MRI properties of a unique hypo-intense layer in degraded articular cartilage

To investigate the characteristics of a hypo-intense laminar appearance in articular cartilage under external loading, microscopic magnetic resonance imaging (μMRI) T1, T2 and T1ρ experiments of a total of 15 specimens of healthy and trypsin-degraded cartilage were performed at different soaking solutions (saline and 100 mM phosphate buffered saline (PBS)). T2 and T1ρ images of the healthy tissue in saline showed no load-induced laminar appearance, while a hypo-intense layer was clearly visible in the deep part of the degraded tissue at the magic angle. A significant difference was found between T2 values at 0° and 55° (from 16.5  ±  2.8 ms to 20.2  ±  2.7 ms, p  =  0.0005), and at 0° and 90° (16.5  ±  2.8 ms to 21.3  ±  2.6 ms, p  <  0.0001) in saline solution. In contrast, this hypo-intense laminar appearance largely disappeared when tissue was soaked in PBS. The visualization of this hypo-intensity appearance in different soaking mediums calls for caution in interpreting the data of relaxation times, chemical exchange and collagen fiber deformation.