An algorithmic approach can be followed in order to make a practical differential diagnosis of ovarian masses. This can first start by confirming the ovarian location to differentiate the lesions from any extra-ovarian pathologies. Once the ovarian location is confirmed, morphological features can be used to reach to a specific diagnosis. These morphological features can categorize ovarian lesions as simple cystic, complex cystic, cystic with solid component (mixed cystic/solid) and solid.

Adnexal lesions can be entirely cystic without any complex features. These can be categorized as ovarian and extra-ovarian according to location. Ovarian simple cystic lesions include functional cyst and serous cystadenoma. Extra-ovarian simple cystic lesions include paraovarian cyst and hydrosalpinx. Multiple simple ovarian cysts can be seen in polcystic ovaries and ovarian hyperstimulation syndrome.

By definition, simple cysts are anechoic unilocular and thin walled (<3 mm) on sonography, with posterior acoustic enhancement (Fig. 28.1) [2]. On CT, these appear as well-circumscribed fluid attenuation lesions with no postcontrast enhancement or solid component (Fig. 28.1). On MRI, simple cyst demonstrate low signal intensity on T1-weighted images and high signal intensity on T2-weighted images similar to simple serous fluid (Fig. 28.1). These are commonly found in premenopausal women and are physiologic or functional in nature. Per the consensus statement of the Society of Radiologist in Ultrasound in premenopausal women, simple cysts <5 cm are benign with no further follow-up needed, whereas those 5–7 cm should be followed up yearly. For cysts >7 cm, further characterization with MRI may be helpful [17, 18]. Simple <1 cm cysts are commonly seen in postmenopausal women and are considered clinically unimportant [18]. Simple cyst >1 cm and <7 cm should be followed up annually to document interval changes in size and morphologic characteristics. For cysts >7 cm, further imaging with MRI or surgical evaluation is recommended [18]. Algorithm 28.2 summarizes imaging differential consideration of simple cystic adnexal masses.

It is one of the most common endocrinopathies in women of reproductive age [19], although not all patients with sonographic features of PCO have clinical PCOS, which is characterized by menstrual irregularities, hirsutism, obesity, and sclerotic ovaries. TVUS is currently the gold standard for diagnosing PCO. The classic sonographic feature is an enlarged ovary with ten or more fairly uniform peripherally arranged cysts of 2–8 mm diameter, with an echogenic central stroma [20]. Although, MRI is rarely used in the primary evaluation of PCO as it does not provide any additional information, the best sequence is T2-weighted images in long and short axes of the uterus, demonstrating peripherally arranged, uniformly sized, high-signal-intensity cysts with hypointense central stroma (Fig. 28.2).

It is a rare complication seen in patient treated for infertility with ovulation induction drugs. Patients may present with abdominal discomfort, nausea and vomiting [21]. Because of exudation of fluid in the extravascular space in the severe form, it may be associated with pleural effusions, intravascular volume depletion, and consequently hemoconcentration and oliguria [22]. Sonography and MRI demonstrate bilateral enlarged ovaries with multiple large peripherally located cysts, producing a “spoke wheel” appearance (Fig. 28.3) [23]. Patients with OHSS have an increased risk of ovarian torsion.

These are common benign epithelial tumors in premenopausal women. On ultrasonography, ovarian cystadenoma are anechoic, thin walled cyst, usually unilocular although rarely may demonstrate thin internal septations. Serous cystadenomas are usually bilateral and smaller than mucinous cystadenoma. They demonstrate homogeneous high signal intensity on T2-weighted images and low signal intensity on T1-weighted images, with no significant postcontrast enhancement of the septae (Fig. 28.4) [24].

These are located in the mesosalpinx, separate from the ovaries, hence the name. They have been described to be arising from mesothelial, paramesonephric (Müllerian), and mesonephric (Wolffian) structures [4]. The paraovarian cyst demonstrate imaging features similar to simple cyst on ultrasonography and MRI (Fig. 28.5).

These are usually functional cysts undergoing hemorrhage or corpus luteum cysts. In premenopausal women, cysts with uniform internal echoes, reticulations, or septations may represent hemorrhagic functional cysts or endometrioma. A follow-up sonogram should be performed in 6–12 weeks in these cases [18]. Functional hemorrhagic cysts demonstrate complete interval resolution, whereas endometrioma persists or even slightly increases in size at intervals. Hemorrhagic corpus luteum cysts have a characteristic appearance of blood products, manifested by relatively high signal intensity on T1-weighted images and intermediate to high signal intensity on T2-weighted image (Figs. 28.6 and 28.7); this may lead to difficulty in differentiating them from endometrioma if no prior images are available [14, 25, 26].

Endometriosis is the ectopic implantation of endometrial tissue outside the uterus, with the ovaries being the commonest site. Sonographically, endometrioma are complex cystic masses with uniform low-level internal echoes and may contain hyperechoic foci secondary to blood clots in the periphery [2]. MRI usually reveals high signal intensity on T1-weighted images (light bulb) and intermediate to low signal intensity on T2-weighted images (known as T2 shading), which is attributed to hemoglobin degradation products in various stages (deoxyhemoglobin, methemoglobin, and iron products) and decreased free water content (Fig. 28.8) [27, 28]. High signal intensity on T1-weighted images can be seen secondary to fat or blood, and a persistently high signal on fat-saturated T1-weighted imaging confirms the absence of fat in the lesion. Other MRI findings may include high signal intensity on T1- and T2-weighted images and adhesion or tethering to adjacent structures. The presence of multiple high-signal-intensity foci on T1-weighted images in both ovaries also favors the diagnosis of endometriosis.

Ovarian torsion results from partial or complete twisting of the vascular pedicle, causing obstruction to the vascular supply of the ovary. It is mostly seen in premenopausal women (can occur in post-menopausal women with large ovarian mass) and usually presents with acute-onset abdominal or adnexal pain. Enlarged ovaries from cysts or masses (>5 cm) remain the predisposing factor, the commonest cause being cystic teratoma [29, 30]. On sonography, ovarian torsion manifests as complex enlarged adnexal mass showing peripherally arranged follicles secondary to displacement by the edematous ovarian stroma [31]. Color Doppler sonographic evaluation is important, although the features depend on the degree of vascular compromise and may range from decreased to completely absent vascular flow (Fig. 28.9) [32]. The presence of central venous flow has been described with spontaneous partial detorsion and are shown to have better outcomes [33]. CT and MRI are rarely used for diagnosis and may reveal nonspecific features such as enlarged adnexal masses, engorged adnexal vessels, and ascites.

These are benign epithelial tumors of the ovaries that present as multilocular cysts (honeycomb-like locules) with thin regular walls and septa with no endocystic or exocystic vegetation or significant postcontrast enhancement. The MRI signal intensity of mucinous cystadenoma depends on the hydration of the mucin: with the watery (hydrated) mucin presenting as low signal intensity of T1-weighted images and high signal intensity on T2-weighted images (Fig. 28.10), while dehydrated thicker mucin results in slightly higher T1- and lower T2-signal intensity [2].

These are common ovarian tumors that are composed of tissue from all three embryogenic germ cell layers and thus are composed of sebaceous material, hair follicles, osseus structure/tooth, and muscles. On sonography, their appearance varies from purely cystic lesions with peripheral nodules to partially echogenic masses with posterior shadowing from adipose material, calcifications, and hair. The hair ball in the teratoma may present as multiple thin echogenic bands [34]. CT demonstrates characteristic fat attenuation, with or without calcification in the wall [35, 36]. On MRI, the sebaceous components have high signal intensity on T1-weighted imaging, similar to that of retroperitoneal fat, and lose signal on fat-suppressed T1-weighted sequences, suggesting macroscopic fat and excluding hemorrhage. Low signal intensity on T1- and T2-weighted images may be secondary to soft tissue protuberances from Rokitansky nodules and calcifications (teeth) (Fig. 28.11). Complications include torsion in large size teratomas and rupture, causing leakage of the sebaceous component into the peritoneum and granulomatous peritonitis.

These are malignant counterpart of teratoma and constitute less than 1 % of all teratoma, commonly encountered in the first two decades of life [37, 38]. These tumors contain immature tissue from all three germ cell. These tumors tend to grow fast and are usually very large at time of presentation and can present with perforation and rupture of the capsule [3]. On imaging, these tumors are seen as large complex masses with cystic and solid components with scattered calcifications (Fig. 28.12).