Endometriosis, defined as the presence of endometrial-like tissue outside of the uterus, is a common gynecologic dis order with a prevalence of approximately 10% of reproduc tive age women and an incidence of approximately 90% and 50% of women with chronic pelvic pain and infertility, respectively [1–4]. Both ultrasound and MRI can be utilized to map deep infiltrating endometriosis in the pelvis [1–3].Deep infiltrating endometriosis (DE) was formerly defined pathologically as greater than or equal to 5 mm inva sion under the peritoneal surface by endometriotic implants [2–5], but any subperitoneal invasion by endometriosis now defines deep disease [1]. DE of bowel may manifest as a mushroom cap or fortune cookie sign [2–6]. Recurrent cyclical hemorrhage of the endometriotic implants in the pelvis can lead to fibrosis, which can manifest morphologi cally on MRI as a T2 hypointense mass with variable irreg ular or spiculated/stellate margins [1–6]. The spiculated margins reflect retraction and architectural distortion of sur rounding tissue due to fibrosis, reminiscent of a sea urchin. These spiculated findings of DE on MRI are not location dependent and can occur in the anterior (Fig. 1), middle (Fig. 2), posterior (Fig. 3), and lateral compartments of the pelvis (Fig. 4) and the anterior abdominal wall (Fig. 5). The spiculated margins often tether adjacent structures to one another and may contain T1 hyperintense foci representing blood products or T2 hyperintense foci reflecting glandular tissue [1–5]. MRI expert consensus identifies fibrotic endo metriosis with less than 25% hemorrhagic or glandular foci as the most common phenotype of DE [1].Fig. 1 a T2 weighted coronal oblique image demonstrates a hypoin tense endometriotic deposit with spiculated margins within the vesico uterine space (circle) in this patient with DE of the anterior compart ment. The endometriotic deposit tethers the bladder wall (arrowhead) and cervix (arrow) leading to a sea urchin sign. b Photograph of a sea urchin in waterFig. 2 a T2 weighted coronal oblique image through the level of the torus uterinus demonstrates an irregular hypointense mass with spicu lated margins (circle) leading to a sea urchin sign extending into the middle compartment with involvement of the parametria. Small T2 hyperintense foci represent glandular tissue (arrowhead). Addition ally, there is a left periureteral endometriotic deposit (arrow), which leads to medial deviation of the ureter and moderate hydroureter (*). b T1 weighted fat suppressed precontrast axial image through the level of the ovaries demonstrates T1 hyperintense hemorrhagic foci (arrowhead) in the deep endometriotic deposit at the torus uterinus. T1 hyperintense bilateral endometriomas (#) are tethered posteriorly and medially at the torus uterinus in a kissing ovaries configuration (circle)Fig. 3 T2 weighted sagittal image demonstrates a hypointense endo metriotic deposit with spiculated margins along the right uterosacral ligament (circle) in a sea urchin morphology resulting in architectural distortion of tissues in the posterior compartmentFig. 4 T2 weighted axial oblique image demonstrates a hypointense endometriotic deposit with spiculated margins centered in the lateral compartment along the left pelvic sidewall (circle). The deposit infil trates and thickens the left levator ani complex musculature (arrow head). Fibrotic spicules extend anteriorly to involve the left vaginal fornix (solid arrow), medially to involve the left rectal wall (*), and laterally to involve the greater sciatic foramen (dashed arrow)Fig. 5 Cropped T2 weighted axial image demonstrates a hypointense endometriotic deposit with spiculated margins in a subcutaneous scar of the right lower quadrant anterior abdominal wall (circle)Anterior compartment DE may cause tethering in the vesicouterine space involving the bladder, ureter, and round ligaments. Middle compartment DE commonly involves the parametrium, fallopian tubes, and ovaries and may extend to involve the posterior compartment structures like the torus uterinus, uterosacral ligaments, and rectosigmoid colon. When the lateral compartment is involved by endometriosis this fibrotic pattern of disease is a common manifestation, often times involving the pelvic side wall and sciatic notch. The lateral aspect of the vagina and low rectum are often involved in continuity.Proper recognition and diagnosis of deep endometriosis is critical in optimizing patient outcomes. Imaging is partic ularly important for surgical planning to identify all sites of disease, particularly those which may require collaboration with surgical subspecialties, such as urology and colorectal surgery for bladder and ureter or extensive bowel disease, respectively [1–3, 7–8]. Detection of nerve involvement can inform surgeons of the need for nerve-sparing surgical tech niques such as laparoscopic neuronavigation [9]. The care of patients with endometriosis requires a multidisciplinary approach to optimize patient outcomes with MRI lending a critical role in surgical planning.ConclusionMRI is increasingly utilized for detection of deep infiltrating endometriosis and surgical planning. We propose the sea urchin sign to describe the appearance of fibrotic endometriotic deposits to heighten awareness and recogni tion of this dominant disease phenotype of DE.References1. VanBuren W, Feldman M, Shenoy-Bhangle AS et al (2024) Radiology State-of-the-art Review: Endometriosis Imaging Interpretation and Reporting. Radiology 312 3:e2334822.Jha P, Sakala M, Chamie LP, et al (2020) Endometriosis MRI lexicon: consensus statement from the society of abdominal radiology endometriosis disease-focused panel. Abdom Radiol 45:1552–1568.3.Mason BR, Chatterjee D, Menias CO, Thaker PH, Siegel CL, Yano M (2020) Encyclopedia of endometriosis: a pictorial rad-path review. Abdom Radiol 45:1587–1607.4.Takeuchi M, Matsuzaki K, Harada M (2024) Endometriosis, a common but enigmatic disease with many faces: current concept of pathophysiology, and diagnostic strategy. 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Radiographics. 44 1:e230106.SourceWarrington NV, Sugi MD, Caserta MP, VanBuren WM, Yano M. ‘Sea urchin sign’: a radiological indicator of fibrotic deep infiltrating endometriosis. Abdom Radiol (NY). 2025 Jul 1.
