Depth and orientational dependencies of MRI T2 and T1ρ sensitivities towards trypsin degradation and Gd-DTPA2− presence in articular cartilage at microscopic resolution

Depth and orientational dependencies of microscopic magnetic resonance imaging (MRI) T2 and T1ρ sensitivities were studied in native and trypsin-degraded articular cartilage before and after being soaked in 1 mM Gd-DTPA2− solution. When the cartilage surface was perpendicular to B0, a typical laminar appearance was visible in T2-weighted images but not in T1ρ-weighted images, especially when the spin-lock field was high (2 kHz). At the magic angle (55°) orientation, neither T2- nor T1ρ-weighted image had a laminar appearance. Trypsin degradation caused a depth- and orientational-dependent T2 increase (4%–64%) and a more uniform T1ρ increase at a sufficiently high spin-lock field (55%–81%). The presence of the Gd ions caused both T2 and T1ρ to decrease significantly in the degraded tissue (6%–38% and 44%–49%, respectively) but less notably in the native tissue (5%–10% and 16%–28%, respectively). A quantity Sensitivity was introduced that combined both the percentage change and the absolute change in the relaxation analysis. An MRI experimental protocol based on two T1ρ measurements (without and with the presence of the Gd ions) was proposed to be a new imaging marker for cartilage degradation.