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[Continued] Hydrosalpinx Approximately 30% of women with endometriosis have associated tubal abnormalities present at laparoscopy [8]. Hydrosalpinges have a tubular, often folded, configuration and can be differentiated from other adnexal masses on MR imaging by the use of multiple imaging planes. Dilated fallopian tubes with high signal intensity on T1-weighted sequences are associated with endometriosis. These fallopian tubes do not always show T2 shortening typical of endometrial cysts. In addition, debris can be present within the dependent portions of the tube (Fig. 4A,4B,4C). A complicated hydrosalpinx may be the only imaging finding indicating endometriosis (Fig. 5A,5B). Although the presence of complicated hydrosalpinges may not influence patient treatment, it does increase the specificity for pelvic endometriosis. Solid Endometriosis Deep nodular (solid) endometriosis is typically found in the rectovaginal septum and in other fibromuscular pelvic structures such as the uterine ligaments and the muscular wall of pelvic organs. The endometrial glands and stroma infiltrate the adjacent fibromuscular tissue and elicit smooth muscle proliferation and fibrous reaction, resulting in solid nodule formation. MR imaging characteristics of these solid masses have been described as low to intermediate in signal intensity with punctate regions of high signal intensity on T1-weighted images, uniform low signal intensity on T2-weighted images, and enhancement corresponding to the abundant fibrous tissue seen in these lesions at histologic examination (Fig. 6). The punctate foci of high signal intensity represent regions of hemorrhage surrounded by solid fibrotic tissue. These solid masses of endometriosis may simulate metastatic peritoneal implants from intraperitoneal malignancies such as ovarian carcinoma. These disease processes can be differentiated by the low signal intensity on T2-weighted sequences of solid endometriosis, often in combination with the presence of endometrial cysts. Solid endometriosis can also develop in cesarian section scars involving Pfannenstiel's incision after cesarian section (Fig. 7A,7B). Some masses of endometriosis are composed of a large proportion of glandular material with little fibrotic reaction that results in high signal intensity on T2-weighted images. This solid glandular material will enhance with contrast material administration, thus distinguishing it from necrosis or intratumoral hemorrhage (Fig. 8A,8B,8C,8D). Visceral Endometriosis Solid endometriosis can involve the alimentary and urinary tracts. Bladder involvement has been described, and similarly the ureter may be involved. Urinary tract disease may present as hydronephrosis caused by ureteral obstruction (Fig. 9A,9B,9C) or as a submucosal lesion within the bladder or ureter (Fig. 10A,10B,10C). The rectosigmoid is the most common segment of bowel involved. The implants adhere to the serosal surface of the bowel and may invade the muscle layers, eliciting marked smooth muscle proliferation. Stricture formation and obstruction may result. Malignant Transformation of Endometriosis Malignant transformation is a rare complication of endometriosis, the exact incidence and prevalence of which is unknown. Criteria for diagnosis include adjacent benign and malignant endometrial tissues without findings to suggest metastatic disease from another primary site. The histologic patterns reflect an endometrial origin and include endometrioid adenocarcinoma and clear cell carcinoma from glandular elements and endometrial stromal sarcoma from stromal tissues (Fig. 11A,11B). Endometriomas with solid components and intermediate or high signal intensity on T2-weighted images or papillary projections are suggestive of malignancy. Summary MR imaging has become an increasingly accepted technique in the diagnosis and characterization of endometriosis. Limitations remain regarding detection of small peritoneal implants and atypical implants, identifying adhesions, and accurate staging of the disease. However, as illustrated in this pictorial essay, the common and less typical manifestations of endometriosis have suggestive findings on MR imaging because of the underlying proteinaceous, hemorrhagic, or fibrous content of these lesions. References 1. Brosens IA. Endometriosis: a disease because it is characterized by bleeding. Am J Obstet Gynecol 1997; 176:263 -267[Medline] 2. Sugimura K, Okizuka H, Imaoka I, et al. Pelvic endometriosis: detection and diagnosis with chemical shift MR imaging. Radiology 1993;188 : 435-438[Abstract] 3. Kier R, Smith RC, McCarthy SM. Value of lipid- and water-suppression MR images in distinguishing between blood and lipid within ovarian masses. AJR 1992;158 : 321-325[Abstract] 4. Togashi K, Nishimura K, Kimura I, et al. Endometrial cysts: diagnosis with MR imaging. Radiology 1991; 108:73 -78 5. Takahashi K, Okada S, Okada M, Kitao M, Kaji Y, Sugimura K. Magnetic resonance relaxation time in evaluating the cyst fluid characteristics of endometrioma. Hum Reprod 1996; 11:857 -860[Abstract] 6. Iizuka M, Igarashi M, Abe Y, Ibuki Y, Koyasu Y, Ikuma K. Chemical assay of iron in ovarian cysts: a new diagnostic method to evaluate endometriotic cysts. Gynecol Obstet Invest 1998; 46:58 -60[Medline] 7. Ascher SM, Agrawal, R, Bis KG, et al. Endometriosis: appearance and detection with conventional and contrast-enhanced fat-suppressed spin-echo techniques. J Magn Reson Imaging 1995; 5:251 -257[Medline] 8. Ott DJ, Fayez JA. Tubal and adnexal abnormalities. In: Ott DJ, Fayez JA, eds. Hysterosalpingography: a text and atlas. 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