Composite metric R2 - R1ρ (1/T2 - 1/T1ρ ) as a potential MR imaging biomarker associated with changes in pain after ACL reconstruction: A six-month follow-up.

This study looked to investigate a new quantitative metric, R2  - R1ρ (1/T2  - 1/T1ρ ), using magnetic resonance (MR) images and voxel-based relaxometry (VBR) for detecting early cartilage degeneration and explore the association with patient-reported outcomes measures (PROMs) in patients 6 months after ACL reconstruction. Sixty-four patients from three sites were bilaterally scanned on a 3T MR with a combined T1ρ /T2 protocol to calculate R1ρ (1/T1ρ ) and R2 (1/T2 ) values at baseline and 6 months after reconstructive surgery. Non-rigid registration was applied to align images onto a template, allowing VBR to determine VBR rate differences and explore cross-sectional and longitudinal differences between injured and uninjured knees, generating Statistical Parametric Maps (SPMs). Baseline R2  - R1ρ differences were further correlated with change in PROMs from the Knee Injury and Osteoarthritis Outcome Score (KOOS) from baseline to 6 months. Cross-sectional results demonstrated low relaxation rate differences in the injured patella (baseline: 21%, p = 0.01; 6-months: 18%, p = 0.02), lateral tibia (baseline: 25%, p = 0.01; 6-months: 24%, p = 0.01), and weight-bearing regions of the tibia and femur. The uninjured patella showed significant longitudinal changes (17%, p = 0.02). R2  - R1ρ differences showed significant correlations with KOOS PROMs, particularly in the lateral tibia, patella, and trochlea. R2  - R1ρ difference VBR analyses provide new and highly sensitive parameters for assessing early cartilage degeneration in patients after ACL injury by integrating findings from both T1ρ and T2 , commonly used relaxation time parameters, into a single metric.