Last Updated on February 19, 2022 by Allison Price
Radiographs can be useful for diagnosing bony structures but ultrasound is the most versatile tool that a modern veterinarian has. A recent graduate of veterinary school may have more ultrasound experience than a typical medical doctor.
The fundamental principle of ultrasound is that an ultrasound transducer emits high-frequency sound waves that are not perceptible to the human ear. The machine emits sound waves for about 0.1 percent and then spends 99.9% of its time listening to those sound waves being reflected off any tissue.
Low frequency probes (3.5 Mhz) can be used to visualize deeper structures. The same principle applies to the bass sound you hear from your neighbor’s stereo. It penetrates walls and buildings. Although lower frequency sounds may not be as clear, they can penetrate deeper. High frequency probes can also be used to visualize shallow structures. High-pitched sound is clear, but it’s not very effective at penetration. High-frequency probes (7.5 Mhz) are best for precise imaging of tendons or objects under the skin.
All probes show fluid-filled structures as dark images, while soft tissue structures are shown as different degrees of white. While ultrasound can be used to view internal organs in large and small animals, it is most commonly used to visualize tendons or reproductive structures in horses. An embryo in development can be seen as early as 12-14 days after gestation. Rectal palpation alone can detect pregnancy between 30 and 35 days. Rectal palpation in miniature mares is not always a safe option due to the risk of manually tearing their rectum. Transrectal ultrasonography is performed by wrapping the tail around the mare and then extricating the rectum. After the probe is inserted rectally, the cross-section is visualized.
It is important to have an ultrasound done to check for impending ovulation. Ovum from the mare is only viable for 12 hours and sperm for 72 hours. This technique allows the mare to see her semen 12 to 24 hours before she ovulates. If the mare is heating up, uteri showing estrogen influence will show typical “wagon wheel”, which is due to the edema in the uterus. It is possible to visualize the follicles of each ovary. A dominant follicle will usually be found on one ovary. To induce ovulation, the mare may be given human chorionic Gonadotropin (HCG), or deslorelin(Ovuplant). Insemination can confirm ovulation by using ultrasound again.
Rectal palpation in full-sized mares is still necessary because it allows us to determine the consistency of the follicle. Rectal palpation is usually not possible in miniature mares so we rely solely on the size and quality of the ultrasound images.
The embryo can be visualized from day 11-14, but by day 15-17 the embryonic vessel has been attached to the wall of your uterus. This makes it much easier to visualize. To confirm the existence of a viable foetus, the heartbeat can now be seen by day 24. It is possible to mistake early fetuses for uterine cysts, and vice versa. You can check the mare for an increase in vesicle sizes periodically. This can help to distinguish cysts from genuine pregnancies. Understanding the importance of ultrasound is easier when you realize that the mare’s heat cycle lasts approximately 21 days.
In the absence of a pregnancy, it is possible to decide to rebreed your mare during the heat cycle. The Fetal Genital Tubercle can be seen closer to the anus for confirmation of a female foal. It can also be visible closer to the umbilicus for confirmation of a male foal. Transabdominal ultrasound is available from day 80. The probe can be placed on the abdomen, just cranially to the udder. To confirm viable fetuses, periodic transrectal or transabdominal ultrasound can be used.
Twin embryos can be difficult to see if they are placed side by side or on top one another in the uterus. However, we usually visualize them as distinct entities. According to some data, 94% of twin pregnancies end in stillbirth or abortion. The mare will usually carry the foals for eight to nine months if the twin pregnancy continues unassisted. After that, both fetuses will be aborted. Only eight percent of twins will survive beyond two weeks, even if the mare carries them to term.
Twin embryos can be separated manually transrectally if they are visible. The smaller embryo is then pinched to cause death. This is typically done before day 17. Standardbreds and Thoroughbreds are the most common for twinning at a rate of one percent. Thoroughbreds are notorious for this (3.5%). Because of the difficulty in pinching (fetal reduction), miniature mares don’t twin.
Although they are usually harmless, uterine cysts can cause problems with the fetal-uterine connection and can be easily seen with ultrasound. Transrectal ultrasound can be used to visualize fluid retention in the uterus. This can occur from infectious or idiopathic ometritis. Endometritis can occur when mares are exposed to stallion semen. Fluid retention can be visualized and treated by intrauterine infusion, flushing the uterus with saline, and/or oxytocin. This will give the uterine body a tone. Ultrasonography can also be used to evaluate post-partum mares for their uterine tone, and ability to “breed back,” on the “foal heat”.