T2 has been investigated as a sensitive metric of collagen integrity in cartilage, as the interaction of the water with the collagen fibrils would be expected to alter T2. However, other macromolecules in cartilage would also be expected to impact T2, and it was also unclear how the collagen molecules alone versus the architectural organization of the fibrils impacted T2. Therefore, we investigated T2 in systems from the simplest to the most complex. We found:

In molecular suspensions of collagen and GAG, T2 and T1rho demonstrated an exponential decrease with increasing [collagen] and [GAG], with [collagen] dominating. T2 varied from 90 to 35 ms and T1rho 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 (Figure 1).

collegengraph

Figure 1

Macromolecules in an unoriented matrix (young bovine cartilage) had T2 and T1rho values consistent with the expected collagen concentration, suggesting that the matrix per se does not dominate relaxation effects. Collagen/GAG matrices (native cartilage) had 13% lower T2 and 17% lower T1rho than collagen matrices, consistent with their higher macromolecular concentration. Complex matrix degradation (interleukin-1 treatment) showed lower T2 and unchanged T1rho relative to native tissue, consistent with competing effects of concentration and molecular-level changes.

Macromolecules in an oriented matrix (mature human tissue) showed T2 and T1rho variation with depth consistent with 16-21% collagen concentration and/or fibril orientation (magic angle effects) seen on polarized light microscopy, suggesting that both hydration and structure comprise important factors.

Overall, these studies illustrate that T2 and T1rho are sensitive to biologically meaningful changes in cartilage. However, they are not specific to any one inherent tissue parameter.

macrograph

Figure 2