A new nuclear medicine imaging scanner might be the key to flagging Thoroughbreds at risk of fetlock failure before they reach the point of catastrophic injury.

By undergoing regular screening with a recently designed positron emission tomography (PET) scanner aimed at viewing the lower limbs while the horse is standing, horses could benefit from early detection of bone changes that could cause the bones to fracture under high pressure during training or while racing, said Mathieu Spriet, DVM, MS, Dipl. ACVR, DECVDI, professor of diagnostic imaging at the University of California, Davis, School of Veterinary Medicine.

“There’s no question that there are way too many horses breaking down, and there are way too many deaths,” Spriet said. “It has been very challenging to identify high-risk horses. Now we have the advantage of being able to monitor fetlocks over time and decide when to race and when not to race.”

Catastrophic leg fractures in racehorses that require euthanasia usually happen in the fetlock, and they usually occur along microscopic lines of weakness in the bone (stress fractures). Finding these weak spots in their early stages could prompt management changes—such as additional rest between races and therapy—in at-risk horses, improving their musculoskeletal health and potentially saving their lives, he said.

“These horses that break down don’t break down on a normal, competent leg,” said Spriet. “The challenge has been to find ways to detect these lesions earlier.”

Finding these microscopic lines before they become serious lesions isn’t easy, said Spriet. They’re impossible to see on X ray. Magnetic resonance imaging (MRI) and computed tomography (CT) scans mainly provide “structural” imaging—meaning they already show physical signs of damage. Scintigraphy, or bone scans, can show “functional” changes in bone using radioactive products that get taken up by the forming lesion. But the spatial resolution is quite low, he said. Also, bone scans are only 2D, which is “a limitation in assessing 3D anatomical structures,” said Spriet.

PET scans successfully find metastatic cancer in humans, through high-resolution 3D images of radioactive-highlighted lesions. Spriet and his fellow researchers believed this technology could find fetlock bone lesions, so they teamed up with engineers at LONGMILE Veterinary Imaging, in Rockville, Maryland, to develop a PET scanner for horses. They developed the system to be usable for standing horses, to avoid having to repeatedly place them under general anesthesia. Instead of the radiopharmaceutical product that gets taken up by cancer tumors, they used ¹⁸F-NaF (sodium fluoride), a bone tracer.

“It’s working at the molecular level, because we inject that small molecule which is going to accumulate at sites of bone turnover,” said Spriet, who decided to become an equine veterinarian after witnessing a racehorse experience a catastrophic injury. “We can see things before they are major structural changes. We have a really exciting tool for high-resolution 3D bone scans.”

Spriet and his colleagues used the new machine at the Southern California Equine Foundation Hospital at Santa Anita Park, in Arcadia, to scan 42 front fetlocks and 12 hind fetlocks in 25 Thoroughbred racehorses at the start, middle, and end of a 12-week layup (resting from racing) period. The horses, averaging 3 years old and having four prior racing starts, had all been referred for bone scans due to lameness or poor performance and were suspected to have fetlock injuries.

The PET scans lasted three to five minutes and allowed for the investigation of 10 specific areas of the fetlock: the lateral and medial dorsal third metacarpal/tarsal bones, the lateral and medial palmar/plantar metacarpal/tarsal condyles, the lateral and medial proximal sesamoid bones, the lateral and medial proximal phalanx subchondral bones, and the lateral and medial dorsoproximal articular proximal phalanxes. The researchers scored the lesions from 0 (normal) to 3 (severe) based on the strength of the contrast provided by the radiopharmaceutical product.

They found 149 areas of radiopharmaceutical uptake (IRU) in the first scan, including 36 that were Grade 3. And their findings were consistent over time: 88% of the IRUs discovered in the first scan were detected again in the second scan, Spriet said. IRUs were most common in the palmar and/or plantar condyles, but lesions also appeared fairly frequently in the proximal sesamoid bones.

By the end of the layup period, 65% of the fetlocks had improved, showing lower grades of IRU, he said. The higher grades of IRU took the longest to improve. In general, about half the horses still had bone healing activity occurring after six weeks of rest and 35% after 12 weeks of rest, suggesting that in some cases, layups for racehorses should be extended longer than 12 weeks.

The team found 25% of the IRUs increased in grade during the rest period. They also discovered 28 new IRU sites that appeared spontaneously after the first scan. It is likely these lesions were actively healing rather than new or worsening lesions, Spriet said. The radioactive marker detects bone remodeling, which begins as soon as injuries start at a microscopic level. Bone remodeling might pick up speed during rest periods before leveling off at the end of healing. “It’s technically an attempt at repairing,” he explained.

Since completing the study, Spriet and his colleagues have scanned more than 1,000 fetlocks at Santa Anita and Golden Gate Fields. “We’ve been busy!” he said.

The study, Longitudinal monitoring of fetlock lesions in Thoroughbred racehorses using standing 18F-sodium fluoride positron emission tomography first appeared in the American Journal of Veterinary Research in 2022.