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. Gadolinium is a chemical element with symbol Gd and atomic number 64. It is a silvery-white, malleable and ductile rare-earth metal. It is found in nature only in combined (salt) form. Gadolinium was first detected spectroscopically in 1880 by de Marignac who separated its oxide and is credited with its discovery. It is named for gadolinite, one of the minerals in which it was found, in turn named for chemist Johan Gadolin. The metal was isolated by Lecoq de Boisbaudran in 1886. Gadolinium as a phosphor is also used in other imaging. In X-ray systems, gadolinium is contained in the phosphor layer, suspended in a polymer matrix at the detector. Terbium-doped gadolinium oxysulfide (Gd2O2S: Tb) at the phosphor layer converts the X-rays released from the source into light. This material emits green light at 540 nm due to the presence of Tb3+, which is very useful for enhancing the imaging quality. The energy conversion of Gd is up to 20%, which means that one-fifth of the X-rays striking the phosphor layer can be converted into light photons. Gadolinium oxyorthosilicate (Gd2SiO5, GSO; usually doped by 0.1-1% of Ce) is a single crystal that is used as a scintillator in medical imaging such as positron emission tomography or for detecting neutrons. Gadolinium is a constituent in many minerals such as monazite and bastnasite, which are oxides. The metal is too reactive to exist naturally. Ironically, as noted above, the mineral gadolinite actually contains only traces of Gd. The abundance in the earth crust is about 6.2 mg/kg. The main mining areas are China, USA, Brazil, Sri Lanka, India and Australia with reserves expected to exceed one million tonnes. World production of pure gadolinium is about 400 tonnes per year. Gadolinium is a silvery-white malleable and ductile rare-earth metal. It crystallizes in hexagonal, close-packed 1- form at room temperature, but, when heated to temperatures above 1235 C, it transforms into its 2- form, which has a body-centered cubic structure. Gadolinium is a strong reducing agent, which reduces oxides of several metals into their elements. Gadolinium is quite electropositive and reacts slowly with cold water and quite quickly with hot water to form gadolinium hydroxide: