PET scanning - reduces catastrophic fractures - latest advance in equine imaging - designed to image horse legs
/By Mathieu Spriet, Associate Professor, University of California, Davis
Santa Anita Park, the iconic Southern California racetrack, currently under public and political pressure due to a high number of horse fatalities during the 2019 season, announced in December 2019 the installation of a PET scanner specifically designed to image horse legs. It is hoped that this one-of-a-kind scanner will provide information about bone changes in racehorses to help prevent catastrophic breakdowns.
What is PET?
Figure 1: The first equine PET was performed in 2015 at the University of California Davis on a research horse laid down with anesthesia. The scanner used was a PET prototype designed for the human brain (piPET, Brain- Biosciences Inc., Rockville, MD).
PET stands for positron emission tomography. Although this advanced form of imaging only recently became available for horses, the principles behind PET imaging have been commonly used at racetracks for many years. PET is a nuclear medicine imaging technique, similar to scintigraphy, which is more commonly known as “bone scan”. For nuclear imaging techniques, a small dose of radioactive tracer is injected to the horse, and the location of the tracer is identified with a camera in order to create an image. The tracers used for racehorse imaging are molecules that will attach to sites on high bone turnover, which typically occurs in areas of bone subject to high stress. Both scintigraphic and PET scans detect “hot spots” that indicate—although a conventional X-ray might not show anything abnormal in a bone—there are microscopic changes that may develop into more severe injuries.
Development of PET in California
The big innovation with the PET scan is that it provides 3D information, whereas the traditional bone scan only acquires 2D images. The PET scan also has a higher spatial resolution, which means it is able to detect smaller changes and provide a better localisation of the abnormal sites. PET’s technological challenge is that to acquire the 3D data in horses, it is necessary to use a ring of detectors that fully encircles the leg.
The first ever equine PET scan was performed at the School of Veterinary Medicine at the University of California in 2015. At the time, a scanner designed to image the human brain was used (PiPET, Brain-Biosciences, Inc.). This scanner consists of a horizontal cylinder with an opening of 22cm in diameter. Although the dimensions are convenient to image the horse leg, the configuration required the horse be anesthetised in order to fit the equipment around the limb.
The initial studies performed on anesthetised horses with the original scanner demonstrated the value of the technique. A first study, published in Equine Veterinary Journal, demonstrated that PET showed damage in the equine navicular bone when all other imaging techniques, including bone scan, MRI and CT did not recognise any abnormality.
A pilot study looking at the racehorse fetlock, also published in Equine Veterinary Journal, showed that PET detects hot spots in areas known to be involved in catastrophic fractures.
Figure 2: These are images from the first horse image with PET. From left to right, PET, CT, MRI, and bone scan. The top row shows the left front foot that has a severe navicular bone injury. This is shown by the yellow area on the PET image and abnormalities are also seen with CT, MRI and bone scan. The bottom row is the right front foot from the same horse, the PET shows a small yellow area that indicates that the navicular bone is also abnormal. The other imaging techniques however did not recognize any abnormalities.
This confirmed the value of PET for racehorse imaging, but the requirement for anesthesia remained a major barrier to introducing the technology at the racetrack. To overcome this, LONGMILE Veterinary Imaging, a division of Brain-Biosciences Inc, in collaboration with the University of California Davis, designed a scanner which could image standing horses. To do this, the technology had to be adapted so that the ring of detectors could be opened and positioned around the limb.
With the support from the Grayson Jockey Club Research Foundation, the Southern California Equine Foundation and the Stronach Group, this unique scanner became a reality and, after the completion of an initial validation study in Davis, the scanner was installed at Santa Anita Park in December 2019.
Figure 3: The two images on the left are bone scan images from a 4-year-old Thoroughbred racehorse. The images on the right are 3D projection of PET images of the same fetlock. The bone scan revealed an abnormality at the bottom of the cannon bone. The PET scan confirmed this abnormality and helped better localize it. In addition, several other abnormalities were found on the PET scan in the sesamoid bones.
PET at the racetrack
The new PET scanner has been used to image the equine limb from the foot to the knee. The current main focus at the racetrack is fetlock imaging, as the majority of catastrophic breakdown in racehorses affects this area. The UC Davis pilot study highlighted the value of PET for detecting abnormalities in the proximal sesamoid bones—the two small bones at the back of the cannon bone—that are commonly involved in catastrophic fractures. Previous necropsy research on horses which suffered breakdowns has shown that changes can be present in the bones prior to the development of major injuries. The goal of the Californian PET project is to detect these warning signs in order to avoid training and racing horses at high risk for catastrophic breakdown.
Alternative imaging techniques
Other imaging techniques are available for examining equine bone. Scintigraphic bone scans are doing an excellent job at detecting stress fractures of the humerus or tibia, and this has helped markedly decrease catastrophic injuries in these areas. Bone scan is also used for fetlocks; but “hot fetlocks” are common on bone scan, and the lower resolution 2D images often do not allow to truly determine whether horses are at high risk of fractures or have normal bone adaptation to training.
Figure 4: The MILE-PET scanner (LONGMILE Veterinary imaging, Rockville, MD) is the first PET scanner specifically designed to image standing horses. An openable ring of detectors allows easy positioning and safe scanning.
MRI is used for fetlock imaging too, and MRI scanners designed for imaging standing horses have been available for over 15 years. Several large racing centers are equipped with such scanners, and MRI excels in particular at detecting changes in the cannon bone that precede condylar fractures. MRI can detect areas of bone densification, or even accumulation of fluid in the bone, typically indicative of microtrauma that can weaken the bone.
Computed tomography (CT) has also recently been used for standing imaging of the fetlock. At the moment, there are a few centers equipped with a CT scanner allowing standing fetlock imaging, but they are only available at, for example, New Bolton Center, Pennsylvania - USA, and the University of Melbourne, Australia. CT uses X-rays to create 3D images. Similar to MRI, CT can detect areas of bone densification or areas of bone loss. …
