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Equine PET Scanning Marks First Anniversary

 | Published on 9/22/2017
In August 2016, the UC Davis VMTH became the first veterinary hospital in the world equipped with a positron emission tomography (PET) scanner to image horses. It has been a busy and productive first year for this unique program. A total of 53 scans were performed, including 26 scans on research horses and 27 scans on equine patients. Over the past year, UC Davis radiologists have continually collaborated with the creators of the PET scanner, BrainBiosciences, Inc., as this work is influencing future veterinary applications of PET.

Data was presented at multiple national and international conferences, including a keynote session at the American College of Veterinary Radiology (ACVR) Annual Scientific Meeting, scientific abstracts at the European Veterinary Diagnostic Imaging Conference and the American Association of Equine Practitioners Convention, and an invited talk at the Lake Tahoe Equine Conference. Manuscripts presenting the pilot data have been published in Veterinary Radiology and Ultrasound and Equine Veterinary Journal. A report was also presented in the local news - Sacramento's ABC afilliate.

The initial research focus was to validate a clinical protocol using a bone tracer (18F-Sodium Fluoride) to detect degenerative, traumatic or stress related lesions. Six Thoroughbred racehorses were included. This project confirmed the safety of the procedure and demonstrated added diagnostic value of PET compared with other imaging modalities. For example, focal proximal sesamoid bone lesions were detected on PET images but not with other modalities (Figure 1). Based on these results, a clinical trial—funded by the Grayson Jockey Club Research Foundation and the UC Davis Center for Equine Health (CEH)—was started in October 2016. Twenty horses that had previously been imaged with magnetic resonance imaging (MRI) or scintigraphy were included in this trial.

Figure 2

A scintigraphic scan (left) on a 14-year-old jumper mare with a right hind limb lameness revealed abnormal uptake in the distal tibia but the radiographs did not find any abnormality.
One of the indications of the 18F-Sodium Fluoride PET scan is to confirm and further characterize abnormalities identified on scintigraphy. For example, the first case in the clinical trial was a jumper mare with hind limb lameness and unusual abnormal uptake in the distal tibia on scintigraphy. The PET scan identified a very focal area of bone loss in the subchondral bone (the bone adjacent to the joint) of the tibia (Figure 2).

Another common indication is for foot lameness when MRI is inconclusive. In several cases, 18F-Sodium Fluoride PET identified a lesion at the attachment of the chondrosesamoidean ligament, a small ligament between the navicular bone and the pedal bone (Figure 3). This abnormality had not been previously identified on MRI. An abstract on this newly identified cause of foot pain will be presented at the upcoming ACVR conference in October 2017.

The suspensory ligaments and the hocks also benefit from 18F-Sodium Fluoride PET scanning in particular in order to determine whether abnormalities identified with other imaging modalities are active (responsible for the lameness) or not. This was illustrated as a case report with an American Quarter Horse mare named Bella.

The clinical trial has now been completed and based on the positive results, 18F-Sodium Fluoride PET scanning is now offered as a regular imaging service at the UC Davis VMTH. Referring veterinarians can contact the UC Davis Large Animal Clinic to schedule a PET scan for one of their patients.

Figure 2 (cont.)

An 18F-Sodium Fluoride PET/CT scan revealed that the uptake was localized to a very focal area of the bone adjacent to the joint. The CT confirmed subtle bone loss in this region.
Several research projects are currently under way with the support of the CEH:

•    Optimization of clinical protocols for bone and soft tissue lesions:
The initial clinical trial was focused on imaging of bone lesions using 18F-Sodium Fluoride. Another commonly used PET tracer (18F-fluorodeoxyglucose [18F-FDG]) allows to also identify injuries to the soft tissue. The association of the two tracers will be of particular interest for foot or suspensory ligament imaging, where bone and soft tissue injuries often coexist. This project assesses different ways of combining the two tracers.

•    Evaluation of new tracers:
In addition to the 18F-Sodium Fluoride and 18F-FDG, multiple other tracers may be useful. A research team from the Stanford School of Medicine has developed a tracer specific for a sigma-1 receptor which appears to play an important role in pain. A collaborative pilot study has been established between UC Davis and Stanford, and two research horses have been imaged with the new tracer showing promising results.

•    Laminitis research:
There remains many unknowns on the development and progression of laminitis. 18F-FDG is an excellent marker of metabolic activity and inflammation and can provide valuable information on the activity of the disease. This project consists of comparing the metabolic activity in the foot of normal horses and horses suffering from laminitis.

•    Stem Cell Research:
Assessing the fate of stem cells after their administration is an important aspect of stem cell research. Stem cells can be labeled with 18F-FDG for tracking purpose. An arterial injection technique was developed a few years ago by the UC Davis Regenerative Medicine research group. Scintigraphy had been used to confirm that stem cells distributed to the foot using this injection technique. PET allows more specific localization of the stem cells. Labeled stem cells were injected in a research horse with an active lesion of the deep digital flexor tendon, and PET imaging demonstrated accumulation of stem cells within the lesion.

Figure 3

13-year-old Warmblood with a chronic left front lameness. An 18F-Sodium Fluoride PET/CT scan showed marked uptake on the pedal bone adjacent to the navicular bone. This is the site of attachment of a small ligament (chondrosesamoidean). The CT confirmed bone loss at the site of attachment of this ligament.
All this certainly contributed to a busy, yet exciting, first year for the equine PET program. These accomplishments were only possible thanks to the financial support of the CEH and Grayson Jokey Club Research Foundation. We are particularly grateful for the efforts of the VMTH’s Anesthesia/Critical Patient Care Service, the Diagnostic Imaging Service’s technical team, and the research herd management team in contributing to the success of these studies.

Year Two is off to a great start with exciting research programs underway and several clinical cases already scheduled.