Positron emission tomography can reveal active processes other diagnostic modalities might missPositron emission tomography has given equine veterinarians a real-time view of pain that helps detect molecular changes before they can cause catastrophic damage. | Courtesy Dr. Mathieu SprietOn a warm, breezy evening at a research center just outside Sacramento, California, in July 2015, a series of nuclear flashes made equine veterinary history.For the previous three months Mathieu Spriet, DVM, MS, Dipl. ACVR, DECVDI, professor of diagnostic imaging at the University of California, Davis, had been experimenting with human brain scanners based on positron emission tomography (PET). At the time doctors mostly used this revolutionary 3D molecular imaging technique to stage cancer in people. Spriet, however, was looking for radioactive glucose uptake in an entirely different target: horses limbs.With only so-so results, though, he considered putting his project on the back burner. Thats when he stumbled across a new paper by researchers trying out another radioactive PET tracer, based on fluoride ions, to diagnose bone pain in peoples feet.And I was like, Whoa, Spriet recalls. 3D bone scans!Shifting gears to the new 18F tracer, Spriets eureka moment didnt disappoint. He scanned the feet of an endlessly and mysteriously lame senior Quarter Horse mare, Fancy Piece of Candy, from the universitys research herd. In a striking moment, a burst of glowing orange radioactivity lit up the ridge at the back of the navicular bone, revealing an active disease process that had not been visible on X ray, computed tomography (CT), magnetic resonance imaging (MRI), and even scintigraphy (bone scan).Thats where everything made sense, he says. A whole new field of opportunities just opened up for equine imaging.Spriets radioactive light-bulb moment sparked a rapidly expanding equine PET program at the universitywhat he affectionately calls PETting horses. The approach has since spread across the U.S. and into Europe, giving veterinarians a vivid, real-time view of pain that helps target treatment and detect molecular changes before they can cause catastrophic damage.Metabolic Maps: Seeing What Other Scans CantIn many ways PET represents the new generation of scintigraphy, Spriet says. While these newer tools dont replace their predecessorsboth still play important roles in diagnosticsthey offer the key advantage of swapping flat, two-dimensional images for high-resolution, cross-sectional 3D views, allowing clinicians to examine scanned areas slice by slice, in fine detail.Unlike other imaging techniques, PET and its predecessor, scintigraphy, dont reveal anatomical structures such as bones, muscles, or ligaments. Instead, they display radioactive tracers that accumulate in areas of increased metabolic activity, notes Holly Stewart, VMD, PhD, Dipl. ACVS-LA, assistant professor of Large Animal Surgery at the University of Pennsylvanias New Bolton Center (Penn Vet), in Kennett Square.When bone remodels, for example, chemically reactive materials in its scaffolding get exposed and attract ions such as those in sodium fluoride (18F-NaF) tracers. And in soft tissues, cells involved in inflammation and repair need excess energy, so they pull in lots of glucosewhich the fluorodeoxyglucose (FDG) tracer flags. Positron emission tomography scanners detect these tracers radioactive signals, transforming hot spots of bone remodeling, soft tissue inflammation, healing, and even tumor growth into detailed, visual maps. The brighter the radioactive glow, the greater the metabolic activity in that area, Stewart explains.Because PET follows metabolic activity rather than physical structures, it doesnt show the lesions in physical context, our sources say. Thats why PET works best with an overlay of MRI or CT images to create co-registered images.Because PET follows metabolic activity rather than physical structures, it works best with an overlay to create co-registered images. | Courtesy Dr. Holly StewartSharper, Quieter, Earlier: PETs Advantages in ActionImportantly, metabolic activity reflects ongoing disease processes rather than healed changes, and it usually precedes structural damage, our sources say. Plus, they add, PET offers comprehensive high-resolution, highly sensitive imaging in a remarkably quiet machine. Together this means PET can help veterinarians:Detect the earliest signs of disease or pathological stressDistinguish between painful, active disease and older, nonpainful changes such as scar tissue or chronic remodelingFind disease not visible on other imagingMonitor treatment efficacy over timeInform vets about when its safe for their patients to return to workProvide a calm, welfare-friendly imaging environment for horsesAnd since 2019, horses dont even have to undergo anesthesia for PET scanning, Spriet adds. Along with an engineering team from a private East-Coast company, he and his colleagues developed the Molecular Imaging of Limbs in Equids PET (MILE-PET)a 25-centimeter-wide ring that moves up and down standing sedated horses limbs as it scans. Designed for horses, the mobile PET system has a quick-release opening and allows vets to scan feet, fetlocks, cannon bones, knees, and hocks in minutes, he says.The scanner is on wheels, so you just open the arm of the scanner and bring the scanner to the horse, Spriet explains. Now that we can scan standing horses, everythings moving faster. Now weve got scanners in California, Pennsylvania, Kentucky, Florida, Texas, New York, Connecticut, Melbourne in Australia, and one is soon to be set up in Germany as well.More recently, Spriets been going back to his original ideausing that first radioactive tracer, FDG, to detect disease in the laminae, the tissues that suspend the coffin bone inside the hoof. Hes shared the technology with laminitis expert Andrew van Eps, PhD, MACVSc, Dipl. ACVIM, also at Penn Vet, whos now using PET to make breakthrough discoveries about the mechanical and chemical workings of laminar disease.With PET a 25-centimeter-wide ring moves up and down standing sedated horses limbs as it scans. | Courtesy Dr. Mathieu SprietAs research advances and clinicians refine techniques, they are getting clearer views of metabolic activity that reveal early stage disease and injuries in muscles, ligaments, tendons, and other soft tissues, Spriet says.Stewart uses the New Bolton scannerthe first installed outside Californiaand calls it an integral part of her process when working up musculoskeletal injuries, especially when traditional imaging yields unclear findings.This really empowers us as diagnosticians to have a better appreciation for what injuries there areand how significant they are, Stewart says.PET Imaging in RacehorsesAt Santa Anita Park, in Arcadia, California, and Golden Gate Fields, now permanently closed, clinicians have run more than 1,400 MILE-PET scans on racehorses. From a research perspective, thats led to a huge dataset revealing clear patterns in how injuries form, evolve, and resolve under real-world training conditions, Spriet says.Notably, he reports hes seen some of the brightest signals in palmar (front) and plantar (back) condyles of the cannon and long pastern bones, and in the proximal sesamoid bones, all within the fetlock. Thats critical, he says, because these structures undergo extreme force at racehorse speeds and frequently represent the sites involved in catastrophic injuries on the track.Data also show how much healing rates can vary from lesion to lesion, ranging from two to six months or more without any clear trends, says Spriet. Some horses have hot spots that tend to reactivate when they return to work.In practice this means racehorses benefit from greater scientific knowledge and receive more accurate and customized treatment, he explains. Clinicians identify at-risk horses and rest them earlyavoiding potential catastrophic injuryand scan them regularly to know when its safe to put them back into training.PET for Performance HorsesA powerful drive to stop catastrophic injuries on racetracks made Thoroughbreds the initial PET priority, Spriet says. But over the past few years the technology has expanded to other equestrian activities such as jumping, dressage, and Western disciplinesparticularly for horses with chronic, vague, or multifactorial lameness.We used to worry about fetlocks in racehorses and feet in sport horses, but it turns out that PET is finding more and more subchondral (bone that sits underneath cartilage in a joint) bone lesions in the fetlocks of sport horses, he says.Combining scans and tracers for both bone and soft tissue lesions has proven especially useful for sport horses, our sources say. In part thats because many injuries involve both tissue types, such as ligament lesions that involve the structures attachment site to bone and lesions deeper in the soft tissue.Its been a great thing for me, both for gauging the seriousness of a structural change found on X ray, for example, and for following patients over time, says Stewart. She usually invites her clients to bring horses back for a re-scan every few months to check on healing and adjust the rehabilitation program if needed.Positron emission tomography scans also help veterinarians make a priority list. So, if a horse has injuries to both soft tissue and bone, you can run a PET scan to understand which of those injuries is more clinically relevant at the time, she says. And then you can direct your therapies to be more targeted in a strategic order.While sport horse vets have used PET primarily in dressage and jumping horses, theyre seeing growing interest in the Western performance world as well, says Gabrielle Solum, DVM, MS, a resident at Colorado State University, in Fort Collins. She recently found abundant bone remodeling in the fetlocks of Western horses using CT scanning and believes PET could help identify active and problematic lesions.What PET Cant (Yet) DoFor all its clarity and clinical value, PET has a few important limitations:Veterinarians can only use it on the carpus (knee) and tarsus (hock) downfor now. The current scanner ring cannot accommodate anything beyond that size. But researchers continue working on scanners for larger body parts.With only 13 PET units available worldwide at press time, the technology remains expensivetypically $900-$2,500 per scanand often requires extensive travel.Tracers include mildly radioactive materials. That means staff need to train in radiation safetyprotecting themselves and other horses through proper handling, stall management, and bedding disposal, Stewart says. With its short half-life of only two hours, horses can safely return home within half a day, Spriet adds.Tracers can be hard to access in certain countries. Government regulations, production rates, and supply chains can make it difficult or even impossible to acquire the radioactive chemicals needed for the scans in different parts of the world. Even so, as nuclear medicine advances for both humans and animals, the products should become easier to obtain, Spriet says.Next Steps: Bright Signals AheadOur sources agree: PET holds strong promise for equine medicine.Beyond the possibilities larger mobile PET rings would offerto scan bigger areas, including the head, and possibly even parts of the neckscientists could also one day detect infection or pain with tracers, Spriet says.Meanwhile, Stewart is investigating the FDGs tracer as a tool for finding joint infections, she adds.On a larger scale, the growing database of scans offers important insight into the most common lesions, how they evolve, and how they affect horses clinically. Currently, scientists are developing standardized metrics, such as the standardized uptake value (SUV) reflecting the intensity of tracer uptake. Spriets team uses these values to track lesions over time and investigate what activity levels correspond to clinical risk.These data also help define whats clinically normal, adds Stewart. Vets often see mild uptake on the upper surface of the long pastern bone (first phalanx, or P1) that isnt necessarily a cause for concern. She says it could stem from healthy bone remodeling caused by repeated stress from galloping and jumpingstress to which a horse can adapt. We can see that activity on PET, but weve learned not to overinterpret that, she says.Take-Home MessageThe story of equine PET began 10 years ago with a scan of a Quarter Horse mares foot. The science and technology have rapidly expanded, now pinpointing precise details of active lesions deep in the limbs of racehorses and performance horses alike. The result: more tailored treatments, clearer guidance for rehab strategies and return to work, and the ability to monitor recovery on a molecular level. By catching subtle changes before they become serious, our sources say, the technology has the potential to keep equine athletes safer and sounder, and make them more durable.This article is from the Winter 2025 issue ofThe Horse: Your Guide to Equine Health Care. We at The Horse work to provide you with the latest and most reliable news and information on equine health, care, management, and welfare through our magazine and TheHorse.com. Your subscription helps The Horse continue to offer this vital resource to horse owners of all breeds, disciplines, and experience levels. To access current issues included in your subscription, please sign in to theAppleorGoogleapps ORclick herefor the desktop version.