Imaging Explained explores the different modalities available to help with lameness diagnosis and what you can expect from each. Seen as the latest advanced functional tool for use in lameness in horses, let’s take a look at Positron Emission Tomography or PET.
What is Positron Emission Tomography (PET)?
PET is the newest advanced functional tool for use in lameness in horses and provides highly sensitive three-dimensional information on tissue metabolic activity. It is particularly useful for detecting areas of increased bone turnover and therefore help distinguish active from inactive lesions. Modern equine PET systems are adapted for standing imaging of the distal limb under light sedation. Increasingly, PET is being used alongside other imaging modalities such as MRI and CT.
PET uses a small amount of a radioactive tracer to identify areas of increased metabolic activity, such as increased bone turnover. Compared to scintigraphy, PET provides three-dimensional images with higher spatial resolution and improved localisation. When used for horses, PET is most commonly used for imaging the distal limb (e.g. foot, fetlock, knee/hock) in the standing, sedated patient. Different tracers can be selected depending on the clinical questions being asked. For example, sodium fluoride for bone and FDG for selected soft tissue or inflammatory processes.
When might PET be used?
- When the source of lameness is hard to localise and first‑line tests haven’t fully explained it.
- PET is particularly useful for foot and fetlock issues in sport and racehorses.
- To spot early, subtle bone changes (e.g., stress response in subchondral bone or at ligament attachment sites)
- And to judge whether findings seen on X‑ray or CT are active or old and inactive.
- For monitoring: repeating scans during rehab to check that activity is settling with treatment and time.
What it shows well
- Bone activity: highlights areas of increased bone turnover before structural changes are apparent on Xrays or CT.
- Entheses (ligament attachment sites): helps identify metabolically active change, more likely to cause pain, compared to more chronic, inactive remodelling.
- Navicular apparatus and fetlock: useful for assessing the distal limb (particularly foot and fetlock). This includes patterns of increased uptake associated with stress-related bone injury in high-risk athletes. For example, racehorse with impending sesamoid bone fracture.
- Integration with anatomic information: PET data can be ‘fused’ or overlayed onto a CT or MRI image. This is useful when combining metabolic activity with detailed anatomical information.
What it doesn’t show well
- Whole‑horse coverage in one go; present systems focus on distal limbs (front: foot to knee; hind: foot to hock).
- Fine anatomical detail on its own. PET is best used alongside X‑rays, ultrasound, CT or MRI for structure and shape.
- Availability isn’t yet universal, and use requires licensed handling of tracers—your vet will advise on local access and suitability.
What to expect from PET
- For a PET scan, the horse remains standing and sedated.
- A small dose of tracer is given via a vein and – after a short uptake period – the scanner tube is clipped closed around the limb.
- Scans of both front feet and fetlocks can often be completed in around 20 minutes.
- Specialist teams follow radiation protocols, and the sedation used is similar to many routine equine procedures.
Where it fits into the bigger picture
Think of PET as the tool that answers the question, “is this area active right now?”
- If X‑rays and/or ultrasound don’t fully explain the lameness, PET can flag which region deserves further imaging.
- This could be targeted MRI for soft tissue and bone marrow detail, or CT for intricate bone structures.
- In athletic horses, PET can help identify early bone stress in fetlocks. Important in supporting safer training decisions, PET has been part of programmes aimed at reducing catastrophic injury on the track.
