Safety
As a free ion, gadolinium is reported often to be highly toxic, but MRI contrast agents are chelated compounds and are considered safe enough to be used in most persons. Free gadolinium ions toxicity in animals is due to interfering with a number of calcium-ion channel dependent processes. The 50% lethal dose is about 100-200 mg/kg. No prolonged toxicities have been reported following low dose exposure to gadolinium ions. Toxicity studies in rodents, however show that chelation of gadolinium (which also improves its solubility) decreases its toxicity with regard to the free ion by at least a factor of 100 (i.e., the lethal dose for the Gd-chelate increases by 100 times). It is believed therefore that clinical toxicity of Gd contrast agents in humans will depend on the strength of the chelating agent; however this research is still not complete. About a dozen different Gd-chelated agents have been approved as MRI contrast agents around the world.
Gadolinium MRI contrast agents have proved safer than the iodinated contrast agents used in X-ray radiography or computed tomography. Anaphylactoid reactions are rare, occurring in approximately 0.03–0.1%.
Although gadolinium agents have proved useful for patients with renal impairment, in patients with severe renal failure requiring dialysis, there is a risk of a rare but serious illnesses, called nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy, that has been linked to the use of four gadolinium-containing MRI contrast agents. The disease resembles scleromyxedema and to some extent scleroderma. It may occur months after contrast has been injected. Its association with gadolinium and not the carrier molecule is confirmed by its occurrence in from contrast materials in which gadolinium is carried by very different carrier molecules.
Current guidelines in the United States are that dialysis patients should only receive gadolinium agents where essential and to consider performing an iodinated contrast enhanced CT when feasible. If a contrast enhanced MRI must be performed on a dialysis patient, it is recommended that certain high-risk contrast agents be avoided and that a lower dose be considered. The American College of Radiology recommends that contrast enhanced MRI examinations be performed as closely before dialysis as possible as a precautionary measure, although this has not been proven to reduce the likelihood of developing NSF.
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