Ultrasound scanning can capture real-time images of various organs and body systems, such as the tendons, ligaments, heart, lungs, intestines, kidneys, liver, spleen, uterus and ovaries. This imaging technique is used across all areas of equine veterinary medicine and its possibilities are nearly endless.
Ultrasound is commonly used to identify musculoskeletal injuries, including damage to structures such as the superficial digital flexor tendon, digital flexor tendon sheath and suspensory ligament, and to monitor the healing of lesions. In internal medicine, ultrasound can identify horses with liver and kidney damage and lung disease.
The technique is invaluable in horses with colic to detect gut abnormalities that may require surgery, such as distension of the small intestine. Cardiologists use ultrasound to check for enlargement of the heart chambers, abnormalities of the heart valves or abnormal flow. Vets involved with reproduction may scan a mare’s ovaries to assess the stage of her cycles and an appropriate time for covering or artificial insemination, before imaging the uterus and embryo to determine the stage of pregnancy and the sex of the foal.
Ultrasound can also be used to image lumps and bumps, giving an idea as to their cause, and to aid biopsy — the sampling of an organ or mass to allow examination under a microscope. It is effective, too, for locating foreign bodies such as blackthorns, which can become lodged under the skin.
How it works
Ultrasound uses high-frequency sound waves, above the range of hearing. These waves are produced from a hand-held transducer, or “probe”, which is connected to a computer.
Ultrasound waves pass easily through fluid, so any fluid in the body shows up on the computer screen as black. Waves cannot pass through bone or air, however, so these waves are bounced back to the probe and appear as bright white. Other tissues of intermediate densities will show up on the screen as variousdifferent shades of grey.
All areas of the resulting two-dimensional image can give diagnostic information.
Higher ultrasound frequencies will give better definition of more superficial structures, while lower frequencies are used to penetrate deeper structures. The frequency is controlled mainly by the probe, so different probes are needed for scanning tendons, the abdomen or the heart. The computer allows alterations for improved image quality, such as in brightness, darkness and focus depth.
Ultrasound units have become smaller over the past 20 years and the quality of images has improved rapidly. Most scans can now be performed on the yard or at a clinic, at the owner’s convenience, using mobile machines that produce high-quality images. All that’s required is a dark area or stable, for better visualisation of the screen and images.
The main limitation in achieving quality images is the operator, rather than the machine. Skill and experience are required to produce the detailed images necessary for accurate diagnosis.
Risks and limitations
This is a painless, non-invasive procedure which is generally tolerated well by horses. No radiation is involved, so it is safe for both horse and operators, and there are very few side effects to worry about.
One limitation is the type of horse. Ultrasound waves do not travel well through areas of fat or thicker skin, so it may not be possible to image an overweight animal or the tendons and ligaments of a thick-skinned cob. Waves will not pass through the hoof capsule, so ultrasound of the foot is very limited. Nor will waves travel through air, so ultrasound of the lungs is restricted to imaging of the lung surface.
Ultrasound can identify roughening on the surface of bone but cannot penetrate the bone itself, so your vet may opt for X-ray or computerised tomography (CT) scanning in certain cases. Magnetic resonance imaging (MRI) is sometimes preferred as structures can be imaged in cross-sectional “slices”.
With acute injuries of tendons and ligaments, the body will take some time to “remove” damaged fibres from the site.
Ultrasound should be delayed until at least 10 days after injury to give a clearer picture of the damaged area.
While sedation is not necessary for most forms of ultrasound, it may make the procedure quicker and safer — especially when scanning the hind legs of a tickly horse who doesn’t want to stand still. There is no need to withhold food or water before sedation or performing ultrasound. A horse will wake up fairly rapidly after sedation, usually within 30mins, and can then be treated normally.
A good contact between the probe and the horse’s skin is essential, so the area to be scanned is brushed and any dirt, scurf or scabs are removed. Ideally, the hair is clipped — this is usually unavoidable if the horse has a thick winter coat. The skin is then cleaned of grease with a surgical scrub, such as chlorhexidine, before gel is applied and scanning can commence.
Surgical spirit is sometimes used as an alternative to the scrubbing and gel, especially where hair is not clipped, but may damage certain types of probe.
The procedure is different for reproductive ultrasonography of the ovaries and uterus. The vet will use a gloved hand to insert a sealed, protected probe into the rectum so that these organs can be scanned across the rectal wall.
This involves obvious risks to the vet, especially with younger or maiden (first-time) mares, so access to stocks and/or sedation may be necessary. A rectal tear is a possible and potentially fatal complication, so adequate restraint protects not only the vet but also the mare.
What’s the damage?
The results are immediate and your vet should be able to give you an answer and a treatment plan there and then.
Ultrasound images can be stored, usually in digital format, and forwarded to colleagues for a second opinion if there is any doubt or question over diagnosis. The images can also be compared to previous scans to monitor injury healing and to structure an exercise programme.
Cost will vary between practices and will depend upon the area being imaged, but is likely to be between £200 and £250.
This would seem good value, given the cost of a decent-quality ultrasound machine (in the region of £20,000 or more) and the expertise of your vet in performing the scans and analysing the images.
Ref Horse & Hound; 27 September 2018