Computed tomography (CT) and magnetic resonance imaging (MRI) have been increasingly applied to diagnosing lameness localised to the fetlock region in standing horses. A full understanding of the benefits and limitations of CT and MRI for detection of bone and soft tissue lesions is essential for the most efficient application in equine practices.

The aim of the work presented in this thesis was to compare cone-beam CT (CBCT), 64-slice fan-beam CT (FBCT), and low-field (0.27 Tesla) MRI for detection of equine fetlock lesions. The first part of the work compared imaging diagnosis and measurement of third metacarpal/metatarsal parasagittal groove and proximal phalanx (P1) sagittal groove fissure, P1 dorsoproximal osteochondral defect, palmar/plantar osteochondral disease (POD), and heterotopic mineralisation in 35 equine cadaver limbs between CBCT, FBCT, and MRI using pathological findings as a gold standard (Chapter 3, 5, 6, and 7). The second part of the work retrospectively reviewed and compared CBCT and low-field (0.27 Tesla) MR imaging findings from 24 fetlock regions of clinical patients.

The diagnosis of fetlock lesions including fissures, dorsoproximal defects, POD lesions, and heterotopic mineralisation was comparable between CBCT and FBCT, with MRI having lower sensitivity but similar specificity compared to CBCT and FBCT (Chapter 3, 5, 6, and 7). The comparison of diagnoses indicated that for detection of bone lesions and heterotopic mineralisation, CBCT and FBCT are equivalent as the diagnostic tool of choice, and although the likelihood of failing to detect the presence of lesions is higher with MRI, lesions detected on MRI are likely to be genuine. MRI also detected increased fluid accumulation related to bone lesions and soft tissue injuries associated with heterotopic mineralisation, providing unique diagnostic information not available from CBCT and FBCT.

The correlation of imaging measurements was strong between CBCT, FBCT, and MRI. The correlation with macroscopic measurements was strong in CBCT and moderate in FBCT and MRI for dorsoproximal defects, and strong in MRI and CBCT and moderate in FBCT for POD lesions (Chapter 5 and 6). The superior correlation with macroscopic measurements in CBCT compared to FBCT could indicate the benefit of superior spatial resolution of CBCT over FBCT for the particular devices used.

Retrospective review of images from clinical patients (Chapter 8) found CBCT detected altered bone attenuation and structural changes in bone lesions including POD lesion, fissure, subchondral and trabecular bone injury, P1 dorsoproximal defect, osteoarthritis, and proximal sesamoid bone injury. In contrast to morphological details provided by CBCT, MRI detected the extent of increased fluid accumulation of bone injuries, providing information relevant to pathological status of bone injuries. Detection of soft tissue injury was superior using MRI compared to CBCT, in which the differentiation of soft tissue injury from normal structure was limited by lower contrast resolution.

In conclusion, the work presented in this thesis shows that CBCT and FBCT are comparable for detection of bone lesions and heterotopic mineralisation, and MRI detects associated increased fluid accumulation and soft tissue injuries. For clinical suspicion of bone injury and heterotopic mineralisation in the fetlock region, combining CBCT or FBCT with MRI provides the most thorough evaluation of lesions. For clinical suspicion of soft tissue injury, MRI alone is sufficient for a thorough assessment of soft tissue lesions.