Key Physical Properties
What are the important physical properties to consider when choosing a contrast agent?
The table below lists several key physical properties of commercially-available gadolinium-based contrast media that should be considered when selecting an agent. In this and other tables I have used the brand names rather than chemical names since these will likely be more familiar to readers.
1. Osmolality. Osmolality reflects the concentration of dissolved particles in a contrast agent's formulation. Values range between 630 and 1970 mOsm/kg of water where 1 mOsm = 1 "milli-osmole" = 1/1000 of a mole. The osmolality of a contrast agent is closely related to whether it is ionic or non-ionic. The clinical differences between low and high osmolality gadolinium agents is not nearly as important as differences between similar classes of iodine-based agents. (See ionic/non-ionic Q&A).
2. Viscosity. Viscosity is a measure of the "thickness" and internal resistance to motion of a fluid. For contrast agents the units are typically given in centiPoise (cP), where 1 cP = 1 g/m-s. The viscosities of contrast agents are in a range similar to that of whole blood with minor differences among them. Generally viscosity is not a major concern for standard doses and injection rates, but could be important for small caliber IVs and rapid injections.
3. Thermodynamic stability. The equilibrium constant (Keq) may be the most important parameter to note as it reflects how tightly the Gd ion is bound to its ligand. This value is presented on a logarithmic scale, with larger values representing exponentially tighter binding. It is no coincidence that the two contrast agents with the lowest Keq's (OptiMARK™ and Omniscan®) also have the highest reported rates of associated nephrogenic systemic fibrosis (NSF), a disease believed due to the deposition of free Gd in tissues. Selecting an agent with a high Keq is especially important in patients with renal insufficiency who are at risk for NSF. The video below explains this concept in a little more detail.
4. Relaxivity. Relaxivity is a measure of the degree to which a given amount of contrast agent shortens T1 or T2. This is obviously a very important metric as accelerated relaxation is the main purpose of administering contrast! Higher values are better. Higher relaxivities are associated with larger ligands and the propensity of the agent to bind with serum proteins. Ablavar®, designed for vascular imaging, attaches readily to albumin and has the highest relaxivity of any commercially available contrast agent.
5. Elimination half-life. Extracellular gadolinium contrast agents all share a common pharmacology. Within minutes of intravenous injection, they distribute rapidly throughout the extracellular interstitium. They are not metabolized and nearly completely eliminated by passive renal filtration, resulting in an biological half-life of approximately 1½ hours in patients with normal renal function. The elimination half-life is shorter for Eovist® (due to its hepatobiliary excretion) and much longer for Ablavar® (due to its albumin binding).
6. Dose. The recommended dose of all gadolinium contrast agents is given on a per kilogram basis. The standard dose is 0.1 mmol/kg for most agents, the only exceptions being for Eovist® and Ablavar® at the much lower values of 0.025 and 0.03 mmol/kg respectively.