Endometriosis is a condition where tissue similar to the lining of the uterus, called the endometrium, grows outside of the uterus. This misplaced tissue responds to hormonal changes during the menstrual cycle by thickening and bleeding, but because it is trapped outside the uterus, it causes inflammation, pain, and the formation of scar tissue. Diagnosing this condition often begins with a detailed assessment of symptoms, followed by imaging to look for physical signs of the disease. Ultrasound is a primary, non-invasive tool used early in this diagnostic process to visualize the pelvic organs and search for characteristic lesions.
How Ultrasound Functions in Pelvic Assessment
Ultrasound technology uses high-frequency sound waves that travel through the body’s tissues and reflect back to create real-time images of internal organs. It is a preferred initial imaging choice for pelvic assessment because it is non-invasive and widely accessible. For evaluating endometriosis, two primary methods are used: transabdominal ultrasound and transvaginal ultrasound (TVS).
Transabdominal ultrasound involves placing a transducer on the lower abdomen, often requiring a full bladder to serve as an acoustic window for better image clarity. While useful for a broad overview of the pelvis, this approach often lacks the resolution needed to spot smaller lesions. Transvaginal ultrasound involves inserting a slender, high-frequency probe directly into the vagina. This proximity to the uterus, ovaries, and surrounding pelvic structures provides superior resolution and detailed visualization.
The higher image quality from the transvaginal approach allows sonographers to detect subtle changes in tissue structure and accurately measure any cysts or nodules present. Dynamic scanning protocols are frequently incorporated, where the organs are gently moved with the probe to check for mobility and the presence of adhesions. These bands of scar tissue can tether organs together, a finding that suggests the presence of the disease.
The Distinct Appearance of Endometriomas
The most common and readily identifiable sign of endometriosis on an ultrasound is the ovarian endometrioma, frequently called a “chocolate cyst.” These cysts form when endometrial-like tissue implants on the ovary, bleeds cyclically, and collects old, thick blood within a closed sac. Endometriomas are typically found as an adnexal mass located near the uterus and ovaries.
The characteristic appearance of these cysts is defined by “ground-glass echogenicity.” This describes a homogeneous, low-level internal echo pattern within the cyst fluid. This hazy, uniform appearance is caused by hemorrhagic debris and old blood products suspended within the fluid, which reflects sound waves distinctly. In premenopausal women, this appearance is a strong indicator of an endometrioma.
Endometriomas are often unilocular, meaning they contain a single chamber, though they can sometimes be multilocular. The cyst walls are typically regular and relatively thick. A sign seen with bilateral ovarian endometriomas is the “kissing ovaries” sign, where the ovaries are pulled together behind the uterus due to extensive adhesive disease. Variations can exist, such as hyperechoic wall foci caused by cholesterol deposits.
Visualizing Deep Infiltrating Lesions
Deep infiltrating endometriosis (DIE) refers to lesions that penetrate more than five millimeters beneath the surface of the peritoneum. These deeply embedded nodules are challenging to visualize but are often responsible for severe pain symptoms. An experienced sonographer uses transvaginal ultrasound to actively search for these lesions in common sites such as the uterosacral ligaments, the rectovaginal septum, and the walls of the bladder or bowel.
These deep lesions typically appear on the ultrasound screen as hypoechoic areas, meaning they look darker than the surrounding healthy tissue. They present as solid nodules with irregular or stellate margins, which cause distortion and thickening of the affected structure.
The visualization of DIE often relies on secondary signs observed during a dynamic scan. Immobile or “fixed” organs, such as an ovary or the uterus that cannot be easily moved with the probe, suggest the presence of restrictive adhesions caused by deep disease. The obliteration of the pouch of Douglas due to adhesions is another strong indicator of deep infiltration. Assessing the depth and spread of these hypoechoic nodules is crucial for surgical planning.