Here is more detailed information courtesy of Richard A. Vitti MD from Medical Affairs at GE Healthcare.

CT contrast, or X-ray contrast for IV use, is usually an iodinated compound, whereas most MRI contrast media contains a heavy metal ion usually gadolinium. Iodine provides contrast by virtue of absorption of X-rays at the K-edge. Gd provides contrast in MR by changing the magnetic moment. Generally these agents distribute themselves from the vascular space to the interstitial space of soft tissues, and are elminated by glomerular filtration by the kidneys.

Omnipaque™ (Iohexol) is a water soluble contrast medium and is available in various concentrations, from 140 to 350 milligrams of iodine per milliliter; it is a tri-iodinated substituted ring compound. Increased ratio of iodine per osmotic particle (3:2 for ionic vs. 3:1 for non-ionic) and the osmolality decreased by half compared with non-ionics, results in an osmolality of 1/3 of ionic contrast media and far fewer side effects. The osmolality of iohexol ranges from 322 mOsm/kg-approximately 1.1 times that of blood plasma-to 844 mOsm/kg, almost three times that of blood, depending on the concentration selected for the procedure.

Image quality depends on iodine concentration and total iodine delivered. Omnipaque is the only contrast media that can be used intravenously, intrarterially, orally, or for intrathecal procedures. It can also be delivered into body cavities. It does not cause irritation to serousal or mucosal linings, and does not cause pneumonitis if aspirated into the lungs. Omnipaque can be used in several modalities within the radiology department including contrast-enahnced CT, Cardiac Cath Lab, Angiography/Interventional, and General Radiology (Fluoroscopy procedures).

The flow chart below illustrates the class of iodinated contrast media for X-ray use.

References: Thomsen HS, Morcos SK. Radiographic contrast media. BJU International. 2000; 86: 1-10. Omnipaque Package Insert, 2011

Gadolinium-based contrast agents (GBCAs) are composed of paramagnetic ions that change T1 (longitudinal) and T2 (transverse) relation rates of hydrogen protons within the body. Specifically they shorten T1 relaxation time increasing signal-to-noise ratios and making abnormalities appear bright compared to surrounding tissue.  Most GBCAs approved use in the U.S. are extracellular agents which are distributed within the extracellular interstitial space. Commercially available GBCAs are either ionic or non-ionic.  When injected an ionic molecule separates (dissociates), it can and will interact with other ions in the body, causing interference with normal physiologic processes (hypertonicity).  Nonionic agents do not dissociate in solution so hypertonicity is avoided.

At one time it was felt that high doses of GBCAs could safely be used instead of iodinated contrast agents for patients with known allergy to iodinated contrast agents or for patients with poor renal function.  With the advent of Nephrogenic Systemic Fibrosis (NSF) this practice is no longer recommended.

“The frequency of all acute adverse events after an injection of 0.1 or 0.2 mmol/kg of gadolinium chelates ranges from 0.07% to 2.4%.”  Although GBCAs are safe, adverse events do occur.  “Severe, life-threatening anaphylactoid or nonallergic anaphylactic reactions are exceedingly rare (0.001% to 0.01%).”  A retrospective analysis of data from the U.S. Food and Drug Administration (FDA) Adverse Event Reporting System (AERS) showed higher immediate adverse event rates with ionic GBCAs when compared with non-ionic GBCAs.

An adverse event rate of 3.7% has been shown in patients with asthma, allergies to other medications, and foods, as well as those with previous reactions to iodinated contrast agents (although cross-reactivity has not been proven).  The American College of Radiology (ACR) recommends 12 to 24 hours of premedication with corticosteroids and antihistamines for patients with prior contrast media reactions. 36% of healthcare providers who responded to a survey study conducted by Murphy et al. did not have policies in place for premedication.

References:

• American College Radiology Manual on Contrast Media Version 7.0.  Accessed online 9/6/2011.
• Prince MR, Zhang H, Zou Z, Staron RB, Brill PW. Incidence of immediate gadolinium contrast media reaction.  Am J Radiol. 2011;1 96: W138-W143.
• Murphy KPJ, Szopinski KT, Cohan RH, Mermillod B, Ellis JH. Occurrence of adverse reactions to gadolinium-based contrast material and management of patients at increased risk: A survey of the American Society of Neuroradiology Fellowship Directors.  Acad Radiol. 1999; 6: 656-664.