Endometriosis is a common condition where tissue similar to the lining of the uterus, called the endometrium, grows outside the uterine cavity. This misplaced tissue often develops in the ovaries, fallopian tubes, and the tissues lining the pelvis, causing inflammation and pain. An ultrasound is typically the first-line imaging tool used to evaluate the pelvis when endometriosis is suspected.
The Role of Transvaginal Ultrasound in Screening
The primary imaging method used to screen for pelvic conditions is the transvaginal ultrasound (TVUS), which offers a significant advantage over the transabdominal approach. The TVUS probe is inserted into the vaginal canal, allowing it to be much closer to the pelvic organs. This proximity provides higher-resolution images of the uterus, ovaries, and the surrounding structures. The transabdominal ultrasound, which scans through the abdominal wall, must contend with a greater distance and intervening tissue, resulting in lower image clarity.
The initial screening purpose of the TVUS is two-fold: to rule out other potential causes of pelvic pain and to assess the pelvic anatomy broadly. Conditions such as ovarian cysts, uterine fibroids, or adenomyosis can present with similar symptoms and are often clearly visible on the scan. As a non-invasive procedure that is widely accessible and relatively low-cost, TVUS serves as the first step in the diagnostic pathway.
Specific Endometriosis Findings Detectable by Ultrasound
A specialized ultrasound performed by an experienced sonographer can reliably identify specific types of endometriosis that form distinct masses or nodules. The most commonly identified finding is the endometrioma, frequently referred to as a “chocolate cyst.” These are blood-filled cysts that form on the ovaries when endometrial-like tissue implants and bleeds within the ovarian tissue. On the scan, endometriomas typically appear as unilocular or multilocular cysts with a characteristic “ground-glass” echogenicity, which describes the uniform low-level echoes caused by old, thick blood within the cyst.
The ultrasound is also effective at detecting Deep Infiltrating Endometriosis (DIE), which involves lesions that penetrate tissues more than five millimeters beneath the surface of the pelvic lining. DIE often manifests as solid, hypoechoic nodules or thick tissue that invades structures such as the uterosacral ligaments, the bowel, or the bladder wall. Visualizing these lesions often requires a dynamic examination, where the sonographer applies gentle pressure with the probe to assess organ mobility. Reduced mobility or a loss of the “sliding sign” between organs, such as the rectum and the uterus, can be an indirect sign of adhesions caused by deep disease.
The distinction between ovarian disease and deep tissue involvement is an important feature of the ultrasound assessment. Endometriomas are fluid-filled masses within the ovary, while DIE presents as solid, fibrotic tissue in other locations. Specialized techniques, sometimes including mild bowel preparation, are occasionally used to enhance the visibility of nodules located on the bowel wall. TVUS accuracy for these larger, more substantial forms of endometriosis makes it invaluable for guiding treatment planning.
Why Ultrasound May Not Detect All Endometriosis
Despite its capabilities, ultrasound has significant limitations in detecting all forms of the disease, meaning a person with severe symptoms may still receive a clear or “normal” scan result. The technology relies on visualizing differences in tissue density and structure, and it is particularly poor at detecting superficial peritoneal endometriosis. This form of the disease involves small, scattered implants on the pelvic lining, or peritoneum, that typically lack the mass or fluid content necessary to create a distinct image on the ultrasound.
These superficial lesions are often only a few millimeters in size, making them too small or diffuse to be reliably visualized with current ultrasound technology. For this reason, patients with minimal or mild stage disease are likely to have a negative ultrasound, even if they experience chronic, debilitating pain. Therefore, a negative ultrasound result should never be used to definitively rule out the presence of endometriosis, especially when a patient’s clinical history suggests the condition.
Diagnostic Steps Following an Inconclusive Ultrasound
When a patient’s symptoms persist but the TVUS is negative or inconclusive, the clinical pathway moves toward more advanced diagnostic methods. The next step in imaging often involves Magnetic Resonance Imaging (MRI), which can provide greater detail and a wider field of view than ultrasound. MRI is particularly useful for mapping the extent of Deep Infiltrating Endometriosis and is frequently employed for pre-surgical planning. It helps clinicians understand the precise location and depth of lesions, especially those involving the urinary tract or bowel.
However, even advanced imaging techniques like MRI may not detect the small, superficial implants. To achieve a definitive diagnosis, laparoscopic surgery remains the established gold standard procedure. This minimally invasive surgery involves inserting a camera into the abdomen to allow for direct visualization of the entire pelvic cavity. Crucially, laparoscopy enables the surgeon to view, biopsy, and confirm the presence of superficial lesions missed by non-invasive imaging. Confirmation of the diagnosis is typically made through histological examination of the biopsied tissue.