High transversal relaxivities of silica coated multicore iron oxide nanoparticles suitable for magnetic resonance imagin
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1257-O05-06
High transversal relaxivities of silica coated multicore iron oxide nanoparticles suitable for magnetic resonance imaging Elena Taboada1, Elisenda Rodríguez2, Anna Roig1* 1
Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Campus de la UAB, 08193 Bellaterra, Spain. * [email protected] 2 Institut d’Investigacions Biomèdiques de Barcelona (IIBM-CSIC), 08036 Barcelona, Spain ABSTRACT We report on high transversal relaxivity values of composite iron oxide-silica nanoparticles. To obtain the material, pre-formed maghemite nanoparticles were coated with silica by sol-gel chemistry, using supercritical fluids as the reaction media. The composite particles were monodisperse and consisted of a core of several maghemite nanoparticles, surrounded by a thick silica shell. The high pressure and high temperature process did not affect the iron oxide particle size but induced an increase on their saturation magnetization values, possibly due to an improvement of the particle crystallinity. These iron oxide-based materials present very high transversal relaxivity values which can be correlated to the magnetic moment and to the silica shell width of the composite particles. Moreover, composite particles are not cytotoxic and they are dispersable in polar solvents.
INTRODUCTION Iron oxide superparamagnetic particles are nanometric entities with high saturation magnetization and no remanence at room temperature. In addition, they are biocompatible, with tunable particle size and high relaxivity values, for what they are very useful as contrast agents for magnetic resonance imaging (MRI)[1]. MRI is a non-invasive medical technique to obtain tomographic images of the body, which allows distinguishing pathological tissues from healthy ones. The physical basis of MRI relies on the nuclear magnetic resonance [2]. Briefly, the sample to be investigated is placed in a magnetic field (parallel to the z axis). The magnetic spins of the protons in the sample are then excited with a radiofrequency (RF) pulse, applied perpendicular to the static magnetic field. When the RF pulse ends, the spins relax to their equilibrium state. The longitudinal relaxation is the recovery of the magnetization in the z axis (with a characteristic time = T1 [s]). The transversal relaxation corresponds to the coherence loss of the spins in the x-y plane (with a characteristic time = T2 [s]). The inverse of these times are defined as the relaxation rates, R1 [s1 ] and R2 [s-1]. The use of contrast agents (CA) accelerates the relaxation processes and consequently, increases the relaxation rates producing an enhancement in the signal intensity. The efficiency of contrast agents is defined as relaxivity (r1 and r2): the relaxation rates referred to the CA concentration [s-1·mM-1]. The higher the ri values, better contrasted will be the images. Contrast agents are classified as positive (T1-CA, r2/r1 < 2) or negative (T2-CA, r2/r1 > 2). The relaxivity values directly depend on the contrast agent magnetization. However, there are important differences
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