Improved magnetic particle spectrometer providing high field amplitudes for investigation of hysteresis effect in superparamagnetic nanoparticle tracers

M. Erbe, T. F. Sattel, T. M. Buzug


The novel and very promising tomographic method magnetic particle imaging (MPI) is capable of detecting the spatial and temporal distribution of a superparamagnetic iron-oxide (SPIO) nanoparticle tracer within a patient in 3D and in real-time. Oscillating magnetic excitation fields are applied imposing a magnetization change on the tracer material, which can be detected in receive coils due to the reciprocity principle. Gradient fields featuring a field free point (FFP) as well as a linearily increasing field strength originating from the FFP are superimposed to the excitation fields. Thus, only the tracer material located at the FFP contributes to the detected signal, since everywhere else the high field strength forces the SPIO particles into saturation. By moving the FFP through the region of interest spatial encoding is achieved. The physical behaviour of the tracer material is one of the crucial aspects regarding the performance of MPI with respect to sensitivity and spatial resolution. The best results were up till now achieved in measurements using the former magnetic resonance imaging (MRI) liver contrast agent Resovist® (Bayer Schering Pharma) as MPI tracer. However, providing tracer particles optimized for MPI will considerably enhance the sensitivity of this method. To analyze the MPI performance of a certain SPIO tracer a magnetic particle spectrometer (MPS) is used. The MPS uses the same physical principles as MPI, but without applying spatial encoding. It is hence possible to analyze the suitability of a SPIO tracer for use in MPI. Since the physical behaviour of Resovist® and comparable tracer particles is not yet fully understood, it is of great importance for the tracer development to provide information about the particle behaviour for a broad range of parameters. Formerly introduced MPS studies allowed for an excitation field strength of not more than 30 mT/μ0. Since hysteresis effects do preponderate especially at higher excitation field strengths, stronger magnetic fields are needed to detect and visualize these effects. An optimized MPS was therefore designed and is presented in this work, which allows for excitation field strengths up to 40 mT/μ0. At these high field strengths hysteresis effects in the particles' magnetization behaviour lead to a phase shift in the detected MPS signal. This is visible in the formation of waves in the particle spectrum. This effect increases with increasing excitation field strength as visible in the measurements presented in Figure 1. These measurements were performed with 10 μl undiluted Resovist®. Analyzing these measurements and the corresponding hysteresis effects will allow for a more detailed characterization and a deeper understanding of the anisotropy properties of SPIO particles. The presented optimized MPS hence provides essential information advancing the MPI tracer development and therefore also the MPI performance. In addition, information about hysteresis effects will be useful for hyperthermia applications as well.
Original languageEnglish
Number of pages1
Publication statusPublished - 07.09.2012


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