Veterinarians can use positron emission tomography (PET), computed tomography (CT), or magnetic resonance imaging (MRI) to help diagnose problems in equine patients, but how do they choose which to use? Timothy Manzi, VMD, Dipl. ACVR-EDI, clinical assistant professor of large animal diagnostic imaging at the University of Pennsylvania’s New Bolton Center, in Kennett Square, and Myra Barrett, MS, DVM, Dipl. ACVR-EDI, associate professor of equine diagnostic imaging at Colorado State University’s College of Veterinary Medicine, in Fort Collins addressed this nuanced question at the 2024 American Association of Equine Practitioners Convention, held Dec. 7-11, in Orlando, Florida.

Positron Emission Tomography in Horses

More veterinarians are using positron emission tomography for its unique capabilities. Paired with motion-correcting software, PET can tolerate minor movement, making the modality ideal for use under standing sedation, rather than anesthesia, they explained.

Used both for screening and follow-up evaluations, PET scans allow the veterinarian to assess all four feet in 20 minutes and structures as high as the proximal suspensory ligament and distal (lower) knee and hock in most horses. The veterinarians agreed the modality excels at detecting subtle issues such as impending sesamoid fractures, which veterinarians might miss on standard radiographs. Positron emission tomography is popular in both racehorse and sport horse practice because it allows practitioners to identify subchondral bone injuries early.

Computed Tomography in Horses

Significant advancements in CT technology have enhanced its use in equine medicine. Specifically, fan-beam CT produces high-resolution images and can be performed under standing sedation. The alternative, cone-beam CT systems, acquire volumetric data (used to display a 2D projection of a 3D sample) but the veterinarians agree these systems are more prone to artifacts (unintended distortions) and less effective for soft tissue evaluation compared to the fan-beam CT.

Small-animal practitioners use cone-beam CT for dental imaging. Manzi said it has more limited use in equine medicine but still offers some good utility.

Veterinarians can also use CT to diagnose vertebral pathologies (disease or damage). Used in combination with myelogram contrast, CT can reveal spinal cord compression undetectable by radiographs. For imaging of the equine pelvis, the modality offers better detail than some other imaging modalities, uncovering conditions such as sacroiliac lysis that were previously only diagnosed post-mortem.

Magnetic Resonance Imaging in Horses

Magnetic resonance imaging remains a cornerstone for diagnosing soft tissue injuries and bone-marrow lesions, including bone bruises with fluid and contusions. While Manzi and Barrett described low-field standing MRI as more accessible, they agreed it is prone to motion artifacts and offers less detail than high-field MRI conducted under general anesthesia.

Diagnostic Imaging in Equine Practice

Veterinarians often use a mixture of imaging techniques to assemble the puzzle pieces they need to reach a diagnosis. Combining MRI or CT with PET can create a comprehensive picture, compensating for the blind spots of each modality. For instance, contrast-enhanced CT outperforms standing MRI in cartilage evaluation, while ultrasound remains superior for some tendon and ligament injuries, the clinicians explained.

Cross-sectional imaging (MRI, CT, and PET, which show slices of the structure imaged) can be particularly valuable when radiographs fail to reveal the source of lameness—in the case of bone marrow lesions, for example, said Manzi. To avoid client disappointment and frustration with inconclusive scans, both veterinarians recommend preprocedural education about the capabilities and limitations of each technology.

The Role of Artificial Intelligence

Artificial intelligence advancements attempt to simplify image acquisition and provide motion control, but that’s where its usefulness currently ends. “Imaging interpretation fully remains in the hands of veterinarians,” Manzi said.

Take-Home Message

Advanced imaging modalities can provide precise answers for equine practitioners, contributing to more precise treatments and better prognoses. Evolving PET, CT, and MRI technologies improve veterinary diagnostic capabilities, offering precise insights into pathologies that were previously difficult to define, and refine the way veterinarians identify and manage lameness in horses.