Peritoneal endometriosis is ectopic presence of functional endometrial tissue (glands and stroma) on peritoneal surfaces. Affects 6-10% of reproductive-age women. Douglas pouch, uterosacral ligaments, ovarian fossa, and bladder peritoneal surface are most commonly involved. Responds to cyclic hormonal stimulation causing pain, adhesions, and infertility. Deep infiltrating endometriosis (DIE) penetrates >5 mm into organ walls and accurate imaging is critical for surgical planning. MRI is the primary imaging modality.
Age Range
20-50
Peak Age
35
Gender
Female predominant
Prevalence
Uncommon
Peritoneal endometriosis is explained by retrograde menstruation theory — during menstruation, endometrial tissue reaches peritoneal cavity through fallopian tubes and implants on peritoneal surfaces. Implanted endometrial tissue undergoes estrogen-dependent cyclic changes: proliferation, secretion, and bleeding. Chronic inflammation from repeated microhemorrhages leads to fibrosis and adhesions. Hemosiderin deposition (from old hemorrhages) produces T1 hyperintense, T2 hypointense signal — this 'T1 shading' is pathognomonic MRI finding for endometriosis. In deep infiltrating form, fibrous tissue penetrates organ walls (rectum, bladder, ureter) creating T2 hypointense nodular lesions. Peritoneal lesions may be small (millimetric) and missed on standard CT — MR T1 fat-suppressed sequences are most sensitive for detecting hemorrhagic content.
Hemorrhagic endometrial implants appearing as hyperintense foci on MR T1 fat-suppressed sequences. Methemoglobin content creates T1 shortening producing high signal despite fat suppression. Pathognomonic MRI finding for endometriosis and most sensitive method for detecting small peritoneal implants.
Hyperintense foci on peritoneal surfaces on T1 fat-suppressed sequences — representing hemorrhagic endometrial implants. Signal preserved on T1 fat-sat (remains bright despite fat suppression) — distinguishes from fat. Foci typically 2-15 mm, multiple, localized in Douglas pouch, uterosacral ligaments, and ovarian fossa.
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Hyperintense foci on peritoneal surfaces on MR T1 fat-suppressed sequences, consistent with hemorrhagic endometrial implants.
T2 hypointense nodular lesion — representing deep infiltrating endometriosis (DIE) plaque. Located in Douglas pouch, uterosacral ligaments, rectovaginal septum, or bladder wall. Fibrotic tissue gives distinctly low signal on T2. T1 hyperintense hemorrhagic foci may accompany within the lesion.
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T2 hypointense nodular lesion in Douglas pouch/uterosacral ligaments, consistent with deep infiltrating endometriosis.
Douglas pouch obliteration — disappearance of Douglas pouch due to adhesions between anterior rectal wall and posterior uterine wall. 'Kissing sign' — rectum and uterus appear adherent. Best assessed on sagittal and axial T2. Degree of obliteration (partial/complete) correlates with surgical difficulty.
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Douglas pouch obliterated with adhesion between anterior rectal wall and posterior uterine wall (kissing sign), consistent with deep infiltrating endometriosis.
Hypoechoic, irregularly marginated nodule in rectovaginal septum or uterosacral ligaments on transvaginal US. 'Tenderness-guided' US — pain at probe pressure site localizes DIE lesion. US sensitivity approaches MRI in experienced hands but limited for deep and posterior lesions.
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Hypoechoic nodule in rectovaginal septum on transvaginal US, consistent with deep infiltrating endometriosis.
Peritoneal thickening, pelvic adhesions, and peritoneal nodularity on CT. CT has significantly lower sensitivity than MRI for endometriosis diagnosis. However may be useful in acute complications (torsion, rupture) or evaluation of large endometriomas. Deep implants may appear as soft tissue density nodules.
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Pelvic peritoneal adhesions and soft tissue density nodules on CT, may be consistent with endometriosis; further evaluation with MRI recommended.
Mild-moderate diffusion restriction in deep infiltrating endometriosis lesions on DWI. Cellular areas may appear hyperintense on DWI. DWI provides complementary information to T1 fat-sat for detecting small peritoneal implants. ADC values in endometriosis are generally higher than malignant tumors.
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Mild diffusion restriction in pelvic peritoneal lesions on DWI, consistent with endometriosis.
Criteria
Implants <5 mm deep on peritoneal surface. Red, black, or white lesions at laparoscopy. Appear as small T1 fat-sat hyperintense foci on MRI. Treatment: hormonal therapy or laparoscopic excision.
Distinct Features
Small superficial foci, may be controlled with hormonal therapy without surgery
Criteria
Lesions penetrating >5 mm deep from peritoneal surface. Rectovaginal septum, uterosacral ligament, bladder wall, ureter or bowel wall involvement. T2 hypointense nodular lesions on MRI. Accurate mapping critical for surgical planning.
Distinct Features
Organ wall invasion, T2 hypointense nodules, form requiring surgery
Criteria
Ovarian endometriosis cyst — 'chocolate cyst'. T1 fat-sat hyperintense, T2 intermediate-low signal (T2 shading). Homogeneous hemorrhagic content. Minimal wall enhancement. Ovarian endometrioma may coexist with peritoneal endometriosis.
Distinct Features
Ovarian cystic lesion, T1 hyperintense/T2 shading, chocolate-colored hemorrhagic content
Distinguishing Feature
Peritoneal carcinomatosis has T1 hypointense nodules with malignancy history in older patients while endometriosis shows T1 fat-sat hyperintense hemorrhagic foci in young women with cyclic pain
Distinguishing Feature
Intra-abdominal abscess shows rim enhancement, central fluid with fever/leukocytosis while endometriosis shows T1 hyperintense foci with cyclic pain pattern
Distinguishing Feature
Peritoneal lymphomatosis shows diffuse homogeneous thickening with B symptoms while endometriosis has focal nodular lesions with cyclic pelvic pain
Distinguishing Feature
IgG4-related disease shows diffuse peritoneal thickening with elevated serum IgG4; endometriosis characterized by focal T1 hyperintense hemorrhagic foci with cyclic symptoms in reproductive-age women
Urgency
routineManagement
surgicalBiopsy
Not NeededFollow-up
6-monthTreatment of peritoneal endometriosis is planned according to symptom control and fertility preservation goals. In mild cases, hormonal treatment (OCP, progestins, GnRH agonists) may suffice. Laparoscopic surgical excision is standard for DIE — preoperative MRI mapping is critical for surgical planning. Multidisciplinary team needed if rectum, bladder, or ureter involved. Surgical timing must be carefully planned in fertility preservation cases. Biopsy usually unnecessary — MRI findings sufficient for diagnosis.
Peritoneal endometriosis is one of the most common causes of chronic pelvic pain and infertility. MRI is the most sensitive imaging modality. Laparoscopic excision and hormonal therapy are applied. Rarely, malignant transformation (clear cell or endometrioid carcinoma) may develop.