Paramagnetic materials attract and repel like normal magnets when subject to a magnetic field. This alignment of the atomic dipoles with the magnetic field tends to strengthen it, and is described by a relative magnetic permeability greater than unity. Paramagnetism requires that the atoms individually have permanent dipole moments even without an applied field, which typically implies a partially filled electron shell. In pure Paramagnetism (without an external magnetic field), these atomic dipoles do not interact with one another and are randomly oriented in the absence of an external field, resulting in zero net moment.
Paramagnetic materials in magnetic fields will act like magnets but when the field is removed, thermal motion will quickly disrupt the magnetic alignment. In general, paramagnetic effects are small (magnetic susceptibility of the order of 10^-3 to 10^-5).
In MRI, gadolinium (Gd) one of these paramagnetic materials is used as a contrast agent. Through interactions between the electron spins of the paramagnetic gadolinium and the water nuclei nearby, the relaxation rates (T1 and T2) of the water protons are increased (T1 and T2 times are decreased), causing an increase in signal on T1 weighted images.

See also contrast agents, magnetism, ferromagnetism, superparamagnetism, and diamagnetism.

Short name: Gd-DTPA, generic name: Gadopentetate dimeglumine, chemical compound: Gadolinium-diethylenetriaminepentaacetic acid
Gadopentetate dimeglumine was introduced in 1981, as the first paramagnetic MRI contrast agent (ionic). The Gd-induced dipole dipole interactions lead to shortening of T1, which results in contrast enhancement on T1 weighted images. The used metal ion Gd3+ (gadolinium) is toxic, and therefore bound in the renally excreted DTPA chelate, a very stable complex. The Gd-complex also induce susceptibility effects, as a result of the magnetic field gradient between the contrast agent in the blood vessels and the surrounding tissue, that lead to shortening of T2 or T2*.
Following intravenous administration, the compound is distributed rapidly in the extracellular space and is eliminated unchanged by glomerular filtration via the kidneys. Up to 6 hours, post injection an average of 83% of the dose is eliminated renal.

See also Magnevist®, Gadolinium and Contrast Agents.

Motion, which is uniform in all directions. This is generally used in reference to molecular diffusion or rotation, which gives rise to relaxation of the spin system through the dipole dipole interaction.

Precession is a wobbling motion that occurs when a spinning object is the subject of an external force. Relevant to MRI, the proton of a hydrogen nucleus spins around its axis giving it an angular moment (quantum mechanics). Through the protons positive charge and its spin it generates a magnetic field and gets a magnetic dipole moment (MDM) parallel to the rotation axis. If placed in a magnetic field the magnetic dipole moment will precess about the direction of the magnetic field with an angular frequency (Larmor frequency). The Larmor equation dictates that the frequency of the precession at higher field strengths is higher.

A measure of the net magnetic properties of an object or particle. A nucleus with an intrinsic spin will have an associated magnetic dipole moment, so that it will interact with a magnetic field (as if it were a tiny bar magnet).