A B-mode real-time ultrasound system consists of a transducer that is connected by a long cord to a base unit containing a display monitor and control panel (Figure 5-1). The transducer transmits and receives high-frequency sound waves to produce images of soft tissues and organs on the display monitor. Tissues vary in echogenicity (i.e., the ability to propagate or reflect sound waves); therefore, the proportion of reflected sound waves is dependent on the innate characteristics of the tissues or fluids being examined. For instance, fluids readily propagate sound waves, whereas air and dense tissues reflect most or all sound waves. Reflected sound waves that are received by the transducer are then converted to electronic impulses and subsequently displayed on the monitor. The monitor consists of a two-dimensional array of closely aligned dots. The brightness of the dots is directly proportional to the amplitude of the echoes (or reflected sound waves). Hence, highly echogenic tissues (e.g., bone or connective tissue) appear white on the ultrasound monitor whereas nonechogenic (anechoic) fluids are black. A continuum of gray shades between white and black allows one to distinguish tissues of intermediate echogenicity. The control panel of the ultrasound unit allows the operator to adjust the quality of the image and label or measure structures of interest. Because air interrupts the transmission of ultrasound waves, when air is present within a viscus, bright echogenic lines or spots are seen on the monitor. When sufficient blockage of ultrasound waves occurs, artifactual shadows or reverberations may be produced below the initially sound-impacted echogenic structure or tissue on the monitor. For example, dark shadows are visible on the monitor below sound-impacted bone, such as fetal ribs. Reverberation echoes are bright artifacts repeated at even intervals, but with lesser reverberation, because blocked sound waves are bounced back and forth between the probe and a gas-filled viscus. Artifacts not only interfere with the ability of the examiner to visualize surrounding tissue but may be falsely interpreted to be pathologic structures.

Linear-array 5-MHz transducers are amenable to most needs encountered in equine reproduction. The higher resolving power of 7.5-MHz transducers allows a more detailed study of structures, but the tissue-penetrating capacity of these transducers is more limited. Transducers with lower sound wave frequencies (e.g., 2.5 to 3.5 MHz) permit greater tissue penetration, so they may be more useful for evaluation of the uterus and its contents during advanced pregnancy; however, image resolution is reduced accordingly. Curvilinear transcutaneous probes available for thoracic or abdominal imaging can be useful for monitoring fetal viability (e.g., heartbeat, fetal activity), character and amount of amniotic or allantoic fluid, uteroplacental thickness, and separation of the placenta.

For ultrasound examination of the mare reproductive tract per rectum, we prefer to advance the transducer over the cervix and body of the uterus until the bifurcation of the uterus is visualized (Figure 5-2, Figure 5-3, Figure 5-4 and Figure 5-5). The transducer is slowly moved toward the tip of one uterine horn, with care taken to ensure the image of the uterine horn in cross section remains in the center of the monitor screen. As the transducer moves beyond the tip of the uterine horn, the ovary is scanned (Figure 5-6) in its entirety. The transducer is then moved slowly back down the uterine horn to the bifurcation, and the opposite uterine horn and ovary are scanned in a similar manner. After scanning the opposite ovary, the transducer is moved slowly back to the bifurcation and is rotated slightly in a back-and-forth motion across the uterine body and cervix as it is withdrawn from the rectum. This systematic scanning procedure ensures that the entire reproductive tract is examined twice, permitting accurate identification of the location of singleton or multiple pregnancies and uterine pathologic conditions, and provides confidence that no conceptus was overlooked during the examination process.

Transabdominal ultrasonography allows maximum visualization of the fetus and placenta, sometimes beginning as early as 60 to 80 days of gestation. Lower frequency (2.5 to 3.5 MHz) curvilinear transducers in various configurations are used for this purpose (Figure 5-7) and may allow assessment of the uterus in the inguinal area in some cases. In later gestation, maximal penetration into the ventral abdomen is necessary. Application of ultrasound gel or alcohol is helpful for improving contact between the transducer (ultrasound probe) and skin, allowing underlying structures to be more easily visualized. Because hair traps air, which interferes with sound wave propagation, it may be necessary to clip the hair closely, from the xiphoid-sternum to the udder, and on the lower flanks on either side of the midline, to improve transducer contact and thus image clarity. For advanced pregnancy, the transducer is initially placed between the sternum and the mare’s udder (Figure 5-8), followed by slow movement from side to side and cranial to caudal, until the fetus is located. Fluid within a viscus (especially the nongravid horn) is searched for to locate the uterus. Fetal structures (e.g., skull, foot, ribs) or membranes are often visualized, which aids in identification of the gravid uterus. Locating the fetal skull or thorax is helpful for orientation. Fetal heart rate, movement, amount and character of fluid (amniotic/allantoic), uteroplacental thickness, and evidence of separation between the uterus and placenta are typically evaluated by the examiner to determine normalcy (see Chapter 9). Examination for two fetuses is also possible if a question about twin status exists.

Per vagina ultrasonography has recently been used in horses for transvaginal aspiration of oocytes (follicular aspirations) and transvaginal aspiration or injection of the yolk sac or allantois of an early twin conceptus (see Figure 8-26). A common apparatus uses a stainless steel needle guide installed in an elongated plastic probe extension handle that accommodates a small curvilinear ultrasound probe in the tip (Figure 5-9). After sterilization and assembly, a 24-inch, 12- to 16-gauge, single- or double-port needle is inserted into the needle guide in the probe handle. The handle is used to insert the ultrasound probe into the cranial vagina; the operator uses the other hand in the rectum to position the structure of interest immediately next to the probe held in the vagina (Figure 5-10). Some ultrasound machines have software installed that delineates a dotted line on the ultrasound monitor and indicates where the needle will pass once inserted through the vaginal tissue. While the structure of interest (ovary with follicle, portion of uterine horn containing the twin conceptus) is held firmly against the ultrasound probe, the needle is forced through the cranial vagina, retroperitoneal fascia, and peritoneum into the structure of interest.

Diagnostic ultrasonography is used in the broodmare for (1) evaluation of ovarian activity, (2) detection and evaluation of pregnancy, and (3) diagnosis of pathologic changes in the reproductive tract. Although transrectal ultrasonography is emphasized in this chapter, transabdominal ultrasonographic images are shown in Chapter 9.

The ovaries of the mare are easily visualized with transrectal ultrasonography. The connective-tissue stroma is uniformly echogenic (white). Follicles are fluid-filled and, hence, represented as circular or irregularly shaped anechoic (black) images on the ultrasound monitor (see Figure 5-6). The ultrasonographic appearance of a corpus luteum is variable and ranges from a uniformly hyperechoic image (Figure 5-11) to a heterogeneous or mottled image, in which only a portion of the gland contains echogenic material (Figure 5-12). Because of the distinct border, many corpora lutea can be distinguished from the surrounding stroma throughout their life span.