Federal health officials in the U.S. are warning doctors that certain types of metallic dye injected for MRI scans have been linked to a rare and dangerous skin disease in kidney patients.

More than two dozen dialysis patients in St. Louis over a four-year period contracted the unusual skin ailment, which causes burning and itching that can lead to discoloration and stiffening of the skin.

The U.S. Centers for Disease Control and Prevention investigated a number of the cases last year and found the illnesses were tied to a contrasting agent — basically a metallic dye — used for magnetic resonance imaging tests. The disease occurred in patients with advanced kidney disease who had undergone an MRI or a similar test.

"To the general public, it's not a big concern. But to somebody with kidney disease, we want to warn them not to get an MRI with the contrasting agent," said CDC spokeswoman Jennifer Morcone.

The dyes in question contain gadolinium and are used in MRI scans that provide detailed pictures of internal organs and in similar scans that image blood vessels. The contrasting agents have been on the market in Canada since 1988.

Last year, the U.S. Food and Drug Administration issued a public health advisory following European reports of a rare skin disease in Austrian and Danish kidney patients who were given the agent.

The skin disease, called nephrogenic systemic fibrosis, is rare, but doctors think it may be under-recognized. A registry at Yale University has recorded about 215 cases to date.

Steroid treatments or kidney transplants have helped some patients recover.

The researchers noted that patients on peritoneal dialysis were more likely to develop the condition than those on hemodialysis. CDC officials believe stricken patients weren't able to clear the contrasting agent from their body.