In reproductive women, various physiologic conditions can cause morphologic changes of the ovary, resembling pathologic conditions. Benign ovarian diseases can also simulate malignancies. Magnetic resonance imaging (MRI) can play an important role in establishing accurate diagnosis. Functional cysts should not be confused with cystic neoplasms. Corpus luteum cysts typically have a thick wall and are occasionally hemorrhagic. Multicystic lesions that may mimic cystic neoplasms include hyperreactio luteinalis, ovarian hyperstimulation syndrome, and polycystic ovary syndrome. Recognition of clinical settings can help establish diagnosis. In endometrial cysts, MRI usually provides specific diagnosis; however, decidual change during pregnancy should not be confused with secondary neoplasm. Peritoneal inclusion cysts can be distinguished from cystic neoplasms by recognition of their characteristic configurations. Ovarian torsion and massive ovarian edema may mimic solid malignant tumors. Recognition of normal follicles and anatomic structures is useful in diagnosing these conditions. In pelvic inflammatory diseases, transfascial spread of the lesion should not be confused with invasive malignant tumors. Radiologic identification of abscess formation can be a diagnostic clue. Many benign tumors, including teratoma, Brenner tumor, and sex-cord stromal tumor, frequently show characteristic MRI features. Knowledge of MRI features of these conditions is essential in establishing accurate diagnosis and determining appropriate treatment.
MRIOvaryPhysiologic changeBenign condition
A wide variety of physiologic and benign conditions develop in the ovaries of reproductive women. In patients with these conditions, treatment of choice may often be problematic, especially in young women who desire to maintain reproductive function. Precise knowledge of clinical and magnetic resonance imaging (MRI) features of these conditions is crucial in establishing an accurate diagnosis and determining appropriate treatment.
In normal circumstances, physiologic changes of the ovaries should not be misdiagnosed as pathologic cysts. In young patients, positive diagnosis of benign physiologic cysts, including follicular cysts and corpus luteum cysts, is important in order to avoid unnecessary laparoscopies. This is especially important during pregnancy, and physiologic changes such as hyperreactio luteinalis or decidual changes of endometrial cysts should be carefully differentiated from cystic ovarian neoplasms. Benign conditions, such as massive ovarian edema and some inflammatory diseases, may simulate solid ovarian neoplasms. In benign cystic lesions, such as endometrial cysts and dermoid cysts, development of secondary malignancy should be carefully evaluated. Occasionally, patients with some benign or physiologic conditions may present with acute abdominal pain. These conditions may include ovarian bleeding from a corpus luteum cyst, ovarian hyperstimulation syndrome, ovarian torsion, pelvic inflammatory diseases, ruptured dermoid cysts, or endometrial cysts and Meigs syndrome in fibrothecomas, etc. Although immediate treatment is necessary in most of these conditions, an indication for surgical intervention (oophorectomy) should be carefully determined.
Transvaginal ultrasonography (TVS) is the initial modality of choice for evaluating ovarian lesions because of its convenience and cost-effectiveness. Nonetheless, TVS may have some limitations in displaying a global image of the whole pelvis and its tissue characterization. When TVS findings are nondiagnostic or equivocal, MRI can be a valuable adjunctive modality for evaluating adnexal lesions, especially owing to its excellent tissue contrast and capability to differentiate benign and malignant conditions. The purpose of this article is to review a spectrum of physiologic and benign conditions of the ovaries and to illustrate clinical and MRI features of these conditions, with an emphasis on the key findings for suggesting accurate diagnoses.
The appearance of the ovaries in women of reproductive age changes during the menstrual cycle (ovarian cycle). The ovarian cycle includes development of an ovarian follicle, rupture of the follicle, discharge of the ovum, formation and regression of a corpus luteum, and formation of corpus albicans. The ovaries are usually seen in the ovarian fossa located anterior to the internal iliac artery although their positions may vary according to parity. In the vast majority of premenopausal women, the ovaries can be easily identified on MRI. This zonal anatomy of the ovaries can be identifiable on T2-weighted images, with the cortex showing lower intensity than the central medulla . The cortex frequently contains small cysts of high intensity on T2-weighted images, which are regarded as physiologic, nonneoplastic cysts, including follicles at various stages, corpus luteum, and surface inclusion cysts (Fig. 1) . The size and number of these cysts can vary during the menstrual cycle. In the preovulatory period, dominant preovulatory follicles can enlarge by 17–25 mm . After ovulation, the dominant follicle becomes the corpus luteum, which may present as a cyst with a thick, enhancing, and occasionally convoluted wall. If conception does not occur, the corpus luteum gradually involutes into corpus albicans, which is not perceptible on MRI. In postmenopausal women, the ovaries can be seen as oval structures of homogeneous low intensity on T2-weighted images. Surface-inclusion cysts are small cystic structures frequently observed in postmenopausal ovaries that no longer undergo formation of cystic follicles and corpus luteum .
Follicular cysts develop when the follicles fail to regress or ovulate. These cysts can range from 3 to 8 cm but rarely exceed 5 cm . They may contain simple fluid or a small amount of blood (Fig. 2) . The cyst wall is often demonstrated as a thin enhancing rim on postcontrast MRI. Since follicular cysts more than 5 cm may be indistinguishable from neoplastic cysts, such as serous cystadenomas, they should be monitored by follow-up ultrasonography. Unlike neoplastic cysts, follicular cysts may spontaneously regress over time, usually within two menstrual cycles. In clinical practice, ovarian cysts greater than 10 cm must be considered to be neoplastic .
Corpus luteum cysts
Corpus luteum cysts develop when the corpus luteum fails to regress after ovulation, and the diagnosis of corpus luteum cyst relies on the date in the menstrual phase. Usually, these cysts are seen at the end of the luteal phase or during pregnancy. A cyst in the early luteal phase may represent a cystic corpus luteum and should not be called a corpus luteum cyst. Corpus luteum cysts are usually single and greater than 2.5 cm and less than 6 cm in diameter . On MRI, corpus luteum cysts tend to have thicker walls than those of follicular cysts. The cyst wall may exhibit slightly increased intensity on T1-weighted images and relatively low intensity on T2-weighted images and show avid enhancement, reflecting increased vascularity of the thick luteinized cell layer (Fig. 3). MR appearance can be variable. Although the signal intensity is usually inferior to fat on T1-weighted images, it can occasionally show high intensity due to hemorrhage that may simulate endometrial cysts. Subsequent ultrasonography may differentiate these conditions by showing regression in cases of corpus luteum cyst .
Corpus luteum cysts may be complicated, with minor leakage with a small amount of fluid in Douglas pouch. However, it is rarely complicated with massive ovarian bleeding causing acute abdominal pain. In this condition, MRI can successfully determine the presence of a corpus luteum cyst with acute hemorrhage, as the cyst will exhibit intermediate signal intensity on T1-weighted images and distinct low intensity on T2-weighted images . Hemoperitoneum (hemorrhagic ascites) can be recognized as fluid of high intensity relative to urine on T1-weighted images (Fig. 4). Treatment of ovarian bleeding is usually conservative unless the patient’s condition is unstable. Since this condition may clinically resemble a ruptured ectopic pregnancy, a pregnancy test is crucial for differentiation.
Nonneoplastic benign conditions
Polycystic ovary syndrome
Polycystic ovary syndrome (PCOS) affects approximately 20% of premenopausal women and is one of the most common endocrinopathies of women . PCOS is usually diagnosed clinically by the presence of characteristic features that include hyperandrogenism and chronic anovulation without a specific underlying adrenal or pituitary gland disease. Although imaging criteria for diagnosis of PCOS have not been universally agreed upon, typical MRI features include slightly enlarged ovaries, with a central stroma and multiple small follicles located peripherally [8, 9]. Enlargement of the central ovarian stroma, which usually show low intensity on T2-weighted images, is not specific but an important sign of this condition, especially to differentiate PCOS from other multicystic ovarian lesions (Fig. 5). In contrast to normal ovaries, follicles seen in PCOS are typically smaller than 1 cm and are uniform in size and appearance [8, 9]. Another role of MRI in this condition is exclusion of other hormone-stimulating ovarian disorders by carefully evaluating the presence of solid components within the lesion .
Ovarian hyperstimulation syndrome and hyperreactio luteinalis
Increased human chorionic gonadotropin (hCG) serum levels can cause a marked enlargement of bilateral ovaries, with multiple theca lutein cysts; this may frequently present as abdominal fullness. When the increased hCG is associated with ovulation induction, the condition is called ovarian hyperstimulation syndrome. Alternatively, if associated with intrinsic causes, such as a normal pregnancy or gestational trophoblastic disease, the condition is called hyperreactio luteinalis (Fig. 6). Typical MR images in these conditions are bilaterally enlarged ovaries with multiple cysts that may resemble cystic ovarian neoplasms with septations, such as mucinous cystic tumors and granulosa cell tumors. However, recognition of the uniform size of each locule can help differentiate them from multilocular cystic ovarian tumors . Additionally, correct image interpretation requires an understanding of the patient’s clinical information.
Endometrial cysts are the most common location of pelvic endometriosis, and they are usually multiple and bilateral. As endometrial cysts are filled with blood products of different ages, they typically appear as high intensity, superior or equal to fat on T1-weighted images and relatively low intensity on T2-weighted images (called “shading”) (Fig. 7) [11, 12]. MRI has been reported to have a sensitivity of 68–90% and specificity of 83–98% for the diagnosis of endometrial cysts [11, 13]. Dermoid cysts can be a major differential diagnostic consideration since fatty content also shows high intensity on T1-weighted images. Fat saturation technique is highly useful for differentiation of these two types of lesions .
Endometrial cysts may occasionally rupture and cause acute pelvic pain. When this condition is associated with severe abdominal pain and unstable vital signs, peritoneal lavage and cystectomy are required. Ruptured endometrial cysts can be detected with MRI as free fluid that is hyperintense on T1- and T2-weighted images with or without shading in the intraperitoneal space. A redundant and disrupted wall may also be visualized (Fig. 8).
Secondary neoplasms, such as endometrioid adenocarcinoma and clear-cell adenocarcinoma, can develop in endometrial cysts, with an incidence of 0.6–1.0% . MRI is a useful modality to diagnose this condition by demonstrating solid components present on the cyst wall. Enhancement of these nodules on postcontrast images may be the key to diagnosing this condition. When the enhancement evaluation is visually difficult due to the hyperintense background of endometrial cysts on T1-weighted images, subtraction images of precontrast and postcontrast images may be useful for evaluating the enhancement . Decidual changes of endometrial tissue in the endometrial cyst during pregnancy can also manifest as mural nodules and may simulate secondary neoplasms. However, these mural nodules characteristically show the same intensity as that of the normal endometrium (placenta) on all sequences (Fig. 9) . Knowledge of this phenomenon helps to obviate unnecessary intervention that may be caused by confusing these nodules with secondary neoplasms.
Peritoneal inclusion cysts
Peritoneal inclusion cysts are a fluid accumulation related to active ovaries and peritoneal adhesions that have developed as a result of pelvic surgery or inflammatory disease. On MRI, these lesions typically present as a unilocular or multilocular cystic mass. One of the characteristic MR findings that can allow specific diagnosis of this condition is configuration of the cyst wall, which is outlined by the pelvic wall, pelvic organs, and bowel loops (Fig. 10). Presence of the normal ovary within the lesion is another suggestive MRI finding. Although the lesion may occasionally be mistaken for a cystic ovarian neoplasm, recognition of a patient’s history of previous surgery and the characteristic MRI features can help avoid confusion. This condition is usually treated conservatively unless the lesion causes pain or abdominal distension .
Solid/solid and cystic lesions
Adnexal torsion is one of the gynecologic emergencies that causes acute pelvic pain. Torsion of a normal ovary is usually seen in children but can occur in young females (Fig. 11) . This condition more commonly occurs in ovaries with an ovarian neoplasm or cyst. If the ovarian tissue does not infarct, the affected ovary should be salvaged by untwisting the adnexa for preserving fertility. Contrarily, if the affected ovary becomes necrotic, salpingooophorectomy is required.
MRI can allow accurate diagnosis of this condition by demonstrating a twisted vascular pedicle between the enlarged ovary and uterus. In the early stages, a torsed ovary may be swollen and contain multiple follicles separated by edematous stroma . Hyperintense signal of enlarged ovarian stroma on T2-weighted images probably related to edema can be useful findings in viable torsions [18, 19]. When hemorrhagic necrosis occurs, the lesion may exhibit high intensity on T1-weighted images [17, 20]. Identification of the enhancement on postcontrast MR images is an important finding for suggesting the viable tissues in the affected ovary . Lack of enhancement in the torsed ovary may indicate complete interruption of blood flow, and it is a delayed sign suggesting hemorrhagic infarction of the lesion .
Massive ovarian edema
Massive ovarian edema is a rare condition caused by partial or intermittent torsion of the ovary with obstruction of venous drainage. This causes enlargement of the ovary due to edema and proliferation of theca cells in the medulla . Patients with massive ovarian edema usually have one or both involved ovaries and present with abdominal or pelvic pain. Unlike ovarian torsion, massive ovarian edema is not accompanied by infarction or necrosis, and the ovary remains viable. Enlarged ovaries may simulate solid ovarian neoplasms when visualized with MRI . However, embedded follicles may help to differentiate an edematous ovary from a neoplastic ovary (Fig. 12). An accurate diagnosis of this condition would allow preservation of ovarian function, as the appropriate procedure would be wedge resection of the affected ovary.
Pregnancy luteoma is a rare non-neoplastic condition characterized by bilateral ovarian enlargement during pregnancy, which may simulate ovarian neoplasms. It may be caused by replacement of normal ovarian parenchyma by solid proliferation of luteinized stromal cells under the influence of hCG. Unlike patients with hyperreactio luteinalis, however, patients with pregnancy luteoma do not have excessive hCG levels, and within the ovary, it is the stromal cells rather than the follicles that are being stimulated. Patients are often asymptomatic although virilization of mother and fetus can occur due to increased androgen produced by the stromal cells. On MRI, pregnancy luteoma may appear as bilateral multinodular solid masses in both ovaries .
Pelvic inflammatory disease (PID) is usually caused by infection ascending through the uterine cavity and is treated with antibiotics. Acute PID is commonly associated with acute pelvic pain and fever. Lesions in the right adnexa frequently mimic appendicitis. A tuboovarian abscess (TOA) may develop at the end stage of acute PID. If medical therapy fails to treat a TOA, surgical drainage of the abscess is required. Delay in diagnosis of a TOA often results in infertility. Although the diagnosis of PID is usually based on clinical information or transvaginal sonography, MRI can provide a more accurate assessment of TOA by demonstrating an irregularly thick abscess wall and linear stranding of the surrounding inflammation (Fig. 13) . The wall typically exhibits low intensity on T2-weighted and high intensity on T1-weighted images. Contents of the abscess may be slightly hyperintense on T1-weighted and slightly hypointense on T2-weighted images due to the presence of hemorrhage or debris. Postcontrast images typically show intense enhancement in the wall and stranding in the surrounding fat planes .
In cases of chronic inflammatory lesions, patients are sometimes less symptomatic and may present with a solid adnexal mass. In patients with long-term use of intrauterine devices, actinomycetes are frequently the causative organism. On MRI, an ill-defined lesion border resulting from transfascial spread of chronic inflammation may simulate extensive malignancy involving the adnexa. Relatively low intensity on T2-weighted images representative of fibrosis may be a clue in the accurate diagnosis of this condition (Fig. 14) [25, 26].
Mature cystic teratomas (dermoid cysts)
Benign germ-cell tumors are mainly represented by mature cystic teratomas (also known as dermoid cysts) and constitute about 95% of all ovarian germ-cell tumors . MRI affords a specific diagnosis of this tumor by demonstrating fat intensity, which is usually demonstrated as high intensity, the same as subcutaneous fat. Although the signal pattern of fat may resemble that of hemorrhagic content in endometrial cysts, frequency-selective fat suppression technique is useful for differentiating these two common entities . In this context, the role of short T1 inversion recovery (STIR) sequence, which is a nonselective fat suppression technique, is rather limited since signal suppression can also be observed in endometrial cysts . Rarely, any visible fat may be present in atypical dermoid cysts. In such cases, chemical shift (in-phase/opposed-phase) imaging has been reported to be useful to detect microscopic lipid .
MRI is also beneficial in identifying complications associated with dermoid cysts, such as torsion, rupture, and malignant transformation. Rupture of dermoid cysts causes an acute abdomen secondary to chemical peritonitis resulting from leakage of the liquefied sebaceous contents into the peritoneal cavity. On MRI, recognition of sebaceous material of fat intensity floating in the peritoneal fluid allows an accurate diagnosis (Fig. 15). Malignant transformation occurs in 1–2% of cases, usually affects postmenopausal women, and is most commonly squamous cell carcinoma. MRI allows recognition of malignant transformation as solid components that exhibit extensive transmural extension, and direct invasion of neighboring organs can be visualized .
Struma ovarii is a rare variant of mature teratoma and is histologically characterized by the presence of mature thyroid tissue. This type of lesion typically presents as a multilocular cyst with varying signal intensity in the locules (“stained-glass appearance”) on MRI. Thyroid colloid in the cystic space exhibits low intensity on both T1- and T2-weighted images (Fig. 16) . This lesion is frequently associated with a focal teratomatous component containing fat tissue, providing a clue for the specific diagnosis of struma ovarii.
Cystadenomas of the ovary are true cystic epithelial ovarian neoplasms. Serous cystadenomas commonly appear as unilocular, thin-walled cysts filled with simple fluid. In contrast, mucinous cystadenomas typically show a characteristic multilocular appearance with varying signals in the locules (stained-glass appearance) and tend to be larger at the time of presentation than their serous counterparts (Fig. 17) . The presence of a thick wall or septa may suggest borderline lesions while the presence of solid components suggests carcinoma.
Cystadenofibroma is an uncommon epithelial tumor, which is pathologically characterized by dense fibrous tissue in association with one or many locules. On MRI, these tumors typically manifest as a complex cystic mass associated with solid components of distinct low intensity on T2-weighted images [1, 31].
Solid/solid and cystic tumors
Brenner tumors are uncommon surface epithelial tumors that account for about 2% of ovarian neoplasms . These tumors are associated with other cystic ovarian neoplasms, often mucinous cystadenomas, in approximately 30% of cases . The solid component of Brenner tumor typically exhibits distinct low intensity on T2-weighted images . Extensive amorphous calcification within the solid components is one characteristic finding of Brenner tumors although MRI may be limited for detection of calcifications (Fig. 18) . Although calcification can be also recognized in serous adenocarcinomas, the combination of calcifications demonstrated by ultrasonography or computed tomography (CT) and low intensity on T2-weighted MR images strongly suggests the diagnosis of Brenner tumors.
Fibromas and thecomas are benign sex-cord stromal tumors commonly seen in both pre- and postmenopausal women. These tumors typically show low intensity on T2-weighted images, reflecting abundant collagenous tissue. The size of these tumors is usually less than 10 cm, with an average of 6 cm . However, fibromas can be so large that they can be mistaken for malignant tumors (Fig. 19). Ovarian fibromas can be infrequently associated with ascites and pleural effusion (Meigs syndrome). This condition may lead to a mistaken impression of a malignant condition although this manifestation is dramatically resolved following tumor removal [36, 37]. Rarely, fibromas may occur in association with basal cell nevus syndrome; they are usually bilateral and multiple and may contain calcifications . Ovarian fibromatosis, which is a rare benign condition, may cause menstrual abnormalities in young women. This condition can manifest as bilateral ovarian masses with predominant fibrous components, which show low intensity on T2-weighted images [39, 40]. The presence of entrapped follicle between the fibrous tissue may be an important differentiating feature of this condition from ovarian fibromas [39, 41].
Sclerosing stromal tumors
Sclerosing stromal tumors are rare, benign, sex-cord stromal tumors that occur predominantly in young women in their second and third decade of life. On MRI, these tumors consist of hyperintense cystic components and heterogeneous solid components with intermediate to high signal intensity on T2-weighted images. Dilated vessels may be observed around the tumor, suggesting tumor hypervascularity. On dynamic contrast-enhanced MRI, the solid component shows striking early enhancement, which is higher than that of the uterine myometrium and which can be a diagnostic clue of the tumor (Fig. 20) [42–44].
Knowledge of MRI features of a variety of physiologic and benign conditions of the ovary facilitates establishment of accurate diagnoses and determination of appropriate treatment strategies.