Gadolinium
gadolinium (Gd) are the most widely used of all MR contrast agents. Because of its unique electronic structure (described below), Gd is strongly paramagnetic.
Paramagnetism is an intrinsic property of certain materials to become temporarily magnetized when placed in an external magnetic field. In fact, Gd is one of only four elements that can be magnetized at room temperature (the other three being iron, nickel, and cobalt).
The powerful paramagnetic properties of Gd make it extremely useful as an MR contrast agent. Gadolinium is not directly seen in an MR image, but manifests its presence indirectly by facilitating the relaxation of nearby hydrogen protons. Gd preferentially shortens T1 values in tissues where it accumulates rendering them bright on T1-weighted images.
Paramagnetism may exist over a wide dimensional range — from subatomic particles to atoms to entire molecules. Nuclear paramagnetism, the form responsible for the NMR phenomenon, is extremely weak except in the immediate vicinity of the nucleus. It plays little or no role in determining the gross paramagnetic properties of entire atoms like gadolinium.
The form of paramagnetism exhibited by gadolinium compounds derives from electrons, not protons, and is known as Curie paramagnetism. Because of electrons have the same spin (½) but a much smaller size than protons, their gyromagnetic ratios are 657 times larger. If these electrons remain unpaired in shells or bonding orbitals, the unbalanced spins produce a strong magnetic moment capable of inducing magnetic relaxation in nearby nuclei. This is the origin of the bulk paramagnetism possessed by elements such as gadolinium.
The form of paramagnetism exhibited by gadolinium compounds derives from electrons, not protons, and is known as Curie paramagnetism. Because of electrons have the same spin (½) but a much smaller size than protons, their gyromagnetic ratios are 657 times larger. If these electrons remain unpaired in shells or bonding orbitals, the unbalanced spins produce a strong magnetic moment capable of inducing magnetic relaxation in nearby nuclei. This is the origin of the bulk paramagnetism possessed by elements such as gadolinium.
Gadolinium has atomic number 64 on the periodic table. It occupies the central position in the lanthanide series of elements. Lanthanides are rare-earth metals grouped chemically because they possess partially filled inner shells of electrons (4f and 5d subshells).
The electronic structure of the neutral Gd atom is shown right. Note the 7 unpaired electrons in its 4f subshell that account for the element's strong paramagnetism. In its ionized state, Gd+3 donates its 6s² and 5d¹electrons for bonding, leaving its 4f7electron shell intact. The powerful magnetic moment of Gd is therefore largely maintained even when chelated to a ligand such as DTPA in a contrast agent formulation.
