Ultrasound imaging and M-echo
To assess the state of the inner mucosal lining of the uterus, ultrasound is used—one of the most informative methods in gynecology. On the sonogram (grayscale visual image), the endometrium forms a midline uterine structure referred to as the M-echo.
The examination is performed using abdominal transducers (requiring a full bladder, as fluid has high sound conductivity and creates an acoustic window) or vaginal transducers. Vaginal transducers have higher resolution and are positioned as close as possible to the target object.
During longitudinal scanning, the physician evaluates five key criteria of the M-echo:
- Total thickness.
- Shape of the structure.
- Echogenicity (the ability of tissue to reflect ultrasound; myometrial echogenicity is taken as the reference standard).
- Sound conductivity (the ability of the wave to propagate into depth).
- Presence of additional echoes (inclusions) within the structure.
Endometrial dynamics during the menstrual cycle
In reproductive age with a normal biphasic menstrual cycle, the structure of the mucosa changes constantly. The ultrasound presentation strictly depends on the cycle phase:
- First week of the cycle: the endometrial echostructure is homogeneous, characterized by low echogenicity (appears darker on the screen).
- Secretory phase: the thickness of the M-echo gradually increases and reaches 0.8–1.0 cm. The area of increased echogenicity transforms, acquiring a characteristic spongy structure.
- Late secretory phase (the last week before expected menstruation): the mucosa reaches its maximum, with the thickness of the echogenic zone measuring up to 1.5 cm.
- Menstruation: during this period, the classical M-echo is not clearly defined. The sonogram visualizes a moderately expanded uterine cavity containing heterogeneous inclusions (sloughing tissues, blood clots).
Age-related evolution of the mucosa
The appearance of the inner uterine lining changes dramatically depending on the patient's age and sex hormone levels.
- Neonatal period: the uterus is enlarged under the influence of maternal estrogens, and the endometrium is visualized very clearly.
- Prepuberty (by 1 year): the uterus decreases in size, assumes a tubular shape, and the endometrium is no longer detected on ultrasound.
- Pubertal period: the uterus enlarges, acquiring the pear-shaped form characteristic of an adult woman. The appearance of the mucosal lining begins to change cyclically.
- Postmenopause: against the background of declining ovarian function and a general decrease in uterine size, the M-echo thins. Normally, it appears as a thin line 3–4 mm thick or has a punctate structure.
Additional diagnostic methods
If standard 2D ultrasound is insufficient for a detailed assessment of the mucosa, additional instrumental techniques are used:
- Three-dimensional (3D) ultrasound: allows obtaining an image in three mutually perpendicular planes, enabling a detailed study of the internal structure and vascular bed.
- Hydrosonography (HSG): a contrast agent is injected into the uterine cavity. Fluid significantly facilitates the visualization of anatomical structures, increasing the diagnostic value of the study.
- Histological examination: a crucial stage of diagnosis. The material is obtained by fractional diagnostic curettage (FDC) of the uterine mucosa under hysteroscopic guidance. Normally, histology should confirm the presence of the secretory phase (its absence indicates pathology, e.g., in dysfunctional bleeding).
To memorize the evolution of the M-echo throughout the cycle, use the rule "from dark to light and fluffy." At the beginning of the cycle, echogenicity is low (darker on the screen), and by the late secretory phase, it increases (becomes whiter), acquires a spongy structure, and thickens up to 1.5 cm.
Class and exam questions
How to correctly measure endometrial thickness on ultrasound?
Measurement of the M-echo, which corresponds to the two combined layers of the mucosal lining, is performed strictly during longitudinal scanning of the uterine body.
Is the endometrium visible in infants?
In newborn girls, it is clearly visualized due to the influence of maternal estrogens. However, by 1 year of age (in prepuberty), the endometrium is normally not detected on ultrasound.
What does the uterine cavity look like during menstruation on a sonogram?
The M-echo itself is not clearly defined during this period. The physician observes a moderate expansion of the uterine cavity with heterogeneous inclusions inside.
What else to study on this topic
- Ultrasound anatomy and normal myometrial dimensions
- Dynamics of ovarian follicular apparatus development
- Doppler ultrasound (CDFI) and blood flow assessment in gynecology
- Histological appearance of the endometrium in dysfunctional uterine bleeding
- Functional diagnostic tests (FDT) for assessing hormonal status
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