X-ray attenuation coefficients of high-atomic-number, hexanuclear transition metal cluster compounds: A new paradigm for radiographic contrast agents

Rationale and Objectives. The purpose of this study was to examine the radiologic attenuation properties of the parent cluster compounds, particularly attenuation as a function of discrete photon energy, before investigating ligand substitutions, which are necessary to improve cluster biocompatibility and to impart desirable physicochemical properties. Materials and Methods. The linear attenuation coefficients for solutions of the cluster compounds Ta6Br14, K8Ta6O19, and ((HO)-O-3)(2)W6Cl4 were determined at 60, 80, 103, 122, and 140 keV from gamma-ray transmission measurements with americium-241, xenon-133, gadolinium-153, cobalt-57, and technetium-99m radioactive sources. Transmission measurements were obtained for a fixed time interval that ensured a statistically accurate count distribution exceeding 20,000 counts through the sample for each trial. Results. On a strictly mole per liter basis, a 0.075 mol/L aqueous solution of K8Ta6O19 showed 1.08 times the attenuation of 0.063 mol/L aqueous iohexol at 60 keV and 3.30 times the attenuation at XO keV. Similarly, a 0.05 mol/L methanolic solution of (H3O)(2)W6Cl14 showed 0.96 times (96%) the attenuation of 0.063 mol/L aqueous iohexol at 60 keV but 3.09 times the attenuation of the iohexol solution at 80 keV. Attenuations of 0.063 mol/L aqueous iohexol and 0.0125 mol/L Ta6Br14, (ie, at approximately one-fifth the iohexol concentration) were comparable at greater than 60 keV. Conclusion. These results confirm the theoretic potential for use of early transition metal cluster compounds as radiographic contrast agents. At higher x-ray energies, cluster compounds demonstrate multiplied x-ray attenuation relative to iodinated contrast agents.