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. The image that we view on the ultrasound machine is created by the sound waves that are reflected back from the tissues. With ultrasonography, fluid-filled structures appear as dark images on the screen, while soft tissue structures appear as varying degrees of white on the screen.
We commonly perform ultrasonography of tendons and ligaments in the field setting. We often select the ultrasound after either our physical exam reveals swelling or pain in a tendon or ligament, or after a lameness exam and nerve blocks have helped localize the source of pain to a particular area. Ultrasonography can also be used to evaluate certain synovial structures such as the digital sheath.
One of the mainstays of reproductive management in the mare is evaluation of the reproductive tract via rectal palpation and ultrasonography. We use it to monitor a mare’s follicular growth, uterine edema, ovulation, and presence of any intra-uterine abnormalities such as endometrial cysts or fluid. With our machine, the equine embryo may be visualized as early as day 12, but we typically suggest the first pregnancy check at day 14 post ovulation. One significant benefit to ultrasound (compared to palpation alone) is the ability to detect and intervene in the case of twin embryos. Some data suggest that 94 percent of twin pregnancies result in abortion or stillbirth of both fetuses. By day 24, the heartbeat can be visualized to verify the presence of a viable fetus.