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Friday, January 25, 2019

Equine imaging milestone at UC Davis: World's first standing PET scan of a horse's foot shows activity of bone or soft tissue at molecular level

The University of California at Davis School of Veterinary Medicine has achieved another milestone in clinical equine imaging with the first successful use of positron emission tomography (PET) to scan the foot of a standing horse. 

"Equine PET", pioneered at UC Davis when the first horse was imaged in April 2015, has until now required patients to be under general anesthesia. The ability to utilize this technology on a standing, sedated horse rather than recumbent under anesthesia, will greatly expand the availability of this imaging technique, allow for more routine use, and open it up to patients that are not able to undergo anesthesia.

PET is a powerful imaging technique because it shows the activity of bone or soft tissue lesions at the molecular level. The πPET scanner from Brain Biosciences, Inc. (Rockville, Maryland), a device originally designed to image the human brain, was used to produce the first equine PET images. 

A clinical program launched in August 2016 with support from the UC Davis Center for Equine Health (CEH) resulted in PET images of more than 100 horses. 

A horse at UC Davis School of Veterinary Medicine stands with its foot inside an improvised scanner ring to achieve a PET scan; a more complex device in development will allow the entire distal limb to the carpus to be imaged. Until this month, general anesthesia was required to obtain a PET image.

Data used for both research and clinical applications has shown PET to be particularly useful for detecting bone lesions that are not recognized using other imaging techniques, and to distinguish between active and inactive injuries in bone or soft tissue. 

For those types of studies, anesthetizing the horses provided easy access to the limbs and prevented motion, which could disrupt the final images. It remains the ideal approach to image multiple areas quickly and safely. However, general anesthesia adds to the cost, requires additional staff and equipment, and, although rare, patients can have adverse reactions to anesthesia drugs.

This horse is under anesthesia so that a PET scan can be performed. Anesthesia is no longer required for distal limb PET imaging; the horse can be sedated and the scan can be performed with the horse standing normally.

In order to develop a PET scanner specifically designed to address the unique needs of equine imaging, the PET research group at UC Davis, led by Dr. Mathieu Spriet, worked with the engineering team from Brain Biosciences, led by David Beylin and Pavel Stepanov. 

PET images are captured using detectors arranged in a ring. For standing PET, the clinician places the foot of the sedated horse into a ring; the horse stands during the image capture. In order to image the horse limb from the foot to the knee, the ring of detectors needs to open freely to allow the horse to remove its leg from the instrument without getting hurt. 

This set of scans of a horse's foot was obtained with a horse under anesthesia. In the top row, far left is a PET scan of a foot. The top row middle image is the CT scan of that foot. In the far right top row image, CT and PET scans are combined. This demonstrates that the active lesion identified with the PET is a fragment at the distal border of the navicular bone. In the bottom row, far left, is another foot's PET scan. It identifies an area of osseous remodeling at a ligament attachment that could not be detected on the CT (middle) image. The far right shows PET and CT combined.

Beylin and Stepanov are working to build this knee-high scanner. In the meantime, Brain Biosciences—through their LONGMILE division dedicated to orthopedic imaging—assembled a foot-only prototype that would allow initial validation of standing equine PET. 

The concept was to use the same full ring of detectors present in the πPET scanner, but to place it horizontally on the ground in a low protective shell. This allows for imaging of the foot, while at the same time keeping the ring low enough for the horse to safely step out of the scanner.

A 10-year-old Thoroughbred gelding from the UC Davis Center for Equine Health with a history of left front lameness stands in the functioning prototype foot scanner designed specifically for equine foot imaging by Brain Biosciences.

Two horses from the vet school's Center for Equine Health research herd were imaged using this  prototype for the first time on January 16, 2019 under the supervision of Spriet, with assistance from UC Davis equine surgery residents, Drs. Jannah Pye and Bridget Nottle, and technical support from Rich Larson. 

Sedation of the horses was as simple as if they were undergoing a radiograph. The clinician positioned the front feet of the sedated horse in the center of the scanner ring, one at a time (see photos above).

High quality PET images, presenting 3-dimensional information, were captured within five minutes for each foot (Figure 2). 

Lateral (above) and dorsal (below) 3D 18F-NaF (bone tracer) PET images of the left front foot obtained without anesthesia. The yellow area indicates marked active remodeling within the navicular bone. (See dorsal view of this foot below.)

Images were obtained on two consecutive days using the two most common PET tracers for equine imaging, the 18F-Sodium Fluoride (bone tracer) and the 18F-Fluorodeoxyglucose (soft tissue tracer). The entire procedure went smoothly, and the data collected confirmed the ability to acquire PET images of the equine forelimb in standing patients without compromising image quality from motion artifacts. 

Another important goal of this initial imaging session was to measure the radiation exposure to staff. Radiation measurements showed that the dose received by the staff was no higher than for scintigraphic bone scans, which are commonly performed in horses, thereby confirming the safety of the procedure.

 Dorsal 3D 18F-NaF (bone tracer) PET image of the left front foot of the sedated horse obtained without anesthesia. Once again, the PET scan is showing activity in the navicular bone. (See a lateral view of this foot above.)

Spriet predicted the use of the scanner for following up on injury healing and screening for lesions in racehorses potentially at risk for catastrophic breakdown, without interrupting the horse's training program, as would otherwise be required while the horse recovered from the procedure. Another important application will be for laminitis evaluation. Preliminary data has shown that PET is valuable in the assessment of laminitis. However, laminitic patients are not good anesthesia candidates.

The ability to perform standing PET will ultimately provide access to this technique for a broader clinical population. The introduction of standing magnetic resonance imaging (MRI) over 15 years ago has had a major effect on the development of the use of the technique in equine imaging. Standing PET similarly has the potential to drastically expand the use of PET in equine lameness evaluation.

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