Endometriosis is a condition where tissue similar to the lining of the uterus grows outside the uterine cavity, often leading to chronic pain and inflammation. Ultrasound is the primary non-invasive imaging tool used to evaluate the pelvis and identify physical signs of this disease. It provides real-time visualization of the reproductive organs and surrounding structures, guiding clinicians toward a potential diagnosis. The effectiveness of the scan depends on the type of ultrasound used and the specific form of endometriosis present.
Types of Ultrasound Used in Diagnosis
The initial assessment often begins with a general pelvic ultrasound, performed either transabdominally or transvaginally. A transabdominal scan uses a probe placed on the lower abdomen, providing a broad view of the pelvic organs. This approach often requires a full bladder and is generally better for viewing larger masses or getting an overview of the entire pelvis.
To achieve the necessary detail for endometriosis detection, Transvaginal Ultrasound (TVUS) is the standard non-invasive method. The TVUS probe is inserted into the vagina, positioning the transducer closer to the uterus, ovaries, and deep pelvic structures. This proximity provides higher resolution images, which are essential for identifying subtle lesions and anatomical distortions caused by the disease.
Specialized TVUS protocols are frequently employed to systematically map the pelvis for signs of deep disease, extending the scan beyond the uterus and ovaries. This targeted approach improves the accuracy of the imaging by assessing commonly affected areas, such as the space behind the uterus. The accuracy of TVUS for moderate to severe disease makes it a foundational tool in the diagnostic workup.
Visual Signatures of Endometriosis
Ultrasound provides distinct visual cues for the two forms of endometriosis it can reliably detect: endometriomas and Deep Infiltrating Endometriosis (DIE). Endometriomas, often called “chocolate cysts,” are cysts on the ovaries filled with old, thick blood. On the screen, they typically appear as smooth-walled masses, unilocular or with few internal compartments, containing homogeneous low-level echoes.
This characteristic appearance is often referred to as a “ground-glass” texture, representing the chronic hemorrhagic debris inside the cyst. While this finding is suggestive of an endometrioma, they can occasionally appear more complex, sometimes resembling other types of cysts. The presence of bilateral ovarian cysts with this appearance, sometimes “kissing” or stuck together, increases the suspicion of endometriosis.
Deep Infiltrating Endometriosis (DIE) appears different, showing up as hypoechoic, or darker, irregular masses or nodules. These lesions represent tissue that has penetrated the surrounding structures by at least five millimeters. They are often found in locations like the uterosacral ligaments, the wall of the bowel, or the bladder.
When DIE involves the bowel, it is seen as an irregular thickening of the bowel wall, often starting in the muscular layer. The tissue can appear rigid and fixed, causing a distortion of the normal anatomy. The visual evidence of these deep nodules helps surgeons plan the complexity and type of procedure required before the patient enters the operating room.
Mapping the Extent of Disease
A specialized endometriosis ultrasound systematically maps the full extent of the disease and its impact on the pelvic anatomy. This process assesses the posterior compartment, including the Pouch of Douglas (the space between the uterus and the rectum). Disease presence here can cause organs to stick together, a condition known as cul-de-sac obliteration.
The sonographer checks for the “sliding sign,” which indicates the free movement of pelvic organs relative to one another. The loss of this sign is a strong indirect indicator of adhesions and scarring that has tethered the uterus or ovaries to the bowel or pelvic sidewall. This immobility suggests a more extensive disease burden.
The dynamic nature of the TVUS allows for the assessment of specific anatomical areas, such as the uterosacral ligaments, which are often infiltrated by DIE. These ligaments, which suspend the uterus, may appear thickened or nodular on the scan. The bladder and bowel are also examined for signs of endometriotic nodules or masses that could be causing symptoms like cyclical pain during urination or bowel movements.
This comprehensive mapping is a crucial step for pre-operative planning, providing a roadmap of the affected structures. By detailing the size, location, and depth of the lesions, the ultrasound findings allow clinicians to determine if specialists, such as colorectal surgeons, need to be involved in a potential surgical intervention.
When Ultrasound Requires Further Investigation
Despite its utility in detecting endometriomas and larger deep nodules, ultrasound has limitations, particularly concerning superficial peritoneal implants. These small lesions, often only a few millimeters in size, are located on the surface of pelvic organs and the peritoneum. They are too subtle to be resolved by ultrasound technology, meaning a scan can be normal even if endometriosis is present.
If symptoms persist despite a negative ultrasound, further steps are often necessary because the disease cannot be definitively ruled out. Magnetic Resonance Imaging (MRI) is a common follow-up, offering superior soft-tissue contrast. MRI can be beneficial for complex cases or for mapping disease in areas difficult to reach with ultrasound, and is especially useful for clarifying potential involvement of the bowel wall or the urinary tract.
Historically, the definitive diagnosis of endometriosis was achieved through laparoscopy, a minimally invasive surgical procedure. During laparoscopy, a surgeon visually inspects the entire pelvis and can take biopsies of suspected lesions for laboratory confirmation. While non-invasive imaging has reduced the need for diagnostic surgery, laparoscopy remains the standard when imaging is inconclusive or when surgical treatment is required.