MR Relaxation, Diffusion, and Stiffness Characterization of Engineered Cartilage Tissue

The primary goal of this thesis is to develop a combined MR relaxation (T2 and T1ρ), diffusion (ADC, apparent diffusion coefficient), elastography (shear stiffness) method to fully characterize the development of tissue-engineered cartilage in terms of the changes in its composition, structure, and mechanical properties during tissue growth. We do this for the purpose of understanding how we may better use MR-based methodologies to noninvasively monitor and optimize the cartilage tissue engineering process without sacrificing the constructs. While conventional T2 and ADC have been widely used in the studies of engineered cartilage tissues, there were few T1ρ and MRE studies related to it. We begin by demonstrating the potential capabilities of T2, T1ρ, ADC, and shear stiffness in characterization of a scaffold-free engineered cartilage tissue. We examine the correlations between MR parameters and biochemical determined macromolecule contents in tissue-engineered cartilage. We show that, in addition to the conventional T2 and ADC, T1ρ and MRE can also be used as potential biomarkers to assess the specific changes in proteoglycan content and mechanical properties of engineered cartilage during tissue growth. Secondly, to increase the efficiency of MR characterization of engineered tissues, we develop two new methodologies for simultaneous acquisition of MRI and MRE data: (1) diffusion and MRE (dMRE) and (2) T1ρ and MRE (T1ρ-MRE), respectively. Conventional T1ρ, diffusion, and MRE acquisitions are performed as separate measurements that prolong the imaging protocols. The dMRE and T1ρ-MRE are developed to overcome this problem by acquiring two pieces of information in one temporally resolved scan. This allows the simultaneous characterization of both biochemical and mechanical properties of engineered cartilage tissues. We carry out dMRE and T1ρ-MRE experiments on tissue-mimicking phantoms to show the feasibilities of two techniques. The results obtained show a good correspondence between simultaneous acquisitions and conventional separate acquisition methods. We expect that the combined MRI/MRE methods will benefit the optimal cartilage tissue engineering process.