Ultrasound imaging has shown an extremely rapid evolution in the last two decades, thanks to the development of highly sophisticated both two-dimensional (2D) and three-dimensional (3D) technology and blood flow mapping, which render ultrasound the first-line imaging modality for the evaluation of the female pelvis.
The interpretation of the ultrasonographic findings requires knowledge of the uterine and ovarian ultrasound anatomy.
3.1 Examination Techniques
The ultrasound examination of the pelvis may be performed either with a transabdominal transducer or with a transvaginal high resolution transducer. The transvaginal approach is the best way of pelvic imaging since it offers the advantage of placing a high-frequency endocavitary ultrasound transducer in close proximity to target pelvic organs, thus improving image resolution and obviating the need for patients to have a full bladder before ultrasound examination (Figs. 3.1 and 3.2).
Fig. 3.1
Midsagittal plane of the pelvis
Fig. 3.2
Transvaginal ultrasound scans for the evaluation of the pelvis
It has the additional advantage of probing pelvic organs to elicit patient’s symptoms and thus correlating symptoms with specific pelvic anatomic locations. The practitioner therefore can gain crucial information by adding ultrasound findings with the physical examination [1].
3D technology offers additional advantages to the evaluation of the pelvic organs, such as multiplanar view, tomographic view, rendering view, and volume calculation [2, 3].
A further advantage is offered by the use of Color and Power Doppler technology which allows the examination of the vascular supply of the uterus and ovaries [4].
3.2 Uterus
In examining the uterus, the following should be evaluated: (1) the uterine shape, orientation, and size; (2) the myometrium; (3) the endometrium and (4) the uterine perfusion.
3.2.1 Shape, Orientation, and Size
The uterus represents the essential landmark of pelvic anatomy. It is located in the middle of the pelvis between the urinary bladder lying before and the large bowel lying behind it.
On a sagittal plane, the uterus has a pyriform shape: the superior two thirds correspond to the uterine body and the inferior third to the cervix. The uterine isthmus is identified where the uterine body and cervix meet.
The uterus may lie in the antiverted position (bent forward in relation to the vaginal axis) (Fig. 3.3) or in retroverted position (bent backward in relation to the vaginal axis).
Fig. 3.3
Transvaginal scan of a normal antiverted uterus during the periovulatory phase: an echo-free strip (arrows) may be seen in the cervical canal due to the fluid mucus production by the cervical glands
The cervix has an oval conic shape and is approximately 2.5 cm in length. The cervical canal appears as an echogenic line; at the time of ovulation, a hypoechoic or echo-free strip may be seen in the cervical canal due to the fluid mucus production by the cervical glands (Fig. 3.3). Sometimes, cystic structures within the cervix are visualized: they refer to Nabothian cysts, retention cysts of the cervical glands within the vaginal portion of the cervix, which have no clinical value (Fig. 3.4).
Fig. 3.4
Multiple Nabothian cysts within the cervix
The overall uterine length is evaluated in a sagittal view from the fundus to the cervix (to the external os, if it can be identified). When the uterus is markedly anteflexed (the corpus is strongly bent forward in relation to the cervical axis), the total length is obtained by adding the corporal and cervical length (Fig. 3.5). The depth of the uterus (anteroposterior dimension) is measured in the same sagittal view from its anterior to posterior walls, perpendicular to the length (Fig. 3.5). The maximum width is measured in the transverse view (Fig. 3.6).
Fig. 3.5
Measurement of the uterine length in an antiflexed uterus, obtained by adding the lengths of the corpus (D1) and the cervix (D2). The depth of the uterus (anteroposterior dimension) is measured in the same sagittal view from its anterior to posterior walls, perpendicular to the longitudinal axis
Fig. 3.6
Measurement of the uterine width in the transverse view
The uterine volume may be calculated by the formula of the ellipsoid prolate: Volume = length (cm) × anteroposterior diameter (cm) × transverse diameter (cm) × 0.523 or can be automatically measured by using the VOCAL (virtual organ computer-aided analysis) function offered by the 3D technology. This measurement, however, is not used routinely in the daily clinical setting. The size of the uterus changes according to age and parity. In the reproductive age, the normal uterus is 7.5–9 cm long, 3–4.5 cm deep, and 4.5–6 cm wide. The volume ranges from 50 to 70 cm3 [5].
3.2.2 Myometrium
The myometrium is the muscular part of the uterus, which is ultrasonically characterized by homogenicity and low echogenicity. The inner part of the myometrium confining with the endometrium is called Junctional Zone (JZ) or subendometrial halo. It is a thin area of lower echogenicity in comparison with the remaining myometrium. It represents a distinct compartment of the myometrium comprising tightly packed muscle cells with an increased vascularity. Such architecture would increase the density of this tissue layer, altering its acoustic impedance, and account for its echopenic appearance on ultrasound [6]. Although the ZJ may be recognized with the two-dimensional imaging, however the best evaluation is obtained by the three-dimensional multiplanar view, which allows the visualization of the sagittal, transverse, and coronal sections of the uterus [7] (Fig. 3.7).
Fig. 3.7
Three-dimensional multiplanar view of the uterus: the Junctional Zone appears as a subendometrial halo in the inner part of the myometrium (arrows)
3.2.3 Endometrium
The sonographic appearance of the endometrium in fertile patients changes with the different phases of the menstrual cycle. Menstruation is characterized by shedding of the functional layer of the endometrium, which is caused by hormonal deprivation and alteration in the spiral arteriolar system. Bleeding is the result of vasoconstriction of the spiral arteries and necrosis of their walls.
During the last menstrual phase, endometrial layers are very thin and ultrasonography shows a single-line with slightly irregular echogenic interphase.
At the beginning of the follicular phase, the endometrium appears as a single hyperechoic line since it is difficult to identify the borders between the two layers (Fig. 3.8).
Fig. 3.8
Early proliferative endometrium appearing as a hyperechoic line
As the ovulation approaches the endometrial glands increase in number and size and the endometrium shows the typical triple-line appearance. The external hyperechoic lines represent the endometrial-myometrial junction; the middle one is the interface between the two endometrial layers (Fig. 3.9). In the periovulatory phase, the endometrial thickness reaches approximately 12–13 mm (range 10–16) [8, 9]. During a normal menstrual cycle, spontaneous uterine contractions usually occur and can influence the measurement of endometrial thickness [10].
Fig. 3.9
Triple-line endometrium typical of the periovulatory phase
Secretory phase is characterized by progesterone secretion by the corpus luteum following ovulation. Glycoprotein secretion from endometrial glands is increased resulting in disappearance of the three lines present in the late proliferative phase. Sonographically, the endometrium appears as a homogeneous and hyperechoic layer measuring 8–14 mm (Fig. 3.10); it has this appearance until the beginning of the menstrual bleeding or pregnancy. In the latter case, echogenicity and thickness are maintained as decidual reaction to implantation.
Fig. 3.10
Thickened and hyperechoic endometrium during the secretory phase of the cycle
The hyperechogenic pattern of the endometrium during the secretory phase, acting as a natural contrast agent, is most suitable for determination of the shape of the uterine cavity and diagnosis of congenital uterine malformations [11, 12]. In this field, three-dimensional sonography plays a fundamental and irreplaceable role, thanks to the reconstruction of the coronal plane which allows to visualize the uterine cavity and surrounding myometrium and recognize Mullerian anomalies [13–15] with results comparable with those obtained with MRI [16]. In normal uterus, the endometrial cavity shows a typical triangular shape (Fig. 3.11); in arcuate uterus, the internal border of the fundus is curvilinear; in septate uterus, a complete or partial septum protrudes from the fundus into the uterine cavity; in hemi-uterus, a single small asymmetrical cavity is present; in bi-corporeal uterus, two uterine cavity and two distinct bodies are present (Fig. 3.12).
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