Endometrium on ultrasound

Physical principles and types of transducers

To understand how the uterine mucosa appears on the monitor, one must understand basic acoustic terms. The ultrasound image (echogram) is formed in a gray-white scale and depends on echogenicity—the ability of tissues to reflect the signal. Myometrial tissue is always taken as the standard of medium echogenicity.

All structures are divided into:

• Anechoic (black on the screen): freely transmit the wave (e.g., fluid in cysts or the urinary bladder).
• Hypoechoic (gray on the screen): create obstacles in a fluid medium (blood, pus, debris).
• Hyperechoic (white on the screen): maximally dense objects (calcifications, bones).

The second important parameter is sound conduction, i.e., the ability of the signal to penetrate deeply. Fluid has excellent sound conduction, which is why a full urinary bladder is used as an acoustic window during the transabdominal approach.

Different types of transducers are used to visualize internal genitalia. Abdominal transducers require a full urinary bladder. Vaginal transducers are positioned as close to the organ as possible and therefore have a higher resolution, although they are not always convenient for examining excessively large masses. In pediatric practice or as an adjunct to the abdominal approach, rectal transducers may be used.

What is the M-echo and how it is measured

During a uterine examination, the physician must evaluate the M-echo—the central uterine structure. Anatomically, it represents two apposed layers of the endometrium (anterior and posterior walls).

To correctly measure mucosal thickness, the transducer must be positioned to obtain a longitudinal section of the organ. When describing the M-echo, the physician analyzes five key criteria:

1. Total thickness.
2. Shape of the structure.
3. Degree of echogenicity.
4. Tissue sound conduction.
5. Presence of additional echoes (inclusions) within the contour.

In addition to the endometrium itself, uterine dimensions are evaluated, which are variable and depend on age, cycle phase, and history of pregnancies. In a healthy non-pregnant woman of reproductive age, the average uterine body length is 52 mm, thickness is 38 mm (measured on a longitudinal scan), and width is 51 mm (measured on a transverse scan). Cervical length varies from 20 to 35 mm. Normally, the myometrium has a fine-grained structure and medium echogenicity.

Endometrial changes across cycle phases

The uterine mucosa is hormonally dependent, so its ultrasound appearance changes drastically throughout a normal biphasic menstrual cycle.

• First week of the cycle: the endometrium appears homogeneous, and its echogenicity is decreased.
• Secretory phase: the thickness of the M-echo gradually increases and reaches 0.8–1.0 cm. The area of increased echogenicity acquires a characteristic spongy structure.
• Late secretory phase (the last week before the expected menstruation): the mucosa reaches its maximum thickness—up to 1.5 cm.
• Menstrual period: the typical M-echo loses its clear outlines and is not defined. Only a moderately dilated uterine cavity filled with heterogeneous inclusions is visualized on the screen, which corresponds to sloughing tissue and blood accumulation.

Features in menopause and detailing methods

With the onset of postmenopause, hormonal stimulation ceases, and the uterus undergoes age-related changes (decreases in size). The endometrium thins: normally, the M-echo appears as a thin line only 3–4 mm thick or is visualized as punctate structures.

If the standard vaginal or abdominal approach is insufficient to evaluate the uterine cavity, refinement techniques are used:

• Saline infusion sonography (SIS): a special contrast agent is injected into the uterine cavity. The fluid separates the walls, creating an ideal acoustic window for more accurate visualization of the endometrial relief.
• Three-dimensional ultrasound (3D): allows for the generation of a 3D image in three mutually perpendicular planes, which is physically impossible with standard 2D scanning. This provides maximum information about the internal structure of the object and its vascular bed.

Class and exam questions

What else to study on this topic

• Pelvic ultrasound anatomy and normal uterine dimensions
• Evaluation of the follicular apparatus and ovarian volume
• Color Doppler ultrasound in gynecology: RI and PI indices
• Ultrasound in cervical pathology and polyps
• Saline infusion sonography: indications and technique

Other topics in the "Gynecology" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.