Gadolinum Shortening T1
In a previous page we saw that hydrogen protons in water molecules (or lipids) which experience an oscillating magnetic field at or the Larmor frequency are efficient at T1 relaxation. This oscillating magnetic field is produced when the molecule in which the hydrogen proton resides moves or "tumbles". In fact, anything which produces an oscillating magnetic field at the Larmor frequency will increase T1 effects. This is what Gadolinium does, as an MRI contrast agent. But how?
The Gd3+ ion affects the T1 of the hydrogen protons of the water molecules it comes into close proximity with, with its electrons. Some nuclei have spin (see 1H Signal in MRI), but electrons can also have spin if they are unpaired (simply an odd number of electrons in any orbital, leaving one left over without a notional partner). Unpaired electrons aren't all that common since most stable molecules have a closed-shell configuration without a suitable unpaired spin. But Gd3+ has seven unpaired electrons, and the interaction of the electron spins with an external magnetic field (electron spin resonance) is equivalent to that of nuclear magnetic resonance. The magnetic field of an electron is 657 times stronger than the magnetic field of a proton spin, and so if the magnetic field the electrons produce oscillates at or near the Larmor frequency, they will have a strong T1 effect (see Shorter T1 in Tissues). This is the case for the unpaired electrons in Gd3+.
The magnetic field of these electrons doesn't extend very far, and so the contrast agent has to come into close proximity with the sources of our MRI signal (hydrogen protons in fat and water molecules). Notice that in MRI, we do not "see" the contrast agent directly. Rather, we only see its effect, indirectly.
So to sum up: the electron spin resonance of Gd3+ matches the Larmor frequency, inducing electron-nuclear dipolar interactions (exchange of spins), increasing the rate of transfer of energy to the lattice, which shortens the T1 relaxation time.
Note: Gd3+ is toxic, so for in vivo use it's wrapped up in a kind of non-toxic jacket, called a chelate molecule, such as DTPA. (This pairs up some of its unpaired electrons but a significant number remain.) Chelation with DTPA ensures its complete elimination from the body and makes it metabolically inert. It does not enter intact cells or get permanently trapped in damaged cells.