Magnetic Structure and Power Absorption in Magnetite Nanoparticles from a MRI Contrast Agent

G F GOYA; ENIO LIMA JR.; M S LANCAROTTE; M RICARDO IBARRA

Dresden, Germany

Congreso; Euromech Colloquium on Magnetic Fluid Research; 2006

Colloidal suspensions based on iron-oxide nanoparticles have gained significance inbiomedical sciences as media contrast agents (CA) for clinical protocols of MagneticResonance Imaging (MRI) and Magnetic Fluid Hyperthermia (MFH). Whereas the basic mechanisms of magnetic CA’s on proton relaxation have been intensively investigated along the last years, the details of the MFH process involved in cell ablation are subject of discussion. The main action of CAs is to change in T1 and T2 relaxation rates of the protons within target tissues, to the extent that a measurable change in signal intensity can be observed. The relaxation effect is proportional to the square of the saturation magnetic moment (MS) of the material to be employed, and therefore for these applications, magnetite (Fe3O4) and maghemite (γ-Fe2O3) are the preferred phases because of their high saturation magnetic moment at room temperature(MS ~ 75 and 82 emu/g, respectively) and the high ordering temperatures. For MFH therapy, however, the ability to absorb radiated power from an ac alternating field at low frequencies (< 100 kHz) depends on the loss component of the magnetic susceptibility, as well as the rheological properties of the target physiological medium (i.e. cell culture or tissue). To improve stability and biocompatibility the particles are coated with a polysaccharide and suspended in water-based solvents, although the understanding of the effects of these coatings on the efficacy in MRI or MFH applications (from which a new material can be designed for specific purposes) is far from complete. In order to scrutinize the relationship between basic magnetic parameters and the efficiency for heating purposes, we have studied the magnetic behaviour of a colloidalsuspension consisting of dextran-coated biocompatible Fe3O4 nanoparticles with average particle core size dC ~ 8 nm (hydrodynamic diameter dH ≈ 100 nm), used as contrast medium in clinical protocols for MRI (ENDOREM®) for the detection of liver lesions.