T2 mapping: an efficient MR quantitative technique to evaluate spontaneous cartilage repair in rat patella11This work was supported by grants from Projet Hospitalier de Recherche Clinique (1998), the Contrat de Projet de Recherche Clinique (2000), ‘Fondation pour la Recherche Médicale’, ‘Région Lorraine’ and ‘Groupement de Recherches CNRS 2237’

Objective : to evaluate the ability of T2 mapping on an 8.5T imager to characterize morphologically and quantitatively spontaneous repair of rat patellar cartilage following full thickness defect. Methods : Patellar cartilage defects were created in 24 rats knees on D0. Eight rats per time-point were killed on D20, D40 and D60 after surgery. T2 maps of repair tissue in patellar defects were obtained from eight different axial spin echo images on an 8.5T imager. Global, superficial and deep T2 values were evaluated in spontaneous repair tissues (3×8 right patellae) vs the opposite patellae (3×8 left patellae) of the same animals. MR data were compared with macroscopic and histological studies. Results : T2 map was able to identify morphologically three types of repair tissue observed macroscopically and histologically: ‘ total ', ‘ partial ' and ‘ hypertrophic ' repair tissue. ‘ Total ' and ‘ partial ' repair tissues were characterized by global T2 values almost similar to controls, whereas ‘hypertrophic' repair tissues were characterized by T2 global values higher than controls. Zonal variation between superficial and deep T2 values observed in controls was not depicted in repair tissue before D60. Conclusion : T2 map is able to characterize quantitatively and qualitatively rat patellar cartilage repair, and thus can be promoted, as a non invasive technique, in clinical longitudinal studies of articular cartilage repair.