Comparison of Ultrasound and Magnetic Resonance Imaging for Identifying Soft Tissue Abnormalities in the Palmar Aspect of the Equine Digit

SIMPLE SUMMARY: Lameness originating from the distal thoracic limb is common in horses and is often due to damage to the soft tissue structures in the heel region. The hoof capsule limits access to this region with ultrasound, and the scan approach can be technically challenging. However, because ultrasound is more readily available and less expensive than MRI, it is often used to assess injuries in this area. We compared ultrasound and high-field MRI findings of the palmar digit of 45 horses. MRI was considered the gold standard for the lesions. We found that ultrasound was a useful screening tool for assessing damage to the deep digital flexor tendon but risked overdiagnosing injuries. It also risks underdiagnosing injuries to other soft tissues in the palmar digit region. ABSTRACT: Damage to the soft tissue structures of the digit is a common source of equine lameness. While magnetic resonance imaging (MRI) allows for the most complete diagnostic imaging of the equine digit, ultrasound is more readily available and less expensive. This prospective diagnostic accuracy study compares ultrasound to MRI for the diagnosis of injuries visible with ultrasound within the digit, including the deep digital flexor tendon (DDFT), collateral sesamoidean ligament (CSL), and navicular bursa. Clinical patients underwent an MRI of the digit and a blinded ultrasound of the digit between the heel bulbs, and results of the two modalities were compared. A total of 70 ultrasound and MRI exams of 45 horses were included. Ultrasound had good sensitivity (85%), moderate specificity (60%), and accuracy of 70% for evaluating the dorsal tearing of the DDFT. Accuracy was lower for navicular bursa effusion (67%), navicular bursa proliferation (61%), and CSL enlargement (61%). Tearing of the DDFT distal to the navicular bone was identified with MRI in 27 limbs, 20 of which also had dorsal damage proximal to the navicular bone identified with ultrasound. Ultrasound evaluation remains a useful screening tool, particularly for the assessment of DDFT tearing proximal to the navicular bone but risks under-diagnosing pathology to the navicular bursa and CSL. Clinically significant concurrent damage to the distal DDFT and other osseous and soft tissues in the hoof capsule is unlikely to be identified without MRI.