Uterine polyps, also known as endometrial polyps, are common, typically benign overgrowths of the endometrium, the tissue lining the inside of the uterus. These masses are composed of endometrial glands, connective tissue, and blood vessels, projecting into the uterine cavity. Although most polyps are non-cancerous, they can cause symptoms like abnormal uterine bleeding or difficulty with fertility. Ultrasound is the preferred, non-invasive imaging method used to locate and characterize these growths.
Ultrasound Techniques Used to Visualize the Uterus
Transvaginal Ultrasound (TVUS) is usually the initial procedure performed to screen for uterine abnormalities. This technique involves placing a specialized probe into the vagina, allowing the sound waves to generate a high-resolution image of the uterus and its internal lining. On a standard TVUS, a polyp may appear as a focal area of thickened tissue or a non-specific mass. However, the minimal fluid naturally present in the uterine cavity often makes it difficult to clearly outline the exact borders of a small polyp.
To overcome this limitation, a more advanced technique called Saline Infusion Sonography (SIS), or sonohysterography, is often performed. SIS is a specialized ultrasound procedure where sterile saline solution is gently injected through the cervix into the uterine cavity. The infused saline acts as an acoustic contrast agent, temporarily distending the cavity.
This fluid-filled environment makes the uterine cavity a dark, anechoic space on the ultrasound screen, allowing any masses, like polyps, to be cleanly outlined. SIS significantly improves the ability to determine the exact size, location, and attachment point of the lesion. SIS offers a much higher diagnostic accuracy for identifying focal lesions compared to a standard transvaginal scan alone.
Distinct Visual Features of Uterine Polyps
When visualized using Saline Infusion Sonography, a uterine polyp typically presents as a smooth, well-defined mass projecting into the fluid-filled uterine cavity. The internal echo pattern, or echogenicity, of a polyp is usually homogeneous and often appears hyperechoic, meaning it is brighter than the surrounding normal endometrium. Sometimes, polyps may contain small, anechoic areas, which correspond to dilated endometrial glands filled with fluid.
The mass is often seen to interrupt the smooth contour of the endometrial lining, a finding sometimes described as the “interrupted mucosa sign.” Polyps connect to the uterine wall either by a slender stalk (pedunculated attachment) or by a broader base (sessile attachment). This distinction in the point of attachment is clearly visible when the polyp is surrounded by saline contrast.
A highly specific diagnostic feature is the presence of a single, dominant blood vessel supplying the mass, detected using Color Doppler imaging. This finding is often called the “feeding vessel sign” or “pedicle artery sign.” Color Doppler highlights the flow of blood, showing a single, central artery extending from the uterine wall directly into the core of the polyp. This single-vessel pattern helps confirm the diagnosis of an endometrial polyp.
Differentiating Polyps from Similar Uterine Findings
The precise visualization techniques are necessary because several other intrauterine findings can mimic the appearance of a polyp on a standard ultrasound. The most common challenge in diagnosis is distinguishing a polyp from a submucosal leiomyoma, a type of fibroid that projects into the uterine cavity. Polyps typically appear homogeneous and hyperechoic, originating from the endometrial lining itself.
Polyps vs. Submucosal Fibroids
In contrast, submucosal fibroids arise from the muscular layer of the uterus, the myometrium. They typically appear broader-based, with a more heterogeneous or varied echo texture. Fibroids often appear hypoechoic, or darker, compared to the surrounding tissue. Their presence can distort the normal interface between the endometrium and the myometrium. Furthermore, fibroids usually lack the distinct single-vessel pedicle seen in polyps, often showing multiple, scattered vessels or a peripheral rim of flow on Color Doppler imaging.
Polyps vs. Endometrial Hyperplasia
Another condition that must be distinguished from a polyp is endometrial hyperplasia, which is an overall thickening of the entire uterine lining. Polyps are focal, discrete masses, meaning they are confined to a single spot. Hyperplasia presents as a more diffuse, uniform thickening across the whole endometrial surface. The focal nature of the polyp, especially when clearly outlined by the saline in SIS, confirms it as a distinct lesion.