Ultrasound Evaluation of the Uterus

Guidelines

The American Institute of Ultrasound in Medicine (AIUM) guidelines for imaging of the uterus have been developed to assist physicians in performing sonographic studies of the female pelvis. 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 examination 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, the following:

• 1.

  Evaluation of pelvic pain

• 2.

  Evaluation of pelvic masses

• 3.

  Evaluation of endocrine abnormalities, including polycystic ovaries

• 4.

  Evaluation of dysmenorrhea (painful menses)

• 5.

  Evaluation of amenorrhea

• 6.

  Evaluation of abnormal bleeding

• 7.

  Evaluation of delayed menses

• 8.

  Follow-up of a previously detected abnormality

• 9.

  Evaluation, monitoring, and/or treatment of infertility patients

• 10.

  Evaluation in the presence of a limited clinical examination of the pelvis

• 11.

  Evaluation for signs or symptoms of pelvic infection

• 12.

  Further characterization of a pelvic abnormality noted on another imaging study

• 13.

  Evaluation of congenital uterine and lower genital tract anomalies

• 14.

  Evaluation of excessive bleeding, pain, or signs of infection after pelvic surgery, delivery, or abortion

• 15.

  Localization of an intrauterine contraceptive device

• 16.

  Screening for malignancy in high-risk patients

• 17.

  Evaluation of incontinence or pelvic organ prolapse

• 18.

  Guidance for interventional or surgical procedures; and

• 19.

  Preoperative and postoperative evaluation of pelvic structures

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 using the transvaginal technique. In specific situations when 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 transducers, as well as sector transducers with a smaller footprint, are most often employed. For transabdominal evaluation, the bladder should be adequately distended to displace bowel superiorly out of the true pelvis and to provide an acoustic window to visualize the uterus and adnexa ( Fig. 28-1 ).

For transvaginal evaluation, the urinary bladder should be emptied and the patient placed in a comfortable position but with her pelvis tilted either with the use of stirrups or by the placement of padding under the patient to elevate the hips. The patient or the sonographer, depending upon the patient's preference, may introduce the vaginal transducer with real-time monitoring. For transvaginal evaluation, the AIUM recommends using probe frequencies of 5 MHz or higher ( Fig. 28-2 ). If a male sonologist is performing the examination, a female member of the staff should be present as a chaperone. However, in some clinical situations, a chaperone is helpful and recommended even for a female sonologist.

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 assessed and documented in both sagittal (long-axis) and transverse (axial or short-axis) 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 external os of the cervix, and the anteroposterior dimension is measured on the same image perpendicular to the long axis. The width is measured on either a transaxial or coronal imaging plane. If the volume of the uterine corpus is assessed, the cervical component should be excluded. Myometrial masses and contour abnormalities should be recorded in two different planes and their locations recorded. Assessment of the endometrium is performed primarily in the sagittal (or sometimes coronal) plane. Variations of the normal appearance of the endometrium during different phases of the menstrual cycle and with hormonal supplementation should be considered ( Fig. 28-3 ) and have been described in detail in preceding chapters. Doppler evaluation of the uterus and endometrium can be of added value. 3D imaging is increasingly available and can also provide valuable additional information, particularly by providing a coronal image of the fundal contour of the uterus in women with suspected uterine congenital malformations and to localize leiomyomas.

SIS, or as it is often referred to, sonohysterography, is an innovative technique used to evaluate a variety of endometrial and myometrial processes that involve the endometrial canal. The most common indications for SIS include, but are not limited to, evaluation of the following:

• 1.

  Abnormal uterine bleeding

• 2.

  Uterine cavity, especially with regard to uterine leiomyomas, polyps, and synechiae

• 3.

  Abnormalities detected on transvaginal sonography, including focal or diffuse endometrial or intracavitary abnormalities

• 4.

  Congenital abnormalities of the uterus

• 5.

  Infertility

• 6.

  Recurrent pregnancy loss

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 premenopausal women during the follicular phase of the menstrual 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.

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 (preferably with saline rather than air to avoid shadowing) so that the catheter does not become dislodged. However, some clinicians prefer to use a catheter without a balloon. 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 introduced is variable, often between 5 and 30 mL. Normal anatomy and abnormal findings should be documented in two separate imaging planes using a high-frequency transvaginal probe, and the endometrium should be fully evaluated from one cornua to the other ( Fig. 28-4 ). Additional techniques such as color Doppler and 3D imaging may be helpful in evaluating both normal and abnormal findings.

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 embedded 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 or corpus and the cervix. The most superior aspect of the uterus is referred to as the fundus and the area where the fallopian tubes enter into the uterus is referred to as the cornua. Anterior to 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 arterial blood supply. The majority of the blood supply comes from the uterine arteries, which arise from the internal iliac arteries, and a minor source of blood supply is the ovarian arteries.

The uterus is most often anteverted and anteflexed ( Fig. 28-5 ), but it may also be retroflexed ( Fig. 28-6 ) or retroverted ( Fig. 28-7 ). Descriptions of flexion refer to the relationship of the body of the uterus to the cervix at the level of the internal os (usually the angle is about 270 degrees), whereas version refers to the cervical relationship to the vagina. The cervix of the uterus is fixed in the midline. However, the body of the uterus can be mobile, and uterine position and orientation may change with varying degrees of bladder and rectal distention. Retroversion and retroflexion are not infrequent in the nongravid state. In such cases the fundus of the uterus is positioned in the sacral hollow. During pregnancy the uterus enlarges and physiologically undergoes reduction by the 14th to 16th weeks of gestation. The fundus of the uterus then rises into the false pelvis. If this fails to happen, the uterus becomes “trapped” in the sacral hollow, often referred to as “incarcerated.” In cases of incarceration of the uterus, the cervix is drawn upward either against or above the symphysis pubis, resulting in distortion of the bladder and urethra as the gestation progresses. The posteriorly positioned fundus can cause pressure on the rectum. Typically, patients present between the 13th to 17th weeks 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:

• 1.

  The pregnancy is deep within the cul-de-sac.

• 2.

  The maternal urinary bladder lies anterior rather than inferior to the uterine corpus and marked bladder distention is noted.

• 3.

  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 is usually possible ( Fig. 28-8 ).

The shape and size of the uterus vary throughout life, affected mostly by hormonal status. The mean measurement of the prepubertal uterus is 2.8 cm in length and 0.8 cm in maximum anteroposterior dimension, with the cervix accounting for two thirds of the total length and contributing to the pear-shaped appearance ( Fig. 28-9 ). 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.

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.

Following menopause, the uterus decreases in size. The decrease in size is related to the number of years since menopause, although the reduction in size is believed to be most rapid during the first decade following menopause. The length of the normal postmenopausal uterus has been reported to range from 3.5 to 6.5 cm and the anteroposterior dimension from 1.2 to 1.8 cm.

The normal myometrium is composed of three layers. The innermost layer, immediately subjacent to the endometrium, is the thinnest and is relatively compact histologically. This layer is also both hypovascular and hypoechoic when compared to the echogenic endometrium and surrounding middle layer of the myometrium. This layer is often referred to as the subendometrial halo and may not always be visualized sonographically. Sometimes, small extremely echogenic foci, usually less than a few millimeters in size and without posterior shadowing, are seen in the inner myometrium at the endometrial/myometrial interface. These foci are thought to represent dystrophic calcifications due to previous intrauterine instrumentation and have no clinical significance. The middle or intermediate layer lies between the subendometrial halo and the arcuate vessels. This is the thickest myometrial layer and is normally uniform and intermediate in echogenicity. The outer layer lies peripheral to or above the arcuate vessels. This layer is relatively thin and slightly less echogenic compared to the intermediate layer in most patients. The cervix is measured from the internal os, identified by narrowing or “waisting” of the uterus at the junction of the lower uterine segment with the cervix to the lips of the external os, which can be seen to project into the lumen of the vagina. The central endocervical canal is echogenic and continuous with the endometrium. The surrounding fibrous cervical stroma is quite hypoechoic and is continuous with the subendometrial halo, if present. The outer cervical muscular layer is continuous with and similar in echogenicity to the intermediate layer of the myometrium ( Fig. 28-10 ).

The arcuate vessels separate the outer layer from the intermediate layer of the myometrium. The arcuate veins are larger than the arcuate arteries and are potentially compressible with excessive probe or manual pressure. The arcuate vessels (particularly the veins) can be prominent and mimic cystic changes. This potential misinterpretation can easily be clarified by using color Doppler imaging ( Fig. 28-11 ). The arcuate arteries branch into radial arteries that penetrate the intermediate layer and reach the level of the inner layer. The arcuate arteries may calcify in postmenopausal women, and this process can be seen earlier in diabetic patients. This change is considered part of the normal aging process ( Fig. 28-12 ).