Magnetic iron oxide nanoparticles
for MR contrast agents

Introduction

Why magnetic nanoparticles?
- Superparamagnetism

How SPIO works as MRI contrast agents?

Commercialized SPIO MRI contrast agents

In vivo detection of biological targets

References

Superparamagnetism

The magnetic anisotropic energy barrier from a spin-up state to spin-down state of the magnet is proportional to the product of the magnetic anisotropic constant (Ku) and the volume (V) of the magnet. While bulk materials have magnetic anisotropic energies that are much larger than the thermal energy (kT) (blue line in figure below), the thermal energy of the nanoparticle is sufficient to readily invert the magnetic spin direction, although it is insufficient to overcome the spin-spin exchange coupling energy (red line). Such magnetic fluctuation leads to a net magnetization of zero, and this behavior is called superparamagnetism. [1]

The transition temperature from ferromagnetism to superparamagnetism with no hysteresis behavior is referred to as the blocking temperature (Tb) and is defined by the relationship Tb=KuV/25k.

• Superparamagnetism is shown when the size of nanoparticles is below Dc, and the temperature is above the blocking temp.
• As particle size is smaller, Tb is lower, i.e. thermal energy (kT) > anisotropy energy barrier, thus, magnetization is easily flipped.
• No remanence, no coercivity, i.e. no hysteresis
• Large magnetic moment in each particle & fast response to applied magnetic field

created by Yoon Kyung Kim

Clinical Medicine: BME240