22.1 Ovaries: Normal, Functional Cysts & Benign Masses
Key Takeaways
- Ovarian volume = length x width x height x 0.523; it is normally under about 10 mL premenopausally and roughly 2-4 mL after menopause, with the internal iliac vessels as the key posterior landmark
- The dominant follicle grows 2-3 mm per day to reach 18-24 mm before ovulation; a visible cumulus oophorus predicts ovulation within about 36 hours
- The corpus luteum shows a circumferential ring of fire on color Doppler, while hemorrhagic cysts show a reticular, lacy fishnet pattern with an avascular retracting clot
- Theca lutein cysts are bilateral, multiloculated soap-bubble ovaries driven by very high hCG states such as molar pregnancy, multiple gestation, and choriocarcinoma
- Classic benign signatures: the dermoid Rokitansky nodule with tip-of-the-iceberg shadowing, the endometrioma ground-glass echotexture, and the fibroma with Meigs syndrome (ascites plus pleural effusion)
Normal Ovarian Anatomy and Position
The ovaries are paired, almond-shaped organs that typically measure about 3 x 2 x 2 cm in a reproductive-age woman. Ovarian volume is calculated with the prolate ellipse formula — length x width x height x 0.523 — and averages about 6-10 mL premenopausally with an upper limit of normal near 18-20 mL, falling to about 2-6 mL after menopause with an upper limit near 8-10 mL (note that the 10 mL figure is the polycystic-ovary-morphology threshold, not the boundary of normal). An ovary more than twice the volume of its contralateral mate is considered abnormally enlarged. The ovaries lie lateral to the uterus in the adnexa, usually in the ovarian fossa (fossa of Waldeyer), which is bounded anteriorly by the external iliac vessels and posteriorly by the internal iliac vessels and ureter. When the ovaries are difficult to find, follow the internal iliac artery and vein — the ovary sits just anterior and medial to them. The ovary is suspended by the infundibulopelvic (suspensory) ligament, which carries the ovarian vessels; the ovarian arteries arise from the aorta just below the renal arteries, the right ovarian vein drains directly into the inferior vena cava, and the left drains into the left renal vein. Blood supply is dual, from the ovarian artery and from branches of the uterine artery.
Cyclical Follicular Changes
In the early follicular phase, several small antral follicles (each under 10 mm) are visible in both ovaries. A dominant follicle is selected around cycle day 5-7 and grows about 2-3 mm per day, reaching 18-24 mm just before ovulation. Visualization of the cumulus oophorus — a tiny echogenic mural mound containing the oocyte — predicts ovulation within roughly 36 hours. Sonographic signs that ovulation has already occurred include a collapsed or crenated follicle, a small amount of new free fluid in the cul-de-sac, and conversion of the endometrium to a secretory pattern. The ruptured follicle becomes the corpus luteum, which produces progesterone and then involutes into the fibrotic corpus albicans if pregnancy does not occur.
Functional Ovarian Cysts
- Follicular cyst: forms when a follicle fails to ovulate or to involute. Simple, unilocular, anechoic, and thin-walled with posterior acoustic enhancement; usually under 5 cm and resolves within one to two menstrual cycles.
- Corpus luteum cyst: has a thicker, echogenic, crenulated wall and may contain internal echoes. Color Doppler shows a prominent circumferential ring of fire with low-resistance arterial flow. The same ring can surround an ectopic pregnancy, so always correlate with the serum hCG level and confirm that the structure is intraovarian.
- Hemorrhagic cyst: bleeding into a functional cyst produces the classic reticular, lacy, fishnet pattern of internal echoes from fibrin strands, often with a retracting clot that has concave margins and no internal Doppler flow. These findings evolve and resolve on short-interval follow-up, which distinguishes them from neoplasm.
Theca lutein cysts develop in states of markedly elevated human chorionic gonadotropin (hCG): gestational trophoblastic disease (molar pregnancy), multiple gestation, Rh isoimmunization, and choriocarcinoma. They are bilateral, large, and multiloculated, producing a soap-bubble or spoke-wheel appearance, and they regress as the hCG level falls. Ovarian hyperstimulation syndrome (OHSS) is the iatrogenic counterpart seen with ovulation-induction drugs, particularly after an hCG trigger; the ovaries enlarge massively (often beyond 5-10 cm) with multiple large, thin-walled cysts, and the patient may develop ascites, pleural effusions, hemoconcentration, and a risk of thromboembolism.
Polycystic Ovaries
The Rotterdam criteria diagnose polycystic ovary syndrome (PCOS) when two of three features are present: oligo- or anovulation, clinical or biochemical hyperandrogenism, and polycystic ovarian morphology. The classic sonographic picture is the string of pearls — twelve or more small follicles (2-9 mm) arrayed around the periphery — combined with increased ovarian volume (over 10 mL) and prominent, echogenic central stroma. Newer high-resolution transducers raise the follicle-count threshold substantially (often 20 or more follicles per ovary), so always document follicle number, follicle size, and ovarian volume. A normal-sized ovary does not exclude the syndrome.
Benign Ovarian Masses
| Mass | Classic Appearance | Distinguishing Clues |
|---|---|---|
| Cystic teratoma (dermoid) | Echogenic Rokitansky nodule; tip-of-the-iceberg shadowing; fat-fluid level | Most common germ cell tumor; may contain hair, sebum, teeth |
| Serous cystadenoma | Simple, unilocular, anechoic cyst | Thin wall, no septations or nodules |
| Mucinous cystadenoma | Large, multiloculated with low-level echoes | Can fill the abdomen (15-30 cm) |
| Endometrioma | Diffuse homogeneous ground-glass low-level echoes | Chocolate cyst; cyclical pain; no internal flow |
| Fibroma | Solid, hypoechoic, attenuating with shadowing | Meigs syndrome: fibroma + ascites + pleural effusion |
The cystic teratoma (dermoid) is the most common benign ovarian neoplasm of reproductive-age women. Its Rokitansky nodule (dermoid plug) is a highly echogenic mural focus that casts an acoustic shadow; the tip-of-the-iceberg sign occurs when the echogenic anterior contents (sebum, hair, calcified teeth) attenuate the beam so severely that the posterior portion of the mass cannot be seen. Fat-fluid levels and fine linear hair echoes (the dermoid mesh or dot-dash pattern) are additional clues. Cystadenomas are epithelial tumors: the serous type mimics a simple cyst, while the mucinous type becomes huge and multiloculated, with low-level internal echoes in its locules from mucin. An endometrioma is endometrial tissue implanted on the ovary, filled with old blood that produces the classic homogeneous ground-glass echotexture; tiny echogenic foci in the wall (cholesterol deposits) support the diagnosis, and there is no internal vascularity. A fibroma is a solid sex cord-stromal tumor that is hypoechoic and strongly attenuating, easily mistaken for a pedunculated leiomyoma; when accompanied by ascites and a (usually right-sided) pleural effusion that resolve after tumor removal, the triad is called Meigs syndrome.
Scanning Technique and Documentation
Begin transabdominally with a comfortably full bladder (a 3-5 MHz curvilinear transducer), then scan transvaginally with an empty bladder and the patient in lithotomy position (a 5-9 MHz endocavitary probe). Measure each ovary in three planes, document follicle number and size, and for any cyst record its greatest dimension, wall thickness, septations, internal echotexture, and Doppler findings; comparison with prior scans is invaluable.
A 27-year-old woman with a negative pregnancy test has a thick-walled intraovarian cyst showing prominent circumferential color Doppler flow. This ring of fire pattern is most characteristic of which structure?
During a pelvic scan, only the anterior portion of an adnexal mass is visible; its intensely echogenic front edge attenuates the beam and obscures everything behind it. This tip-of-the-iceberg sign most strongly suggests:
A 34-year-old with cyclical pelvic pain has a unilocular adnexal cyst filled with diffuse, homogeneous, low-level ground-glass internal echoes and no internal Doppler flow. The most likely diagnosis is: