# Magnetic Resonance Roadmap in Detecting and Staging Endometiosis: Usual and Unusual Localizations
## 4 Ovarian Endometroma  
The endometrioma, also known as \"chocolate cysts\", develops when superficial endometriic lesions invade the surface of the ovaries. As a result, the blood produced at each menstrual cycle fails to flow properly and accumulates in the context of the ovary, forming a cyst that can grow over time reaching up to 10-20 cm in exceptional cases. They are often responsible for adhesions and, therefore, dislocation and fixity of the ovaries. The typical symptoms of an endometriosis cyst are pain and dysmenorrhea. In some cases, the presence of cysts interferes with proper ovulation, leading to reduced chances of conception and/or an increased risk of ectopic pregnancy [48; 49]. Rupture of an endometrioma is considered a medical emergency with severe pain, dark blood loss from the vagina, fever, nausea, and vomiting. Treatment can be pharmacological or surgical. Typically, small cysts require a medical approach, while large cysts require surgery [50]. On an MRI, endometriosis appear as cystic lesions with different signal characteristics according to the age of the blood and the type of hemoglobin present. Generally, they appear hyperintense on fat-suppressed T1 WI and T1 WI (Figures 7 and 8). The hyperintensity on fat-suppressed T1 WI helps differentiate endometriomas from dermatoid cyst and teratoma, which usually contain fat [51]. On T2 WI, a variable signal can be obtained: a hypointense signal can affect variable portions of the cyst, sometimes also presenting a stratification, until a complete loss of the signal. This is called the shading sign and is correlated to the different state of hemoglobin degradation [52] (Figure 8). The T2 dark spot sign refers to hypointense spots in the wall of the cyst due to the presence of macrophages (Figure 9). Contrast agents may exclude the presence of enhancing nodules that can be indicative of malignant transformation. MRI is also a valuable aid in achieving a proper differential diagnosis between endometrioma and other ovarian cystic lesions, such as mature cystic teratoma, hemorrhagic cyst, cystadenoma, cystadenofibroma and malignant cystic neoplasm. We can distinguish the main ovarian cystic lesions based on the following magnetic resonance characteristics [53]. Figure 7: Ovarian endometrioma in a 37-year-old female. (**A**) Axial T1 WI; (**B**) Axial fat-suppressed T1 WI. Typical aspect of an ovarian endometrioma (white arrows). Mature cystic teratoma is macroscopically characterized by a multicystic mass containing hair, teeth, and/or skin. Occasionally, a solid component known as Rokitansky's nodule can be observed within it [54; 55]. The abundant presence of adipose tissue allows for a differential diagnosis from endometriosis, as a signal reduction will be observed in sequences with adipose signal suppression [56]. Hemorrhagic ovarian cysts (HOCs) develop when bleeding occurs within a follicle or follicular cyst. On an MRI, hemorrhagic cysts exhibit high signal intensity on pre-contrast fat-suppressed T1 WI and low signal intensity on T2 WI. They do not show enhancement after gadolinium administration [54]. In some cases, a correct differential diagnosis can be aided by considering the patient's clinical presentation, which may involve acute abdominal symptoms, contrasting with the chronic and cyclic pain associated with endometriosis [57]. Cystadenoma and cystadenofibroma are benign ovarian tumors that can exhibit either serous or mucinous characteristics. They may manifest as cystic formations containing fluid or nematic contents, accompanied by the presence of septations and/or enhancement of the cyst wall following contrast administration [56; 58]. Malignant cystic neoplasm typically demonstrates signal intensity distinct from fat and blood, and they may be accompanied by features such as lymphadenopathy, abdominal  
Figure 8: Right tubo-ovarian endometriosis in a 25-year-old female patient with reported localized abdominal pain in the right iliac fossa, which increases intensity during the menstrual cycle.