Uterine fibroids, also known as leiomyomas, are non-cancerous growths that develop within or on the uterus. These growths originate from the smooth muscle cells and connective tissue of the uterus. Fibroids are common, affecting an estimated 20% to 50% of women of reproductive age, and up to 70% to 80% by age 50. While many individuals experience no symptoms, others may have painful or heavy periods, or other issues depending on the fibroid’s size and location.
General Characteristics
Fibroids typically present as distinct, well-defined masses. They are usually round or oval in shape and can vary significantly in size, from as small as a grain of rice to as large as a melon. The texture of a fibroid is generally firm and rubbery, composed of dense muscle and fibrous tissue.
Fibroids are typically paler than the surrounding uterine muscle tissue, often appearing whitish or pale pink. Their solid consistency differentiates them from fluid-filled structures. These characteristics help distinguish fibroids from the healthy myometrium, the muscular wall of the uterus.
Appearance on Diagnostic Imaging
Diagnostic imaging provides a detailed view of fibroids, revealing their internal structure and relationship to the uterus. Ultrasound is often the initial method due to its accessibility and non-invasive nature. On ultrasound, fibroids commonly appear as solid, well-defined masses. They are often described as hypoechoic (darker than surrounding uterine tissue), though they can also be isoechoic (similar in brightness) or hyperechoic (brighter).
Fibroids may exhibit a heterogeneous, or uneven, echotexture, particularly if they have undergone degenerative changes. Calcifications (calcium deposits) can appear as bright spots with acoustic shadowing. Color Doppler ultrasound may show blood flow around the fibroid, often in a circumferential pattern.
Magnetic Resonance Imaging (MRI) offers greater detail, used for comprehensive assessment, especially for complex cases or surgical planning. On T1-weighted MRI images, non-degenerated fibroids typically show low to intermediate signal intensity, similar to or slightly darker than the normal myometrium. On T2-weighted images, they usually appear with low signal intensity, attributed to their dense fibrous and muscular composition. Fibroids can display a high-intensity rim on T2-weighted images, possibly due to dilated veins, lymphatic vessels, or edema. Areas of degeneration within a fibroid can alter its MRI appearance, potentially leading to higher signal intensity on both T1 and T2 images depending on the type of degeneration.
Calcifications within fibroids appear as signal voids on MRI. This means they produce no signal.
Variations by Location and Size
The appearance of fibroids on imaging can vary significantly based on their specific location within the uterus and their overall size. Intramural fibroids are the most common, developing directly within the muscular wall of the uterus (myometrium). On ultrasound, they appear as masses embedded within the myometrium, often causing the uterine wall to thicken or bulge.
Subserosal fibroids grow on the outer surface of the uterus, beneath its outermost layer. They can project outward, sometimes appearing as discrete masses extending from the uterus. If a subserosal fibroid is attached to the uterus by a thin stalk, it is called a pedunculated subserosal fibroid. They can sometimes be confused with ovarian masses, but a “bridging vessel” sign connecting the fibroid to the uterus on Doppler ultrasound or MRI helps confirm its uterine origin.
Submucosal fibroids are less common, but they grow just beneath the inner lining of the uterus and protrude into the uterine cavity. These fibroids can significantly distort the shape of the endometrial cavity, which is often visible on imaging. Their presence can be challenging to assess with standard ultrasound, sometimes requiring specialized techniques like saline infusion sonography for better visualization.
Smaller fibroids may be uniform in texture, but as they grow larger, they can become more complex and heterogeneous due to internal changes like degeneration, hemorrhage, or calcification. Large fibroids can distort the overall shape of the uterus, which is evident on imaging scans. Calcification is more frequently observed in larger, older fibroids, and these dense calcium deposits can appear very bright on ultrasound or as signal voids on MRI.