Ultrasound is the primary, non-invasive imaging tool used to detect and characterize uterine fibroids, which are common growths. The accessibility, safety, and effectiveness of the technology make it the preferred initial step in the diagnostic process. This imaging technique provides detailed information about the size, number, and exact location of the growths within the uterus. Uterine fibroids, also known as leiomyomas, are almost always non-cancerous and affect a significant portion of women during their reproductive years.
What Uterine Fibroids Are
Uterine fibroids are benign tumors that originate from the smooth muscle tissue of the uterus, known as the myometrium. Their growth is linked to the hormones estrogen and progesterone, which explains why they typically develop during the reproductive years and often decrease in size after menopause. Fibroids can range in size from being practically microscopic to large masses that can alter the shape of the uterus.
The classification of a fibroid is determined by its specific location within the uterine wall, which also influences the type of symptoms a person may experience. Intramural fibroids are the most common type, growing within the muscular wall of the uterus itself. Subserosal fibroids develop on the outer surface of the uterus, sometimes growing on a stalk, which may cause pressure on nearby organs like the bladder or rectum.
Submucosal fibroids, though less common, grow just beneath the uterine lining and project into the uterine cavity. Even when small, these fibroids are often responsible for disruptive symptoms, such as heavy and prolonged menstrual bleeding. Other symptoms associated with fibroids include pelvic pressure, back pain, and a feeling of fullness in the lower abdomen.
How Ultrasound Identifies Fibroids
Ultrasound is the initial imaging method for evaluating the uterus because it uses high-frequency sound waves to create real-time images without using radiation. The sound waves bounce off internal structures and are converted into an image, allowing a technician to visualize the uterine wall and any growths within it. This technique is highly effective and can often detect fibroids as small as 3 to 5 millimeters.
The two main variations of the test used for detection are transabdominal and transvaginal ultrasound. Transabdominal ultrasound involves placing the probe on the lower abdomen, providing a broad overview of the entire pelvic region and being particularly useful for locating large fibroids. Conversely, transvaginal ultrasound involves inserting a smaller probe into the vagina, which offers a closer, higher-resolution view of the uterus.
The transvaginal approach is superior for visualizing smaller fibroids and those located deep within the pelvis or near the uterine lining. On the ultrasound screen, fibroids typically appear as solid, well-defined masses that look different from the surrounding uterine muscle. Radiologists characterize these masses by their echogenicity—how bright or dark they appear—often presenting as darker (hypoechoic) than the surrounding tissue. The sonographer measures the fibroid’s dimensions, notes its exact location, and counts the total number of growths. This detailed mapping is crucial because the size, number, and position determine the likelihood of symptoms and future treatment decisions.
Interpreting Results and Further Testing
The ultrasound report provides specific details that inform management, including the size, number, and precise location of the fibroids (e.g., intramural or subserosal). This information helps the healthcare provider understand which symptoms are most likely caused by the growths. The report also notes any distortion of the uterine cavity.
While ultrasound is a robust diagnostic tool, it has limitations in challenging cases. Very small fibroids or those obscured by bowel gas may be difficult to visualize clearly. A key limitation is the difficulty in reliably differentiating a benign fibroid from a rare, malignant tumor (leiomyosarcoma), as their appearance can be similar on ultrasound.
When the ultrasound results are inconclusive, or if the fibroid is submucosal and impacting the uterine cavity, further testing is necessary. A specialized technique called hysterosonography, or saline infusion sonogram, involves filling the uterine cavity with sterile saline during a transvaginal ultrasound. This saline distends the cavity, providing a clearer outline of any growths projecting into it. For more complex cases, such as very large or numerous fibroids, Magnetic Resonance Imaging (MRI) may be recommended. MRI provides superior soft-tissue contrast, offering greater detail about the tissue composition and relationship of the fibroids to surrounding structures. Finally, hysteroscopy, which involves inserting a thin telescope directly into the uterus, can be used to visually confirm the presence of submucosal fibroids and polyps.