Depth-dependent characterization of cartilage nanostructures using MRI signal decays

Objective: The multi-exponential nature of echo decay in nuclear magnetic resonance exam of cartilage complicates the determination of relaxation times. In this study, a novel method has been developed and applied to analyze the cartilage nanostructure using multi-exponential signals. This approach eliminates the need for relaxation time determination, avoids sample rotation, and removes the requirement for multiple experiments. A key feature of this method is its ability to provide detailed insights into the nanostructures of the sample. Methods: Quantitative T₂ imaging method was used to examine the signal delays in mature and healthy canine articular cartilage, at a transverse resolution of 35.1 μm. A modeling method was used to analyze the multi-exponential echo decay for each resolved tissue depth along the full thickness of articular cartilage. Results: The developed approach provides detailed information on the nanostructure in the tissue, which varies with cartilage depth. The information contains the volumes of the water-filled nanocavities created by the fibril structure and their orientation. This information reveals that the superficial and transitional anatomic zones of cartilage contain two distinct types of nanocavities, while the radial zone contains only one type. Discussion: The proposed voxel-based method of echo decay analysis enables the estimation of nanocavities, their angular distribution, and spatial variations of the nanocavity characteristics throughout the sample. This newly developed approach demonstrated that detailed structural tissue information can be obtained as a depth function, representing a significant advancement in understanding cartilage nanostructures and holds potential for future medical applications.