by: Heather Smith Thomas
Bone scan (nuclear scintigraphy) as a means for imaging the musculoskeletal system of horses was first reported as a method for use in diagnosing lameness in 1977. Since then, many researchers and a growing number of clinicians have been using this technology. As equipment has become more affordable, the value of nuclear scintigraphy for the lameness clinician has become more apparent.
Dr. Patty Doyle, equine veterinarian at the Marion Dupont Scott Equine Medical Center in Virginia, where nuclear scintigraphy has been in use since 1994, says this technology "has helped us localize lameness in horses that have multiple sites, multiple limbs involved, or in areas such as the back that are difficult to evaluate with other modalities. We've been able to evaluate subtle lamenesses, and very severe lamenesses where you don't really want to do a lameness exam. I've used it for many horses that resent needles (for nerve blocks), which really limits your ability to do a proper lameness exam. I've used it as a reevaluation to monitor healing of fractures, etc. and have used it in a few pre-purchase exams, though I am not a big proponent of that."
Doyle says that nuclear scintigraphy has changed her outlook on how she looks at horses' lamenesses. "It's made me scrutinize radiographs much more closely than I was ever taught to do in school. It makes me scrutinize how the horse travels, because now I have very good riders (such as dressage riders, three day event people, and jockeys) telling me what they are feeling with the horse, and I am having to look at the bone scan and radiographs afterward to try and figure out what would cause the clinical signs they are seeing. So it has really helped develop my ability to look at a horse and say, yes, I see that he's not pushing off on his right hind strongly, or he's lame in three legs and not just one, or it's a bilateral lameness (both fronts or both hinds) rather than just a short stride," she says.
"I like to use the bone scan once we localize an area of a lameness, because sometimes it pinpoints a site that no other modality can differentiate that there's an abnormality there. It's not the 'gold standard' but it's a good identifier for problems I can't see on x-rays or ultrasound. These are some of the things I quiz my students on; I ask them, if you're the veterinarian out there, why would you send me a horse for a bone scan?" says Doyle.
"Use of nuclear scintigraphy has really helped me look at horses. I don't just assume that because this horse is a dressage horse that he'll have the typical coffin joint, hock and suspensory problems. I take care to also look at the oddball joints like the elbow and the hipthe things that you just don't really do when you're out there thinking you have to treat every horse's hocks. So it keeps you a little bit more open minded," she says.
"Scintigraphy is a nice way to evaluate those lamenesses that we can't figure out with other modalities, and I'm not opposed to evaluating and going through easier, less expensive technologies, but I like to do the bone scan fairly early in the exam. I get a lot of horses that have been lame for two, three or four years and it's really difficult to diagnose them, because by then the whole pattern of bone metabolism will have either resolved or changed, and it makes it difficult to say exactly what's going on. So an early examination is betterlike three to six weeks into a lameness rather than 8 to 24 months," explains Doyle.
"It's always easier if we're not having to do a whole body scan. It's easier to do if we know it's the left front lower limb and the right hind upper limb, for instance; then we don't have to do as many views. It's also cheaper for the client. I like to have fairly focal areas to evaluate rather than the whole body, because the whole body scan is very tedious and time consuming. It's difficult to get really good views over that two to three hour exam, rather than just a view of the hind limbs, for instance, or the front limbs, or something more specific like shoulders and below, stifles down (not having to worry about the pelvis, back or neck)."
She says it helps to have the problem area localized as much as possible when you do a bone scan, though often you might have to do a whole body scan "because you just don't know what's going on with the horse. I'm always a believer in the idea that you won't find it unless you look. So it is reasonable at times to do that."
The problem with a bone scan is if your local veterinarian does not have the equipment for it, then the horse must be referred into a university setting. "It's often the last thing a horse owner wants to do, so many people wait a long time before doing it. Then you don't get the nice response you expected from it, because you waited so long," says Doyle.
"Another thing people expect is that the bone scan should be the end-all answer, and it's not. It's kind of a vague, blurry outline of a horse's skeletal system, and it takes a lot of experience to know which pixel is significant, especially in these horses like dressage horses or older hunter-jumpershorses that (unlike racehorses) don't do a lot to initiate a tremendous stress reaction. So they expect us to find a definitive cause and be able to fix it, on these horses that may have 10, 12 or 15 sites of arthritis. That's really hard, because a little bit of bone metabolism in one horse may not be creating the actual lameness, whereas in another horse, two pixels will mean a grade 2 lameness."
"So even though we are doing the bone scan in horses like dressage horses, older hunter-jumpers, and backyard trail horses, we're not always as successful in localizing the lameness as we are in a two year old race horse that comes off the track dead lame and we have a pretty good idea that it's either his knee or his cannon bone in a specific leg. In these cases we're going to get a flaming hot spot and there's almost no doubt in our mind that this is what's hurting him," explains Doyle.
"It can be a frustrating modality for some horse owners to chase after, because their horse's problem may not create enough evidence (even though this is a very sensitive exam). The bone metabolism caused by their horse's lameness may not be enough for me or any other nuclear scintigraphy person to pick it out and say, these two pixels (out of 100, 000) in this picture are the reason this horse is lame. That's what makes this very difficult, especially for the chronic lamenesses. So it's really ideal for looking at acute traumas, such as racehorses, three day eventersthose horses that are really stimulating their bone to metabolize and change over rather rapidly. When we do those bone scans, the isotope absorbs onto the bone very strongly. When we have the horse up against the camera you get a definitive flaming 'hot spot'. Those are the ones that are easy to see," she says.
"A lot of times, when we are doing sport horses from disciplines other than speed-type events, we still end up doing a lameness exam the next day after the bone scan. The good thing about doing the bone scan (and perhaps we had to do a whole body scan and we got 15 'hot spots' and are asking which one is causing the problem) , is that this is still much better than saying, 'Hmmm, he just doesn't look right anywhere!' Maybe we are able to focus on ten of the most significant-looking 'hot spots' and narrow it down that way. In other cases we will get just one 'hot spot' and we know that we can block that. So if it's a horse that doesn't like needles, we only have to do that one spot. If the nerve block resolves it, then we know that place is where we need to take the x-ray," explains Doyle.
"We like to do racehorses, and we don't like doing dressage horsesbut we will do them. The number of dressage horses that I can come up with an answer for are fewer, however, than with the racehorses. That's why I am sometimes not all that keen on doing bone scans on some of these horses, because I feel like I'm just spending the client's money," she says.
"In a way, however, we are helping them, in that we are ruling out a lot of things. The client has to be willing to spend the $500 to $1,200 for the bone scan (the upper figure would be a typical whole body scanthe total expense for that horse in and out of here) and has to be prepared to hear the veterinarian say 'you don't have this problem, nor this problem, but I still don't know what it is. For a lot of people, this is hard to understand, and they are not ready to hear that. They think, because this is an expensive modality (and we keep saying it is sensitive for bone metabolism) that it should show where the lameness is. Yet here they are, after spending $1,200, and I am shrugging my shoulders. That's very frustrating for a lot of clients," says Doyle.
"When they come to any university, they have to kind of brace themselves for that possibility. We may say, 'yes, we've ruled out a fracture, but no, we still don't know why your horse is lame on that leg, or why he can't compete at training level dressage, or whatever.' Horsemen need to know that we use bone scan in combination with all the other modalities. Sometimes when I do a bone scan, I can look at the outline of the hot spot and I can say that it's consistent with a fracture, or this is consistent with navicular disease, or this is consistent with arthritis. But other times all I can say is that your horse has a hot spot in the ankle joint and now we need to take an x-ray to see if we can figure out what's going on. If we don't see anything on x-ray, then we recommend arthroscopy. So it's still a combination of diagnostic tools, in order to give the client an answer. Of course, that runs up the bill even more, but it does give us one more tool for diagnosisand a very good tool, for the most part."
Using scintigraphy and the other tools at a clinician's disposal can give a more complete approach to a lameness problem, often helping determine not only where the injury is located but also how serious it might be. This can be very helpful not only for the veterinarian, but also for the horse owner, to make the necessary decisions for treatment.
Nuclear scintigraphy as a means to look at the horse's bones involves intravenous injection of a radioactive compoundan isotope attached to a bone tracer, 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 the blood flow to body tissues. This phase is rarely used in equine diagnostics except for establishing whether or hot a horse has circulation problems. Occasionally a veterinarian will use this phase, to see if the horse has impaired circulation in a leg, for instance.
The next phase is the soft tissue or pool phase; the compound "pools" in the extracellular spaces and soft tissues, and can show areas of inflammation. This examination must be done during the first 30 minutes after the injection because it doesn't last long.
The final phase, and probably the one most used by equine veterinarians, is the bone phase, where the imaging is performed two to three hours after the injection. This is the amount of time it usually takes for the radioactive agent to be cleared from the soft tissues surrounding the bone; the bone-seeking agent then binds to the exposed crystals of remodelling bone. If there is any increased remodelling (as occurs with a bone injury) , a greater amount of the agent binds there, forming a "hot spot".