Effects of Magnetic Nanoparticles on Magnetic Resonance and Spin Relaxation in Systems of Different Viscosity
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Effects of magnetic nanoparticles on magnetic resonance and spin relaxation in systems of different viscosity Natalia Noginova1, Aleksandr Andreyev2, Julia Noginova3, Joseph C. Hall1, Vani Ramesh1 and Vadim A. Atsarkin4 1 NSU, Norfolk, Virginia, USA 2 Virginia Tech, Blacksburg, Virginia USA 3 PAHS, Virginia Beach, Virginia USA 4 IRE, Moscow, Russia ABSTRACT Nuclear Magnetic Resonance (NMR) technique is a convenient method to monitor magnetic nanoparticles in different biomedical applications and observe changes induced by the particles. To better understand the specifics of the magnetic resonance and spin relaxation in the systems with magnetic nanoparticles, the NMR spectra and magnetization dynamics of the host protons were studied in the model systems of different viscosity and some biological systems in the presence of magnetic nanoparticles. The results confirmed that nanoparticles affect the proton relaxation kinetics of liquid solutions, changing the relaxation time (T1) significantly, whereas in systems of high viscosity the relaxation times are unchanged. The kinetics in intermediate systems was multi-exponential. A complicated picture was observed in biological systems, demonstrating contributions of liquid-like and solid-like behavior. INTRODUCTION Magnetic nanoparticles (MNPs) are of a great interest for many promising applications in medicine and biology. The information about the specifics of the nuclear magnetic resonance and relaxation in the media with magnetic nanoparticles is very important for such applications; it is directly related to the use of MNPs in magnetic resonance imaging (MRI) as well as for other applications, where magnetic resonance and relaxivity methods can be used for the remote monitoring of MNPs concentrations and related effects. As was demonstrated earlier [1,2], the effects of MNPs to 1H NMR spectra and spin relaxation of host systems are very different in solid and liquid systems. Significant acceleration of the relaxation kinetics (decrease in T1) and homogeneous broadening (decrease in T2) on the proton NMR spectra are observed in liquids, while solid hosts demonstrate inhomogeneous broadening of the spectra and no changes in T1 and T2 with increase in MNPs concentration. Such a behavior is explained with the molecular motion as a predominating mechanism of the spin relaxation in the systems with MNPs [2]. The spin relaxation is caused by variations of magnetic field seen by nuclei due to rapid molecular motion in a spatial non-uniform field [3]. The standard outer sphere model [4] (slightly modified to take clustering of MNPs into account) fairly well describes acceleration of the spin-lattice and spin-spin relaxation recorded in liquid solvents such as chloroform, toluene and water [1,2]. In solids, in the absence of molecular motion and due to almost 100% polarization of MNPs, the effects of MNPs are reduced to broad distribution of static local fields only and subsequent inhomogeneous broadening of the spectra without any significant effects on spin rel
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