3D Correlation Mapping between Changes in Cartilage T1ρ and Thickness Over 1-year after ACL Reconstruction using Atlas-based Methods

Abstract

Purpose: Subjects with anterior cruciate ligament (ACL) injury are at risk for post-traumatic osteoarthritis (PTOA). T1ρ MRI has been used to detect early degeneration after ACL injury and reconstruction. Previous studies also revealed significant changes in cartilage thickness after ACL injury, with thickness increasing in some compartments while decreasing in other compartments. However, the correlation between the change in T1ρ and the change in cartilage thickness has not been demonstrated. A better understanding of these correlations will help with staging the cartilage degeneration status after ACL injury. Very recently, a novel atlas-based method was developed which permits voxel-based relaxometry analysis (VBR). The goal of this study was to apply the atlas-based method to evaluate the local voxel-by-voxel correlation between changes in T1ρ and changes in thickness from baseline to 6-months and 1-year after ACL reconstruction (ACLR). Methods: Bilateral knees were scanned using a 3T MRI scanner for twenty-five patients with ACL injuries prior to surgical reconstruction (baseline, BL), and 6-months (6M) and 1-year (1Y) after ACLR (age = 28.4 ± 8.2 years, BMI = 23.8± 2.7 kg/m2, 10 female). The imaging protocol included the 3D MAPSS T1ρ/T2 sequence used for cartilage relaxation times quantification and high-resolution 3D FSE (CUBE) used for WORMS grading and semi-automatic cartilage segmentation. Those segmentations were used to define a voxel based thickness map for each subject. An atlas based registration approach was then used to morph all the T1ρ-weighted images and the thickness maps to the same reference space, and T1ρ maps were computed in the atlas space allowing for voxel-by-voxel correspondence between T1ρ and thickness. Longitudinal changes in cartilage thickness and T1ρ, as well as the local Pearson correlation coefficients between changes in T1ρ and changes in thickness were calculated voxel-by-voxel. Both analyses were adjusted for age, gender and BMI. The level of significance was set at p<0.05. Percentage of significant voxel (PSV, %) with positive or negative correlation and the mean value of correlation coefficients (R) were calculated in each compartment. Results: Significant increases of T1ρ in MFC and trochlea, and significant decreases of T1ρ in patella were observed; while significant increases of cartilage thickness in patella and significant decreases of cartilage thickness in trochlea were observed (Fig. 1 A–B). The most significant correlation between changes in T1ρ and changes in thickness was observed in patella, LFC, trochlea and MFC (Fig. 1C). In the patella, significant negative correlation was found between changes in T1ρ and changes in thickness from BL to 6M (38.4% voxels, R=−0.536) and from BL to 1Y (15.1% voxels, R=−0.590) (Tab1). This correlation was primarily the result of a decrease in T1ρ and an increase in thickness, while in other compartments, the negative correlations primarily resulted from an increase in T1ρ and a decrease in thickness. From 6M to 1Y, in LFC, a large decrease of the positive correlation PSV (from 11.2% to 2.5%) and a slight increase of negative correlation PSV were observed. There is an increasing trend of the total cartilage WORMS scores from BL (2.0±2.1) to 6M (2.2±2.1) and 1Y (2.4±2.3), but the increase was not significant. Conclusions: In this study, the local correlation between changes in cartilage matrix composition and changes in cartilage thickness was analyzed voxel-by-voxel using a novel 3D atlas-based method. Significant correlations were observed between ΔT1ρ and ΔΤhickness from BL to 6M and 1Y after ACLR. A positive correlation with increase in T1ρ and thickness may suggest the cartilage is at very early stages of degeneration with tissue swelling; while a negative correlation with increase in T1ρ and decrease in thickness may suggest later stages that cartilage begins to experience tissue loss. From 6M to 1Y, there is a general trend of decreasing positive correlation percentage while an increasing negative correlation percentage (Figure 2) in the femoral condyles and trochlea, suggests a progression of cartilage degeneration, confirmed by change in WORMS. In patella, the negative correlation was driven by the decrease of T1ρ and increase of thickness, which may be associated with changes in loading to patella-femoral joint (PFJ) as well as changes in activities after ACLR. Longer follow-up analysis is warranted to better understand cartilage degeneration progression after ACL injury. Funding from NIH/NIAMS P50 AR060752.