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 Ms/Mo, 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. Ms/Mo was quantified with a 12-microT saturation pulse applied 6 kHz off resonance. Both glycosaminoglycans (GAG) and collagen exhibited concentration dependent effects on Ms/Mo, being approximately linear for GAG solutions (Ms/Mo = -0.0137[% GAG] + 1.02) and exponential for collagen suspensions (Ms/Mo = 0.80 x exp[-(%collagen)/6.66] + 0.20); the direct saturation of water could not account for the measured Ms/Mo. Although the effect of collagen on Ms/Mo is much stronger than for a corresponding concentration of GAG, Ms/Mo is not very sensitive to changes in collagen concentration in the physiological range. Tissue degradation with 25 mg/ml trypsin led to an increase in Ms/Mo 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-1 beta (IL-1 beta) caused a 19% decrease in Ms/Mo (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 Ms/Mo is sensitive to macromolecular structure as well as concentration. In conclusion, whereas the baseline Ms/Mo value in cartilage may be primarily due to the tissue collagen concentration, changes in Ms/Mo may be due to physiological or pathophysiological changes in GAG concentration and tissue structure, and the measured Ms/Mo may differentiate between various pathomimetic degradative procedures.