By Dan Ross
In the lead up to this year's Breeders' and Melbourne Cups, PET and CT units have been at the crux of dashed dreams, with several horses scratched from these contests due to unsatisfactory PET and CT scans. But how do veterinarians decide what's satisfactory and what isn't?
It's in the last few years these technologies have emerged at the cutting edge of efforts to determine the root cause of subtle lameness in racehorses. This period has proven a fast learning curve for diagnosticians–just ask Ryan Carpenter, a SoCal-based vet who has been instrumental in solidifying PET as an invaluable diagnostic tool in the region.
Here, Carpenter discusses these technologies, what they illustrate and the sorts of factors regulatory veterinarians must juggle when reading a PET or CT scan.
TDN: In a nutshell, can you explain what do PET and CT scans do and show?
RC: A PET scan is basically a more sensitive bone scan than nuclear scintigraphy. What we see with PET scans are areas of bone inflammation, of bone turnover. And we know that those areas are often associated with pre-fracture.
Basically, what we're looking for are hot spots, locations–like a hot sesamoid bone or a hot area on the fetlock–where we know that if you continue to train that horse, that'll likely lead to a condylar or a sesamoid bone fracture.
[A CT unit uses X-rays to create detailed cross-sectional images of the body]
TDN: How do they compare to some of the more established diagnostic tools, like X-ray and nuclear scintigraphy?
RC: PET and CT are obviously much newer than nuclear scintigraphy and radiographs. They're also more detailed and more sensitive–you can see smaller, more fine-tuned changes. Basically, while nuclear scintigraphy and radiography are highly useful and beneficial, they don't have the level of precision that a CT or a PET scan would have.
TDN: What are some of the things regulatory vets need to grapple with when they're looking at a PET or CT scan, and suspect they're seeing something of potential concern?
RC: You always want to use your diagnostic imaging with your clinical exam. That's what they're doing. I don't think anybody is just randomly picking things to PET scan or CT scan–there's usually a clinical suspicion or indication to do so.
For example, if you were looking at a horse and it may be a little short-strided on the right front, maybe there's a little soreness to flex in the ankle. Maybe it just looks a little different than the left. So, you want more information.
Oftentimes in that case, a radiograph isn't going to be highly useful because it's oftentimes going to be what we consider clean. We know that, in order for radiographs to be useful, there actually has to be something broken. So, you have to see a chip fracture, or you have to see a condylar fracture. You're not going to pick up the subtle changes.
That's where things like PET scan and CT can come in because they can look beyond what a radiograph can see, provide you with a lot more information.
TDN: How do factors like age, racing history play into the reading of a PET or CT scan?
RC: I think history plays an important role. We know some things–i.e. diseases or pathology or fractures–occur more commonly in some horses than in others, different ages and training styles.
If you have a 2-year-old, a 2-year-old is often going to have different diseases and problems than what a 3- or 4-year-old will have. Again, you're just trying to put all the information together so that you can make a more accurate diagnosis.
TDN: In an ideal world, therefore, would it be helpful for a regulatory vet to have a series of these scans over a period, so they have a historical record of a horse's limbs, see what's normal bone remodeling in that horse, what's not?
RC: Sequential imaging is extremely important because obviously [with one lone scan], you're just getting a snapshot in time. If you had the ability to monitor or track changes, then I think you would be able to make more accurate diagnoses.
We're trying to be risk-adverse. If are some things that we know just shouldn't be there–for example, if there's uptake on the sesamoid bone–it's hard to, without sequential studies, be able to predict where we're at, how close we are to breaking, how close we are for it to fracture. We can't necessarily predict that.
We often use the level of intensity on a PET scan to determine what's more severe than less. But if you had a spot that hasn't changed in the last three PET scans over the last four or five months, then I think you could have some level of confidence that, if it's not in a particular location, that might just be this horse and he's handling it fine as long as his clinical exam looks good.
But also, if you examine this horse and that spot is intensifying over the course of time and changing, then you're going to be more concerned because you're going to worry that this horse is getting closer to it fracturing.
There have been some horses that I've followed along that I've been able to stop at the right time because I have sequential imaging to know that the disease is progressing or not.
We once looked at imaging like a PET scan as good for a short period of time. But as we've done these sequential studies, if you have a clean PET scan, you can feel very confident for the next four, six, eight weeks that you're not going to have a problem.
We always have thought that some of these problems occur relatively quickly. But as we've learned and understood bone pathology more, they actually occur relatively slowly over time. They just might not be clinically apparent initially. And so, that's where it's helpful to have sequential imaging to help understand where you're at in the disease process.
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