Over the past several years, transvaginal sonography (TVS) of the endometrium has assumed an integral role in the evaluation of women with possible endometrial disorders.^1,2 TVS allows detailed delineation of endometrial thickness and texture in most patients. The sonographic findings have important implications in determining which patients need endometrial biopsy or dilatation and curettage, observation, or alteration of medication. The more extensive use of sonohysterography (SHG) (fluid instillation into the endometrial lumen) has also greatly enhanced sonographic evaluation of patients with endometrial disorders related to subfertility (please refer to Chapter 35) and polyps. Similarly, the use of 3D and color Doppler sonography has enhanced the evaluation of endometrial disorders such as those associated with polyps, distortion due to fibroids, or pathologic process such as adenomyosis, which can disrupt the inner myometrial/basal layer of endometrium interface or be associated with textural changes such as punctate cysts.

This chapter discusses and illustrates the clinically important applications of TVS in the evaluation of the endometrium. It emphasizes the technical and operator dependent factors as well as presenting the limitations of the technique.

One of the most common disorders evaluated by gynecologists is unexpected or excessive uterine bleeding. It should be emphasized that only 10% to 15% of women who present with irregular bleeding have endometrial cancer; the vast majority have bleeding secondary to benign disorders such as atrophy or hyperplasia.³ The role of sonography is to determine which patients can forgo endometrial biopsy and to monitor endometrial changes in patients receiving tamoxifen or other forms of hormone therapy (HT). Approximately 30% of women receiving combined HT experience bleeding. It is the role of TVS to determine which patients need biopsy, dilatation and curettage, alteration in their medicines, or observation.

Although endometrial cancer is more common than ovarian cancer, this disease is associated with far less mortality. This is clearly related to the fact that one of the earliest signs of endometrial cancer is vaginal bleeding. In addition, endometrial cancer is usually still confined to the uterus when the patient presents, whereas two-thirds of patients with ovarian cancer have extensive disease at time of presentation (see Chapter 36 on early detection of ovarian and endometrial cancer).

It is important conceptually to differentiate causes of uterine bleeding in the premenopausal from the peri- or postmenopausal woman. In the woman of child-bearing age, bleeding is usually associated with anovulation. It may also be related to poor corpus luteum function and support of endometrium with progesterone. Fibroids are also a very common cause of bleeding in the premenopausal age group. It is thought that fibroids result in an irregular endometrial surface, resulting in excessive bleeding. Postmenopausal women have bleeding due to a variety of disorders. These include atrophy, hyperplasia, polyps, cancer, and fibroids. Atrophy is the most common cause of bleeding and is related to excessive thinning of the endometrium, making it friable and prone to bleeding. Sonographic detection of a polyp by TVS with or without SHG enables its hysteroscopic removal. Conversely, endometrial biopsy is sufficient for diagnosis if the endometrium appears diffusely thickened on SHG.

Approximately 30% of patients who receive HT may experience bleeding during the first few months of treatment. It is also not uncommon for some patients who receive cyclical HT to have bleeding in days 12 to 14 of the “pseudocycle” related to decreasing amounts of progesterone. Bleeding outside of these times can be associated with endometrial pathology. TVS can help assess the endometrium for abnormal thickness and/or texture in these patients.

Endometrial evaluation by TVS is helpful in patients taking tamoxifen because this medication is associated with an increased risk of hyperplasia and cancer in these patients. However, not all of these patients with thickened endometrium on TVS will have positive biopsies. This may be attributed to a sampling error or the fact that cystic atrophy of the endometrium can produce multiple interfaces that appear to thicken the sonographic depiction of the endometrial “stripe.” In addition, tamoxifen exposure may be associated with the development of punctuate cystic areas in the inner myometrium, thought to represent reactivated adenomyosis.⁴

Recently, three-dimensional (3D) sonography has provided a means to better visualize the entire endometrium due to its ability to depict it in a coronal plane.⁵ 3D ultrasonography (3D US) can be performed using a freehand sweep or, better, with an automated sweep transducer. With either technique, multiplanar images are obtained including one in the coronal plane. The angle of the scan plane can be altered to best depict the entire endometrial width. For a more detailed discussion of 3D US in gynecology, please refer to Chapter 49.

The endometrium has a nongeometric volumetric shape. As such, measurement errors can occur if the endometrium is not evaluated in proper plane. In addition, up to a 2-mm interobserver error involved in its measurement has been reported.^6,7 Over measurement may occur if the endometrium is measured obliquely on a short-axis image. Inclusion of the hypoechoic inner myometrium may also erroneously overestimate measurement of the endometrial thickness. Therefore, every effort should be made to obtain the endometrial bilayer in its true long axis.

The texture and thickness of the endometrium must be evaluated. Significant changes in texture of the endometrium can also be observed in women of child-bearing age from relatively hypoechoic in the early follicular phase to echogenic in the midsecretory phase. Enhanced through transmission can be seen in the secretory or luteal phase endometrium due to increased fluid content (edema) of the stroma. The sonographic texture of the endometrium of normal postmenopausal women is typically mildly echogenic compared with the spiral (middle) layer of the myometrium.

Evaluation of the endometrium begins after sonographic identification of the long axis of the uterus. Assessment of the arrangement of the myometrial muscle bundles into the inner, spiral or middle, and outer muscle bundle layers may be helpful to determine that the endometrium is imaged in its true long axis (Figure 35-1). The optimal measurement of the endometrium is obtained in its long axis in anteroposterior (AP) dimension, or “height.” This measurement excludes the hypoechoic inner myometrium and represents the bilayer thickness. Once the endometrium is imaged in its thickest sagittal plane, the probe should be turned approximately 90 degrees, and endometrium can be imaged in short axis. This is a more variable measurement and may image the endometrium obliquely, resulting in overestimation of endometrial thickness. One can observe the endometrium extending into the area of the tubal ostia, particularly during the secretory phase. This provides an anatomic landmark for assessment of the endometrium. The endometrial “width” describes the thickness of the endometrium in the coronal plane at the level of the tubal ostia. If taken as the “height,” it can lead to overestimation of the endometrial thickness. However, this measurement may be helpful if an endometrial volume is calculated by using the length, AP dimension (height), and width of the endometrium.

The sonographer must be skilled at empirically adjusting the scan plane according to the uterine flexion (angle of the fundus relative to the cervix) and version (angle of the cervix relative to the vagina). For very anteflexed uteri, the probe must be placed in the posterior fornix with a sharp anterior angulation; for a retroflexed uterus, the probe needs to be directed sharply posteriorly. Assessment of the endometrium may be difficult in patients with fibroids that displace the endometrium.

Andreotti and coworkers reported that 3D US is most useful if an abnormality is suspected in two-dimensional (2D) US; therefore, 3D US is not required in studies that appear normal on 2D. 3D US is particularly helpful in depicting polyps, submucosal fibroids, and adhesions.⁵

The endometrial thickness and texture seen in women of child-bearing age is quite different from that seen in postmenopausal women (Figures 35-1, 35-2, and 35-3). In a woman of child-bearing age, the endometrium consists of an outer basal layer that is not shed and an inner functional layer that has stromal and glandular elements. Refluxed mucus from the cervix typically produces a thin and echogenic median echo. In contrast, the endometrium of most postmenopausal women is comprised mostly of the basal layer and should be thin (<5 mm) and regular in texture.

In the patient of child-bearing age, the endometrium is shed during the first 3 to 5 days of the cycle. During this time, the endometrium may appear as an irregular, broken, echogenic interface, with focal hypoechoic intraluminal collections representing unclotted blood and sheets of sloughing endometrium. Organized clots may appear as echogenic intraluminal tissue, but are not attached to the basal layer like polyps. It is interesting to note that the subendometrial contractions directed from the fundus to the cervix can be observed during menses on real-time scanning if the videotaped segment is played back in fast-forward mode. After menses, the endometrium gradually thickens and becomes isoechoic to the surrounding myometrium. At this stage, the endometrium measures approximately 4 to 6 mm in bilayer thickness. In the periovulatory period, the endometrium may demonstrate a multilayered appearance consisting of the outer echogenic basal layer in the inner hypoechoic functionalis layers. The innermost echo represents refluxed mucus and has a variable echogenicity depending on the amount of refluxed mucus. The periovulatory endometrium usually measures between 5 and 8 mm in bilayer thickness and may contain a sliver of intraluminal fluid. At this time, subendometrial contractions can be seen directed from the cervix to the fundus. These peristaltic waves have a role in sperm transport. Under the influence of progesterone, the endometrium becomes echogenic relative to the basal layer. The increased echogenicity is probably related to stored mucus and glycogen within the more tortuous glands, and to stromal edema that produces more interfaces and is enhanced through transmission. The luteal phase endometrium can measure up to 14 mm in bilayer thickness.

Normal endometria in postmenopausal women are thin (<6 mm) and regular. In fact, the thickness of the endometrium can be related to the number of years that the patient is postmenopausal in that the endometrium becomes much thinner as the postmenopausal patient ages.^8,9 In some instances, the echogenic interface in postmenopausal patients may be related to trapped mucus or secretions rather than to the endometrium itself.

In the patient receiving HT, the endometrium reflects the type of therapy administered to the patient. The patient who receives unopposed estrogen may have a thicker endometrium than those who take combined estrogen and progesterone.⁸ The sonologist should be aware that the endometrium in women taking estrogen and cyclic progesterone may differ up to 3 mm, depending on which part of the pseudocycle is being examined. The thinnest endometrium is usually observed after the progesterone phase (day 12); the thickest is later in the cycle after maximal estrogen exposure. Initial experience with a selective estrogen receptor modulator (SERM), such as raloxifene or Evista®, or idoxifene, indicate that the endometrium typically maintains a thin appearance. However, in some patients in whom the endometrium may appear thickened, related probably to cystic atrophy. This is discussed in greater detail in the section on serial monitoring. Women on tamoxifen should demonstrate a thin (<6 mm) and regular endometrium. Changes in texture may require further evaluation by SHG and/or biopsy.

On 3D US obtained in the coronal plane, the entire surface of the endometrium is depicted including the endometrium that invaginates into the cornua (cornual portion of the endometrium).