By Benjamin Espy, DVM, DACT
Although radiographs are valuable for detecting bony structures, ultrasound is probably the most versatile tool available to the modern veterinarian. Even a recent veterinary school graduate may have more hands-on ultrasound experience than your typical medical doctor.
The basic principle behind ultrasound is that the ultrasound transducer sends out high frequency sound waves that are imperceptible to the human ear. About 0.1 percent of the time the machine emits sound waves and then spends 99.9 percent of the time listening for those sound waves to be reflected back off of whatever tissue to which the machine is exposed.
Deeper structures are visualized with low frequency probes (3.5 mhz). This is the same theory of that bass on your neighbor’s stereo, penetrating walls and even buildings away. Lower frequency sounds are not as clear but are able to penetrate much deeper. Likewise, shallow structures are visualized with high frequency probes. High-pitched noise is very clear but not able to penetrate well. For tendons and objects just under the skin that require very precise imaging, high-frequency probes (7.5 mhz) are most useful.
With all probes, fluid-filled structures appear as dark images on the screen, while soft tissue structures appear as varying degrees of white on the screen. Although ultrasound can be used in both large animal and small animal medicine to view internal organs, the most common uses in the equine species is to visualize tendons and reproductive structures. A developing embryo can be visualized as early as day 12 to 14 days of gestation, where rectal palpation alone can detect pregnancy by 30 to 35 days of gestation. Unfortunately, in miniature mares, rectal palpation is usually not a very safe option when weighing the danger of manually tearing the rectum. Mares are prepared for transrectal ultrasonography by wrapping the tail and then evacuating the rectum. The probe is then inserted rectally and the cross-section of the uterus is visualized.
Because ova from the mare are only viable for about 12 hours, and sperm is viable for up to 72 hours, it is wise to ultrasound for impending ovulation. Using this technique, it is possible to have the semen in the mare 12 to 24 hours before she ovulates. Uteri that are showing the influence of estrogen when the mare is in heat, show a typical “wagon wheel” appearance that is caused by the folds of the uterus becoming filled with edema. The follicles on each ovary may be visualized. Usually a follicle on one ovary will become dominant. When it grows to 35 to 40 mm in diameter, human chorionic gonadotropin (HCG) or deslorelin (Ovuplant) may be administered to the mare to cause her to ovulate. Ultrasound can then be used again after insemination to confirm ovulation.
In full-size mares, rectal palpation is still important because of the necessity of being able to detect the consistency of the developing follicle. The majority of the time rectal palpation in miniature mares is not an option so we rely on the size of the ultrasound image alone.
Although the embryo may be visualized by day 11 to 14, by day 15 to 17 the embryonic vesicle has become fixed to the wall of the uterus and is much easier to visualize. By day 24, the heartbeat can be visualized to verify the presence of a viable fetus. Early fetuses can be mistaken for uterine cysts and vice versa. The mare can periodically be examined for an increase in vesicle size. This may differentiate cysts from true pregnancies. The importance of ultrasound can be best understood when realizing the mare’s heat cycle is approximately 21 days in duration.
The absence of a pregnancy can allow the decision to be made to rebreed the mare on the ensuing heat cycle. By day 60 to 78 days of gestation the Fetal Genital Tubercle (FGT) can be visualized closer to the anus to confirm the presence of a female foal, or closer to the umbilicus to confirm a male foal. From day 80 onwards, transabdominal ultrasound can be utilized by placing the probe on the abdomen just cranial to the udder. Periodic transrectal and transabdominal ultrasound for fetal heartbeats can be utilized to confirm viable fetuses.
Although twin embryos may be hard to visualize if they are side by side or on top of one another in the uterus, usually we can visualize them as separate entities. Some data suggest that 94 percent of twin pregnancies result in abortion or stillbirth of both fetuses. If the twin pregnancy is allowed to progress without intervention, the mare usually carries the foals to eight or nine months and then both fetuses are aborted. Even if the mare carries the twins to term, only eight percent will be born and live more than two weeks.
If twin embryos are visualized, they may be manually separated transrectally and the smaller one is pinched to cause death. This procedure is usually performed before day 17. Thoroughbreds are most notorious for twinning (3.5 percent), and Standardbreds are next at one percent. Luckily, miniature mares rarely twin because fetal reduction (pinching) is difficult in this breed.
Uterine cysts, although usually innocuous, can interfere with fetal-uterine attachment and are easily visualized with ultrasound. Retention of fluid in the uterus from either infectious or idiopathic endometritis can also be visualized via transrectal ultrasonography. Some stallion semen can be irritating enough to the mare’s uterus to cause endometritis. If fluid retention is visualized, then the mare can be treated with intrauterine infusion, saline flushing of the uterus and/or oxytocin to lend tone to the uterine body. Finally, using ultrasonography, post-partum mares can be evaluated for uterine tone and their ability to “breed back” on the ensuing “foal heat.”
Reviewed by original author in 2016.