Ultrasound signs of a normal uterus

Echographic characteristics of the normal anatomical structure of the uterus

The ultrasound begins with a study of the location of the uterus, which is of great importance when performing invasive procedures.

Position of the uterus. In transabdominal ultrasound, longitudinal scanning allows diagnosing the displacement of the uterus along the sagittal axis based on the angle of inclination between the body and the cervix: with hyperanteflexion, the angle decreases, with retroflexion, the value of this angle in relation to the bladder exceeds 180°. A cross-sectional examination allows identifying the deviation of the uterus to the left or right.

During transvaginal ultrasound scanning, determining the topography of the uterus presents certain difficulties, which is associated with a decrease in the projection area of ultrasound waves. As a result, depending on the position of the uterus in the pelvic cavity, its various sections are examined sequentially; detection of the fundus of the uterus indicates retroflexion of the uterus, and the cervix indicates anteflexion.

In the anteroposterior section, transvaginal ultrasound determines the condition of the cervix: the direction of the axis of the cervical canal, the condition of the endocervix and internal os.

The cervical canal is very easily visualized and is defined as a continuation of the endometrium. The endocervix is represented on the echogram by a linear echo with a high level of sound absorption. The ultrasound picture depends on the quantity and quality of cervical mucus and varies depending on the phase of the menstrual cycle: from a thin echogenic structure to a very pronounced hypoechoic cavity, especially in the preovulatory period.

In some cases, at some distance from the endocervix, closer to the external os, there are cystic thin-walled round cavities reaching 20-30 mm in diameter (Ovulae Nabothi). In the immediate vicinity along the cervical canal, it is possible to detect liquid structures of various sizes, which, according to most researchers, are endocervical glands dilated due to obstruction.

Normally, the size and shape of the uterus vary widely depending on parity and the state of the reproductive system. By the reproductive period, the uterus on the echogram is a pear-shaped formation, its length reaches 6 cm, the anteroposterior size is 4 cm.

In women who have given birth, all dimensions of the uterus are increased by 0.7-1.2 cm. In postmenopause, a decrease in the size of the uterus is observed.

Assessment of the condition of the myometrium. The myometrium is divided into 3 zones.

The inner (hypoechoic) zone is the most vascularized part of the myometrium, surrounding the echogenic endometrium. The middle (echoic) zone is separated from the outer layer of the myometrium by blood vessels.

An important indicator is the so-called midline uterine echo (M-echo), which is the reflection of ultrasound waves from the endometrium and the walls of the uterine cavity. Its shape, contours, internal structure and anteroposterior size are assessed - a parameter that has the greatest diagnostic value in pathological conditions of the endometrium. When interpreting this criterion, one should take into account the patient's age, the phase of the menstrual cycle in women of reproductive age, and, in the presence of uterine bleeding, its duration and individual characteristics.

There are 4 degrees, corresponding to the ultrasound picture characterizing the physiological processes in the endometrium:

• Grade 0. The midline structure of the uterus is revealed as a linear echo with high acoustic density; it is determined in the early proliferative phase of the menstrual cycle and indicates a low content of estrogens in the body.
• Grade 1. Linear M-echo is surrounded by an echo-positive rim caused by edema of the stroma of the mucous membrane of the uterine cavity; determined in the late follicular phase: under the influence of estrogens, there is a sharp increase in the size of the tubular glands with thickening of the endometrium.
• Grade 2 is characterized by an increase in the echogenicity of the distal M-echo zone (immediately adjacent to the endometrium). Typically, this type of echogram occurs in the preovulatory period and reflects the completion of the maturation of the dominant follicle, coinciding with an increase in progesterone content.
• Grade 3. The median M-echo is defined as a homogeneous, pronounced hyperechoic structure and corresponds to the secretory phase of the ovarian-menstrual cycle; the ultrasound picture is explained by an increased concentration of glycogen in the endometrial glands, caused by the effect of progesterone

A simpler interpretation of echograms in accordance with the phases of the menstrual cycle was proposed by Timor-Trisch and Rottem (1991). During menstruation, the endometrium is represented by a thin, intermittent echogenic line; dense hypoechoic structures (blood clots) are visualized in the uterine cavity. In the proliferative phase of the menstrual cycle, the thickness of the endometrium, isoechoic in relation to the myometrium, is 4-8 mm. In the periovulatory period, the endometrium may be represented by a trilinear echo. In the secretory phase of the menstrual cycle, the thickness of the echogenic endometrium is from 8 to 14 mm.

After menopause, the endometrium is usually thin (less than 10 mm in the anteroposterior section). Atrophic endometrium is characterized on the echogram by a thickness of less than 5 mm. In postmenopause, M-echo can be visualized during transabdominal examination in 27-30% of cases, during transvaginal examination - in 97-100%. Sometimes a small amount of fluid (2-3 ml) can be determined in the uterine cavity.

The main vessels of the small pelvis, accessible for visualization by transvaginal ultrasound and used in diagnostics of uterine pathology, are the uterine arteries and veins, as well as the endometrial vessels. The uterine vessels are usually easily visualized at the level of the internal os, closer to the lateral walls of the uterus. Doppler studies of blood flow in these vessels allow us to assess uterine perfusion.

Numerous studies have shown changes in the blood flow velocity curves in the uterine artery depending on the menstrual cycle: a marked decrease in the pulsatility index and resistance index in the luteal phase. There is no consensus on changes in blood flow indices in the uterine artery in the periovulatory period to date. However, for the correct interpretation of blood flow study data, the circadian rhythm of the pulsatility index in the uterine artery in the periovulatory period deserves attention: the pulsatility index is significantly lower in the morning than in the evening (increases during the day).

The intra- and subendometrial vessels of the endometrium are accessible for visualization using transvaginal ultrasound and color Doppler mapping. Establishing the presence or absence of blood flow is the simplest study, which nevertheless provides the most valuable information about the state of the endometrium. Thus, it is precisely the absence of blood flow in the subendometrial vessels that Zaidi et al. (1995) explain the failures in embryo transfer during in vitro fertilization.

The depth of vascular penetration of the endometrium is assessed by the larger internal part of the endometrium with pulsating vessels. In the presence of a three-layer endometrium (periovulatory period), the Applebaum classification (1993) by zones is used to assess the degree of vascular penetration of the uterus:

• Zone 1 - vessels penetrate the outer hypoechoic layer of the myometrium surrounding the endometrium, but do not penetrate the hyperechoic outer layer of the endometrium.
• Zone 2 - vessels penetrate the hyperechoic outer layer of the endometrium.
• Zone 3 - vessels penetrate the hypoechoic inner part of the endometrium.
• Zone 4 - vessels reach the endometrial cavity.