Magnetization transfer in cartilage and its constituent macromolecules

The goal of this work was to investigate magnetization transfer (MT) in cartilage by measuring water proton signals M_s/M_o, as an indicator of MT, in (i) single-component systems of the tissue's constituent macromolecules and (ii) intact cartilage under control conditions and after two pathomimetic interventions. M_s/M_o was quantified with a 12-μT saturation pulse applied 6 kHz off resonance. Both glycosaminoglycans (GAG) and collagen exhibited concentration dependent effects on M_s/M_o, being approximately linear for GAG solutions (M_s/M_o = -0.0137[% GAG] + 1.02] and exponential for collagen suspensions (M_s/M_o = 0.80 x exp[-(%collagen)/6.66] + 0.20); the direct saturation of water could not account for the measured M_s/M_o. Although the effect of collagen on M_s/M_o is much stronger than for a corresponding concentration of GAG, M_s/M_o is not very sensitive to changes in collagen concentration in the physiological range. Tissue degradation with 25 mg/ml trypsin led to an increase inM_s/M_o from the baseline value of 0.2 (final/initial values = 1.15 ± 0.13, n = 11, P < 0.001). In contrast, a 10-day treatment of cartilage with 100 ng/ml of interleukin-1β (1L-1β) caused a 19% decrease in M_s/M_o (final/initial values = 0.81 ± 0.08, n = 3, P = 0.085). The changes in hydration and macromolecular content for the two treatments were comparable, suggesting that M_s/M_o is sensitive to macromolecular structure as well as concentration. In conclusion, whereas the baseline M_s/M_o value in cartilage may be primarily due to the tissue collagen concentration, changes in M_s/M_o may be due to physiological or pathophysiological changes in GAG concentration and tissue structure, and the measured M_s/M_o may differentiate between various pathomimetic degradative procedures.