EPR of powders, crystals, and clays of MRI contrast agents: Zero field splitting and proton relaxation

MRI (magnetic resonance imaging) has brought great advances to diagnostic medicine, especially with the use of contrast agent (CAs) for comparison of images with and without contrast. Since the first use of Gd III CAs in imaging in the early 1980.s, the chemistry and physics of these CAs have been studied extensively. These agents increase the longitudinal proton relaxivity of protons in waters hydrogen-bound to the Gd III and indirectly affect nearby water protons.;Gd DTPA (Magnevist) is the first Gd CA and has been widely used in imaging throughout the body. Gd DOTA (Dotarem) has been intensively used in neurological applications. Both are vital to diagnostic MRI. Due to advances in CA chemistry, both are bound to a variety of macromolecules to increase rotational correlation time. This increases the proton relaxation rate, providing better contrast. In current use, the electron spin correlation time of Gd III is the critical time in determining the proton relaxation rate. This electron correlation time is dependent on the zero field splitting (ZFS) of Gd III, particularly the static ZFS component.;This static ZFS of Gd III has been determined to an order of magnitude by various researchers. This work utilizes multifrequency EPR (X, Q and W Band) of dilute powders of both Gd DTPA and Gd DOTA to give a narrow range for the ZFS of Gd DTPA and Gd DOTA. Also, simulations of peak positions of two planes of Q Band EPR of dilute Gd DOTA crystals were conducted. This allowed precise determination of the static ZFS of Gd DOTA including quartic terms. This knowledge will help in analyzing the effectiveness of current CAs and in developing future Gd CAs.;In a second project, Gd DTPA has been intercalated into layered double hydroxides (LDHs) of magnesium and aluminum salts for potential use as a CA. Particularly the Gd DTPA LDHs were intended to target cancerous vasculature and cells. During the project, it was discovered that uptake of the LDHs into cells did not occur. However, these Gd DTPA LDHs were measured at X Band EPR to show intercalation of the CA.;Also, studies on rotational dynamics of Gd DTPA in LDH were attempted. John Chen's study of the rotational dynamics of Gd DTPA in solution involved the use of vanadyl as a substitute for Gd III. VO II has strong hyperfine coupling which reveals changes in rotational dynamics in EPR. The VO II DTPA in LDH was used in a multi-temperature, X Band EPR study of the rotational dynamics of Gd DTPA in LDH. The study shows the likelihood of a motionally (temperature) dependent mixture of anisotropic and isotropic motional states of the Gd DTPA in LDH. Unfortunately, simulations did not confirm this.