‘Tis the season! Breeding season is once again upon us and whatever your plans may be this year, it never hurts to brush up on a little equine reproduction education before making those vet appointments for your mare. And speaking of which – during a routine ultrasound examination of your mare, have you ever felt like Rachel in the episode of Friends, following her own ultrasound exam? You know, the old episode depicting Rachelʼs frustration when she was unable to see her own pregnancy on the ultrasound monitor… yet all her friends could pick it out without a problem?
If you ever felt the same way while having your vet ultrasound your mare for breeding, keep reading. Ultrasonography is a complex procedure that uses sound vibrations with an ultrasonic frequency, for medical imaging. But when the ultrasound process is broken down, it isnʼt as difficult to understand, nor are its resulting images confusing to view. Ultrasonography is particularly beneficial for monitoring a mareʼs cycle, ovarian or uterine disease and pregnancy.
If you have a craving to better understand what your vet is explaining about your mareʼs cycle and perhaps her subsequent pregnancy, this two-part article will explain how ultrasonography works and why it is beneficial for equine reproduction. Plus you can also follow the growth of a follicle in a typical pregnancy along, through the use of ultrasound imaging.
1. Transducer – usually a flat piece, connected by a long cord, placed directly on/in the
horseʼs body in the area intended for ultrasonographic imaging.
4. TV echo display
6. Digital scan converter
HOW IT WORKS:
Sound waves are created by electrical stimulation of crystals in the transducer. When trying to determine where a mare is at in terms of her cycle or pregnancy, because of the sheer size of the horse, the transducer is placed inside the mareʼs rectum directly overtop of the reproductive tract. When the electric current is applied to crystals in the transducer, vibrations are produced that result in sound waves.
These waves are then spread through the horseʼs tissue and some of them are reflected back to the transducer. (The number of reflected sound waves is directly proportional to the density of the horseʼs tissue). The reflected waves are converted to electrical impulses and displayed on a screen (most often, a black and white monitor).
The reflected waves are represented on the screen by shades of gray, extending from black to white. Fluid is a great transmitter for sound waves, therefore, liquid structures such as follicular fluid or yolk sac fluid within the horseʼs body appear black on the screen. Dense tissue in the body reflect most of the ultrasonic beam and appear white on the screen – these structures include the cervix and pelvis. Other tissues appear in varying shades of gray, depending on their ability to reflect sound waves. Air and gas are poor propagators of the signals, which is why the vet must maintain a close contact between the transducer and the examined tissue.
Modern ultrasound machines used for examining the reproductive tract are “B-Mode”, real time scanners. B-mode means “brightness modality” and the imaging from these machines results in a two-dimensional display of dots of the screen. The brightness of the dots is proportional to the amplitude of the returning echoes. And the real time imaging means a “live” or moving display of the echoes that are being recorded, can be seen.
Although there are two types of ultrasound transducers used for reproductive exams in horses, typically, our vets use linear-array transducers. Linear array means there is a side-by-side arrangement of the rectangular crystals, along the length of the transducer. The transducer is positioned in the longitudinal plane of the mareʼs body, which means images of the cervix and the uterine body are longitudinally oriented while the uterine horns are cross sectional. Tissues that are closest to the transducer can be seen at the top of the ultrasound screen.
ANATOMY OF THE MAREʼS GENITAL TRACT
The red lines on this diagram represent typical linear array transducer orientations, over the genital tract during an exam. When the transducer is over the cervix and uterine body (1), a lengthwise (longitudinal) image is produced. The uterine horn is generally seen in short-axis cross-section (2), which allows assessment of the endometrial folds. Sections through the ovary show follicles and Corpus luteums cut in different planes (3).
Echogenic – if an item is echogenic, it will appear white on the ultrasound screen. Dense tissues reflect most of the ultrasonic beam.
Non-echogenic – if an item is non-echogenic, it will appear black on the ultrasound screen. Fluid and liquid containing structures are excellent transmitters of sound waves.
Estrous Cycle – the repetitive sequence of hormonal cycling that prepares the mare for conception. It may be divided into estrus (follicular phase – 5 to 7 days)) and diestrus (luteal phase – 14 to 16 days.)
Conceptus – the embryo in the uterus, especially during the early stages of pregnancy.
Stay tuned to My Stable Life to follow a follicle along in ultrasound images, plus a pregnancy story in pictures! Thank-you to Dr. Tammi Roalstad of Scottsdale, AZ, for providing the ultrasound images and information used in this article.