Modern science has given us a number of new tools for medical examinations, and several of these are now being used in veterinary medicine as an aid in diagnosing lameness problems. “Bone scan” or nuclear scintigraphy as a means for imaging the musculoskeletal system of horses was first reported as a tool for use in diagnosing lameness in 1977. Since then, many researchers and an increasing number of clinicians have been using this technology. As the equipment to do it has become more affordable, the value of nuclear scintigraphy for the lameness clinician has become more apparent.

Kent Allen, an equine veterinarian in private practice at Middleburg, Virginia (specializing in lameness, diagnostic imaging and prepurchase exams), says lameness in performance horses is often subtle in nature, and bone scan technology is very useful in arriving at a diagnosis. These are often not really lamenesses, per se. The horse may be just a little off or not performing as it should. He says most veterinarians are more comfortable dealing with actual lameness problems, where it is not as difficult to pinpoint the area of concern. The hard part, says Allen, is when someone brings in a horse that’s not performing at peak ability. Perhaps it’s “not quite right behind”. In earlier days, the veterinarian would do some diagnostic tests such as flexing the leg to try to see if the problem was in the hock, and maybe you’d never be really sure. Without a good way to pinpoint the problem, sometimes all a veterinarian could say to the horse owner was to bring the horse back when it was lamer. In the past dozen years, however, we’ve developed more advanced diagnostic technology such as ultrasound and better methods of radiology (digital radiographs), but the biggest move forward has been in physiological realms such as thermography (detecting temperature differences in certain areas of the body) and nuclear scintigraphy or bone scan, according to Allen.

The reason for using bone scan, he says, is that it’s one of the ways to look at subtle performance problems and sort out what is wrong–especially in situations where a horse might have multiple leg lamenesses or occult lameness (the horse is definitely lame but there’s no apparent cause). Perhaps the veterinarian has localized the problem and narrowed it down to the front heels. But radiographs show nothing wrong there. The use of scintigraphy at that point gives you a chance to see what the bone feels like, and can show you where the problem lies, explains Allen. The technique itself is simple. A radioactive compound–an isotope attached to a bone tracer–is injected intravenously into the horse, with the horse in front of a gamma camera. There are 3 phases of the process. In the first phase (vascular phase), the imaging immediately shows the blood vessels, highlighting blood flow to the body tissues. This phase is rarely used in equine diagnostics except for establishing whether or not a horse has circulation problems. Occasionally a veterinarian will use this phase, perhaps to see if the horse has impaired circulation in a leg; he can give the injection and then watch, says Allen. The second phase is the soft tissue or pool phase, which lasts about 10 to 15 minutes. This can show areas of inflammation in the soft tissues. This phase is used occasionally in horses, to look at an increase in circulation around a particular soft tissue area. “You can only do a few views of that, because this phase lasts only a short time,” says Allen.

The final phase, and probably the most useful for the equine clinician, is the bone phase–in which the imaging is performed 2 to 3 hours after the injection. This is the amount of time it usually takes for the radioactive compound to be cleared from the soft tissues and for the bone-seeking agent to bind to the areas of remodelling bone. If there is increased remodelling (as occurs with a bone injury) a greater amount of the agent binds there, forming a “hot spot”. As Allen explains, “the terms nuclear scintigraphy and bone scan are used interchangeably. Nuclear scintigraphy is the proper term for the technology, but bone scan is what we usually do in horses. We use that third phase about 99 percent of the time. The other term you often hear is ‘uptake of radioisotope’ but the more common term used by horsemen is ‘hot spots’. This is where there is a concentration of the radio-labelled bone tracer in a particular area of the body, bone or joint.” Bone scans are typically started within 2 hours after the injection. “Then you have several hours in which to do the scans, but once you get 8 to 12 hours out from the injection you can’t do any more. This particular isotope has a very short half-life in the body. That’s why we can release the horse the next day and it can go home quite safely. The compound is cleared out by then. The only radioactivity is sitting there on the stall floor, rapidly decaying to nothing. So you must do the scans within a finite period of time. It’s not like a radiograph where you could say, ‘let’s just x-ray the right front leg today, because if we don’t get the answer we’ll go back and get the left front tomorrow.

Once you give the injection for a bone scan, the clock starts ticking. You’ve got to do everything you want to do within this 6 to 8 hour time frame, but the best time to do it is within the 2 to 4 hour period after the injection. In this phase you get to look at where the bony inflammation is, and what bones are affected,” he says. Veterinarians often use bone scan in sport horses because these horses tend to have subtle performance problems. “We are looking for subtle complaints and problems, or lamenesses that are multi-limb. With sport and performance horses, one reason we pay a lot of attention to diagnostics is because it’s often frustrating trying to correct a subtle problem,” explains Allen. There are many cases where the horse owner is never sure what is actually wrong with the horse. “We see people who bring horses in after a long course of trying to figure out what’s wrong. They had a veterinarian look at the horse for a problem (but no definitive diagnosis was made); maybe they’ve had a chiropractor treat it, and they’ve had two corrective farriers work on the horse, or an acupuncture person treat it. The client shows up with the horse, and there’s already been a lot of money expended over a period of time on this problem, but no one has ever gotten to the diagnosis–and that’s where some of our modern imaging technology really shines, says Allen. “Occasionally I have an economic discussion with a horse owner. The point is, the diagnostic dollar provides a much better return on investment than a lot of shots in the dark that may or may not fix the horse. If people can think about it, this makes a lot of sense. But a lot of horsemen try one thing after another to resolve the problem, or end up turning the horse out for 6 months to see if it will recover. The rest might do him some good, but if this is a performance horse and he could have competed safely and soundly, that’s a big loss out of your pocketbook and out of his athletic career,” he says. “Some of the reason we look to these modern diagnositic imaging techniques is that they can show us fairly quickly an area to zero in on and pursue.

Of course, all the imaging technologies are always done in conjunction with good detailed clinical lameness exams. You can’t have one without the other. None of these things exist alone. They are all just pieces of the puzzle.” He explains that the veterinarian will use each tool (which is basically what the imaging technologies are) to help zero in on what the problem is. “If you do that, and you are accurate and determined, then you have spent the client’s money very wisely. The horse owner should be very happy with the return. Before the advent of these advanced imaging technologies (back when we just had x-rays) veterinarians could probably resolve about 60 to 70 percent of lamenesses. Now with advanced imaging technologies and a very persistent approach to these cases, we are into the lower 90 percent range by now, and that’s a big jump. By contrast, equine surgery has never made that big a jump, except for the advent of arthroscopy. So that’s a huge increase. It’s really exciting, being able to use some of these tools.” The anatomic imaging technologies include radiographs and ultrasound, looking at the tissues (x-rays view the hard tissues and ultrasound the soft tissues). “The other category is physiological–instead of what the tissues look like, now we’re talking about what the tissues feel like, such as whether they are inflamed.