Discussion

Tranquille, C.A., Pinilla, M.J., Blunden, A.S., Parkin, T.D.H., Murray, R.C.

Introduction: Magnetic resonance imaging (MRI) is increasingly being used in the examination of the equine limb for lameness diagnosis. However, what changes in signal intensity (SI) on MR images represents histologically is not fully understood. Aims: To determine what is represented histologically by SI changes on MR images in the distal third metacarpal bone (McIII). Methods: Thirty- eight McIIIs underwent MRI and histological examination. T1, T2* and fat suppressed (FS) (STIR and fat saturated) MRI sequences were acquired for each bone. Selected standard sites from each bone underwent histological preparation and examination, which was blinded to MRI findings. A histological grading system was used to describe features of subchondral (SCB) and trabecular bone, adapted from OARSI, 2009; larger scores signifying more severe change. Histological grades and features were compared with MRI findings. Results: Linear high SI areas on MRI proximal to the parasagittal grooves represented vascular channels histologically. Low signal intensity on T1, T2* and FS images represented dense lamellar bone with small vascular spaces. High signal intensity on FS with low signal intensity on T1 represented areas of trabecular microdamage, bone collapse, haemorrhage and the presence of haematoidin, suggesting lack of oxygenation, was present in most cases. Focal high SI on T1, T2* and FS images was seen with cartilage hypertrophy and invagination or SCB plate collapse with intralesional necrotic bone or haemorrhage. These could often be differentiated by the degree of surrounding bone reaction. Conclusions and practical significance: These results give a guide to the MRI interpreter of structural changes associated with patterns of MR SI. Particular features to note are the appearance of vascular channels, which should not be confused with fracture lines, and that focal high SI on all sequences could represent either cartilage repair mechanisms or severe damage. Acknowledgements: HBLB for funding and Mr Ray Wright for histological preparation.