Chemical Properties of Cartilage Studied Using Charged Ions

Articular cartilage is a soft tissue built mainly from collagen and highly negatively charged biological polyelectrolytes. The high negative charge is responsible for crucial properties of cartilage, such as tissue hydration and biomechanical load resistance. As a consequence, breakdown and loss of the polyelectrolytes in cartilage has an adverse effect on its function and there is a need for methods to determine the amount of polyelectrolytes in cartilage. Here we discuss one such method, the delayed gadolinium-enhanced MRI of cartilage (dGEMRIC). dGEMRIC builds on the fact that a negatively charged MRI contrast agent will distribute in an inverse relation to the concentration of polyelectrolytes on account of the electrostatic repulsion between the two. We introduce the method and discuss some of the key assumptions. In particular, we focus on the relaxivity parameter that should be used to convert the measured water spin-lattice relaxation times to a concentration of the contrast agent, the time for reaching equilibrium with respect to penetration of the contrast agent into cartilage and the use of ideal Donnan equilibrium in the further analysis of dGEMRIC data. Finally, we present some examples of clinical applications of the dGEMRIC method.