Image 4 - Transverse Uterus (click here to restore original image)
Observations

This image is obtained through the upper portion of the uterus (same patient as Image 3). The transverse imaging plane can be inferred by the squared-off or trapezoidal shape of the bladder as well as the oval shape of the uterine corpus. As noted previously, the endometrial stripe is not visible, indicating that this patient has just undergone menstruation. During the first half of the menstrual cycle (at which time estrogen secretion by the developing ovarian follicles is steadily increasing) there is a gradual change in the appearance of the endometrium. What appears first is a thin echogenic line which gradually thickens to form a multilayered configuration. This reflects growth of the endometrial glands under the influence of estrogen synthesis by the ovarian follicles. This multilayered or proliferative (transvaginal, example 1, transvaginal, example 2) endometrium is preparing for possible implantation by a fertilized egg. Following mid-cycle ovulation, in the second half of the menstrual cycle, the endometrium is under the influence of progesterone secretion by the corpus luteum. The endometrial glands fill up with mucus, changing the endometrium into a thick echogenic configuration (secretory phase) (example 3). The endometrium usually obtains it thickest dimensions at this point (< 14mm). As luteinizing hormone levels decrease, the level of progesterone secretion by the corpus luteum falls, signaling the onset of menstruation. Thus, the sonographic appearance of the endometrium (invisible, thin echogenic, multi-layered, thick echogenic) is frequently an excellent indicator of the phase of the menstrual cycle. Additionally, sonography is a useful tool for the evaluation of dysfunctional uterine bleeding. An abnormally thickened endometrium may indicated hyperplasia, polyps, or endometrial carcinoma, while a normal appearing but displaced endometrial stipe would indicate a submucosal fibroid.

Following menopause the endometrium gradually atrophies and should be no thicker than 4 mm in A/P dimension, making ultrasound a useful tool for the evaluation of post-menopausal bleeding. Since the commonest cause of post-menopausal bleeding is endometrial atrophy, the sonogram in such cases should show a barely perceptible endometrial stripe, no thicker than 4 mm. If a thickened endometrial stripe is seen, further investigation (including aspiration and biopsy) should be performed.

On this transverse image note the echogenic interface where the sound waves are reflected off the sacrum leading to an acoustic shadow behind. By contrast, the two hyperechoic areas (one directly posterior to the uterus and the other to the left side of the uterine fundus) show multiple reverberation artifacts due to bowel gas within the sigmoid colon.