Division of Radiology, Department of Pathophysiological and Therapeutic Science, Faculty of Medicine, Tottori University, Yonago, Japan
MR imaging has played a significant role as a noninvasive method, although laparoscopy is the gold standard for diagnosis. We describe MR imaging findings about endometriotic cyst, endometriotic implant, adhesion, deep endometriosis, decidualized endometriotic cyst, and malignant neoplasms arising from endometriosis.
19.1 Endometriotic Cyst
The signal intensity of endometriotic cysts is hyperintense on T1-weighted imaging similar to that of fat. T2-weighted imaging finding of endometriotic cyst is hypointense (shading) in most cases (Fig. 19.1), but can be hyperintense. Besides, multiplicity of the cysts is another important finding. MR imaging for the diagnosis of endometriotic cysts has a sensitivity of 90–92 %, a specificity of 91–98 %, and an accuracy of 91–96 % [1–4]. Therefore, MR imaging is very important and useful for the diagnosis of endometriotic cysts. This characteristic signal intensity is considered to reflect aged blood and its viscosity . The signal intensity of endometriotic cysts on T2-weighted imaging has a significant relationship with the iron concentration, although there is no significant relationship with the signal intensity on T1-weighted imaging . Fat-suppressed T1-weighted imaging should always be performed for not only the differentiation from mature cystic teratoma showing hyperintensity on T1-weighted imaging but also the detection of small lesions such as endometriotic implants. Another characteristic finding is thick wall with hypointensity on both T1- and T2-weighted imaging. This finding reflects the fibrous nature of the cyst wall with hemosiderin-laden macrophages . Susceptibility weighted imaging can detect this hemosiderin deposit . Meanwhile, 3.0 T MR imaging is more useful for the diagnosis of endometriotic cysts than that of 1.5 T, because 3.0 T MR imaging can demonstrate shading sign on T2-weighted images  (Fig. 19.2).
A 39-year-old woman with bilateral ovarian endometriotic cysts. Axial T1-weighted MR image (a) shows bilateral hyperintense ovarian masses (arrows). T2-weighted image shows shading with hyperintensity in both lesions (arrows) (b). On fat-suppressed T1-weighted image, these masses remain hyperintense (c)
A 32-year-old woman with left ovarian endometriotic cyst. Axial T2-weighted images demonstrate left ovarian hypointense cystic mass (arrow). The shading sign is more prominent at 3 T (a) than at 1.5 T (b)
19.2 Endometriotic Implants
Fat-suppressed T1-weighted imaging is mandatory sequence for the detection of endometriotic implants [2, 3, 9–11]. The sensitivity in detecting peritoneal implants is significantly higher with fat-suppressed T1-weighted imaging (61 %) than with conventional T1-weighted imaging (27 %) due to better visualization of hemorrhagic implants with signal suppression of surrounding fat tissue . Additionally, fat-suppressed T1-weighted imaging can detect endometriotic implants that are not detected during surgery because of overlying adhesions [12, 13]. However, the sensitivity is not so high because even fat-suppressed T1-weighted images cannot detect tiny peritoneal lesions and the lesions which do not contain enough methemoglobin ; some small endometriotic implants can show hypointensity on T1-weighted images and hyperintensity on T2-weighted images .
19.3 Adhesion, Posterior Cul-de-sac Obliteration
Adhesion is diagnosed when there is obliteration of fat planes with a lack of a clear interface between adjacent organs, spiculated hypointensity stranding between the organs, and angulation and distortion of adjacent bowel loops . These findings are usually subtle, and we should detect the findings carefully.
The following findings are related to posterior cul-de-sac obliteration: retroflexed uterus, elevated posterior vaginal fornix, intestinal tethering and/or a tethered appearance of the rectum in the direction of the uterus, faint strands between the uterus and intestine, and fibrotic plaque and/or nodule covering the serosal surface of the uterus  (Fig. 19.3). Particularly, the following findings are proposed as the major criteria for diagnosing cul-de-sac obliteration because of their good specificity: intestinal tethering and/or a tethered appearance of the rectum in the direction of the uterus, strands between the uterus and intestine, and a fibrotic plaque covering the serosal surface of the uterus. Additionally, the most accurate combination of two findings is intestinal tethering in the direction of the uterus and fibrotic plaque in the uterine serosal surface, and the next highest combination is retroflexed uterus and intestinal tethering in the direction of the uterus .
A 34-year-old woman with bilateral ovarian endometriotic cysts and posterior cul-de-sac obliteration. The uterus is retroflexed on sagittal T2-weighted image (a). An irregular hypointense area at posterior serosal surface of uterus is found (curved arrows). The posterior vaginal fornix (arrow) is elevated toward this area. Axial T2-weighted image (b) shows faint strands between uterus and rectum (arrowhead), tethered appearance of the rectum in the direction of the uterus (bold arrow)
19.4 Deep Pelvic Endometriosis
Deep infiltrating endometriosis is defined as the presence of endometriotic implants, fibrosis, and muscular hyperplasia penetrating >5 mm into the peritoneum. Associated symptoms include dysmenorrhea, dyspareunia, noncyclical pelvic pain, dysuria, and lower gastrointestinal symptoms. The lesion can involve the posterior cul-de-sac, uterosacral ligaments, rectovaginal septum, ureters, bowel, and bladder. Endometriotic implants usually elicit an intense desmoplastic response in the surrounding tissues, leading to the formation of adhesions, fibrotic bands, and plaques. The histologic finding of deep pelvic endometriosis is mainly characterized by fibromuscular hyperplasia surrounding foci of endometriosis. Previous report has demonstrated the high accuracy in the prediction of deep pelvic endometriosis with a sensitivity of 90.3 %, specificity of 91 %, and accuracy of 90.8 % . Additionally, 3.0 T MR imaging has high accuracy in the diagnosis and staging of deep endometriosis .
Deep endometriosis shows typically hypointensity with punctate hyperintensity on T1-weighted imaging, hypointensity on T2-weighted imaging, and contrast enhancement, which findings correspond to fibrous tissue. Punctate foci of hyperintensity reflect hemorrhage surrounded by solid fibrous tissue. Additionally, tiny hyperintensities within the lesion, which represent endometrial glands, can be shown on T2-weighted imaging.
Irregular fibrotic thickening and nodularity with regular or stellate margins along the course of the uterosacral ligament suggests deep pelvic endometriosis (Fig. 19.4) . MR imaging for the diagnosis of uterosacral ligament endometriosis has a sensitivity of 69.2–90 % and a specificity of 76–94.3 % . Thin-section oblique axial T2-weighted imaging can improve the depiction of uterosacral ligament endometriosis . However, 3D T2-weighted imaging in combination with a multi-planar reconstruction technique has no significant different accuracy from that of conventional 2D axial T2-weighted imaging .
A 29-year-old woman with deep pelvic endometriosis of bilateral uterosacral ligaments. Axial T2-weighted image (a) demonstrates hypointense fibrotic plaque (arrow) with punctate hyperintense foci on fat-suppressed T1-weighted image (arrow) (b) and irregular thickening of the right uterosacral ligament (arrowheads)
Endometriosis of rectovaginal septum is mainly classified to massive lesion of the deepest portion of the pouch of Douglas and retroperitoneal deep endometriotic lesion originating from the metaplasia of müllerian remnants located in the rectovaginal septum. Laparoscopic diagnosis is difficult because of complete obliteration of the cul-de-sac. The lesion demonstrates ill-defined hypointense mass between posterior vaginal fornix and rectum and can cause rectal stenosis and hydronephrosis.
The rectum and distal sigmoid colon account for most of the bowel endometriosis. The endometriotic implants are usually serosal, but can eventually erode through the subserosal layers and cause marked thickening and fibrosis of the muscularis propria. Inflammatory response to cyclic hemorrhage can lead to adhesions, bowel stricture, and gastrointestinal obstruction. Bowel endometriosis shows mushroom or fan-shaped configuration, demonstrating isointensity compared to muscle and slightly hyperintense at the luminal side of the bowel wall on T2-weighted imaging [19, 20] (Fig. 19.5). The lesions sometimes demonstrate hemorrhagic foci that are hyperintense on fat-suppressed T1-weighted imaging. MR imaging has a sensitivity of 100 % and specificity of 75 % for predicting muscular infiltration . Diffusion-weighted imaging depicts the lesion as hypointensity and is useful for the differentiation from colorectal cancer . Additionally, contrast-enhanced imaging can allow easier recognition of colorectal endometriosis .
A 30-year-old woman with deep pelvic endometriosis infiltrating the rectosigmoid. Rectosigmoid involvement shows hypointense fan-shaped configuration on sagittal T2-weighted image (a) and contrast enhancement on enhanced T1-weighted image (arrow). An irregular hypointensity area at posterior serosal surface of uterus is also found (arrowhead)
Involvement of bladder is seen in less than 1 % of patients with endometriosis. Bladder endometriosis is mostly found on the posterior wall or in the dome of the bladder. Bladder endometriosis can be demonstrated as localized or diffuse bladder wall thickening showing hypointensity on T1- and T2-weighted imaging. Hyperintense foci on fat-suppressed T1-weighted and T2-weighted imaging indicative of hemorrhage and endometrial glands may be seen within the lesion [23, 24].
Parametrial involvement can also occur, representing a severe form of endometriosis, and is important for surgical planning for deep infiltrating endometriosis because a parametrectomy can be required for the complete removal of deep infiltrating endometriosis. Recent report demonstrates the MR findings indicating parametrial involvement as follows: the presence of a hypointense area in the paracervical or paravaginal region on T2-weighted imaging or pelvic wall involvement and ureteral dilatation .
19.5 Prediction of Hormone Therapy Response
MR imaging can predict the hormone therapy response of endometriotic cyst. Response of the therapy is not good for endometriotic cysts with shading on T2-weighted imaging, while endometriotic cysts without shading and multiplicity indicate a good response to hormone therapy  (Fig. 19.6). The signal intensity of responding endometriotic cysts decreases after the therapy on T2-weighted imaging.
A 34-year-old woman with bilateral ovarian endometriotic cysts. Bilateral endometriotic cysts (arrows) show hyperintensity with subtle shading on axial T2-weighted images obtained before hormone therapy (a). The size and signal intensity of the cysts (arrows) decreased after hormone therapy (b)
19.6 Decidualized Endometrioma
Decidual changes of the ectopic endometrial stroma during pregnancy are well known. Decidual changes of endometrial tissue in endometriomas during pregnancy may manifest as mural nodules and mimic malignant transformation. The mural nodules indicating decidualization are demonstrated as linear, small nodular, broad-based nodular, or polypoid structures, which show isointensity with the nomotopic decidualized endometrium on T1- and T2-weighted images, balanced fast field echo images, and diffusion-weighted images [27–29]. Additionally, the apparent diffusion coefficient of decidualized mural nodules is significantly higher than that of ovarian cancers . These MR imaging characteristics can help to differentiate decidualized endometriotic cyst from malignant transformation.
19.7 Malignant Tumor Arising in Endometriotic Cyst
Malignant transformation is a rare complication of endometriosis and is estimated to occur in 0.6–0.8 % of women with ovarian endometriosis. Approximately 75 % of malignancies arising in endometriosis are found in ovarian endometriosis, and 25 % are found in extraovarian endometriosis . Malignancies arising in ovarian endometriosis are composed of endometrioid carcinomas in about 70 %, clear cell carcinoma in 13.5 %, and sarcoma in 11.6 %, while malignancies arising in extraovarian endometriosis are composed of endometrioid carcinomas in 65 %, sarcoma in 25 %, and clear cell carcinoma in 4.5 % .
The following findings suggesting endometriotic cysts with malignant transformation are endometrioma with increasing size, larger size than those of contralateral endometriotic cysts, loss of shading on T2-weighted images, and enhancing mural nodules on contrast-enhanced images  (Fig. 19.7). The enhancement of mural nodules is sometimes difficult to evaluate on conventional contrast-enhanced T1-weighted images. Dynamic subtraction imaging can improve the detection of enhancing mural nodule against intrinsically hyperintense background signal of endometriotic cyst  and is useful for the differentiation of the tumors from intracystic blood clots. Meanwhile diffusion-weighted images can provide a little information about its differentiation because some clots can show hyperintensity . Additionally, we should recognize that the small enhancing nodules have the possibility of benign conditions such as granulomatous tissue and benign endometriotic tissue .
A 45-year-old woman with endometrioid adenocarcinoma associated with endometriotic cyst. Left endometriotic cyst shows hyperintensity on axial T1-weighted image (a) and absence of shading on T2-weighted image (b). The solid component (arrows) demonstrates contrast enhancement on contrast-enhanced T1-weighted image (c) and hyperintensity on diffusion-weighted image (d)
Müllerian mucinous borderline tumor (MMBT) should be recognized as malignant tumor arising from endometriosis because of coexistence with ovarian or pelvic endometriosis in about 30 %. MMBTs are unilocular or paucilocular in 80 % of cases, bilateral in 7.7–40 %, and show prominent hyperintensity on T2-weighted images reflecting intraluminal mucinous material and stromal edema .
Malignant tumors arising in extraovarian endometriosis occur in various anatomic locations, such as rectovaginal sites, colorectal sites, and bladder, which seem to reflect the distribution of extraovarian endometriotic implants. Malignant tumors arising from extraovarian endometriosis typically manifest as solid lesions showing intermediate signal intensity on T1- and T2-weighted images, enhanced by contrast material, and hyperintensity on diffusion-weighted images .
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