Correlation of Positron Emission Tomography and Low-Field Standing Magnetic Resonance Imaging in Horses With Forelimb Lameness Localized to the Foot.

Low-field magnetic resonance imaging (LF-MRI) and positron emission tomography (PET) imaging can be obtained in the standing horse. There is potential for increased diagnostic capacity combining PET with MRI, given that PET assesses the metabolic activity of injured tissues, whereas MRI provides structural and functional details on both bone and soft tissue anatomy. This study aimed to describe 18-fluorine-fluorodeoxyglucose (18F-FDG)-PET and 18-sodium fluoride (18F-NaF)-PET imaging of horses with forelimb lameness localized to the foot and compare PET with LF-MRI. Fourteen horses with lameness localized to the foot underwent standing MRI and PET with a dual-tracer protocol. MRI abnormalities and PET isotope uptake were subjectively graded. Weighted kappa statistics were used to compare the agreement between the ordinal grading of MRI and PET images. Maximum, minimum, and mean standard uptake values (SUV) were calculated at increased radiopharmaceutical uptake (IRU) regions. Metabolic volume (MV) within a cuboidal area of the IRU was calculated. No clinically significant kappa results were obtained comparing MRI to 18F-FDG-PET or 18F-NaF-PET. The mean SUV of background radiation was significantly lower than the mean SUVmax for subjectively mild and moderate-severe 18F-NaF-PET (p < 0.001). When comparing the SUV ratio for subjectively mild and moderate-severe groups, there was no significant difference between the two (p = 0.546). The major limitation of the study was the small population size. Low-field MRI may underrepresent bone remodeling and potentially misrepresent the source of lameness. We report various standardized uptake values and MVs for regions within the foot with IRU for 18F-FDG and 18F-NaF.