Feasibility of noninvasive evaluation of biophysical properties of tissue-engineered cartilage by using quantitative MRI

The application of tissue-engineered cartilage in a clinical setting requires a noninvasive method to assess the biophysical and biochemical properties of the engineered cartilage. Since articular cartilage is composed of 70–80% water and has dense extracellular matrixes (ECM), it is considered that the condition of the water molecules in the tissue is correlated with its biomechanical property. Therefore, magnetic resonance imaging (MRI) represents a potential approach to assess the biophysical property of the engineered cartilage. In this study, we test the hypothesis that quantitative MRI can be used as a noninvasive assessment method to assess the biophysical property of the engineered cartilage. To reconstruct a model of cartilaginous tissue, chondrocytes harvested from the humeral head of calves were embedded in an agarose gel and cultured in vitro up to 4 weeks. Equilibrium Young's moduli were determined from the stress relaxation tests. After mechanical testing, MRI-derived parameters (longitudinal relaxation time T1, transverse relaxation time T2, and water self-diffusion coefficient D) were measured. The equilibrium Young's modulus of the engineered cartilage showed a tendency to increase with an increase in the culture time, whereas T1 and D decreased. Based on a regression analysis, T1 and D showed a strong correlation with the equilibrium Young's modulus. The results showed that T1 and D values derived from the MRI measurements could be used to noninvasively monitor the biophysical properties of the engineered cartilage.