This class of MRI contrast agents are the most commonly used form in the enhancement of vessels of MR angiography. Dosage of gadolinium can be as low as 0.1 mmol per kg body mass for large vessels such as the aorta and its branches but higher concentrations are used for finer vasculature [1]. Due to the molecule size, Gd chelates do not pass blood-brain barrier and are thus useful in enhancing lesions and tumors where the Gd leaks out. In the rest of the body, the Gd will initially remain in circulation but then distribute into interstitial space or be eliminated from the body through the kidneys.

As a free ion, gadolinium is highly toxic but is generally regarded as safe when administered as a chelated compound. The compounds can be classified by whether they are macro-cyclic or linear geometry and whether they are ionic or not. Cyclical ionic Gd compounds being considered the least likely to release the Gd ion and hence the most safe[2]. FDA approved Gd chelated contrast agents include:[3]