Despite the prolific increase in new technology in interventional cardiology, x-ray imaging combined with radiographic contrast media continues to be the mainstay in imaging technology. Visualization of vascular structures using x-rays requires the use of a radiographic contrast medium in order to distinguish them from surrounding tissues, which aside from bones, absorb x-rays poorly. Iodinated radiographic contrast agents have been used for this purpose since the 1950s. Since then, just as digitalization and evolution of imaging equipment technology have made significant strides in improving image quality and reducing exposure to ionizing radiation, refinements in the design of and judicious use of iodinated contrast has improved patient safety and reduced adverse outcomes.
The iodine atom, with its relatively high atomic weight, attenuates x-rays and is used in most intravascular contrast agents today. Iodine’s K shell binding energy of 33.2 keV is ideal for x-ray photon absorption. Covalent bonding of 3 iodine atoms to a benzene ring at the 2, 4, and 6 position—the basic structure of all iodinated contrast agents (Fig. 18-1)—allows iodine to be delivered intravascularly, free of the side effects of free iodine, and also increases the effective molecular concentration of iodine, improving the ability to attenuate x-rays.1 An iodine concentration of 320 to 370 mg/mL is optimal for angiographic studies. Although all iodinated contrast agents use this basic benzene ring structure, the number of rings per molecule (monomers or dimers) and the side chains at the 1, 3, and 5 positions give each of them unique chemical properties.2
Schematic of iodinated benzene monomers and dimers used in radiographic contrast media.
Contrast agents are classified based on whether they are monomers or dimers, ionic or nonionic, and high, low, or iso-osmolar (Table 18-1). Ionic agents have 2 osmotically active particles per molecule, whereas nonionic agents have 1 osmotically active particle per molecule. High-osmolar agents are generally 4 to 6 times the osmolarity of blood, low-osmolar agents are 1.5 to 3 times the osmolarity of blood, and iso-osmolar agents are usually the same osmolarity of blood.
Table 18-1Iodinated Intravascular Contrast Media |Favorite Table|Download (.pdf) Table 18-1 Iodinated Intravascular Contrast Media
|Monomers ||Dimers |
|1 benzene ring per molecule ||2 benzene rings per molecule |
|Ionic high-osmolar agents ||Ionic high-osmolar agents |
|2 iodinated molecules with 2 osmotically active particles or ratio 1:5 agents ||6 iodine molecules for 2 osmotically active particles; 6:2 agents, or ratio 3.0 |
|Low viscosity ||Low viscosity |
|Diatrizoate (Hypaque and Renografin); osmolality: 1500–1800 mosmol/kg/water ||Ioxaglate (Hexabrix); osmolality: 580 mosmol/kg/water |
|Nonionic low-osmolar agents ||Nonionic iso-osmolar agents |
|3 iodine atoms with 1 osmotically active particle; 3:1 agents, or ratio 3.0 ||6 iodine molecules for 1 osmotically active particle; 6:1 agents, or ratio 6.0 |
|Intermediate viscosity ||High viscosity |