| Abstract: | Ultrasonography is useful for monitoring the dynamic follicular and luteal changes of equine ovaries, since it permits rapid, visual, non-invasive access to the reproductive tract. A 5 MHz transducer has greater resolving power and is far more suitable for evaluation of ovaries than a 3–3.5 MHz transducer. Follicles as small as 2–3 mm can be seen and the corpus luteum can usually be identified throughout its functional life. In a study involving daily ovarian examinations, there was a pronounced change in shape of the preovulatory follicle from a roughly spherical to a pear-shaped or oblong form in 66% of the ovulatory periods, This change usually occurred on the day preceding ovulation. The occurrence of ovulation was detectable by the disappearance of a large follicle. In addition, the ovulation site on day 0 was characterized by an intense echogenic area in 88% of 32 ovulations. The developing corpus luteum retained the echogenicity for a mean of 2.4 days. In a blind study, the location of the corpus luteum, as determined by ultrasound, agreed with a previous independent determination of the side of ovulation by palpation in 88% of the 40 bred mares on days 0–14. In all of the 12 mares that were in estrus, the location of the corpus luteum could not be ascertained. In another study, the corpus luteum was identified for a mean of 16 days in 14 estrous cycles. One or more days before the corpus luteum became ultrasonically unidentifiable, it developed increased echogenicity in 36% of the mares, indicating greater tissue density. It is concluded that ultrasonic evaluation of the corpus luteum is superior to digital evaluation by rectal palpation. Some of the potential applications of ultrasonic examination of the ovaries include: 1) obtaining important, sometimes definitive, information by a single examination for judging whether a mare has entered the ovulatory season, 2) aiding in estimating the stage of the estrous cycle, 3) detecting double preovulatory-sized follicles which are in close apposition and difficult to discern by palpation, 4) detecting failure of ovulation or anovulatory estrus by the absence of a corpus luteum, 5) differentiating a persistent corpus luteum from anovulatory or anestrous conditions, 6) diagnosing certain pathological conditions such as peri-ovarian cysts and ovarian tumors, and 7) diagnosing anovulatory hemorrhagic follicles. |
