Gd-Albumin Relaxivity in the Rat Thalamus In Vivo at 11.1 T

Introduction The relaxivity of an MR contrast agent (CA) parameterizes how efficiently a CA alters neighboring water proton longitudinal (T1) and transverse (T2) relaxation times. A priori knowledge of a CA relaxivity can be used in conjunction with preand post-contrast T1-weighted scans and native T1 values to determine the concentration of CA in a sample. CA relaxivity in solution can be readily determined by measuring the T1 and T2 dependence on CA concentration; however, the relaxivity of the CA in solution may differ from that in tissue due to compartmentalization of water, binding of the CA to macromolecules, and changes in the microenvironment of the CA (e.g. pH, temperature, microviscosity). Convection enhanced delivery (CED) is the direct infusion of an agent into the extracellular space of tissues via a positive pressure gradient and has emerged as a promising local drug delivery method for treating neurological diseases. As a surrogate for therapeutic agents, the macromolecular CA, gadolinium-labeled albumin (Gd-albumin), has been infused into the rat dorsal and ventral hippocampus to determine the influence of tissue architecture on infusate distribution; however since the relaxivity of Gd-albumin in tissue is unknown, only the final distribution volume of the CA could be compared between ventral and dorsal distributions following CED. Measuring the relaxivity of Gd-albumin in the rat brain, in vivo, would enable the direct measure of CA concentration profiles, rather than just the spatial distribution, which are more closely related to therapeutic efficacy in a targeted region. In this study, the relaxivity of Gd-albumin was measured in the rat brain thalamus in vivo at 11.1 T. These tissue relaxivity values were compared to relaxivity values in solution to assess the error associated with applying solution-based relaxivity values to in vivo tissue CA concentration calculations. The measured CA relaxivity in tissue can be used to directly determine CA concentration in similar brain regions.