Objective-To evaluate tendon injuries in horses over a 16-week period by use of ultrasonographyand low-field magnetic resonance imaging (MRI).Sample-Tendons of 8 young adult horses.Procedures-The percentage of experimentally induced tendon injury was evaluated incross section at the maximal area of injury by use of ultrasonography and MRI at 3, 4, 6, 8,and 16 weeks after collagenase injection. The MRI signal intensities and histologic characteristicsof each tendon were determined at the same time points.Results-At 4 weeks after collagenase injection, the area of maximal injury assessed oncross section was similar between ultrasonography and MRI. In lesions of > 4 weeks'duration, ultrasonography underestimated the area of maximal cross-sectional injury by approximately18%, compared with results for MRI. Signal intensity of lesions on T1-weightedimages was the most hyperintense of all the sequences, lesions on short tau inversionrecovery images were slightly less hyperintense, and T2-weighted images were the mosthypointense. Signal intensity of tendon lesions was significantly higher than the signal intensityfor the unaltered deep digital flexor tendon. Histologically, there was a decrease inproteoglycan content, an increase in collagen content, and minimal change in fiber alignmentduring the 16 weeks of the study.Conclusions and Clinical Relevance-Ultrasonography may underestimate the extent oftendon damage in tendons with long-term injury. Low-field MRI provided a more sensitivetechnique for evaluation of tendon injury and should be considered in horses with tendinitisof > 4 weeks' duration.
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