CHAPTER 20: Magnetic Resonance in the Assessment of Tissue Engineered Cartilage

Magnetic resonance spectroscopy (MRS) and imaging (MRI) are routinely used for non-invasive monitoring and assessment of cartilage regeneration in vitro and in vivo. Cartilage tissue engineering utilizes a combination of three-dimensional porous scaffold, chondrocytes or stem cells, growth factors such as transforming growth factor-β, and growth stimulating conditions to obtain a neocartilage tissue that contains a high level of chondrogenic extracellular matrix proteins, proteoglycans and collagen, type II. Water proton (¹H) parametric MRI is commonly applied for monitoring and assessing tissue-engineered cartilage growth at the bench and for in vivo settings. The change in relaxation times (T₁, T₂ and T_1ρ) and apparent diffusion coefficient are correlated with the change in the amount of proteoglycan and collagen in tissueengineered cartilage. In stem cells and scaffold-based engineered cartilage, it has been shown that once the scaffold's contribution is removed, both T₁ and T₂ correlate with the amount of matrix regeneration. The cartilage tissue's functional properties depend on its special composition of extracellular matrix proteins. This arrangement of extracellular matrix is highly anisotropic and one that is the source of cartilage health. In engineered cartilage, tissue anisotropy can be measured using the sodium triple quantum coherence nuclear magnetic resonance-based average quadrupolar coupling (ω_Q) or the diffusion tensor imaging based fractional anisotropy parameters. Using these techniques, it has been shown that the engineered cartilage tissues are less anisotropic than the natural cartilage. Glycosaminoglycan (GAG) of proteoglycan is negatively charged and sodium MRI can be used for assessing the GAG amount. The sodium MRI-based fixed charge density (FCD) is found to strongly correlate with the FCD derived from the GAG assay in a tissue-engineered matrix created from stem cell chondrogenesis in polymer-hydrogel hybrid scaffolds. In summary, magnetic resonance technologies offer tools to non-invasively assess the engineered cartilage tissue growth at all stages, in vitro and in vivo, from cell seeding to post-implantation.