How SPIO works as MRI contrast agents?

Magnetic iron oxide nanoparticles
for MR contrast agents

MRI is based on NMR signal of protons from water in tissues, membrane lipids, proteins, etc., through the combined effect of a strong static magnetic field B0 up to 2 T in current clinical apparatus and a transverse radiofrequency-field (5-100 MHz). After the rf-sequence, the net magnetization vector is once again influenced by B0 and tries to realign with it along the longitudinal axis as protons attempt to retrun to a state of equilibrium: this phenomenon is called relaxation. This relaxation can be divided into two different processes, T1 and T2. Owing to their different T1 and T2 relaxation, tissues may be differentiated. [5]

Longitudinal relaxation (‘spin-lattice’ relaxation): loss of energy, T1-recovery
Transverse relaxation (‘spin-spin’ relaxation): loss of phase coherence, T2-decay.

MRI contrast agents are also classified as following by the different changes in relaxation times after their injection.

Paramagnetic contrast agents (T1-agents), usually Gd3+-chelates
Superparamagnetic iron oxide (SPIO) nanoparticles with a size between 3 and 10 nm (T2-agents).

Since iron oxide nanoparticles have markedly higher value of the magnetic moment and thus, present much higher relaxivities than Gd-chelates, SPIO will be explored in this website.

Under an applied magnetic field, induced magnetic spins in magnetic nanoparticles perturb the nuclear spin relaxation processes of protons of water molecules surrounding magnetic nanoparticles. This effect leads to the shortening of spin-spin relaxation time (T2) of the proton due to inhomogeneities in local magnetic field and fluctuating magnetic fields at molecular level, which results in darkening of MR images, called as negative contrast agents.

created by Yoon Kyung Kim

Clinical Medicine: BME240