Understanding Equine Diagnostic Imaging
What to expect when your horse undergoes advanced imaging exams
If your horse has been experiencing a complex lameness, your veterinarian might recommend advanced imaging to further investigate the source of the problem. The most common modalities are magnetic resonance imaging (MRI), computed tomography (CT), and nuclear scintigraphy (aka a bone scan).
Both MRI and CT are cross-sectional modalities, meaning they produce images of the horse’s anatomy in “slices,” giving us the ability to look inside. Each of these modalities provides information about both bone and soft tissue structures, although, generally speaking, CT is used more often to examine bone and MRI more often for soft tissue injury. Typically, CT and MRI equipment can only capture images of certain regions of the horse’s body that fit in the scanner—not the whole horse. The veterinarian will need to have pinpointed the location of the injury prior to the scan. For both MRI and CT, some scanners enable the exam to be performed under standing sedation and others require the horse to be placed under general anesthesia. Usually, images obtained under general anesthesia are of a higher quality, so there’s a trade-off between the greater chance of an accurate diagnosis and the financial cost and (relatively small) risks associated with anesthesia.
Nuclear scintigraphy is an imaging technique veterinarians typically use to image large regions of the horse, such as the forelimbs, hind limbs and pelvis, or even the whole body. It involves using a radioactive compound that highlights regions of bone injury. Practitioners use it less commonly to diagnose soft tissue injury. Situations in which this modality is most useful include when your veterinarian suspects:
- Severe bone damage, such as a fracture.
- Pathology (disease or damage) in regions difficult to fully image with X ray and ultrasound, such as the pelvis.
- Multiple regions of injury, because the bone scan can give information about all these locations in one imaging study.
Nuclear scintigraphy is always performed under standing sedation.
The first thing to know about performing these imaging studies is they all require bulky, expensive equipment and specific expertise to operate and interpret. For this reason, they are often available in larger hospitals. Scheduling an appointment with your closest suitable clinic and arranging the necessary transportation are, therefore, the first steps in preparing for these examinations. Once your appointment is scheduled, here is what to expect from your horse’s imaging field trip.
Because veterinarians perform standing MRI scans with the horse under sedation, you might be able to drop off and collect your horse on the same day the scan is performed, depending on the clinic’s schedule. However, because the exam requires the horse to stand very still, he might receive heavy doses of sedation. In these instances the clinic might choose to keep your horse overnight following the procedure to monitor for any complications.
With this modality a giant magnet generates images (The clue is in the name!). Therefore, metal horseshoes can interfere with the exam and your farrier or clinic staff should remove all shoes prior to the study. The veterinarians also need to take hoof X rays to ensure no metallic nail fragments from the horseshoe remain.
The practitioner typically places a catheter within your horse’s jugular vein (in the location of an intravenous injection) so he or she can give multiple doses of sedation without repeated pokes. After being sedated, technicians lead the horse into the MRI room and position the region of interest within the magnet. They also place a “coil” around the horse’s limb. The scan proceeds once the horse is standing still and in position and typically takes a couple of hours, since some repositioning associated with horse movement is often necessary. Scanning is safe, and the horse does not feel anything during the procedure. Once the scan is complete, the techs return the horse to his stall, remove the catheter, and give the horse time to wake up prior to leaving the clinic.
Additional tip: A clean, dry horse is the perfect patient for MRI, since the presence of water on the skin and hair can interfere with the scan. Blue or purple antimicrobial sprays also severely affect the image and should be avoided.
MRI Under General Anesthesia
MRI under general anesthesia also necessitates shoe removal, X rays to rule out residual metal debris, and intravenous catheter placement. Any equipment used to position the horse in the magnet, such as halters, should not include metal components. Metal within the body, such as plates and screws, is generally safe, so long as the scan is not being performed in the immediate postoperative period—although it’s wise to inform the clinic of the presence of such implants so staff can consider the risk on a case-by-case basis. If the metal implant is in proximity to the region being scanned, it will create a pronounced imaging artifact that will limit interpretation.
In contrast to standing MRI, imaging examinations performed under general anesthesia usually require a short, several-day stay at the hospital. This is so clinic staff can withhold food from the horse the night prior to the anesthesia and monitor him for complications the night following the procedure.
On the day of scanning, your horse is anesthetized, then hoisted onto a table used to position the region of interest within the magnet. The scan typically takes less time to complete than a standing scan because the horse remains perfectly still throughout. Once the scan is over, the horse is placed in a padded stall to recover from anesthesia. Recovery is typically uneventful, although there are always risks associated with getting a woozy 1,000-pound animal back on his feet.
Fortunately, studies investigating the risks of anesthesia in healthy horses have revealed fatal complications occur in fewer than 1% of cases (Johnston et al., 2002).
Standing CT units come in several configurations, so the exact scanning procedure varies depending on the version your hospital has, as well as the region being imaged. The technology used to generate a CT image is the same as that used for X rays: Beams of radiation pass through the horse, and the detected beams generate an image. Because this technology does not use a magnet, horseshoes can remain in place unless the foot itself is being scanned, in which case the shoe does interfere with the image.
As with standing MRI, the scans are performed under sedation and the veterinarian might place a jugular vein catheter for easy administration. CT scans take much less time than MRIs, so horses customarily receive comparatively lower doses of sedatives. As a result, standing CT scans can be performed as outpatient procedures.
CT Under General Anesthesia
Your veterinarian might recommend a CT under general anesthesia if there is a chance your horse needs immediate surgery as a result of the imaging findings (e.g., in cases of fracture). A CT under anesthesia might also be the only option available regionally. Although these scans are quick to perform, horses usually stay at the clinic for at least two days to enable appropriate patient monitoring. Intravenous catheter placement is also necessary. In this instance the horseshoes must be removed to reduce the chance of the horse hurting himself during recovery.
While CT scans use radiation, which can be harmful in high doses, the exposure associated with a single or even several CT scans is extremely unlikely to cause your horse any health issues.
Scintigraphy involves injecting a radioactive tracer into the horse, which the bloodstream carries to the skeleton. The tracer “sticks” best to bones in horses performing regular exercise, so keep your horse in work for two weeks leading up to the scan, if possible. This might be contraindicated in some instances, such as when a horse is extremely lame, so follow your veterinarian’s recommendations. Shoes do not have to be removed for bone scans unless the feet are the primary region of interest.
Because the radiotracer needs good blood flow to distribute properly, many clinics will longe the horse in the morning prior to injecting the radiotracer. Therefore, it’s helpful if you have a horse that is used to longeing. In particularly cold parts of the world, longeing alone might not increase blood flow enough to the extremities. At these clinics, staff might blanket the horse and apply standing wraps the night prior to scanning. Because of the steps required to prepare the horse for scintigraphy, clinics often request you drop the horse the day before scanning.
Veterinarians perform scintigraphy with the horse sedated, and this, in addition to radiotracer administration, means they must place a jugular catheter. Once they’ve injected the radiotracer, the horse remains in the stall for up two hours before scanning commences. To obtain images of the standing horse, the veterinarian places a large unit in various positions to capture the rays emitted from the radiotracer within the horse’s body. The scanning process can take several hours; therefore, the horse might receive heavy doses of sedation.
The radioactive tracer does pose a low health risk to people handling the horse following injection. For this reason clinic staff aim to minimize interactions until radiation levels have decreased. Of course, they’ll still monitor your horse closely and provide emergency first aid if warranted. These precautions are in place to protect handlers who perform these scans on a regular basis. The one-time dose of radiation the horse receives does not pose a risk to his health. It gets excreted in his urine and accumulates in the stall, so clinics often house horses undergoing bone scans in stalls separate from the remainder of the hospital. Each clinic is subject to national and regional regulations regarding handling of radioactive materials, and the exact rules relating to when the horse is considered safe to leave the premises can vary geographically. In most cases radioactivity levels are assessed the morning following the scan, and the horse can leave if these fall below a predetermined threshold.
It’s important to note the images obtained from scintigraphy are not highly detailed, and veterinarians often recommend following the scan with more detailed imaging (i.e., X rays, ultrasound) of the abnormalities detected.
For all the imaging studies mentioned, veterinarians might review the images on site or remotely, which can take some time. You are unlikely to receive the results of the study on the same day. Typically, a full imaging report is available within a few days to a week.
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