Effectively measuring exercise-related variations in T1ρ and T2 relaxation times of healthy articular cartilage

Background: Determining the compositional response of articular cartilage to dynamic joint-loading using MRI may be a more sensitive assessment of cartilage status than conventional static imaging. However, distinguishing the effects of joint-loading vs. inherent measurement variability remains difficult, as the repeatability of these quantitative methods is often not assessed or reported. Purpose: To assess exercise-induced changes in femoral, tibial, and patellar articular cartilage composition and compare these against measurement repeatability. Study Type: Prospective observational study. Population: Phantom and 19 healthy participants. Field Strength/Sequence: 3T; 3D fat-saturated spoiled gradient recalled-echo; T_1ρ- and T₂-prepared pseudosteady-state 3D fast spin echo. Assessment: The intrasessional repeatability of T_1ρ and T₂ relaxation mapping, with and without knee repositioning between two successive measurements, was determined in 10 knees. T_1ρ and T₂ relaxation mapping of nine knees was performed before and at multiple timepoints after a 5-minute repeated, joint-loading stepping activity. 3D surface models were created from patellar, femoral, and tibial articular cartilage. Statistical Tests: Repeatability was assessed using root-mean-squared-CV (RMS-CV). Using Bland–Altman analysis, thresholds defined as the smallest detectable difference (SDD) were determined from the repeatability data with knee repositioning. Results: Without knee repositioning, both surface-averaged T_1ρ and T₂ were very repeatable on all cartilage surfaces, with RMS-CV <1.1%. Repositioning of the knee had the greatest effect on T_1ρ of patellar cartilage with the surface-averaged RMS-CV = 4.8%. While T_1ρ showed the greatest response to exercise at the patellofemoral cartilage region, the largest changes in T₂ were determined in the lateral femorotibial region. Following thresholding, significant (>SDD) average exercise-induced in T_1ρ and T₂ of femoral (–8.0% and –5.3%), lateral tibial (–6.9% and –5.9%), medial tibial (+5.8% and +2.9%), and patellar (–7.9% and +2.8%) cartilage were observed. Data Conclusion: Joint-loading with a stepping activity resulted in T_1ρ and T₂ changes above background measurement error. Evidence Level: 2. Technical Efficacy Stage: 1.