18F-sodium fluoride positron emission tomography provides pertinent additional information to computed tomography for assessment and management of tarsal pain in horses.

Objective: To assess the value of 18F-sodium fluoride (18F-NaF) positron emission tomography (PET) for imaging the tarsus and proximal metatarsus and compare it with CT and lameness evaluation.
Animals: 25 horses with lameness localized to the tarsal and proximal metatarsal regions that underwent 18F-NaF PET/CT between 2016 and 2021.
Methods: 18F-NaF PET and CT images were retrospectively independently evaluated by 3 observers. Standardized uptake values (SUV) were used to characterize 18F-NaF uptake. Correlation between PET and CT findings with subjective and objective maximum (Max-D) and minimum pelvic height lameness data was estimated.
Results: The inter-observer Kappa-weighted value (κ) was higher for PET (κ = 0.66) than CT (κ = 0.6). CT and PET scores were fairly correlated (R = 0.49; P < 0.05). PET SUVratio (SUV of the main lesion/SUV talus) had the highest correlation with Max-D (R = 0.71; P < .05). PET and CT scores for the plantar region were significantly higher in Quarter Horses (P < .05) and showed consistently higher correlation with objective lameness data (CT plantar grade - Max-D [R = 0.6; P < .05], PET plantar grade - Max-D [R = 0.47; P = .04]) than other regions of the distal tarsal joints. Three Warmbloods presented marked uptake at the medial cochlea of the distal tibia.
Clinical Relevance: PET had a moderate correlation with CT for assessment of tarsal lesions. The degree of PET uptake can help differentiate active versus inactive lesions. Specific location of the uptake is important in determining clinical relevance.