Transvaginal sonography (TVS) affords improved resolution of the uterus and ovaries over the conventional transabdominal sonography (TAS) approach. Although TVS allows a closer proximity of the transducer to the pelvic organs and more detailed depiction, it may be more 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 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 transducer with side-to-side angulation within the upper vagina for sagittal imaging. (Figures 2-1, 2-2, 2-3)

   
   
2. 
   

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

   
   
3. 
   

   Variation in depth of transducer insertion for optimal imaging of the fundus to the cervix by gradual withdrawal of the transducer 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 (broadband 5.5-7.8 MHz). Higher-frequency (>8 MHz) transducers may limit the field of view to within only 6 cm of the transducer.

The major types of transducers 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 transducers 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 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 element. Reverberation artifacts can be created by suboptimal coupling of the condom/transducer/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.

Transvaginal equipment that utilizes a mechanical transducer is relatively rarely used today when compared to electronic transducers.

Practitioners should follow the AIUM guidelines for the disinfection of transvaginal transducers. These guidelines are included as Appendix 2-2. The more recent widespread use of the Trophon device has extended disinfection capabilities to include the human papilloma virus (HPV) virus. There is evidence that some disinfectants such as glutaraldehyde and ortho-phthalaldehyde are ineffective against HPV16, the leading cause of cervical cancer.^1-3 The Trophon system has been shown to be effective against HPV16 and HPV18.⁴ This is considered a major advantage since HPV contamination was identified in up to 7% of disinfected transducers used in TVS.² HPV has been shown to account for up to 5% of all cancers worldwide and is responsible for almost all cases of cervical cancer. The HPV virus is a leading cause of oral, throat, anal, and genital cancers. In addition, the design of the Trophon unit affords disinfection of the handle of the transvaginal transducer, which has been shown to be a reservoir for pathogens.^4-6

For infection control purposes, a disposable protective sheath is used to cover the transducer. After completely covering the transducer with a sheath such as a condom and securing the sheath to the shaft of the transducer 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 directed 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.

Examination of the uterus begins with its depiction in long axis (Figures 2-4, 2-5, 2-6, 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.⁷

It may be difficult to determine the flexion of the uterus with static 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 needed to obtain optimal images of the uterus. For example, retroflexed uteri are best depicted when the transducer is in the anterior fornix and angulated in a posterior direction. The fundus of the retroflexed uterus is directed to the inferior right corner of the image. Conversely, the anteflexed uterus will demonstrate the fundus directed to the upper left corner of the image.

The endometrium has a variety of appearances depending on its stage of development. The stages of endometrial development can be described in relation to oocyte maturation (follicular vs luteal) or endometrial development (proliferative vs secretory). In the proliferative phase, the endometrium measures 5 to 7 mm in anterior-posterior (AP) dimension. This measurement includes the 2 layers of endometrium. A hypoechoic interface can be seen within the luminal aspects of echogenic layers of endometrium in the peri-ovulatory phase and likely 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.⁸ 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.⁹ Alternatively, volumetric measurements can be made using 3D (see Chapter 49). One can use the axial plane landmark where the endometrium invaginates into the area of ostia in the region of the uterine cornu. This is also a useful landmark to denote the proximal portion of the tube.

Because of the close proximity of the transducer to the cervix, the cervix is not as readily visualized as the remainder of the uterus. This may make exact measurement of the long axis of the uterus difficult due to the imprecision of its measurement. If one slightly withdraws the transducer into the vaginal canal, however, images of the cervix can easily be obtained. The mucus within the endocervical canal usually appears as an echogenic interface. This interface may become hypoechoic during the peri-ovulatory period because the cervical mucus has a higher fluid content.

Ovaries are typically depicted as oblong-shaped 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.