Ionic versus nonionic MR imaging contrast media: operational definitions.

An experimental rationale is provided to differentiate between the terms ionic and nonionic for magnetic resonance (MR) imaging contrast media such as gadodiamide and gadopentetate dimeglumine. Four independent types of physical measurements (electric conductivity, osmolality, electrophoresis, and ion exchange) were performed on a range of test compounds, including D-glucose, iohexol, gadopentetate dimeglumine, and gadodiamide. Iohexol, D-glucose, and gadodiamide are shown to be nonionic species at physiologic pH (7.4), not measurably dissociating in solution. A range of gadopentetate salts behave as electrolytes, dissociating into constituent charged ions in aqueous media. Operational definitions for the terms ionic and nonionic are provided, and the terms neutral and net zero charge are compared with nonionic for accuracy. The nomenclature nonionic and ionic is deemed appropriate for differentiating MR imaging contrast media.