et al. 2004b; Di Paola et al. 2015; Roy et al. 2009; Del Frate et al. 2006; Chassang et al. 2010; Bazot et al. 2013; Kruger et al. 2013; Scardapane et al. 2014) should be acquired. An additional oblique axial T2w can be used to check for uterosacral and parametrial implants (Bazot et al. 2011a; Bazot et al. 2012). The protocol can be optionally supplemented by a 3D T2w sequence, which has high potential for detection of deep endometriotic lesions (Manganaro et al. 2012; Bazot et al. 2013).

T1-weighted (T1w) spin echo (SE) sequences in axial and sagittal planes with and without fat suppression (FS) are the sequences of choice/ gold standard for the detection of ovarian endometriotic cysts, comparatively rated with T2w sequences for definitive diagnosis (Togashi et al. 1991). FS with a Dixon sequence enables simultaneous acquisition of four different T1w contrasts and stronger fat suppression, which is advantageous primarily when imaging is performed at 3 tesla (Cornfeld and Weinreb 2008; Cornfeld et al. 2008). Preliminary results suggest that T1w sequences with FS have advantages in the detection of peritoneal endometriosis (Ha et al. 1994; Tanaka et al. 1996).

Deep infiltrating endometriosis continues to be a challenge for morphologic imaging, which is why there is a long controversy about which additional pulse sequences are most beneficial. Currently available data suggest that contrast-enhanced T1w sequences, diffusion-weighted imaging (DWI), and susceptibility-weighted sequences do not improve detection of deep infiltrating endometriosis (Busard et al. 2010; Bazot et al. 2011b). Single-shot fast spin echo (SSFSE) or half-Fourier acquisition single-shot turbo spin echo (HASTE)

imaging can be used to evaluate uterine function by assessing uterine peristalsis and possible adhesions to other organs (Hodler et al. 2014).

The radiologist interpreting the MRI dataset should use a checklist of all potential localizations of endometriosis for structured interpretation of images. These are:

1.Ovaries

2.Vesicouterine pouch and urinary bladder

3.The vaginal wall and in particular the posterior fornix of the upper vaginal wall

4.The uterine ligaments including the uterosacral ligaments and the round ligaments, the lateral and anterior pelvic wall, the parametrium, and the peritoneum

5.Bowel, specifically the anterior rectum and the sigmoid, the cecum, the ileum, and the appendix

6.Rare localizations, special types, and associated complications

3\ MR Imaging Findings

3.1\ Endometriosis of the Ovaries:

Endometriotic Cysts or

Endometriomas

Endometrial cysts or endometriomas in this location are hyperintense or isointense to subcutaneous fat on plain T1w images without a decrease in signal on T1w with FS; hyperintense foci on T1w images with FS indicate hemorrhagic deposits; the T2 appearance is characterized by a shading effect (Togashi et al. 1991) (Figs. 1 and 2). The wall appears thickened, and bilateral or multifocal lesions are common. It is important to identify any changes suspicious for malignancy

Fig. 1  (a–d) A 49-year-old woman with chronic left pelvic pain and an office ultrasound suspicious for bilateral ovarian cancer. (a) Oblique view of the right ovary at color Doppler transvaginal ultrasound demonstrates a heterogeneous right ovarian mass with small hyperechoic foci in the wall of the cyst (arrowhead). The intraor extracystic location of peripheral color Doppler flow (arrow) is difficult to diagnose. Pelvic MRI is performed to exclude ovarian cancer. (b) Axial T2-weighted FSE image of the pelvis shows two right-sided ovarian cysts of intermediate signal intensity and one left-sided ovarian cyst with shading of signal intensity (arrow). (c) Axial

T1-weighted image shows bilateral T1 hyperintense content of all three cysts and a speculated nodule between the two ovaries (arrow) with one hypointense line extending from the nodule toward the anterior rectum. (d) Axial T1-weighted fat-suppressed T1-weighted image at the same level as c confirms the hemorrhagic nature of all ovarian cysts and the interovarian peritoneal implant, suggesting endometriosis. At surgery, bilateral endometriomas were attached to each other (kissing ovaries) and associated with severe adhesions toward the rectosigmoid. Pathology confirmed bilateral endometriomas without malignancy

Fig.2  (a–d) A 27-year-old woman undergoes MRI of the pelvis for characterization of a complex left ovarian mass with elevated serum CA 125 level. (a) Coronal T2-weighted image shows a hypointense mass in the left ovary (white star) and additional tissue between both ovaries and below the uterine isthmus (black arrow). (b) Coronal T1-weighted image at the same shows a hyperintense content of the left ovarian mass (black star) that remains hyperintense in the fat-suppressed T1-weighted image in (c). (c) In addition to the hemorrhagic cyst of the left ovary, the hyperintense spots (black arrows) within the fibrous structure below the uterus and at the periphery

of the left ovary suggest peritoneal implants of endometriosis and possible deep endometriosis of the retrocervical region. (d) Axial T2-weighted image through the cervix shows the typical T2 hypointense shading of the left endometrioma (white star) and abnormal thickening of the right uterosacral ligament (black arrow). The left uterosacral ligament (white arrows) is displaced medially toward the right uterosacral ligament with a fibrous structure at the level of the torus uterinus. Subsequent surgery and pathology confirmed a left endometrioma and endometriosis of the torus uterinus and right uterosacral ligament

such as solid nodules of intermediate T2 signal intensity within the cyst, peritoneal metastases, or thickened intracystic septa (> 3 mm). If con- trast-enhanced T1w images have been obtained, suspicious changes may be suggested by contrast enhancement within the cyst (Wu et al. 2004). According to the new ESUR guidelines for complex adnexal masses, contrast enhancement is mandatory (Bazot et al. 2016; Forstner et al. 2016).

most often located in the bladder wall at the level of the vesicouterine pouch forming an obtuse angle with the bladder. Hemorrhage within a nodule has high T1 signal intensity (Bazot et al. 2004b) (Fig. 3). In the event of bladder endometriosis, the distance to the ureterovesical junction and potential hydronephrosis can be assessed with both T2-weighted sequences and URO-MRI sequences.

As described above, it is helpful to prepare the patient to ensure moderate urinary bladder filling for optimal detectability of endometriotic tissue in the bladder wall. Manifestations appear as nodules of low T1 and T2 signal intensity and are

Endometriosis of the vagina predominantly involves the upper posterior third of the vaginal wall. The extent of disease and locations in the posterior vaginal pouch profit from contrast filling of the vagina with sonographic gel (Dessole

Fig. 3  (a–e) A 36-year-old woman with a painful retrocervical nodule and a history of surgery for endometriosis 1 year ago. (a) Sagittal T2-weighted image demonstrates a retrocervical mass extending into the rectosigmoid and the posterior vaginal fornix (white ellipse). The posterior bladder wall has a hyperintense nodule (black arrow). (b) Axial T2-weighted image through the center of the retrocervical nodule shows extension into the initial portion of the right uterosacral ligament (white arrow) and the anterior rectal wall (white star). (c) Coronal T2-weighted image through the rectal wall demonstrates an extension

of the rectal nodule into the pararectal fat (white arrows). (d) Sagittal T1-weighted fat-suppressed image after intravenous injection of paramagnetic contrast confirms vascularity of the vaginal and rectal extension of the retrocervical nodule (black star) and shows small subserosal leiomyomas (white arrows). (e) Axial T1-weighted fat-suppressed image through the nodule of the bladder wall demonstrates a hemorrhagic portion (white arrow). Surgery and pathology confirms bladder, vaginal, retrocervical, and rectosigmoid deep endometriosis

c

e

Fig. 3  (continued)

et al. 2003). Vaginal endometriosis is hypointense on T2w images and isointense on T1w images (Bazot et al. 2009), and the lesions may contain hyperintense spots in both sequences. Endometriosis of the posterior vagina is often associated with a thickened posterior cervical wall (Bazot and Darai 2005). Most vaginal lesions are associated with an obliteration of the pouch of Douglas extending in the upper retrocervical region, the lower or anterior rectosigmoid, or both (Figs. 3 and 4).

d

Endometriotic tissue in the rectovaginal septum can be associated with endometriotic lesions of the vagina, the rectosigmoid or uterosacral ligaments most notably in association with deep infiltrating endometriosis (Bazot and Darai 2005).

In up to 61% of cases, endometriotic lesions of the rectouterine pouch have high-signal-inten- sity areas on T1w images, which correspond to cystic hemorrhagic components in pathologic correlation (Bazot et al. 2004b; Kinkel et al.

Fig. 4  (a–d) A 30-year-old woman with persistent dysmenorrhea 3 years after laparoscopic removal of a left endometrioma. (a) Sagittal T2-weighted image shows additional tissue at the posterior wall of the uterus (white star) extending into the inferior wall of the sigmoid. (b) Sagittal T1-weighted image (same level as a) indicates a small hemorrhagic portion (white arrows) in the submucosal of the abnormal bowel wall and within the adhesion between the sigmoid and the retroisthmic part of the uterus. (c) Coronal T1-weighted fat-suppressed image through the midportion of the uterus demonstrates a tubu-

lar hemorrhagic right adnexal structure compatible with hematosalpinx (white arrow). Other hyperintense spots in contact with the uterus suggest peritoneal implants. (d) The T2-weighted image at the same level as c shows multiple incomplete septa (white arrow) and confirms the tubular origin of the hemorrhagic right mass. A normal follicule can be seen in the left ovary (black star). Laparoscopic resection of the thickened sigmoid wall and the right tube confirmed deep endometriosis of the sigmoid and endometriosis of the right tube

1999). However, the absence of T1 hyperintense areas does not exclude the diagnosis.

3.4\ Endometriosis of the Uterine

Ligaments

Including the Uterosacral

Ligaments and the Round

Ligaments, the Lateral

and Anterior Pelvic Wall,

and the Parametrium

and the Peritoneum

Endometriotic infiltration of the uterine ligaments such as the uterosacral ligament or the round ligament is seen as unilateral or bilateral nodular lesions (regular or with stellate margins) and/or fibrotic thickening of the affected ligaments (Bazot et al. 2004b; Novellas et al. 2010) (Figs. 4, 5, 6, and 7). Endometriotic lesions of the upper posterior cervix are seen as band-like structures of low T2 and T1 signal intensity extending laterally to one or both uterosacral ligaments (Bazot et al. 2004b) (Figs. 5, 6, and 7). Nodular thickening with regular or stellate margins at the initial uterine portion of the uterosacral ligament is easier to identify on T2-weighted images obtained in an oblique orientation per-

pendicular to the long axis of the cervical channel (Bazot and Darai 2005; Kinkel et al. 2006).

Parametrial and pelvic wall endometriosis (with muscle infiltration) is characterized by low T2 signal intensity, and the lesions may contain hyperintense spots (Bazot et al. 2012). Pelvic wall endometriosis may affect the pelvic muscles such as the piriformis, coccygeus, or obturator muscles leading to extension of the endometriosis to the sciatic or pudendal nerve.

3.5\ Endometriosis of the Bowel,

Specifically the Anterior

Rectum and the Sigmoid,

the Cecum, the Ileum,

and the Appendix

Deep infiltrating endometriotic lesions are defined by their morphologic appearance and signal intensity and may be found anywhere in the body. Most endometriotic lesions of the bowel are of a deep infiltrating nature and detected by abnormal wall thickening (Figs. 4 and 6). They are isointense to muscle on T2w and T1w images (Busard et al. 2012). Hyperintense foci on T1w images (+/− FS) correspond to hemorrhagic spots (Chamie et al. 2011b). Cavities have high

Fig.5  (a–e) A 40-year-old woman with unexplained dysmenorrhea, perimenstrual hematuria, and infertility. (a) Sagittal T2-weighted image shows bladder wall thickening (white star), small cystic spaces within the myometrium, and nabothian cysts within the cervical stroma. (b) Coronal oblique T2-weighted image confirms a cystic and solid mass of the bladder wall (black arrow) typical of bladder endometriosis. (c) Axial oblique T2-weighted image through the upper portion of the bladder mass (white star) shows an irregular contour of the anterior uterus with a fibrous nodule (white arrow) in the pouch between the bladder and the uterus as well as cystic spaces in the myometrium suggesting adenomyosis. The enlarged right uterosacral ligament (black arrow) and anterior rec-

tal wall close to the posterior uterus (white circle) suggest deep endometriosis of the rectum and the right uterosacral ligament. (d) Contrast-enhanced T1-weighted sagittal image of the mid-pelvis shows the abnormally thickened bladder wall (white star) and a small retrocervical nodule (black arrow). There are indistinct margins between the anterior sigmoid and the posterior myometrium (white arrow). (e) The axial fat-suppressed T1-weighted image through the cervix shows hemorrhagic spots (white arrows) within the right uterosacral ligament, the torus uterinus, and the bladder wall. Surgery performed partial cystectomy and ablation of the right uterosacral ligament. Associated bowel endometriosis and adenomyosis were treated medically

Fig. 7  (a–c) A 36-year-old woman with painful defecation during menstruation and a sonographic suspicion of retrocervical endometriosis. (a) Sagittal T2-weighted image with gel filling of the vagina shows a “butterfly”- shaped mass behind the cervix with the posterior wing in the anterior rectosigmoid junction (short white arrow) and the anterior wing at the torus uterinus and the posterior vaginal cuff (long white arrow). Ectasia of the cesarean scar (black arrow) displays T2 hyperintensity due to colonization by normal endometrium. (b) Sagittal T1-weighted fat-suppressed image at the same level confirms the exten-

sion of deep endometriosis into the upper posterior vagina (white arrow). The hypointense portion of the cesarean section is due to artifacts. (c) Axial T2-weighted image through the cervix shows the larger part of the hypointense mass behind the cervix at the torus uterinus (white star) and the second smaller portion in the anterior rectal wall (white arrow). Extensive surgery with transvaginal and laparoscopic approach allowed complete resection of all portions of deep endometriosis, confirmed by pathology