To determine the effects of mechanical loading on T1 relaxation times in cartilage in the presence of Gd ions, in several early stages of OA using the anterior cruciate ligament transection (ACLx) model of canines.Thirteen skeletally mature dogs underwent ACLx in one knee. Six animals were sacrificed 8 weeks and 7 dogs were sacrifices 12 weeks post surgery. Animals were handled according to the protocols approved by the Institutional Review Boards. A non-metallic compression device was built as previously described¹. Five rectangular blocks were harvested from the central load-bearing area of fresh medial tibia from both knees. Each block was immersed in 1mM gadolinium contrast agent and stored at 4˚C until experiment. Quantitative µMRI T1 imaging at the magic angle were carried out on a Bruker AVANCE II 300 NMR (7-Tesla/89-mm) and micro-imaging accessory (Bruker instrument, Billerica, MA) and a custom-made 5-mm solenoid coil. Specimen positioning and MR imaging plane orientation received particular attention to enable a comparison of the results of quantitative cartilage analysis on T1 maps across the different time points. The average T1 values and standard deviations were calculated for both bulk and sub-tissue zones. One-way ANOVA with Bonferroni was used to test the differences among locations and zones. Difference of T1 values before and after loading (total of 89 specimen) was measured for both bulk and sub-tissue zones at each time point. Paired student t-test was performed to determine the loading effect. A p value of less than 0.05 was considered as an indication of statistical significance. (Additional data of the normal controls from a related project (N=7) was used for statistical comparison².)Fig 1 showed a representative set of T1 images of the medial tibia joints of unloaded (a) and loaded (b) at five different stages: N (23%), 8C (40%), 12C (13.3%), 8X (30.2%) and 12X (22.7%). (The percentage inside each bracket is the amount of strain in the compression.) T1 profiles showed an increasing trend monotonically from the surface to deep cartilage when it was not loaded (Fig 2, solid) and a significant increase of T1 values near the surface when the tissue was loaded (Fig 2 open) in all specimens. By averaging the profiles in each tissue zones, the mean T1 values in the meniscus-covered area were significantly higher than those from the uncovered area, for most sub-tissue zones except RZ2 as well as all zones of 12X. Loading increased the mean T1 values significantly in most surface zones (SZ, TZ)³. The amount of changes in T1 as the function of mechanical strain showed a trend towards smaller values from the articular surface (SZ) to the TZ, RZ1, and further to RZ2, for both covered and uncovered areas at all OA-time-points. Fig 3 showed the statistic result at SZ between unloaded and loaded at OA-time-points (***: p<0.001). At all sub-tissue zones, the mean T1 values among the OA-time-points were significantly different. This study characterized the knee cartilage using T1 in at different stages of OA, reflecting early degradation and loading effects. We measured mean T1 not only in bulk but also in each sub-tissue zone before and after loading. With the number of sample at nineteen, we observed significant differences in most of the sub-tissue zones as well as in bulk, among the OA time-points as well as before and after loading at each time-point. We found topographical variations of T1 between the meniscus-covered and uncovered areas at each OA time-point at most sub-tissue zones except RZ2, while the difference was not significant at bulk and 12X. Further, this study showed direct evidence that the T1 values were significantly changed with compression of the cartilage. More importantly, the magnitude of these changes was highly dependent on the sub-tissue zone and level of strain, with larger increased in the SZ and TZ as compared to the RZs.This study statistically analyzed both topographical and zonal variations of T1 under different strains and at different time-points of early OA, using µMRI at 17.6µm-resolution. The results suggest that the progression of early OA in medical tibia cartilage varies topographically and the mechanical compression could alter the T1 relaxation time of articular cartilage at each sub-tissue zone and topographically. The detailed knowledge of the topographical maps of T1 and the strain-modified T1 of cartilage could provide a better understanding of the mechanical compliance of cartilage during joint loading, and could help to design effective protocols clinically by detecting the early stage of OA.