Contrast Enhanced Imaging of Cartilage Reflects Changes in Both Water and GAG Content

INTRODUCTION: Contrast enhanced imaging of articular cartilage is predicated on the use of anionic contrast agents that function as mobile ionic probes that partition themselves in inverse proportion to the glycosaminoglycan (GAG) content of the cartilage matrix. GdDTPA (charge -2, 548 g/mol) has been used for delayed gadolinium enhanced magnetic resonance imaging of cartilage (dGEMRIC) and Ioxaglate (charge -1, 1269 g/mol) or Iothalamate (charge -1, 809.17 g/mol)has been used for contrast enhanced computed tomographic imaging of cartilage (CECT). dGEMRIC is able to differentiate between healthy and arthritic cartilage both in vitro and in vivo. Changes in T1 relaxation time measured by MRI in the presence of GdDTPA reflect variations in the structure and composition of the cartilage extracelluar matrix (ECM) including GAG. Similarly, changes in the x-ray attenuation of cartilage measured by CT in the presence of Ioxaglate or Iothalamate have been used to quantify changes in the GAG content and structure of normal and degraded articular cartilage. According to Donnan equilibrium, partitioning of these anionic contrast agents are affected both by the GAG content and permeability of the cartilage ECM, making it difficult to ascertain what changes are specifically measured by contrast enhanced imaging of cartilage. Assuming that water content reflects the porosity (and permeability) of articular cartilage, the aim of this study was to evaluate the affect of naturally occurring changes in the water and GAG content of bovine articular cartilage on the variation of the xray attenuation of cartilage measured by microCT in the presence of the anionic contrast agents GdDTPA and Ioxaglate.