This is Part II of our two-part series on imaging technologies in equine medicine. Find Part I, which explains what each type of imaging does, here.
Horsemen and racing fans always ask the question, “Why can't high-tech diagnostics be used before a race to detect if a horse has a condition that might cause it to break down?”
The modalities described in Part One of this series—digital X-ray, nuclear scintigraphy, magnetic resonance imaging (MRI), thermography, ultrasonography, and computed tomography (CT) — give clinicians an unprecedented look inside the horse that wasn't possible 20 years ago. Yet science and technology still are not to the point where a normal horse can be walked through a scanner, like a traveler at an airport, to reveal an area of concern.
“People still have the perception that there is a Holy Grail of imaging, and there just is not,” said Dr. Larry Bramlage, surgeon at Rood & Riddle Equine Hospital in Lexington, Ky.
These diagnostics share two common drawbacks that would make them unfeasible for routine scanning of horses before a race: the most sensitive images are not specific and the most specific images can't be broadly applied. The clinician first has to know what part of the horse needs to be investigated. They usually require sedation to image the horse, and the most sensitive require general anesthesia, which is not practical on a repeated basis.
“Virtually any [imaging] is going to require some sort of sedation to protect the horse and the equipment, so that gives you a problem to overcome right from square one,” Bramlage said. “You would raise the specter of positive tests if you got in close to the event.”
Bramlage said a high-field MRI is the most sensitive and specific of imaging diagnostics, but it requires general anesthesia and is not feasible for a whole-body scan because you have to aim it at a specific spot.
“It takes about an hour to scan one specific, limited area, such as a pair of fetlocks, so you need to know where to look,” he said. “Standing low-field MRI also requires you to know the site of concern, is less sensitive than the high-field MRI, requires multiple sedations and two to three times as long to perform than the high-field MRI.”
Scintigraphy can be used for a whole-body scan to search for an area of concern. It is very sensitive, but not very specific. For instance, it can be difficult to discern between normal bone remodeling in response to training and a brewing problem.
“Every horse, when he is training, has to adapt his skeleton so he has an amount of activity level that's normal for the process,” Bramlage said. “In really young horses there is also activity in multiple locations because their growth plates haven't fully matured yet. So there is a fair amount of background 'fog.' Scintigraphy rarely yields a specific diagnosis without accompanying radiographs and a clinical exam.
“If you know, for instance, what leg the horse is lame on, it helps because asymmetry from side to side makes a big difference in how you assess a bone scan. To give you an example, if a horse has a soft-tissue lameness in the one side, let's say the left hind, his activity level on the right hind increases. So if you just look for increased activity level, you're going to be looking at the wrong leg. Scintigraphy is very sensitive, but it's not very specific.”
Bramlage is most excited about the developing standing CT technology, but he said its present use is limited by a learning curve and the practicality of the equipment. Similar to when digital X-ray technology appeared, clinicians are discovering details on CT scans that they didn't see on other imaging modalities, and they must discern if the changes are normal or pathology. Increased use of CT and more practical equipment will enhance clinicians' knowledge of the technology.
“This will likely be the imaging modality of the future,” Bramlage said.
Imaging a Normal Horse
The conundrum with imaging a normal horse with any of these modalities is what to do if an anomaly appears. If the trainer observes nothing of concern in the horse's performance, it's happy, eating well, and looks good, with no heat or swelling anywhere, what justifies scratching the horse? Someone would have to make that call.
“How do you take someone's horse out of the Belmont [Stakes] that has never had any clinical problems because you found something on the CT that you're suspicious about?” Bramlage said. “If I were scanning normal horses for an owner that is coming up to a stakes race and I found some questionable lesion in a perfectly normal horse, I wouldn't know what to do. Some degree of low-level pathology is how bone trains. We use the clinical exam and local anesthesia to assess the imaging findings.
“It would be the same way if you scanned a whole field of Belmont horses. You're going to find some pathology, but it may not at all be important to the horse, so it puts you in a very difficult situation. If you suspect that there is something wrong with the horse and you have an idea of where it's located, then your perspective on what the images are showing you is a little different, and that's a different situation.”
Bramlage's advice to an owner with deep pockets as well as an owner with claiming horses would be the same: “I would not recommend that they scan normal horses because you're going to get so much background 'fog' that it's just going to confuse the issue.”
He added, “There's nothing better than a good trainer, good observation, and knowing if the horse is happy training. … For the most sensitive diagnostics you have to know where to scan or image the horse. For routine injuries this would most often be the fetlock. You have to do that far enough in front of the race to be able to acquire the images but close enough to be relevant, and someone has to say, 'Something's not right, this horse can't run.' That will be a very tough assignment. If you're just looking at horses with no other information, you're going to get so much 'fog' that I don't think any current technology would be useful as a routine screening tool.”
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