Abstract
T~2~ and T~1ρ~ have potential to nondestructively detect cartilage degeneration. However, reports in the literature regarding their diagnostic interpretation are conflicting. In this study, T~2~ and T~1ρ~ were measured at 8.5 T in several systems: 1) Molecular suspensions of collagen and GAG (pure concentration effects): T~2~ and T~1ρ~ demonstrated an exponential decrease with increasing [collagen] and [GAG], with [collagen] dominating. T~2~ varied from 90 to 35 ms and T~1ρ~ from 125 to 55 ms in the range of 15–20% [collagen], indicating that hydration may be a more important contributor to these parameters than previously appreciated. 2) Macromolecules in an unoriented matrix (young bovine cartilage): In collagen matrices (trypsinized cartilage) T~2~ and T~1ρ~ values were consistent with the expected [collagen], suggesting that the matrix per se does not dominate relaxation effects. Collagen/GAG matrices (native cartilage) had 13% lower T~2~ and 17% lower T~1ρ~ than collagen matrices, consistent with their higher macromolecular concentration. Complex matrix degradation (interleukin‐1 treatment) showed lower T~2~ and unchanged T~1ρ~ relative to native tissue, consistent with competing effects of concentration and molecular‐level changes. In addition, the heterogeneous GAG profile in these samples was not reflected in T~2~ or T~1ρ~. 3) Macromolecules in an oriented matrix (mature human tissue): An oriented collagen matrix (GAG‐depleted human cartilage) showed T~2~ and T~1ρ~ variation with depth consistent with 16–21% [collagen] and/or fibril orientation (magic angle effects) seen on polarized light microscopy, suggesting that both hydration and structure comprise important factors. In other human cartilage regions, T~2~ and T~1ρ~ abnormalities were observed unrelated to GAG or collagen orientation differences, demonstrating that hydration and/or molecular‐level changes are important. Overall, these studies illustrate that T~2~ and T~1ρ~ are sensitive to biologically meaningful changes in cartilage. However, contrary to some previous reports, they are not specific to any one inherent tissue parameter. Magn Reson Med 51:503–509, 2004. © 2004 Wiley‐Liss, Inc.