Superficial endometriosis lesions are composed of peritoneal implants, generally up to 10 mm long, and adhesions secondary to the inflammatory process. Their most frequent location is the Douglas pouch and the broad ligaments.The diagnosis is still made by direct observation of lesions via laparoscopy or biopsy. Transvaginal ultrasound has no diagnostic value in this form of the disease. Adhesions may eventually be found by ultrasound, but is not specific [5].

Finding an impairment in the abdominal wall in MR occurs in less than 2% of the endometriosis cases, yet it is considered the most common location outside the pelvic cavity. The cyclic pain related to menstrual flow is important information from the clinical examination, with palpable lesions in 96% of the cases. The lesion at its greatest diameter, the depth of the abdominal wall invasion, and its relation to anatomical markers, must be described.

The morphological aspect of ovarian endometriosis is seen as small implants or hemorrhagic cysts limited by an endometrial epithelium, i.e., the endometriomas [5].

The identification of small hyperechoic foci, distributed on the ovarian surface (Figs. 5.8 and 5.9), was assigned to endometriosis in some publications [6]. More recent studies have shown that hyperechoic ovarian foci are common in transvaginal ultrasounds and may correspond to various histological findings, including small inclusion cysts, hemosiderin, calcifications, and small, dense cortical nodules [7]; thus it is a non-specific signal.

Ovarian endometriosis is often associated with dense adhesions in the pelvis, either in the ovarian fossa or in other places, among them the uterosacral ligaments, the retro-cervical space, or rounded ligaments [8]. Ultrasound is of great value for diagnosing adherence processes because it is a dynamic test in which the ovaries’ mobility can be tested in real time. For this, the examiner can use one hand by abdominally compression and/or gently press the transducer vaginally.

The ovaries may be fixed in the posterior or anterior compartment to the uterus (no sliding maneuvers) and painful to touch by the transducer (Figs. 5.10, 5.11, 5.12, and 5.13). When bilateral, both ovaries may be medianized and there may be adhesions between them, commonly in the retro-uterine region (the so-called “Kissing-ovaries.” [9] (Figs. 5.14 and 5.15).

Endometriomas have a brownish content and are therefore known as chocolate cysts. This hemorrhagic content is an essential criterion in ultrasound diagnosis, providing a typical sonographic appearance: a rounded cyst containing internal fine echoes with low echogenicity of homogeneous appearance, known as the “frosted glass” aspect (Figs. 5.16, 5.17, 5.18, and 5.19) [10]. It is estimated that this characteristic is present in about 95% of endometriomas [10].

Hyperechoic parietal foci can be seen in more than 30% of endometriomas and can be a useful criterion for their distinction because they are rarely seen in other benign ovarian cysts [11 ] (Figs. 5.20 and 5.21). Therefore, a cyst with thin internal echoes, homogeneous and hypoechoic, with no other signs of malignancy, is 32 times more likely to be an endometrioma than another anexial mass [11]. Septacan also be found in endometriomas, giving them a multilocular aspect [11] (Figs. 5.22 and 5.23).

Denser areas of the content or intracystic bleeding may appear as fixed parietal hyperechoic nodularity that can simulate solid malign projections and cause doubts [9]. Amplitude Doppler can help show the absence of flow in cystic content [12] (Figs. 5.24, 5.25, 5.26 and 5.27). Endometriomas’ walls are usually thin and smooth and are clearly visible, but can rarely be true papillary projections resulting from inflammation, necrosis, or even endometriosis focus pericystic proliferation.

Transvaginal ultrasound showed a positive predictive value of 91% for the diagnosis of endometriomas [13]. In this same study, the positive predictive value was 97%, when the endometrioma had a classic look (rounded cyst, regular margins, thin and smooth walls, homogeneous smooth internal fine echoes, and hypoechoic), and 70% for those with an atypical appearance (anechoic, internal septa, irregular margins, or solid projections) [13]. Most often we see atypical endometriomas in the oldest diseases; they are more associated with adhesion processes (Figs. 5.28, 5.29, 5.30, and 5.31).

The multicentric study (IOTA) evaluated 3511 patients with adnexal masses, of which 713 were endometriomas; ultrasonography showed 81% sensitivity and 97% specificity for their diagnosis [8]. The true positive cases were more often represented by unilocular cysts with “frosted glass” content than any other category of Axial mass [8].

In magnetic resonance imaging, the endometrioma typically manifests as a hyperintense lesion (the same signal or stronger than fat) in sequences weighted in T1, with a tendency to hypointensity (usually with hyperintense foci, causing the shading effect on sequences weighted in T2) (Figs. 5.32 and 5.33).

This aspect of the image reflects the chronicity of endometrioma, which has very “thick” hematic content, due to the high concentration of intracystic methemoglobin and other proteins, which promotes shortening of T2 relaxation time. Large endometriomas may contain multiple thin septa and often present levels with blood (Fig. 5.34).

Complex lesions, mainly represented by large cystic formations and located in the ovaries, present an often parietal solid component, and this makes it difficult to characterize the tumor mass, requiring correlation with contrasted phase, to exclude malignant transformation (endometrioid carcinoma). The differential diagnosis for endometrioma on ultrasound includes hemorrhagic luteum cyst, cystadenoma, hematosalpinx or pyosalpinx, abscess, dermoid lesion, and ovarian cancer. All these lesions may have thick content (blood, mucus corpus) that eventually present a “frosted glass” on ultrasound [9].

The hemorrhagic corpus luteum cyst, because it is more recent and has the greatest amount of hemorrhage, usually has a more heterogeneous appearance with echogenic are as intermingled with liquid content, or with thin echogenic lines corresponding to fibrin networking aspect. This heterogeneous pattern in grayscale, unlike the homogeneous and hypoechoic pattern of endometriomas, is usually the best parameter for differentiation (Figs. 5.35, 5.36, 5.37, and 5.38). Doppler can also be helpful by showing the abundant classic peripheral vascular halo of the luteum cyst, while endometriomas usually have little and peripheric vascularization [12] (Figs. 5.39 and 5.40). Hemorrhagic cysts are usually present in the second phase of the cycle, associated with endometrium of a secretory aspect, and are absorbed spontaneously. The ultrasound control 5-6 weeks later can, therefore, demonstrate their regression.

Tubal dilatation, mainly hematosalpinx, usually has a serpiginosum aspect and is more tubular (Fig. 5.41). The transducer pressure may help demonstrate the non-ovarian origin (Fig. 5.42).