What Does an Ovarian Cyst Look Like on Ultrasound?

Most ovarian cysts appear on ultrasound as dark, fluid-filled circles with thin, smooth walls and no internal structures. This classic appearance belongs to the simple cyst, which is by far the most common type and almost always benign. But not every ovarian cyst looks the same, and radiologists rely on a set of specific visual features to distinguish harmless functional cysts from endometriomas, dermoids, cystadenomas, and the rarer malignant masses. Understanding those visual differences can make your ultrasound report a lot less confusing.

The Simple Cyst

A simple cyst is the baseline image that all other cyst types are compared against. On ultrasound, it shows up as a round or oval structure filled with completely black (anechoic) fluid, surrounded by a paper-thin wall. There are no internal partitions, no solid bumps, and no debris floating inside. The back wall of the cyst typically appears brighter than the surrounding tissue because sound waves pass through the fluid easily and reflect strongly off the far side. Radiologists call this “posterior acoustic enhancement,” and it is one of the hallmarks of a fluid-filled structure.

Simple cysts are extremely common, and most require no treatment or follow-up. A consensus update from the Society of Radiologists in Ultrasound recommends that in premenopausal women, simple cysts smaller than about 3 cm don’t even need to be mentioned in the report, and follow-up imaging is reserved for those larger than roughly 5 to 7 cm. In postmenopausal women, the thresholds are a bit lower: reporting starts at 1 cm, but follow-up is only suggested for simple cysts over about 3 to 5 cm.¹ If a simple cyst shrinks by even 10 to 15 percent on a repeat scan, further monitoring is unnecessary. The ones that grow, however, may turn out to be cystadenomas and deserve closer attention.

Corpus Luteum Cysts

After ovulation each month, the follicle that released the egg transforms into a structure called the corpus luteum. Sometimes it fills with fluid and becomes a corpus luteum cyst. On ultrasound, these look quite different from simple cysts. The hallmark is a thick, irregular (often described as “crenulated”) wall that lights up with blood flow when Doppler is applied. The cyst itself is typically 1 to 3 cm and filled with fluid that may contain low-level echoes from recent bleeding.² That thick, scalloped wall is the key feature that separates a corpus luteum cyst from a simple cyst. It can occasionally alarm patients who see the image and mistake the wall thickening for a solid mass, but it is entirely normal and expected during the second half of the menstrual cycle and early pregnancy.

Hemorrhagic Cysts

When a cyst bleeds internally, the ultrasound picture changes dramatically. Fresh blood doesn’t look like clear fluid. Instead of a clean black circle, a hemorrhagic cyst often contains a web of thin, strand-like echoes that radiology reports sometimes describe as “cobweb” or “fishnet” patterns. These internal strands are fibrin clots. Over time, the blood products settle and the cyst’s appearance evolves: areas of bright (hyperechoic) clot gradually separate from darker fluid, and eventually the whole thing can resolve into a simple cyst pattern before disappearing entirely.³

The evolving nature of hemorrhagic cysts is actually useful diagnostically. If a cyst with internal echoes is scanned again a few weeks later and has changed in appearance or shrunk, that behavior strongly suggests it was hemorrhagic rather than something more concerning. A solid tumor doesn’t reorganize itself over the course of a menstrual cycle.

Endometriomas

An endometrioma, sometimes called a “chocolate cyst,” forms when endometrial tissue grows on the ovary and fills a cyst with old, thick, dark blood. On ultrasound, the classic appearance is strikingly uniform: the cyst is filled with homogeneous, low-level echoes that give it a slightly hazy, grayish look throughout. Radiologists describe this as “ground glass” echogenicity because the image resembles frosted glass rather than clear fluid. About half of endometriomas present as single-chambered cysts with this ground glass pattern.⁴

The ground glass look is so characteristic that experienced sonographers can often spot an endometrioma at a glance. However, the pattern isn’t foolproof. Some hemorrhagic cysts can mimic the appearance early in their evolution, and some endometriomas develop small wall nodules that need to be evaluated for other pathology. When a woman with known or suspected endometriosis has a cyst with this uniform hazy fill, though, the diagnosis is usually straightforward.

Dermoid Cysts

Dermoid cysts (also called mature cystic teratomas) produce some of the most visually striking ultrasound images because they can contain fat, hair, teeth, and even bits of bone. These mixed tissues create a jumble of ultrasound appearances. The characteristic features include a visible line where fat floats on top of denser fluid (a fat-fluid level), a tangled echogenic mass sometimes called a “dermoid mesh,” and a phenomenon known as the “tip of the iceberg” sign.⁵

The tip of the iceberg sign deserves special mention because it can cause confusion. Hair and sebum inside the dermoid reflect ultrasound waves so strongly that they cast a dense acoustic shadow behind them, completely hiding the back wall of the mass. On the screen, the sonographer sees a bright, highly echogenic area at the top of the mass with nothing visible behind it, as though most of the cyst is hidden below the surface.⁶ A less experienced reader might mistake this for bowel gas or fail to recognize a mass is present at all. Despite their dramatic imaging appearance, dermoids are benign in the vast majority of cases.

Cystadenomas

Cystadenomas are benign tumors of the ovary’s surface lining that come in two main flavors: serous and mucinous. Their ultrasound appearances differ enough that radiologists can often tell them apart. A large international database of ovarian tumors found that serous cystadenomas tend to be single-chambered or have just a few compartments, with clear (anechoic) fluid inside and little to no blood flow on Doppler. Their median size was about 68 mm. Mucinous cystadenomas, by contrast, tend to be larger (median about 93 mm), are more often multilocular with several compartments, and their fluid content shows low-level echoes rather than being perfectly clear.⁷

One distinctive feature of some mucinous cystadenomas is a “honeycomb” pattern, where multiple small locules cluster together in a way that resembles a cross-section of honeycomb. This pattern showed up in about 14% of mucinous cystadenomas in that database and was not found in serous cystadenomas at all. Both types are benign, but because they can grow large and because their compartmentalized appearance can mimic borderline or malignant tumors, they often end up getting surgically removed for a definitive diagnosis.

Features That Raise Concern for Malignancy

The vast majority of ovarian cysts are benign. But radiologists are trained to look for specific features that shift the probability toward something more serious. The ultrasound signs that raise a red flag include solid components within or beside a cystic area, thick internal walls or partitions (septations), papillary projections growing from the cyst wall into the fluid, and irregular borders. No single feature is a slam dunk for cancer, but certain combinations are particularly concerning.

Papillary projections are among the most studied warning signs. In one study comparing benign, borderline, and malignant ovarian masses, the projections found in malignant tumors averaged about 35 mm in size, compared to roughly 10 mm in benign tumors. In malignant masses, these projections tended to have irregular surfaces, obtuse angles where they met the cyst wall, and they were scattered across the wall rather than isolated in one spot. When a papillary projection was 10 mm or larger and showed blood flow on color Doppler, it was present in all the malignant tumors studied but absent in all the benign ones.⁸

What Doppler Adds to the Picture

Standard grayscale ultrasound shows the structure of a cyst. Doppler ultrasound adds a layer of information by detecting blood flow within and around it. Malignant tumors tend to recruit new blood vessels to fuel their growth, so the pattern of blood flow can help distinguish benign from malignant masses. One study found that color Doppler detected blood flow in nearly all malignant ovarian tumors (about 98%), compared to roughly two-thirds of benign ones.⁹

The location of blood flow matters as much as its presence. Central blood flow within a solid component is far more suspicious than flow limited to the periphery of the cyst wall. A study evaluating Doppler predictors of malignancy found that new blood vessels with a central location carried a high predictive value for cancer, second only to a solid tissue component itself as a risk indicator.¹⁰ Simple cysts and most functional cysts show no internal blood flow at all, which is part of why they are so reassuring.

The O-RADS Scoring System

If you’ve had a pelvic ultrasound and your report mentions an “O-RADS” score, it refers to the Ovarian-Adnexal Reporting and Data System. This is a standardized framework developed by the American College of Radiology to reduce vague or confusing ultrasound reports. Instead of a radiologist writing something subjective like “possible complex cyst, clinical correlation recommended,” the O-RADS system assigns a score from 0 to 5 based on specific imaging features, with each number corresponding to an estimated risk of malignancy and a management recommendation.¹¹

In practice, the system works well. A U.S. validation study of over 1,000 adnexal lesions found that the malignancy rate was less than 1% for O-RADS 2 (essentially benign-looking), about 5% for O-RADS 3 (low risk), roughly 12% for O-RADS 4 (intermediate risk), and about 66% for O-RADS 5 (high risk). Using a cutoff of O-RADS 4 or higher to flag potential cancer yielded a sensitivity above 90% and a specificity above 80%.¹² An O-RADS 2 score on your report is very reassuring. An O-RADS 4 or 5 doesn’t necessarily mean cancer, but it does mean your doctor will likely recommend further workup.

Transvaginal Versus Transabdominal Scanning

Two types of ultrasound are used to evaluate ovarian cysts: transabdominal (where the probe goes on the belly, using a full bladder as a sound window) and transvaginal (where a smaller probe is placed in the vagina, much closer to the ovaries). For cyst characterization, transvaginal scanning is consistently superior. In a prospective comparison, transvaginal images provided more detailed information about a mass’s internal architecture in about three-quarters of patients, and transabdominal scanning did not provide better information in any patient.¹³ Another study found that transvaginal scanning yielded new diagnostic information in 60% of cases and improved visualization in an additional 22%.¹⁴

The advantage is straightforward: the transvaginal probe sits just centimeters from the ovaries, so it uses higher-frequency sound waves that produce sharper images. This makes a real difference when the question is whether a cyst has thin septations, tiny solid nodules, or subtle internal echoes. A more recent comparison found transvaginal ultrasound had a sensitivity of 90% for detecting and characterizing pelvic masses, compared with 80% for transabdominal scanning.¹⁵ Transabdominal scanning still plays a role, particularly for very large masses that extend out of the pelvis or in patients who cannot undergo transvaginal examination. In many clinical settings, both approaches are used during the same visit.

The “String of Pearls” in Polycystic Ovaries

Polycystic ovary syndrome (PCOS) produces a distinctive ultrasound appearance that is quite different from a single ovarian cyst. Rather than one large fluid-filled space, the ovary contains many small follicles, typically arranged around the periphery like beads on a necklace. This is the well-known “string of pearls” pattern. The ovary itself is often enlarged, and the central tissue (stroma) may appear unusually bright or echogenic.¹⁶

It is worth noting that the string of pearls pattern, while often discussed, is not formally part of the diagnostic criteria for PCOS. The distribution of follicles varies among individuals, and two main patterns have been described: the peripheral cystic pattern (the classic string of pearls) and a more general cystic pattern where small follicles are scattered throughout the ovary.¹⁷ PCOS is diagnosed using a combination of clinical, hormonal, and ultrasound criteria, not ultrasound alone. A polycystic-appearing ovary on ultrasound without accompanying symptoms doesn’t mean you have PCOS.

When a Cyst Isn’t a Cyst

One of the trickier aspects of pelvic ultrasound is distinguishing an ovarian cyst from other structures that can look similar. A hydrosalpinx (a fluid-filled, blocked fallopian tube) can mimic a cyst, particularly when it folds on itself and appears round. Radiologists look for specific signs to tell them apart: a tubular shape, a “waist sign” where the tube appears pinched at intervals, and tiny projections from the wall that represent the remains of the tube’s inner folds. One study found that the combination of a waist sign and a tubular shape was present in hydrosalpinges but in no other adnexal masses, making it essentially diagnostic.¹⁸

Paraovarian cysts (cysts that sit next to the ovary rather than within it) and peritoneal inclusion cysts can also be mistaken for ovarian cysts if the sonographer doesn’t carefully identify the ovary as separate from the mass. Bowel loops filled with fluid occasionally create confusion as well, though their peristaltic movement usually gives them away on real-time scanning. These are the kinds of distinctions that experienced sonographers make routinely but that are invisible to the patient watching the screen.

Ovarian Torsion on Ultrasound

A cyst can become a medical emergency if it causes the ovary to twist on its blood supply, a condition called ovarian torsion. On ultrasound, the most definitive sign is the “whirlpool sign,” where the twisted vascular pedicle (the stalk carrying blood vessels to and from the ovary) appears as a spiraling structure on real-time scanning.¹⁹ The affected ovary typically appears swollen and edematous, often significantly larger than the other side, and Doppler may show reduced or absent blood flow.

A systematic review of ultrasound signs for detecting torsion found that the whirlpool sign, ovarian edema, and decreased Doppler flow all had good specificity, meaning that when these signs are present they reliably point to torsion. However, their sensitivity was only moderate, meaning torsion can sometimes be present without all the classic signs on ultrasound.²⁰ Simply finding a mass or some free fluid in the pelvis, by contrast, was not particularly helpful for confirming or ruling out torsion. If clinical suspicion is high and the patient is in acute pain, surgery may proceed even when the ultrasound is equivocal.

Ovarian Cysts in Children and Adolescents

Ovarian cysts are not limited to adults. They can occur at any age, including in newborns and young children, though the types of cysts and their implications differ by age group. In the pediatric population, germ cell tumors make up the majority of ovarian neoplasms, whereas in adults, epithelial tumors (like cystadenomas) are far more common.²¹ Simple functional cysts in adolescents who have started menstruating are generally managed the same way as in adult premenopausal women. In younger children or in cases where ultrasound features are complex, the threshold for further workup tends to be lower because the spectrum of possible diagnoses is different.

Neonatal ovarian cysts deserve a brief mention because they are surprisingly common. They are usually detected on prenatal ultrasound and result from stimulation by the mother’s hormones. Most are simple and resolve on their own in the first few months of life. The concern in a newborn is torsion, since even a benign cyst can cause the tiny ovary to twist. If follow-up ultrasound shows a cyst that has become complex (developed internal echoes or lost its simple appearance), it may indicate that torsion has already occurred.

Contrast-Enhanced Ultrasound

Standard ultrasound and Doppler are the workhorses of ovarian cyst evaluation, but contrast-enhanced ultrasound (CEUS) is an emerging option for difficult cases. In CEUS, tiny gas-filled microbubbles are injected into a vein and travel to the blood vessels within and around a mass, dramatically increasing the ability to see blood flow patterns in real time. A recent meta-analysis found that CEUS had a pooled sensitivity of 93% and specificity of 91% for distinguishing benign from malignant ovarian masses, while the O-RADS ultrasound system alone achieved 94% sensitivity but lower specificity at 81%.²² The specificity advantage matters because it means fewer false alarms and potentially fewer unnecessary surgeries. CEUS is not yet standard practice for ovarian cyst evaluation in most settings, but it may become a useful add-on when conventional imaging leaves doubt.