Chapter 2: NORMAL PELVIC ANATOMY AS DEPICTED WITH TRANSVAGINAL SONOGRAPHY

Transvaginal sonography (TVS) affords improved resolution of the uterus and ovaries over that which can be obtained with the conventional transabdominal sonography (TAS) approach. Although TVS allows a closer proximity of the transducer/probe to the pelvic organs and more detailed depiction, it may be more, rather than less, difficult for the sonographer to become oriented to the images when compared with conventional TAS because of the limited field of view and unusual scanning planes depicted with TVS. As one develops a systematic approach to the examination of the uterus and adnexal structures with TVS, however, the examination becomes much easier to perform. Appendix 2-1 lists the American Institute of Ultrasound in Medicine (AIUM) guidelines for a complete pelvic sonogram.

In this chapter, the sonographic appearances of the uterus, ovary, and other adnexal and pelvic structures will be described, with particular emphasis on how they are best depicted in a real-time TVS examination.

The 3 scanning maneuvers that are used in TVS include:

1. Vaginal insertion of the probe with side-to-side movement within the upper vagina for sagittal imaging (Figures 2-1,2-2, and 2-3).

2. Transverse orientation of the probe for imaging in various degrees of semiaxial to semicoronal planes.

3. Variation in depth of probe insertion for optimal imaging of the fundus to the cervix by gradual withdrawal of the probe into the lower vagina for imaging of the cervix.

In contrast to conventional TAS, bladder distention is not necessary for TVS. In fact, overdistention can hinder TVS by placing the desired field of view outside the optimal focal range of the transducer. Minimal distention is useful in a patient with a severely anteflexed uterus to straighten the uterus relative to the imaging plane.

As is true for conventional sonographic equipment, the highest-frequency transducer possible should be used that allows adequate penetration and depiction of a particular region of interest. Thus, transducers with a high-central frequency are preferred (broad band 5.5 to 7.8 MHz). Higher-frequency (>8 MHz) transducers may limit the field of view to within only 6 cm of the probe.

The major types of transducer/probes that are used for TVS include those that contain a single-element oscillating transducer, multiple small transducer elements that are arranged in a curved linear array, and those that consist of multiple small elements steered by an electronic phased array. All of these depict the anatomy in a sector format that usually encompasses 100 to 120 degrees. In our experience, the greatest resolution is achieved with a curved linear array that contains multiple (up to 200) separate transmit-receive elements. Mechanical sector transducers may be subject to minor image distortions at the edges of the field due to the hysteresis (lag in effect when stopping and starting) that occurs with an oscillating transducer. Reverberation artifacts can be created by suboptimal coupling of the condom/probe/vagina surfaces. Although degradation of image quality by side-lobe artifacts can occur in the far field in a phased array transducer, they do not significantly degrade the image in the near field. Therefore, phased array transducers have similar resolution capabilities to sector as curved linear array transducers for use in transvaginal examinations.

Practioners should follow the AIUM guidelines for the disinfection of transvaginal probes. These guidelines are included as Appendix 2-2.

For infection control purposes, a disposable protective sheath is used to cover the transducer. After completely covering the transducer/probe with a sheath such as a condom and securing the sheath to the shaft of the probe with a rubber band, the transducer is lubricated on its tip and periphery and then inserted into the vagina and manipulated around the cervical lips and into the fornix to depict the structures of interest in best detail. When the transducer is oriented in the longitudinal or sagittal plane, the long axis of the uterus can usually be depicted by slight angulation off midline. The uterus is used as a landmark for depiction of other adnexal structures. Once the uterus is identified, the transducer can be angled to the right or left of midline in the sagittal plane to depict the ovaries. The internal iliac artery and vein appear as tubular structures along the pelvic side wall. Low-level blood echoes can occasionally be seen streaming within these vessels. The ovaries typically lie medial to those vessels. After appropriate images are obtained in the sagittal plane, the transducer can be turned 90 degrees counterclockwise to depict these structures in their axial or semicoronal planes.

Particularly in larger patients, it is helpful for the sonographer to use one hand to scan while the other is used for gentle abdominal palpation to move structures, such as the ovaries, as close as possible to the transducer/probe.

Examination of the uterus begins with its depiction in long axis (Figures 2-4,2-5,2-6, and 2-7). The endometrial interface, which is typically echogenic, is a useful landmark to depict in long axis. The actual sonographic texture of the endometrium varies according to its consistency, which is elaborated upon in other sections of this chapter. Once the endometrium is identified in long axis, images of the uterus can be obtained in the sagittal and semiaxial/coronal planes.¹

It may be difficult to determine the flexion of the uterus on the hard-copy images obtained solely from transvaginal scanning except in extreme cases of anteflexion or retroflexion; however, one can obtain an impression of uterine flexion during the examination by the relative orientation of the transducer/probe needed to obtain the most optimal images of the uterus. For example, retroflexed uteri are best depicted when the probe is in the anterior fornix and angulated in a posterior direction. The fundus of the anteflexed uterus is directed to the upper left corner of the image. Conversely, the retroflexed uterus will demonstrate the fundus directed to the inferior right corner of the image.

The endometrium has a variety of appearances depending on its stage of development. In the proliferative phase, the endometrium measures 5 to 7 mm in anterior-posterior (AP) dimension. This measurement includes 2 layers of endometrium. A hypoechoic interface can be seen within the luminal aspects of echogenic layers of endometrium in the periovulatory phase and probably represents edema and increased glycogen and mucus in the inner layers of endometrium. In the few days after ovulation, a small amount of secretion into the endometrial lumen can be seen. During the secretory phase the endometrium typically measures between 6 and 12 mm in bilayer thickness; is homogeneously echogenic, most likely as a result of multiple interfaces resulting from stromal edema; and is surrounded by a hypoechoic band, representing the inner layer of the myometrium. This inner layer of myometrium appears hypoechoic on TVS and corresponds roughly to the "junctional zone" seen on magnetic resonance imaging (MRI). The junctional zone, however, may be thicker than the hypoechoic band seen in TVS, perhaps because of different physical interaction with the myometrium in this area.² This layer is hypoechoic probably due to the longitudinal arrangement of the myometrial fibers.

Endometrial volume may be calculated by measuring its long axis and multiplying by the AP and transverse dimension.³ One can use the axial plane landmark where the endometrium invaginates into the area of ostia in the region of the uterine cornu.

Because of the proximity of the transducer/probe to the cervix, the cervix is not as readily visualized as the remainder of the uterus. If one withdraws the probe into the vagina, however, images of the cervix can be obtained. The mucus within the endocervical canal usually appears as an echogenic interface. This interface may become hypoechoic during the periovulatory period because the cervical mucus has a higher fluid content.

Ovaries are typically depicted as oblong structures measuring approximately 3 cm in long axis and 2 cm in AP and transverse dimensions (Figure 2-8). On angled long-axis scans, they are immediately medial to the pelvic vessels. They are particularly well depicted when they contain a mature follicle that is typically in the 1.5- to 2.0-cm range. It is not unusual to depict multiple immature or atretic follicles in the 3- to 7-mm range.

The size of an ovary is related to the patient's age and phase of follicular development. When the ovary contains a mature follicle, it can become twice as large in volume as one that does not contain mature follicles. The greatest dimension of a normal ovary, however, is typically less than 3 cm.⁴ The ovaries of postmenopausal women may be difficult to recognize because they are relatively small and usually do not contain follicles that enhance their sonographic recognition.

Ovarian volumes can be estimated by measurement of the greater transverse, longitudinal, and AP dimensions. The average ovarian volumes measured in menstruating women were 9.8 cm³, in postmenopausal women were 5.8 cm³, and in premenarchal females were 3.0 cm³.⁵ There is a gradual decrease in ovarian volume after menopause except in women receiving hormone replacement.⁶ Echogenic foci can be seen on TVS within the center and/or periphery of the ovary. Most of the central foci are due to tiny cysts or calcifications within atretic follicles. Those that are peripherally located are probably of no clinical significance and represent calcified foci within superficial epithelial inclusion cysts.⁷

Recent studies have further elucidated the origin of echogenic foci within the ovary. Those without an associated shadow may represent specular reflections from unresolved microscopic (<0.5 mm) cysts.⁸ Ones that have shadowing may represent hemosiderin or calcified foci associated with benign histologic changes. They do not appear to be associated with endosalpingiosis, which is a histology correlated of benign overgrowth of epithelial cells derived from coelomic peritoneum, or endometriosis.⁹ Further studies correlating the sonographic findings with histologic findings is needed, particularly in light of an unknown histologic precursor of ovarian eptithelial cancers.

Transvaginal sonography can depict several other pelvic structures besides the uterus and ovaries (Figures 2-9 and 2-10). These include bowel loops within the pelvis, iliac vessels, and occasionally distended fallopian tubes. Even small amounts (1 to 3 cc) of intraperitoneal fluid can be detected in the cul-de-sac or surrounding the uterus.

The pelvic vessels appear as straight tubular structures on either pelvic side-wall. The internal iliac arteries have a typical width of between 5 and 7 mm and tend to pulsate with expansion of both walls. The iliac vein is larger (~1 cm) but does not demonstrate this pulsation. Occasionally, low-level blood echoes will be seen streaming within the vein. The transducer can be manipulated or pivoted to demonstrate these vessels in their long axis. Occasionally, a distended distal ureter may have this appearance but does not demonstrate pulsations. The distal ureter and urethra course toward the base of the urinary bladder. In most patients, the larger branches of the uterine vessels will be demonstrable by TVS as tubular structures coursing in the paracervical area.

Distended uterine veins can be traced back into the myometrium, where the arcuate veins that course in the outer third of the myometrium lie. Ovarian veins tend to be located superior to the ovary. When normal, these vessels do not measure more than 5 mm. When there is valvular incompetency, however, the ovarian vein can be distended (>5 mm). Whether or not this finding is associated with a distinct clinical entity, such as "pelvic congestion syndrome," is controversial because many women with distended veins do not experience pain.

The nondistended fallopian tube is difficult to depict on TVS, which is probably related to its small intraluminal size and serpiginous course. Occasionally, one can identify the origin of the tubes by finding the invagination of endometrium depicting the area of the tubal ostia and following these structures laterally in the axial or coronal plane. The ovarian and infundibulopelvic ligaments usually cannot be depicted.

Sonographic delineation of the tubes is facilitated by intraperitoneal fluid that may be present in the cul-de-sac.¹⁰ Placing the patient in a reverse Trendelenburg position (head higher than hips), may augment intraperitoneal fluid around the fallopian tubes. When surrounded by fluid, the normal tube appears as a 0.5- to 1-cm wide tubular echogenic structure that usually comes from the lateral aspect of the uterine cornu posterolaterally into the adnexal regions and cul-de-sac. The flaring of the fimbriated end of the tube can be appreciated in some patients because it approximates its nearby ovary. Transvaginal sonographic depiction of the tubes is also facilitated when they contain intraluminal fluid. Rarely, small (<1 cm) rounded structures can be seen projecting from the fimbriated end of the tube representing cysts of Morgagni.¹¹

The transvaginal sonographic appearances of the round ligaments are somewhat similar to that arising from a nondistended tube, except that its course is straighter and more parallel to the uterine cornu.

The bowel typically can be recognized as a fusiform structure that frequently contains intraluminal fluid and changes in configuration due to active peristalsis. If there is fluid within the lumen, periodic intraluminal projections—resulting from the valvulae conniventes—can be recognized from small bowel or the haustral indentations that are characteristic of large bowel. Nondistended bowel appears as a fusiform structure that consists of an echogenic center, representing mucus and enteric contents, surrounded by a hypoechoic rim, representing the muscularis of the bowel wall.

Transvaginal sonography affords detailed depiction of the uterus and ovaries; however, it requires a systematic evaluation of these pelvic structures for their complete delineation because of the limited field of view of transvaginal transducer/probes. This can be achieved by understanding the anatomic relations of these structures from previous experience with TAS combined with anticipated findings from prior palpation of these structures during pelvic examination.

I. INTRODUCTION

The clinical aspects of this guideline (Indications, Specifications of the Examination, and Equipment Specifications) were developed collaboratively by the American Institute of Ultrasound in Medicine (AIUM), the American College of Radiology (ACR), and the American College of Obstetricians and Gynecologists (ACOG). Several sections of this guideline (Qualifications and Responsibilities of Personnel; Documentation; and Quality Control and Improvement, Safety, Infection Control, and Patient Education and Concerns) vary among these organizations and are addressed by each separately.

This guideline has been developed to assist physicians performing sonographic studies of the female pelvis. Ultrasound of the female pelvis should be performed only when there is a valid medical reason, and the lowest possible ultrasonic exposure settings should be used to gain the necessary diagnostic information. In some cases, additional or specialized examinations may be necessary. While it is not possible to detect every abnormality, adherence to the following guideline will maximize the probability of detecting most of the abnormalities that occur.

II. INDICATIONS

Indications for pelvic sonography include but are not limited to:

1. Pelvic pain

2. Dysmenorrhea (painful menses)

3. Menorrhagia (excessive menstrual bleeding)

4. Metrorrhagia (irregular uterine bleeding)

5. Menometrorrhagia (excessive bleeding irregularly)

6. Follow-up of previously detected abnormality (eg, hemorrhagic cyst)

7. Evaluation and/or monitoring of infertile patients

8. Delayed menses or precocious puberty

9. Postmenopausal bleeding

10. Abnormal pelvic examination

11. Further characterization of a pelvic abnormality noted on another imaging study (eg, computed tomography or magnetic resonance imaging)

12. Evaluation of congenital anomalies

13. Excessive bleeding, pain, or fever after pelvic surgery or delivery

14. Localization of an intrauterine contraceptive device

15. Screening for malignancy in patients with an increased risk

III. QUALIFICATIONS AND RESPONSIBILITIES OF PERSONNEL

See the AIUM Official Statement Training Guidelines for Physicians Who Evaluate and Interpret Diagnostic Ultrasound Examinations and the AIUM Standards and Guidelines for the Accreditation of Ultrasound Practices.

IV. SPECIFICATIONS OF THE EXAMINATION

The following guideline describes the examination to be performed for each organ and anatomic region in the female pelvis. All relevant structures should be identified by the transabdominal or transvaginal approach. In many cases, both will be needed. A transrectal or transperineal approach is useful in patients who cannot tolerate a vaginal probe (eg, virgins and postmenopausal women).

A. General Pelvic Preparation

For a pelvic sonogram performed transabdominally, the patient's urinary bladder should, in general, be distended adequately to displace the small bowel and its contained gas from the field of view. Occasionally, overdistention of the bladder may compromise evaluation. When this occurs, imaging may be repeated after the patient partially empties the bladder.

For a transvaginal sonogram, the urinary bladder should preferably be empty. The patient, the sonographer, or the physician may introduce the vaginal transducer, preferably under real-time monitoring. When possible, a female member of the physician's or hospital's staff should be present as a chaperone in the examining room if a male examiner is performing the examination.

B. Uterus

The vagina and uterus provide anatomic landmarks that can be used as reference points for the remaining normal and abnormal pelvic structures. In evaluating the uterus, the following should be documented: (1) the uterine size, shape, and orientation; (2) the endometrium; (3) the myometrium; and (4) the cervix. The vagina may be imaged as a landmark for the cervix and lower uterine segment.

Uterine length is evaluated in the long axis from the fundus to the cervix (the external os, if it can be identified). The depth of the uterus (anteroposterior dimension) is measured in the same long axis view from its anterior to posterior walls, perpendicular to the length. The width is measured from the transaxial or coronal view. If volume measurements of the uterine corpus are performed, the cervical component should be excluded from the uterine measurement.

Abnormalities of the uterus should be documented. The endometrium should be analyzed for thickness, focal abnormality, and the presence of fluid or a mass in the endometrial cavity. Assessment of the endometrium should allow for variations expected with phases of the menstrual cycle and with hormonal supplementation. If the endometrial stripe is difficult to image or ill defined, a comment should be added to the report. The myometrium and cervix may be evaluated for contour changes, echogenicity, and masses. Masses, if identified, should be measured in at least 2 dimensions and their locations recorded.

C. Adnexa (Ovaries and Fallopian Tubes)

When evaluating the adnexa, an attempt should be made to identify the ovaries first since they can serve as a major point of reference for assessing the presence of adnexal pathology. The ovaries should be measured, and ovarian abnormalities should be documented. Ovarian size can be determined by measuring the ovary in 3 dimensions (width, length, and depth), on views obtained in 2 orthogonal planes. It is recognized that the ovaries may not be identifiable in some women. This occurs most frequently after menopause or in patients with a large leiomyomatous uterus.

The normal fallopian tubes are not commonly identified. This region should be surveyed for abnormalities, particularly dilated tubular structures.

If an adnexal mass is noted, its relationship to the ovaries and uterus should be documented. Its size, echogenicity, and internal characteristics (cystic, solid, or complex) should be determined. Doppler or color Doppler ultrasound may be useful in select cases to identify the vascular nature of pelvic structures.

D. Cul-de-Sac

The cul-de-sac and bowel posterior to the uterus may not be clearly defined. This area should be evaluated for the presence of free fluid or a mass. If a mass is detected, its size, position, shape, echogenicity, internal characteristics (cystic, solid, or complex), and relationship to the ovaries and uterus should be documented. Differentiation of normal loops of bowel from a mass may be difficult if only a transabdominal examination is performed. A transvaginal examination may be helpful to distinguish a suspected mass from fluid and feces within the normal rectosigmoid.

V. DOCUMENTATION

Adequate documentation is essential for high-quality patient care. A permanent record of the ultrasound examination and its interpretation should be included in the medical record. Images of all appropriate areas, both normal and abnormal, should be recorded. Variations from normal size should be accompanied by measurements. Images are to be appropriately labeled with the examination date, facility name, patient identification, image orientation, and, whenever possible, the organ or area imaged. Relevant history for each patient should include obstetric history and/or relevant menopausal history. Retention of the permanent record of the sonographic examination should be consistent with both clinical needs and the relevant legal and local health care facility requirements.

Reporting should be in accordance with the AIUM Standard for Documentation of an Ultrasound Examination.

VI. EQUIPMENT SPECIFICATIONS

The sonographic examination of the female pelvis should be conducted with a real-time scanner, preferably using sector, curved linear, or endovaginal transducers. The transducer or scanner should be adjusted to operate at the highest clinically appropriate frequency, realizing that there is a trade-off between resolution and beam penetration. With modern equipment, studies performed from the anterior abdominal wall can usually use frequencies of 3.5 MHz or higher, while scans performed from the vagina should use frequencies of 5 MHz or higher.

VII. QUALITY CONTROL AND IMPROVEMENT, SAFETY, INFECTION CONTROL, AND PATIENT EDUCATION CONCERNS

Policies and procedures related to quality, patient education, infection control, and safety should be developed and implemented in accordance with the AIUM Standards and Guidelines for the Accreditation of Ultrasound Practices.

Equipment performance monitoring should be in accordance with the AIUM Standards and Guidelines for the Accreditation of Ultrasound Practices.

Acknowledgments

This guideline was developed by the American Institute of Ultrasound in Medicine (AIUM) in collaboration with the American College of Obstetricians and Gynecologists (ACOG) and the American College of Radiology (ACR), according to the process described in the ACR Practice Guidelines and Technical Standards Book.

The purpose of this document is to provide guidance regarding the cleaning and disinfection of transvaginal and transrectal ultrasound probes.¹

All sterilization/disinfection represents a statistical reduction in the number of microbes present on a surface. Meticulous cleaning of the instrument is the essential key to an initial reduction of the microbial/organic load by at least 99%. This cleaning is followed by a disinfecting procedure to ensure a high degree of protection from infectious disease transmission, even if a disposable barrier covers the instrument during use.

Medical instruments fall into different categories with respect to potential for infection transmission. The most critical level of instruments are those that are intended to penetrate skin or mucous membranes. These require sterilization. Less critical instruments (often called "semi-critical" instruments) that simply come into contact with mucous membranes, such as fiberoptic endoscopes, require high-level disinfection rather than sterilization.

Although endocavitary ultrasound probes might be considered as even less critical instruments because they are routinely protected by single-use disposable probe covers, leakage rates of 0.9% to 2% for condoms and 8% to 81% for commercial probe covers have been observed in recent studies. For maximum safety one should therefore perform high-level disinfection of the probe between each use and use a probe cover or condom as an aid to keeping the probe clean.

There are 4 generally recognized categories of disinfection and sterilization. Sterilization is the complete elimination of all forms or microbial life including spores and viruses. Disinfection, the selective removal of microbial life, is divided into 3 classes:

• High-level disinfection—Destruction/removal of all microorganisms except bacterial spores

• Mid-level disinfection—Inactivation of Mycobacterium tuberculosis, bacteria, most viruses, most fungi, and some bacterial spores

• Low-level disinfection—Destruction of most bacteria, some viruses, and some fungi. Low-level disinfection will not necessarily inactivate M. tuberculosis or bacterial spores.

The following specific recommendations are made for the use of endocavitary ultrasound transducers. Users should also review the Centers for Disease Control and Prevention (CDC) document on sterilization and disinfection of medical devices to be certain that their procedures conform to the CDC principles for disinfection of patient care equipment.

1. Cleaning—After removal of the probe cover, use running water to remove any residual gel or debris from the probe. Use a damp gauze pad or other soft cloth and a small amount of mild, nonabrasive liquid soap (household dishwashing liquid is ideal) to thoroughly cleanse the transducer. Consider the use of a small brush especially for crevices and areas of angulation, depending on the design of your particular transducer. Rinse the transducer thoroughly with running water, and then dry the transducer with a soft cloth or paper towel.

2. Disinfection—Cleaning with a detergent/water solution as described above is important as the first step in proper disinfection since chemical disinfectants act more rapidly on clean surfaces. However, the additional use of a high-level liquid disinfectant will ensure further statistical reduction in microbial load. Because of the potential disruption of the barrier sheath, additional high-level disinfection with chemical agents is necessary. Examples of such high-level disinfectants include but are not limited to the following:

   • 2.4% to 3.2% glutaraldehyde products (a variety of available proprietary products including Cidex, Metricide, or Procide).

   • Non-glutaraldehyde agents including Cidex OPA (o-phthalaldehyde) and Cidex PA (hydrogen peroxide and peroxyacetic acid).

   • 7.5% hydrogen peroxide solution.

   • Common household bleach (5.25% sodium hypochlorite) diluted to yield 500 parts per million chlorine (10 cc in 1 L of tap water). This agent is effective but generally not recommended by probe manufacturers because it can damage metal and plastic parts.

   Other agents such as quaternary ammonium compounds are not considered high-level disinfectants and should not be used. Isopropanol is not a high-level disinfectant when used as a wipe, and probe manufacturers generally do not recommend soaking probes in the liquid.

   The US Food and Drug Administration (FDA) has published a list of approved sterilants and high-level disinfectants for use in processing reusable medical and dental devices. That list can be consulted to find agents that may be useful for probe disinfection.

   Practitioners should consult the labels of proprietary products for specific instructions. They should also consult instrument manufacturers regarding compatibility of these agents with probes. Many of the chemical disinfectants are potentially toxic, and many require adequate precautions such as proper ventilation, personal protective devices (gloves, face/eye protection, etc) and thorough rinsing before reuse of the probe.

3. Probe Covers—The transducer should be covered with a barrier. If the barriers used are condoms, these should be nonlubricated and nonmedicated. Practitioners should be aware that condoms have been shown to be less prone to leakage than commercial probe covers, and have a 6-fold enhanced AQL (acceptable quality level) when compared to standard examination gloves. They have an AQL equal to that of surgical gloves. Users should be aware of latex-sensitivity issues and have available nonlatex-containing barriers.

4. Aseptic Technique—For the protection of the patient and the health care worker, all endocavitary examinations should be performed with the operator properly gloved throughout the procedure. Gloves should be used to remove the condom or other barrier from the transducer and to wash the transducer as outlined earlier. As the barrier (condom) is removed, care should be taken not to contaminate the probe with secretions from the patient. At the completion of the procedure, hands should be thoroughly washed with soap and water.

Note: Obvious disruption in condom integrity does NOT require modification of this protocol. These guidelines take into account possible probe contamination due to a disruption in the barrier sheath.

In summary, routine high-level disinfection of the endocavitary probe between patients, plus the use of a probe cover or condom during each examination, is required to properly protect patients from infection during endocavitary examinations. For all chemical disinfectants, precautions must be taken to protect workers and patients from the toxicity of the disinfectant.