The normal ovary in premenopausal women typically contains small cysts, less than 3 cm in diameter, representing follicles or functional cysts. Larger cysts, those greater than 3 cm, sometimes develop in the ovary. These cysts may represent unusually large follicles, nonfunctional ovarian cysts, or a cystic neoplasm of the ovary. Simple cysts are virtually always benign ovarian lesions. Except for a cystic neoplasm of the ovary, simple ovarian cysts tend to resolve on their own without intervention.

Very large, simple-appearing ovarian cysts, those measuring greater than 7 cm in diameter, may be difficult to evaluate completely by ultrasound. Further imaging with magnetic resonance imaging (MRI) or referral for surgical evaluation may be warranted for these very large cysts.

In postmenopausal women, simple cysts up to 1 cm in size are not uncommon and generally require no further assessment or follow-up. Simple cysts measuring between 1 cm and 7 cm, while most often benign, should undergo sonographic follow-up to resolution or until the cyst proves stable in size after 1–2 years. For simple cysts greater than 7 cm, MRI or surgical evaluation is often recommended.

A simple cyst of the ovary is an anechoic lesion with thin, smooth walls and enhanced through transmission (Figure 31.1.1). No blood flow should be detected within the cyst or its wall (Figure 31.1.2). Normal ovarian tissue is usually visible around a portion of the cyst, proving its intraovarian location.

A hemorrhagic ovarian cyst results when a functional ovarian cyst bleeds into itself. Hemorrhagic cysts, like simple ovarian cysts, are benign lesions of the ovary that most often resolve spontaneously, without requiring surgical intervention. Occasionally, acute hemorrhage into a cyst will cause sudden onset of pelvic pain, and, rarely, a hemorrhagic cyst ruptures.

Hemorrhagic cysts typically occur in women of reproductive age and not in postmenopausal women.

Hemorrhagic cysts appear as complex ovarian lesions, often with fine septations that form a reticular pattern throughout the cyst (Figures 31.2.1 and 31.2.2). The pattern of septations
is sometimes described as web-like or lacy. The fluid within the cyst often contains scattered echoes. The walls of a hemorrhagic cyst may be thin and smooth, or they may be focally or diffusely thickened. There is typically no blood flow seen within the cyst or in its septations, but flow may be seen in its outer walls (Figure 31.2.2).

The appearance of a hemorrhagic cyst typically evolves after the initial hemorrhage. With time, after the initial reticular pattern, the echoes within the cyst may conglomerate to one side of the cyst (Figure 31.2.3), sometimes with a concave contour. The conglomeration represents retracting clot. With continued involution, the cyst decreases in size and the septations and conglomeration of echoes become smaller or disappear (Figure 31.2.4).

The most common benign neoplasm of the ovary is a dermoid tumor, also called a mature cystic teratoma. This tumor is of germ cell origin and is often found in women of reproductive age. It is bilateral in 10% to 15% of cases. Most dermoid tumors are asymptomatic, but occasionally they cause lower abdominal pain, swelling, and irregular menses. Ovaries containing dermoids are at risk of torsion. If an ovarian mass with features suggestive of dermoid is detected, it is generally surgically excised. At pathology, the tumor may be found to contain fat and sometimes bone, teeth, or hair. Rarely, an ovarian teratoma is malignant.

Mature cystic teratomas (dermoid tumors) of the ovary often have a sonographic appearance that allows them to be differentiated from other ovarian neoplasms. One characteristic appearance is a complex, partially cystic mass in the ovary that contains one or more highly echogenic regions (Figure 31.3.1). Echogenic lines and dots (Figure 31.3.2) may be seen within the cystic areas of the mass, representing hair. Some of the highly echogenic regions represent fat within the tumor. The fat may be seen floating on top of other fluids in the lesion, leading to the sonographic finding of a fluid–fluid level (Figure 31.3.3). There may be echogenic regions with shadowing (Figure 31.3.4) or densely calcified structures representing teeth or bone. Dermoid tumors typically have little or no detectable internal flow on color Doppler imaging.

Ovarian neoplasms that arise from epithelial cells and surrounding stromal cells can be malignant or benign. The most common benign neoplasms are mucinous or serous cystadenomas. Less common benign tumors arising from these cells include transitional cell (Brenner) tumors. Benign ovarian neoplasms can also arise from granulosa, theca, Sertoli, and Leydig cells. These neoplasms include ovarian fibromas, granulosa cell tumors, thecomas, and Sertoli–Leydig cell tumors.

Serous and mucinous cystadenomas of the ovary are typically complex ovarian lesions with septations separating areas of anechoic fluid (Figure 31.4.1). Modest blood flow can often be identified in the septations with color Doppler. Mucinous cystadenomas tend to have more septations than serous cystadenomas, and the fluid is more likely to contain low-level echoes (Figure 31.4.2). Some benign tumors are solid, such as fibroma (Figure 31.4.3) or thecoma. These tumors are typically homogeneous in echotexture and hypoechoic, sometimes causing acoustic shadowing, and their appearance may be similar to a uterine fibroid. Other benign tumors may contain both solid and cystic areas (Figures 31.4.4 and 31.4.5). In some benign cystic ovarian neoplasms, solid tumor nodules, with internal vascularity visible with color Doppler, are seen in the wall of the tumor (Figure 31.4.4), although this finding is more frequently seen with malignant than benign neoplasms.

Color Doppler is an important part of the sonographic evaluation of ovarian masses, because it provides information about the amount and location of blood flow within the

lesion. Spectral Doppler, however, is of limited value. While benign tumors tend to have high-resistance patterns of arterial flow, with a resistive index above 0.6 (Figure 31.4.5) or pulsatility index above 1.0, and malignant tumors often have low-resistance flow, with a resistive index below 0.4 or pulsatility index below 1.0, there is too much overlap in resistance patterns for spectral Doppler to differentiate benign from malignant lesions reliably.