Measuring the microenvironmental temperature around magnetic nanoparticles
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Measuring the microenvironmental temperature around magnetic nanoparticles Daniel B. Reeves1 and John B. Weaver1,2 1 Department of Physics and Astronomy, Dartmouth College, 6127 Wilder Hall, Hanover, NH 03755, U.S.A. 2 Department of Radiology, Geisel School of Medicine, Hanover, NH 03755, U.S.A. ABSTRACT It is important to measure the temperature of magnetic nanoparticles during hyperthermia therapy to develop safe practices. We theoretically demonstrate a method for measuring the temperature of magnetic nanoparticles using induction coils and nanoparticle magnetization harmonics. A geometrically decoupled sensing coil is described that enhances the sensitivity to small amounts of iron and also could possibly be used to eliminate sensing challenges created by the high-powered hyperthermia drive field. INTRODUCTION Magnetic fluid hyperthermia has been studied extensively as a promising addition to current cancer therapies [1]. The method uses dissipative losses from magnetic nanoparticles (MNPs) to selectively heat malignant tissue. The technology shows promise, but the methods to monitor the success of the therapy (often conventional needle thermometry) are insufficient for accurate heating measurements. The unfortunate consequence of the strong field needed to achieve heating is that many methods for nanoparticle temperature measurements (e.g., MRI [2]) that use sensitive magnetic or electrical measurements become impossible [3]. A possibility is that through spectroscopy of nanoparticle magnetizations that rotate physically (i.e., with Brownian relaxation) and thus are coupled to their surroundings, we can infer properties of the nanoparticle microenvironments like temperature. So called “magnetic nanoparticle spectroscopy” is attractive because the magnetic nanoparticle heating agents double as their own thermometer [4]. We describe a new spectroscopic technique for measuring the temperature of nanoparticles that employs a sensing coil which is geometrically decoupled from the drive field but coupled to the nanoparticles through an additional static field. Because the perpendicular sensing coil does not receive flux from the excitation field it is more sensitive to small nanoparticle concentrations, and can more accurately measure MNP properties, and moreover, since it is decoupled from the excitation field, it is a possible candidate for concurrent use during hyperthermia. THEORY Magnetic nanoparticles suspended in a fluid thermal bath can be described by Langevin equation dynamics [5]. We model the particles as non-interacting magnetic dipoles that do not accelerate (in the small Reynolds number regime) but feel a drag force linear in fluid viscosity. The simulation scheme for Brownian nanoparticles is detailed Ref. [6]. Simulations of statistical
particle dynamics are developed by integrating a stochastic differential equation millions of times. To mimic the perpendicular spectroscopy, we choose the field (1) with the excitation field frequency ƒ and amplitude Ho and static field amplitude Hs. We also often
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