IMAGING

The American Institute of Ultrasound in Medicine (AIUM) guidelines for imaging of the uterus have been developed to assist sonologists in performing sonographic studies of the female pelvis.¹ Knowing the potential but also the limitations of ultrasound helps us to maximize the probability of detecting most significant abnormalities. As with any clinical test, ultrasound of the pelvis should be performed only if there is a valid clinical reason. Following the AIUM guidelines, the indications for pelvic sonography include, but are not limited to

Techniques

All relevant anatomic structures in the pelvis should be identified first by transabdominal technique, and then, more detailed evaluation of the deep pelvic structures should be performed by transvaginal technique. In specific situations in which transvaginal evaluation cannot be performed or tolerated, transrectal or transperineal evaluation can be very useful.

The transducer should be selected to operate at the highest clinically appropriate frequency that will allow adequate visualization of deep pelvic structures. For transabdominal evaluation, a 3.5-MHz or higher transducer is employed. Curved linear array, as well as sector, transducers with a smaller footprint are often employed. For transabdominal evaluation, the bladder should be adequately distended to displace bowel superiorly out of the true pelvis and provide an acoustic window to visualize the uterus and adnexa (Fig. 27-1)

FIGURE 27-1 A. Sagittal-transabdominal view of the normal uterus. B. Transverse-transabdominal view of the normal uterus. Notice the acoustic window provided by the distended urinary bladder.

For transvaginal evaluation, the urinary bladder should be emptied and the patient in comfortable position but with her pelvis tilted either with the use of stirrups or with an elevation under the hips. The patient or the sonographer, depending on the patient’s preference, may introduce the vaginal transducer with real-time monitoring. For transvaginal evaluation, the AIUM recommends using probes of 5 MHz or higher (Fig. 27-2) If a male sonologist is performing the examination, a female member of the staff should be present as a chaperone.

FIGURE 27-2 A. Sagittal-transvaginal view of the normal uterus. B. Transverse-transvaginal view of the normal uterus.

The vagina, uterus, and the urinary bladder are used as reference points for identification of the remaining normal and abnormal pelvic structures. The uterine size, shape, and orientation should be identified and documented in sagittal and transverse axial planes. The endometrium, myometrium, and cervix should be carefully evaluated, and their appearance documented. The uterine length is measured in long axis from the fundus to the cervix, and the anteroposterior dimension is measured on the same image perpendicular to the length. The width is to be measured on a separate image on either a transaxial or coronal plane of section. If the volume of the uterine corpus is assessed the cervical component should be excluded.¹

Assessment of the endometrium is performed in a sagittal plane. Variations of the appearance of the endometrium with different phases of the menstrual cycle and with hormonal supplementation should be considered (Fig. 27-3). Myometrial masses and contour abnormalities should be recorded in two different planes and their locations recorded.¹ Doppler evaluation of the uterus and endometrium can be of added value.

FIGURE 27-3 A. Diagram showing development of the endometrium during the menstrual, proliferative, and secretory phases. In the menstrual phase, the endometrium appears as a thin, irregular interface. The central echogenicity probably arises from sloughed tissue and blood. In the proliferative phase, the endometrium is relatively hypoechoic, likely a reflection of the straight and orderly arrangement of the glandular elements. The central thin, echogenic line is likely a specular reflection from the endometrial surfaces. In the secretory phase, the endometrium achieves its maximum thickness and echogenicity. This appearance is from the distended and tortuous glands, which contain secretions. B. Postmenstrual, early proliferative endometrium. C. Periovulatory endometrium. A three-layered endometrium is seen: the collapsed endometrial lumen is demonstrated by the central echogenic line (thin arrow). A hypoechogenic layer representing the edematous endometrium functionalis (thick arrow) and an outer hyperechogenic layer representing the endometrium basalis (arrowheads) is seen. D. Secretory endometrium.

(A, From Fleischer AC, Kalemeris GC, Entman SS: Sonographic depiction of the endometrium during normal cycles. Ultrasound Med Biol 12:271, 1986, Pergamon Journals Ltd. Reprinted by permission of Elsevier Science. Copyright 1986 by World Federation of Ultrasound in Medicine and Biology.)

Saline infused sonography (SIS) or, as it is often referred to, sonohysterography, is an innovative technique to evaluate a variety of endometrial and myometrial processes, which involve the endometrial canal. The most common indications for SIS are abnormal uterine bleeding in both pre- and postmenopausal women. Other indications include

1. Infertility and habitual abortion

2. Congenital abnormalities or anatomic variants of uterine cavity.

3. Preoperative and postoperative evaluation of uterine cavity, especially with regard to uterine myomas, polyps, and cysts

4. Suspected uterine cavity synechiae

5. Further evaluation of suspected abnormalities as seen on transvaginal sonography, including focal or diffuse endometrial thickening or debris

6. Inadequate imaging of endometrium by transvaginal sonography

SIS is contraindicated in women who could be pregnant or have an active infection. Because the normal secretory endometrium may be thick and simulate endometrial disease, the examination should be scheduled in the follicular phase of the cycle, after menstrual flow has ceased but prior to ovulation, no later than the 10th day of the menstrual cycle. Active vaginal bleeding is not generally a contraindication but can make imaging challenging or even nondiagnostic.²

At our institution, we perform a preliminary transabdominal and transvaginal sonogram before SIS. After the procedure is explained to the patient, the external os is cleansed before catheterization of the cervical canal using aseptic technique. A sonohysterography catheter (flushed with saline to remove any air bubbles) is then advanced into the endometrial canal. Once in the endometrial canal, the balloon is inflated so that the catheter does not become dislodged. The speculum is removed, and the transvaginal probe is reinserted adjacent to the catheter. Under ultrasound guidance, the balloon is gently retracted to occlude the internal os. Sterile saline should be administered under real-time sonography. The amount of saline one introduces is variable, often between 5 and 30 mL. Normal anatomy and abnormal findings should be documented in two separate planes using the high-frequency transvaginal (TV) probe extending from one cornua to the other (Fig. 27-4)

FIGURE 27-4 A. Sagittal-transvaginal image of balloon positioning in the cervix. B. Sagittal-transvaginal image after saline infusion. The normal thin endometrium is well seen.

ANATOMY

The uterus is a hollow organ in which the myometrium is firmly adherent to a thin internal layer of endometrium. Externally the uterus is imbedded between the two layers of the broad ligament. Anatomically, the uterus lies between the bladder anteriorly and the rectosigmoid colon posteriorly. The uterus is divided into two major parts, the body and cervix. The most superior aspect of the uterus is referred to as the fundus, and the entrance of the fallopian tubes into the uterus are referred to as the cornua. Anterior to the origin of the fallopian tubes are the round ligaments, one on each side, which extend anterolaterally, coursing through the inguinal canals and inserting onto the fascia of the labia majora. The uterus has a dual blood supply. The majority of blood supply is from the uterine arteries arising from the internal iliac arteries and a minor blood supply is from the ovarian arteries.

The uterus is physiologically most often anteroverted and anteroflexed (Fig. 27-5) but may also be retroflexed (Fig. 27-6) or retroverted (Fig. 27-7). The cervix of the uterus is fixed in the midline but the body of the uterus can be mobile, and may change with varying degrees of bladder and rectal distention. Descriptions of flexion refer to the relationship of the body of the uterus to the cervix (usually the angle is about 270 degrees), whereas version refers to the cervical relationship to the vagina.

FIGURE 27-5 A. Illustration demonstrating an anteflexed, anteverted normal uterus. B. Midline sagittal sonogram demonstrating anteversion (cervix [Cx] to the vagina [V]) and anteflexion (fundus [F] to cervix). A distended urinary bladder slightly displaces the fundus posteriorly.

(Illustration by James A. Cooper, MD, San Diego, CA.)

FIGURE 27-6 A. Illustration demonstrating an anteverted, retroflexed uterus. B. Midline sagittal sonogram demonstrating uterine retroflexion. The relationship of the cervix (Cx) to the vagina (V) is normal, but the fundus (F) is angled posteriorly.

(A, Illustration by James A. Cooper, MD, San Diego, CA.)

FIGURE 27-7 A. Illustration demonstrating a retroverted uterus. B. Midline sagittal sonogram demonstrating retroversion. The cervix (Cx) is almost parallel to the vagina (V). There is often mild concomitant retroflexion as well. F, fundus.

(A, Illustration by James A. Cooper, MD, San Diego, CA.)

Retroversion and retroflexion are not infrequent in the nongravid state. In these cases the fundus of the uterus is positioned in the sacral hollow. During pregnancy, by the 14th to 16th week of gestation, the uterus enlarges and physiologically undergoes reduction. The fundus and uterus then rise into the false pelvis. If this fails to happen, the uterus becomes “trapped” in the sacral hollow, often referred to as “incarcerated.” As the gestation evolves, the cervix is drawn upward either against or above the symphysis pubis, resulting in distortion of bladder and urethra. The posteriorly positioned fundus can cause pressure on the rectum. Usually, patients present between 13th to 17th week of pregnancy with symptoms of bladder outlet obstruction. A history of multiple trips to the emergency room for bladder outlet obstruction should raise suspicion. A constellation of three findings on sonography is diagnostic of an incarcerated uterus. First, the pregnancy is deep within the cul-de-sac. Second, the maternal urinary bladder lies anteriorly rather than inferiorly to the uterus and marked bladder distention is noted. Third, a soft tissue structure (the cervix) is seen between the bladder and pregnancy. This appearance can be misconstrued as an empty uterus associated with an ectopic or abdominal pregnancy. Failure to recognize an incarcerated uterus can result in compromise of the uterine circulation, leading to spontaneous abortion or even uterine rupture. If recognized early, manual uterine repositioning usually is accomplished (Fig. 27-8)

FIGURE 27-8 A. Incarcerated uterus. B. Sonogram of 14-week-old gestation. Patient presented with inability to empty urinary bladder. Sagittal-transabdominal images of an incarcerated uterus. The uterine fundus (F) is trapped in the sacral hollow. The cervix (arrows) is drawn anteriorly and superiorly, and can be misconstrued as an empty uterus with an ectopic or transabdominal pregnancy. C. Sagittal view of the same patient demonstrating the degree to which the bladder (Bl) is drawn superiorly. D. Postmanual reduction view demonstrates physiologic relationship of the uterus and cervix (calipers). Note the normal anteriorly positioned placenta (Pl) that appeared posterior when the uterus was incarcerated.

(A, Illustration by James A. Coooper, MD, San Diego, CA.)

The shape and size of the uterus varies throughout life, affected mostly by hormonal status. The mean measurement of a prepubertal uterus is 2.8 cm in length and 0.8 cm in maximum anteroposterior dimensions, with the cervix accounting for two thirds of the total length and contributing to the pear-shaped appearance (Fig. 27-9).³ It is important to remember that in the immediate postdelivery state, the neonatal uterus can be slightly larger owing to the effects of residual maternal hormones. For the same reason, the echogenic endometrium is well seen and a small amount of fluid can be present in the endometrial cavity.

FIGURE 27-9 Sagittal-transabdominal view of the normal prepubertal uterus. Notice that the cervix (Cx) is significantly more prominent than the body or fundus (F). Bl, bladder.

From birth until 4 years of age, the uterus decreases in size. At approximately 8 years of age the uterus starts to grow preferentially in the fundus. The uterus continues to grow for several years after menarche until it reaches the mean dimensions of a reproductive age uterus, which are approximately 7 cm long and 4 cm wide.

Parity increases the size of the uterus, with a multiparous uterus measuring approximately 8.5 cm by 5.5 cm.⁴

The postmenopausal uterus is often small. The decrease in size is related to years passed after menopause,⁴ although the reduction in size is believed to be most rapid during the first decade after menopause. The range can be from 3.5 to 6.5 cm in length and 1.2 to 1.8 cm in anteroposterior dimensions.⁵

The normal myometrium is composed of three layers. The innermost layer is the thinnest, relatively hypovascular and hypoechoic when compared with the adjacent echogenic endometrium. This appearance is often referred to as the subendometrial halo. The next layer is the intermediate layer, which is the thickest layer and demonstrates uniform echotexture in a normal uterus. The outer layer is thin again and slightly less echogenic. The arcuate vessels separate the outer layer from the intermediate layer. The arcuate arteries branch into radial arteries that penetrate the intermediate layer and reach the level of inner layer. The arcuate vessels (particularly the veins) can be prominent and mimic cystic changes. This potential misinterpretation can easily clarified by using color Doppler imaging (Fig. 27-10)

FIGURE 27-10 Sagittal (A) and transverse sonograms (B) of the uterus with prominent arcuate vessels (arrows) that can mimic cystic changes. C. These cystic appearing spaces are confirmed to be arcuate vessels by applying color Doppler.

The arcuate arteries can calcify in postmenopausal women; this process can be seen earlier in diabetic patients. This is considered part of normal aging process (Fig. 27-11). Sometimes, small hyperechogenic foci are seen in inner myometrium (usually a few millimeters and nonshadowng in appearance), which are thought to represent dystrophic calcifications due to previous intrauterine instrumentation and have no clinical significance.

FIGURE 27-11 Sagittal-transvaginal view of the uterus demonstrates arcuate artery calcifications (arrows).