Because of the multifocal nature of the disease, often foci of endometriosis are simultaneously observed at different sites. Owing to the possibility to perform a complete assessment of all pelvic compartments at one time , MRI represents the best imaging technique for preoperative staging of endometriosis.
Endometriotic lesions may affect the urinary tract in up to 20% of cases; these implants are associated with lesions in other pelvic locations in up to 50–75% of cases [2, 14]. The bladder is the most frequently involved organ (85%), followed by ureter (9%), kidney (4%), and urethra (2%) . Different locations of urinary tract endometriosis may coexist, especially the bladder and ureters. Bladder endometriosis is often multifocal, the trigone and the dome being the most frequently affected sites . The distal ureter, 3–4 cm above the vesico-ureteral junction, is the most common ureteral segment involved [14, 30].
According to the degree of wall infiltration, bladder and ureteral involvement may be classified as extrinsic or intrinsic. In extrinsic involvement, the most common form, implants are confined to the serosal surface (or the ureteral adventitia) or the surrounding connective tissue; it can be treated by bladder or ureteral shaving. In intrinsic involvement lesions infiltrate the muscular layer manifesting as mural masses; rarely implants infiltrate and ulcerate the mucosal layer. Preoperative differentiation between intrinsic and extrinsic ureteral endometriosis is not always easy; furthermore, the two types may coexist.
Regards to its origin, bladder endometriosis may be classified as primary when it occurs spontaneously, or as secondary when it is related to iatrogenic lesions due to pelvic surgery (caesarean delivery, hysterectomy) .
At MR imaging bladder endometriosis may manifest as localized or diffuse wall thickening and signal intensity abnormalities . The appearance is of low signal intensity on T2-weighted and intermediate signal intensity on T1-weighted images, with or without spots of high signal intensity on T1-weighted images, representing hemorrhagic content  (Figs. 8 and 9). Implants minimally enhance after injection of a gadolinium-based contrast material. The maximum lesion diameter varies between 1 and 5 cm. MRI reaches sensitivity up to 88%, specificity up to 99% and diagnostic accuracy of about 98% for the diagnosis of bladder endometriosis .
Clinical management of bladder endometriosis may be conservative, using hormonal therapies, or surgical. For planning a correct surgical treatment, it is important: to ascertain the precise location of bladder nodule (distance between the ureteral meatus and the caudal border of the endometriotic lesion) and to define the ureteral status .
The differential diagnosis of bladder endometriosis includes urachal remnant, epithelial tumours (bladder carcinoma) and mesenchymal tumours (angiomas, leiomyoma) [3, 15]. Malignant transformation of bladder endometriosis is extremely rare .
Ureteral endometriosis may be defined as any situation where endometriosis or surrounding associated fibrosis causes compression or distortion of the normal ureteral anatomy, even when hydroureteronephrosis is not yet present [33, 34]. All patients with urinary tract endometriosis should be assessed for renal function by blood creatinine measurements, since when endometriosis involves the ureters a silent loss of renal function may occur [15, 29]. Ureteral endometriosis is most often unilateral, with a left predisposition; bilateral involvement is present in approximately 10–20% of cases . Ureteral involvement is often associated with an ipsilateral endometrioma or with a recto-vaginal nodule larger than 3 cm [29, 30]. Even if ureteral endometriosis is now increasingly recognized because of the improvement in diagnostic tools , preoperative diagnosis is challenging mostly when it is not complicated by obstruction and proximal dilatation .
MR is the best imaging technique for ureteral evaluation. On T2-weighted MR images ureteral implants appear as solid nodules with spiculated margins, showing low signal intensity, that envelop the ureter, causing dilatation of the ureter upstream  (Fig. 10).
In a retrospective series including 77 patients affected by ureteral endometriosis Seracchioli et al.  described two different histological pattern of ureteral involvement: endometriotic ureteral endometriosis characterized by endometrial glands and/or stroma cells within the wall of the ureter or within periureteral tissue, and fibrotic ureteral endometriosis where only fibrosis tissue was observed. They found that the endometriotic pattern (77% of patients) was significantly associated with hydroureteronephrosis at pre-operative uro-CT scan, whereas the fibrotic pattern (23% of patients) was significantly associated with concomitant endometriosis in the recto-vaginal septum. According to author’s point of view this new classification of ureteral endometriosis based on tissue composition and histological pattern could be useful in both diagnostic and therapeutic fields. Indeed, fibrotic tissue typically does not respond to hormonal therapy .
When deep pelvic endometriosis involves the uterosacral ligaments and rectovaginal septum, ureteral involvement may occur even if not suggested preoperatively. Hence these patients should undergo retroperitoneal laparoscopic inspection and isolation of both ureters in order to avoid complications .
The differential diagnosis of ureteral endometriosis includes ureteral invasion by cervical cancer.
The ovaries are the most common site of endometriosis (20–40% of cases) .
Ovarian endometriosis may show the following patterns :
superficial implants associated with fibrous adhesions;
micro intra-ovarian endometriomas;
deep implants with repeated cyclic haemorrhage resulting in endometriotic cysts (endometriomas).
Peritoneal implants confined to the ovarian surface are often underdiagnosed at imaging due to their small size (< 5 mm) .
Micro intra-ovarian endometriomas are small size (< 1 cm) implants within the ovaries. They show hyperintense signal on T1-weighted images and variable signal intensity on T2-weighted images. Often multiple and bilateral, these endometriotic foci may be overlooked at laparoscopy .
Endometriomas are pseudocysts with hemorrhagic content, formed by the invagination of endometriosis within the ovarian cortex ; they are frequently multilocular and bilateral (50% of cases).
Even though in the differential diagnosis of ovarian endometriosis, transvaginal US is highly sensitive and specific, MR imaging is considered by some authors as the best imaging modality for diagnosing endometriomas . At MR imaging the pathognomonic feature of endometriomas is the “shading” sign, which can be seen on T2-weighted images. It reflects the chronic nature of endometriomas and is the result of cyclic bleeding occurring over time. Old blood products contain high iron and protein concentrations which determine a decrease in T2-relaxation time. Therefore, on T2-weighted images endometriomas will show a gradual loss of signal within the lesion with low signal intensity till complete signal void in the declivous portion (“shading”). Endometriotic cysts show high signal intensity on T1-weighted images. Fluid-fluid levels may also be observed within the lesion [3, 14, 38, 39] (Fig. 11).
The most specific pathologic feature of endometrioma is the thick fibrous capsule containing a cluster of hemosiderin-laden macrophages due to repeated haemorrhage. Takeuchi et al.  evaluated hemosiderin deposition within the walls of endometriomas on susceptibility-weighted MR images at both 3.0 T and 1.5 T. They found punctate or curved linear signal voids due to hemosiderin deposition along the cyst wall in 92.9% of the endometriomas of their series, thus concluding that this imaging sign may be diagnostic of endometrioma and that susceptibility-weighted imaging can contribute to its diagnosis.
In certain cases, the ovaries may be joined together behind the uterus in the pouch of Douglas due to adhesion formation between the adjacent peritoneal surfaces, a sign described at US as “kissing ovaries” and suggestive of severe pelvic endometriosis .
In a recently published meta-analysis the sensitivity and specificity of MRI for the diagnosis of endometrial cysts were 95% and 91% respectively .
The differential diagnosis of endometriomas includes lesions with high signal intensity on T1-weighted images: dermoids, mucinous cystic neoplasms, and hemorrhagic masses.
Fat saturated T1-weighted sequences are helpful to rule out a fat-containing lesion (such as dermoids) and to confirm the presence of blood .
Mucinous lesions may show hyperintensity on T1-weighted images, but signal intensity is lower than that of blood.
Hence, the most challenging differential diagnosis is with other hemorrhagic masses. To differentiate endometriomas from functional hemorrhagic cysts is important in order to prevent unnecessary surgical interventions. Functional hemorrhagic cysts (i.e. hemorrhagic follicular cysts and hemorrhagic corpus luteum cysts) are usually unilocular and unilateral, do not display shading on T2-weighted images, and mostly disappear on follow-up examinations (generally in 4–6 weeks) .
The role of DWI sequences in differentiating endometriomas from functional hemorrhagic ovarian cysts is still debated. Balaban et al.  found significantly lower ADC values in endometriomas compared with functional hemorrhagic ovarian cysts in all b values. On the other hand, in a retrospective study by Lee et al.  the mean ADC values of endometriomas (1.06 ± 0.38 × 10−3 mm2/s) was significantly higher than that of functional hemorrhagic cysts (0.73 ± 0.29 × 10−3 mm2/s).
Endometrioid and clear cell tumours may be associated with endometriosis. Large lesions with wall nodularity, thick septations and enhancing solid components may be suggestive of malignancy.
Surgical excision of an ovarian endometrioma is effective in managing pain. Nevertheless, operative treatment of endometriomas is controversial in women desiring future fertility since it may lead to reduced ovarian reserve in the short term and premature ovarian insufficiency; bilateral and recurrent endometrioma excision may further reduce ovarian reserve .
Uterine serosa, round ligaments, broad ligaments, fallopian tube
The vesicouterine pouch or anterior cul-de-sac is a common site of endometriotic involvement . Either peritoneal or deep endometriotic implants involving the serosal surface of the uterus often determine adhesions between the peritoneal folds of the bladder dome and the uterus with anteflexion of the uterus and obliteration of the anterior cul-de-sac.
At MR imaging deep endometriotic implants involving the anterior uterine serosa demonstrate infiltrative pattern with indistinct margins and show low signal intensity on T2-weighted images, with small cystic areas (Fig. 12) .
At MRI the round ligaments can be identified as thin structures with hypointense fibrous signal on T1- and T2-weighted images, extending from the uterine horns to the pelvic sidewall, passing anteriorly to the external iliac vessels. They have an intra- and an extra-pelvic portion, the latter being the distal part of the ligament in the canal of Nuck .
When involved by endometriosis, round ligaments appear thickened (more than 1 cm), nodular, shortened and irregular (Fig. 13). Usually endometriotic implants are a mixture of fibrous tissue and haemorrhage. Fibrous tissue shows hypointense signal on T1- and T2-weighted images; small hemorrhagic foci displays hyperintense signal on fat-suppressed T1-weighted images . The presence of free fluid around the intra-pelvic portion of the round ligaments may represent an indirect sign of endometriosis .
Endometriosis of the broad ligaments usually manifests as thickening and nodularity of these peritoneal folds extending between the uterus and the lateral walls of the pelvis .
Asymmetry of the morphology or signal intensity at MR imaging of the parametrium may represent a sign of endometriotic involvement .
Endometriosis is the most common cause of hematosalpinx and peritubal adhesions in women of reproductive age. Endometriotic involvement of the fallopian tubes is strongly associated with infertility. Serosal or subserosal implants involves the peritoneal surface of the fallopian tubes, where repeated haemorrhages lead to fibrosis and retraction of the tube with hydrosalpinx. Intraluminal implants determine cyclic haemorrhage thus causing hematosalpinx.
At MR imaging hematosalpinx appears as a tortuous enlarged tubular adnexal structure filled with hemorrhagic fluid. Endoluminal content shows high signal intensity on fat-suppressed T1-weighted images and intermediate signal intensity, with or without internal fluid-fluid level, on T2-weighted images  (Fig. 14). According to Siegelman the presence of T1-weighted hyperintensity within a dilated fallopian tube is suggestive of endometriosis .
Nevertheless, more rarely hematosalpinx can be associated with other pathologic conditions such as tubal torsion, tubal ectopic pregnancy or malignancy.
Retrocervical region, uterosacral ligaments
The retrocervical area is a virtual extraperitoneal space behind the cervix, located above the rectovaginal septum . It is a common site of deep pelvic endometriosis.
Retrocervical implants are often associated with USL involvement and may extend inferiorly to the posterior vaginal fornix or posteriorly to the anterior rectal wall (Fig. 15). Usually such lesions determine obliteration of the pouch of Douglas and uterine retroflexion. In more severe cases, endometriosis of this anatomical region may determine multiple adhesions and distortion of pelvic anatomy resulting in a frozen pelvis .
Deep endometriotic lesions of the retrocervical area frequently appear as ill-defined infiltrative tissue, hypointense on T2-weighted images, extending from the posterior uterine serosa to the retrocervical region [2, 3]. Nevertheless, some lesions may contain abundant glandular component and little fibrotic reaction, thus showing high signal intensity on T1-weighted images and variable signal intensity on T2-weighted images [2, 12]. Small cystic areas, hyperintense on T2-weighted images may also be seen (Fig. 16) . The solid glandular component enhances after intravenous administration of contrast material [2, 12].
In a recently published meta-analysis the sensitivity and specificity of MRI for the diagnosis of endometriosis of the pouch of Douglas were 89% and 94%, respectively .
The differential diagnosis of retrocervical lesions includes peritoneal metastases from intraperitoneal malignancies (i.e. gastrointestinal and ovarian neoplasms). Peritoneal metastases usually show intermediate to high signal intensity on T2-weighted images and, as the primary cancer site, high signal intensity on DWI ; moreover, ascites and a tumour mass into the abdominal cavity may be identified. On the other hand, solid endometriosis shows low signal intensity on T2-weighted images.
USL are the most frequent location of deep endometriosis. Bilateral USL involvement is often associated with other posterior deep endometriotic locations, mostly the rectosigmoid colon .
At MR imaging normal USL are not visible  or are depicted as thin, regular, semicircular hypointense cords that originate from the lateral aspect of the uterine cervix and the vaginal vault and course dorsocranially toward the sacrum . USL endometriosis is depicted as nodularity within the ligament or as unilateral or bilateral hypointense thickening of the ligament, with regular or irregular margins [2, 48]. Hyperintense spot on fat-suppressed T1-weighted images, representing punctate foci of haemorrhage, may also be observed  (Figs. 17 and 18). The proximal medial portion of the USL is most commonly affected by endometriosis . According to Bazot et al.  thin-section oblique axial T2-weighted sequences (3 mm thick, perpendicular to the long axis of the cervix) can improve the capability of conventional MRI to assess USL endometriosis. Left uterosacral endometriosis may be more difficult to diagnose than the right one, because of the frequent location of the rectosigmoid colon in the left part of the pelvic cavity .
In a recently published meta-analysis the sensitivity and specificity of MRI for the diagnosis of endometriosis of USL were 85% and 80%, respectively .
In patients undergoing surgery, to establish whether USL endometriosis is unilateral or bilateral is particularly relevant. Indeed, the risk of urinary dysfunction and dysuria is significantly higher in patients after bilateral than unilateral USL resection . Other potential complications of the resection of USL are bleeding, ureteral lesions, and pelvic support disorders .
The rectovaginal space is the anatomical region located between the posterior vaginal wall and the anterior rectal wall. It extends from the deepest part of the pouch of Douglas to the top of the perineal body. The inferior two thirds of this space constitute the rectovaginal septum, a thin membranous partition usually filled with fat [2, 3].
Usually rectovaginal implants represent extensions from retrocervical or posterior vaginal lesions, but may also involve the rectovaginal septum alone, without any link to the cervix. These latter implants usually manifest as small nodular lesions palpable at vaginal examination. At MR imaging they show low signal intensity on T2-weighted images [2, 3].
In a recently published meta-analysis the sensitivity and specificity of MRI for the diagnosis of rectovaginal septum endometriosis were 82% and 77%, respectively .
Vaginal endometriosis is usually associated with implants in other pelvic locations, mostly retrocervical and rectal lesions; seldom isolated involvement of the vagina may occur. The upper one-third of the vagina and the posterior fornix are the most commonly affected sites.
Generally, the vaginal wall implants show a thickened or nodular appearance [2, 3], but may also have a polypoid structure. Distention of the vaginal lumen with gel may facilitate the identification of the lesion . At MR imaging vaginal endometriotic implants show low signal intensity on T2-weighted images. They often have a multiloculated internal appearance because of the presence of cystic areas. These locules can show hyperintense content on T1-weighted images due to subacute blood products  (Fig. 19). Polypoid variant may have a T2 hypointense rim corresponding to surrounding fibrous tissue associated with endometriosis . Rectovaginal fistulation represents a complication of vaginal endometriosis.
In a recently published meta-analysis the sensitivity and specificity of MRI for the diagnosis of vaginal and posterior vaginal fornix endometriosis were 82% and 82%, respectively .
Differential diagnosis includes epithelial neoplasms arising from the uterine cervix or vaginal wall. DWI may be useful in this setting demonstrating no restricted diffusion within the endometriotic mass, thus avoiding invasive surgery . On the other hand, vaginal carcinoma displays restricted diffusion on DWI.
Among the bowel segments the rectosigmoid is the most commonly involved by endometriosis (65.7%) , followed by vermiform appendix, terminal ileum, cecum and descending colon, in order of frequency . Rectosigmoid endometriosis is often associated with other pelvic locations and with a second intestinal lesion in 55% of cases .
Anatomically the rectosigmoid wall is characterized by four intraperitoneal layers: serosa, outer longitudinal muscularis, inner circular muscularis and mucosa . The implants generally involve the serosal surface but may invade the underlying muscular and submucosal layers; only rarely implants erode the mucosa causing cyclic rectal bleeding. Typically, endometriotic lesions infiltrating the anterior rectal wall have a characteristic “fan shaped” configuration (or a pyramidal shape, with the base adhering to the rectal wall and the apex oriented anteriorly toward the retrocervical region). The core of the lesion shows isointense signal compared to muscle on T2-weighted and T1-weighted sequences and at histopathology corresponds to thickening and distortion of the muscularis propria and smooth muscle hyperplasia. The overlying layer, hyperintense on T2-weighted images, at the luminal side of the bowel wall corresponds to (sub)mucosal thickening, as a consequence of “non-specific inflammation” with or without infiltration of endometriosis (Figs. 20 and 21) . When the longitudinal extent of the parietal lesion along the bowel wall is short, a pattern of intraluminal endophytic growth, called “mushroom cap”, may be observed .
In a recently published meta-analysis the sensitivity and specificity of MRI for the diagnosis of rectosigmoid colon endometriosis were 83% and 88%, respectively .
MR imaging is useful to predict infiltration of the muscular layer of the bowel with a sensitivity of 100% and specificity of 75%. On the other hand, it is of limited value in diagnosing (sub)mucosal infiltration, as (sub)mucosal thickening may be caused by edema without infiltration of endometriosis. Nevertheless, extensive irregularities of the (sub)mucosal layer may raise suspicion of (sub)mucosal involvement . Transvaginal US after bowel preparation is the best imaging modality for determining which bowel wall layers are affected .
Adhesions, strictures, and bowel obstruction may occur representing complications of intestinal endometriosis.
The surgical procedure depends on the lesion size (>2 cm or 3 cm), degree of infiltration (muscularis invasion), percentage of circumference involvement, number and location of intestinal lesions [3, 17, 51, 53]. The distance between the inferior margin of the nodule and the anal border in another important information for surgical planning and can be easily estimated at MRI on sagittal T2-weighted sequences .
The multilayer structure of the rectosigmoid wall lead to several possibilities of intestinal resection depending on the degree of invasion (Table 3) . Regarding clinical symptoms, segmental bowel resection is performed when patients do not respond to hormonal therapy and/or there is suspicion of a clinically relevant stenosis of the bowel .
Differential diagnosis includes rectal cancer and metastatic implants to the bowel.
Rectal endometriosis may be difficult to differentiate from colorectal carcinoma when presenting with non-specific clinical and imaging features or in cases of incidental bowel wall thickening on MR imaging . In this regard it is important to remember that endometriosis is an extrinsic lesion which starts at the serosa, infiltrates the muscular layer and only rarely invades the mucosa , whereas colorectal carcinoma is an intrinsic lesion starting at the mucosa. Busard et al.  have proposed qualitative assessment of high b-value diffusion-weighted images as a valuable, non-invasive tool to facilitate differentiation between endometriosis infiltrating the bowel and colorectal carcinoma. Both colorectal carcinoma and endometriosis infiltrating the bowel demonstrate low ADC values; nevertheless, colorectal carcinoma shows high signal intensity on DWI images due to high cellularity, whereas endometriosis displays hypointense signal intensity on DWI images. The low signal intensity of endometriosis infiltrating the bowel on DW images could be due to the “T2-blackout effect” of these lesions, that are very hypointense on T2-weighted imaging because of smooth muscle hyperplasia and fibrous tissue; their restricted diffusion (low ADC) might be explained by low water content and fibres blocking diffusion.