accounts for 3-6% of spontaneous cases in women and is associated with endometriotic diaphrag - matic foci extending into the thorax. These foci can irritate and perforate the pleura. Endometriotic nodules are found in 78% of catamnestic cases. This disease mainly affects the right side (90%) 43. The main challenge in the diagnosis of diaphragmatic endometriosis is its potential rarity, in addition to its asymptomatic nature, which can reach almost 70% of cases44. When symptomatic, it manifests by pain in the arm, shoulder or right upper quadrant and is often diag - nosed incidentally during surgical procedures, although the exact mechanism is still unclear 45. Some theories such as retrograde menstruation, hematogenous spread, lymphatic dissemination, and coelomic metaplasia have been proposed as possible causes 46. On MRI, endometrial foci may appear hyperintense in all sequences. Figure 6 of an abdomino-pelvic MRI of a 36-year-old woman with dyspareunia, and dysmen- orrhea and cyclic right upper quadrant pain shows a subcapsular lesion with intermediate signal intensity and internal hypointense septa on T2-weighted images. On T1-weighted fat-suppressed (LAVA) sequences, a hyperintense nodule within the lesion suggests an endometrioma with blood derivatives. Post-contrast Figure 4. MRI of the pelvis of a 39-year-old woman with chronic pelvic pain associated with constipation and dyspareunia. A-B: sagittal T2-weighted MRI shows irregular, hypointense lesions involving the rectovaginal septum (white arrows). The distended vagina is visible due to the presence of intravaginal gel. The MRI findings are consistent with the diagnosis of DIE involving the rectovaginal septum. DIE: deep infiltrative endometriosis; MRI: magnetic resonance imaging. A B J Mex Fed Radiol iMaging . 2025;4(3):142-153 150 T1-weighted imaging with subtraction shows heteroge - neous enhancement of the lesion. The MRI findings are consistent with the diagnosis of extra pelvic extension of the endometriosis to the diaphragm and the perihe - patic region. Susceptibility artifacts in the thorax can lead to dis - tortions that obscure the diagnosis due to the proximity to the air in the lungs. Recognizing the typical linear shape, that runs parallel to the diaphragm, can be help - ful in distinguishing true endometriosis lesions from artifacts47. Extrapelvic endometriosis: sciatic nerve Isolated, DIE of the sacral nerve roots or pelvic nerves such as the sciatic nerve is a rare entity that usually causes back pain, buttock pain with radiation to the dor- sal region of the thigh and lateral aspect of the leg. Physical examination may reveal a positive Lasegue’s sign, loss of sensation, reflex changes, muscle weak - ness and paresis18. The sciatic nerve is rarely affected by endometriosis. It is usually a case of cyclic sciatica that does not respond to the usual conservative treat - ment48. Cyclic sciatica is a reactive, self-limiting inflam - matory reaction to deposits and bleeding of endometrial glands and stroma in the sciatic nerve. Early and timely diagnosis is essential, as untreated and unrecognized cases carry the risk of permanent neuronal damage if left untreated 27. Given the complex- ity of the condition, treatment requires a multidisci - plinary approach, involving the specialties of gynecology, orthopedics, neurology and radiology. MRI and/or myelography are of paramount importance for preoper - ative assessment, especially for the localization of the lesion, which must be confirmed by immunohistochem - istry22. A study by Kale et al. 42 showed that endometri - osis isolated to the sciatic nerve root is more commonly found on the right side. MRI findings suggestive of neural involvement include neural thickening, abnormal signal intensity, and increased enhancement on contrast-enhanced MRI. Sciatic endo - metriosis foci can be identified by the stage of hemor - rhage, as they typically have high signal intensity on T1-weighted images, and variable signal intensity on T2-weighted images49. Figure 7 shows an MRI of the pelvis of a 36-year-old woman with chronic pelvic pain, and unilateral shooting pain that extended from the but - tock down to the posterior thigh, suggesting sciatic nerve involvement. Axial T2-weighted images show an ill-de - fined, spiculated, hypointense lesion in the right lateral wall of the pelvis with involvement of the sciatic nerve Figure 5. MRI of the pelvis of a 40 -year-old woman with pelvic pain associated with rectal bleeding during menstruation. A: sagittal and B: axial T2-weighted MRI views show a lesion of the mucosa of the mid-rectum with thickening of the muscularis propria forming the mushroom cap sign (white arrows). The distended vagina and rectum are visible due to the presence of intravaginal and intrarectal gel. The MRI findings are consistent with the diagnosis of DIE with involvement of the rectosigmoid colon and show the mushroom cap sign. DIE: deep infiltrative endometriosis; MRI: magnetic resonance imaging. A B L.M. Olarte-Bermudez et al. Endometriosis in MRI 151 roots. The MRI findings are consistent with the diagnosis of DIE with sciatic nerve. It is important to note that the absence of pelvic endo- metriosis does not exclude the diagnosis of sciatic nerve endometriosis, which emphasizes the importance of high clinical suspicion in patients. In women of child - bearing age who complain of sudden sciatic symptoms that correlate in time with the menstrual cycle and exac- erbation of symptoms, nerve involvement secondary to endometriosis should be considered 50. MRI is a helpful tool in determining the cause. ENDOVALIRM group MRI consensus lexicon and compartment-based approach According to the ENDOVALIRM group, the pelvis can be divided into 9 compartments based on important structural landmarks 41. Two horizontal lines divide the pelvis into an anterior, a middle and posterior region (Figure 8). The anterior horizontal line is drawn anterior to the cervix or vagina, while the posterior line is located anterior to the rectum. In addition, vertical lines divide the pelvis into a central, left and right compartment. These lines run from posterior to anterior, through the uterosacral ligament, the mesorectal fascia, the lateral walls of the cervix and the bladder. This anatomical framework creates 9 compartments: right anterolateral, right mediolateral, right posterolateral, anterocentral, mediocentral, posterocentral, left anterior, left medial, and left posterior with additional extrapelvic areas 41. The Deep Pelvic Endometriosis Index (dPEI) is an MRI-based system, developed for DIE, the aims to com- prehensively describe all localizations of deep endome - triosis by creating structured reports with diagrams and scores. This scale is valuable for predicting operative time, length of hospital stay and possible subsequent complications, making it an excellent tool for clinical and surgical management22,24,41. This scale has shown high diagnostic performance, with a sensitivity of 91% and a specificity of 90% 22,24. According to the dPEI, one point is awarded for each affected compartment, resulting in a total of 10 points, as extrapelvic sites are also taken into account. The severity is interpreted as follows: mild ( 5 points)41. Although the sensitivity of MRI is considered to be better than that of TVUS or laparoscopy, its diagnostic performance varies depending on the type of endometri- osis present. Accordingly, laparoscopy was found to be the better method for detecting superficial endometriosis compared to MRI, TVUS, or physical examination41. Figure 6. MRI of the abdomen and pelvis of a 36-year-old woman with right upper quadrant pain, dyspareunia, and dysmenorrhea, and cyclic right upper quadrant pain. A: T2-weighted image shows a subcapsular hepatic lesion with intermediate signal intensity and multiple internal hypointense septa (white arrow). B: T1-weighted image with fat suppression (LAVA) with a hyperintense nodule inside the lesion, showing an endometrioma with blood derivatives (white arrow). C: post-contrasted T1-weighted image with subtraction shows heterogeneous enhancement of the lesion (white arrow). The MRI findings are consistent with the diagnosis of extrapelvic extension of the endometriosis to the diaphragm and the perihepatic region. MRI: magnetic resonance imaging; LAVA: Liver Acquisition with Volume Acceleration. A B C J Mex Fed Radiol iMaging . 2025;4(3):142-153 152

Endometriosis is a disease with a wide spectrum of symptoms, signs and imaging findings, represents a major burden worldwide due to its high prevalence, debilitating effects, and usually delayed diagnosis. MRI plays an invaluable in the non-invasive assessment of endometriosis, providing unmatched anatomical detail and sensitivity in the localization of DIE. The introduc - tion of standardized reports and protocols for MRI, such as the MRI consensus lexicon and the ENDOVALIRM group’s compartment-based approach to the assess - ment of dPEI, strengthens understanding between radiologists and clinicians, and provides tremendous help in surgical and clinical planning, improving patient outcomes, especially when combined with clinical find - ings and additional imaging modalities. It is important that radiologists are familiar with the anatomical land - marks, potential pitfalls and key imaging signs, as this is essential for disease compromise. This review high - lights the key MRI features of endometriosis and emphasizes the importance of close collaboration between radiologists and clinicians to optimize the out - come for the patient. Acknowledgment The authors thank Professor Ana M. Contreras- Navarro for her guidance in preparing and writing this scientific paper. Funding The authors declare that they have not received funding. Conflicts of interest The authors have no conflicts of interest to disclose. Ethical considerations Protection of humans and animals. The authors declare that no experiments involving humans or ani - mals were conducted for this research. Confidentiality, informed consent, and ethical approval. The study does not involve patient personal data nor requires ethical approval. The SAGER guide - lines do not apply. Declaration on the use of artificial intelligence . The authors declare that no generative artificial intelli - gence was used in the writing of this manuscript.

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