Benign Ovarian Tumors
Most ovarian cysts have low signal intensity on T1-weighted images and very high signal intensity on T2-weighted images, as they contain simple fluid. Cyst walls are thin and featureless on T1-weighted images and clearly delineated on T2-weighted images. These are most commonly physiologic cysts, such as follicular cysts that occur because of failure of ovulation.
Corpus luteum cysts formed after ovulation are usually larger than follicular cysts and often have thick, irregular walls. Hemorrhagic corpus luteum cysts have relatively high signal intensity on T1-weighted images and intermediate to low signal intensity on T2-weighted images. They do not demonstrate the significant T2 shortening that is seen with many endometriomas.
Peritoneal Inclusion Cysts
Peritoneal inclusion cysts are usually seen in premenopausal women with a history of prior pelvic or abdominal surgery. Failure of peritoneal resorption of ovarian exudates and peritoneal adhesions following surgery results in fluid collection around the ovary. This usually conforms to the shape of the pelvis. The ovary is frequently entrapped within the fluid collection, which demonstrates thick enhancing septation. Identification of the normal ovary within the fluid collection allows confident diagnosis of peritoneal inclusion cyst.
Polycystic Ovarian Disease
Polycystic ovarian disease is characterized by bilaterally enlarged polycystic ovaries, secondary amenorrhea or oligomenorrhoea, and infertility. About 50% of patients are hirsute, and many are obese. Many cases of female infertility secondary to failure of ovulation are due to polycystic ovarian disease. The classic appearance on MRI is enlarged ovaries with intermediate signal intensity central stroma on T2-weighted images and more than 10 peripherally placed follicles less than 9 mm in diameter. The uterus may demonstrate thickened endometrium due to hyperplasia or even early endometrial carcinoma.
Endometrioma and Endometriosis
Endometriosis is the presence of endometrial epithelium and stroma outside of endometrium and myometrium. It is a syndrome that can present with endometriomas, adhesions, or endometrial implants. Staging of endometriosis requires a laparoscopic procedure to look for any of these elements.
Common locations of endometrial tissue include ovary, uterine ligaments, Fallopian tube, recto-vaginal septum, pouch of Douglas, bladder wall, and umbilicus. Imaging is most commonly used for the diagnosis and follow-up of endometriomas. Laparoscopy, however, remains the gold standard, as it can provide a complete evaluation for endometrial implants within the abdomen as well as the pelvis. Transvaginal US is the first imaging modality, with MRI reserved for masses atypical on US.
The most specific (91–98%) MRI findings in endometriomas are multiple cystic masses of high signal intensity on T1-weighted images and low signal intensity on T2-weighted images. A range of low signal intensities (shading) within the lesion or a low-signal-intensity ovarian cyst wall (hemosiderin deposition) can also be seen on T2-weighted images. Occasionally, endometriomas may demonstrate high signal intensity on both T1- and T2-weighted images. They retain high signal intensity on fat-suppressed T1-weighted images.
Endometriomas larger than 1 cm are routinely seen, but imaging small implants remains problematic. Implants commonly appear as solid masses or spiculated bands of intermediate-to-low signal intensity on both T1- and T2-weighted images due to fibrosis surrounding the glandular island. Small hemorrhagic endometrial implants become more obvious on fat-saturated T1-weighted images (Woodward et al. 2001).
Mature Cystic Teratoma
Mature cystic teratomas or dermoid cyst is the only benign germ cell tumors. They are most common in young women with a median age at presentation of 30 years. MRI can be used to diagnose dermoid cysts with confidence as the signal intensity of the fat or sebum within the cyst parallels that of fat on all pulse sequences (i.e., high signal intensity on T1-weighted images and intermediate signal intensity on T2-weighted images).
Typical findings of mature cystic teratomas include fat–fluid and/ or fluid–fluid levels; layering debris; low-signal-intensity calcifications (e.g., teeth) and soft-tissue protuberances (Rokitansky nodules or dermoid plugs) attached to the cyst wall.
Both endometriomas and mature cystic teratomas show high signal intensity on T1-weighted images and therefore must be distinguished from each other. The fat in mature cystic teratomas results in chemical shift artifact at the fat fluid interface, which appears as bright or dark bands along the frequency-encoding direction.
Use of frequency selective fat-suppression allows differentiation of hemorrhagic lesions such as endometriomas or hemorrhagic cysts from mature cystic teratomas. Immature teratomas are malignant ovarian tumors that also demonstrate fat on MRI.
They are usually very large at presentation and have significant solid components that contain coarse calcification and multiple small foci of fat (Outwater et al. 2001).
Fibromas, thecomas, and fibrothecomas are all fibrotic tumors of gonadal stromal origin and account for approximately 5% of all ovarian tumors. They are usually asymptomatic and typically detected in middle-aged women during routine gynecologic examination. Fibrotic tumors appear solid on imaging, thus mimicking malignant ovarian tumors. They are associated with ascites in 15% of the cases and pleural effusion (Meigs syndrome) in 1%.
Fibromas show homogeneous low signal intensity on T1-weighted images. On T2-weighted images, they appear as well-defined masses of low signal intensity, which contain scattered areas of high signal intensity, representing cystic degeneration or edema. The low signal intensity in all sequences is a reflection of the predominantly collagen content of these tumors and is diagnostic of fibromas.
Thecomas without prominent fibrotic components have MR imaging appearances similar to those of malignant tumors. The prominent lipid content of thecomas demonstrates intermediate to high signal intensity on both T1- and T2-weighted images.
Cystadenofibromas appear as cystic masses with a solid fibrotic component that shows marked enhancement after administration of gadolinium. They are less likely to be borderline or malignant compared with other serous or mucinous tumors.
Benign Epithelial Tumors
All epithelial ovarian neoplasms can be classified as benign, borderline (low malignant potential) or malignant based on their pathologic features and clinical behavior. Benign types of serous and mucinous tumors are common while benign forms of endometrioid and clear cell tumors are exceptionally rare.
Serous cystadenoma is the most common benign ovarian epithelial tumor. It appears as a thin-walled ovarian unilocular cyst and is usually indistinguishable from a follicular cyst on MRI. The signal intensity of cyst contents is variable but is usually low to intermediate on T1-weighted images and high on T2-weighted images. The cyst wall may contain small nodules, which are of high signal intensity peripherally and have thin low-signal-intensity cores on T2-weighted images due to fibrosis and/or calcification. If these solid projections are prominent, then a borderline tumor should be suspected.
Mucinous cystadenoma contributes to 15–25% of all ovarian tumors and is the second most common epithelial tumor. The most common appearance of a mucinous cystadenoma is a very large multilocular cyst with no solid components. In mucinous cystadenomas, the signal intensities of the loculi often vary depending on the amount of proteinaceous or mucinous fluid and whether hemorrhage is present. Endometrioid cystadenomas are often malignant and difficult to distinguish from endometriosis.
Brenner tumors are usually benign and account for only 1–2% of all ovarian tumors. They arise from Wolffian metaplasia of the surface epithelium. Thirty percent of Brenner tumors are associated with cystic teratomas or cystadenomas. Brenner tumors may be solid, cystic, or mixed. When solid, they demonstrate low signal intensity on both T1- and T2-weighted images.