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1. Imaging by Magnetic Particles with a Nonlinear Field Response Gareth Kafka Adviser: Dr. Robert Brown Department of Physics, Case Western Reserve University, Cleveland, OH 44106 Relaxation Simulations MPI Spectrometer Motivation Current and Future Work Previous simulations assumed immediate particle responses. We modeled two relaxation mechanisms, Neel and Brownian. Neel relaxation is the relaxation of the magnetic moment of a single particle and is proportional to exp(K*d3), where d is the particle diameter and K is a constant. Brownian relaxation is the randomization of magnetic moments with respect to each other and is proportional to d3. Tomographical imaging is essential in Biomedical fields. Magnetic Resonance Imaging is one such imaging technique, but it is plagued by large noise and requires large magnetic fields. Magnetic Particle Imaging (MPI) utilizes Superparamagnetic Iron Oxide (SPIO) particles with large magnetization responses. MPI thus provides potential for high signal to noise ratio and may have applicability in temporal applications, such as angiography and functional MPI. Currently, the Amplifier/Band Pass Filter Network shown in Figure 5 is being constructed. This network will have a band pass filter, an amplifier, and another band pass filter to generate the purest possible 25 kHz sine wave with a 1 A amplitude. Each of the inductors will be designed by wrapping wire in the shape of a toroid around a ferrite core. Figure 5: MPI Spectrometer apparatus. The input wave and two coils have been constructed. An MPI spectrometer is being built to measure the best frequency response and efficacy of our SPIO sample. The apparatus schematic is shown above. We are using 8 nm sample particles. To saturate these particles, we built the transmit coil shown in Figure 6. The SPIOs are dissolved in a solution and placed in a container which fits inside the receive coil shown in Figure 7. This coil is then inserted into the transmit coil. Neel Relaxation Brownian Relaxation Introduction MPI uses the nonlinear response of SPIOs to magnetic fields to generate a signal. Figure 8: Band Pass Filter Network. The inductors values will range from a few µH to a few mH, and the capacitors values will range from a few nF to a few µF. Avenues for Future Work Our simulations assumed a relaxation time dependent only on the particle size. The time constants could be modified to be field dependent as well. Simulations testing the temporal applications of MPI suggested in the Motivation section could be run. With the completion of the MPI spectrometer, the actual MPI apparatus could be built. Tests of different coil configurations, FFP trajectories, etc. could be conducted. 25 kHz Sine Wave Input Amplifier/Band Pass Filter Network Figure 3: Relaxation time vs particle size. The total relaxation time can be found from 1/τTotal = 1/ τNeel + 1/τBrownian. Previous results showed that larger particles had better resolution. We found that smaller particles had faster relaxation times. Consequently, there should be an optimal mid-sized particle. Transmit Coil Receive Coil (nm) Figure 1: The magnetization signal of SPIOs. On the top, an oscillating magnetic field is applied resulting in large harmonics. On the bottom, the oscillating field is offset by a constant value which damps out the signal [1]. Band Stop Filter Data Collection Figure 6: Transmit coil. The coil is 2.9 cm long, has 430 turns, has an inner radius of 0.8 mm, and is made of 30 gauge wire. The magnetic field at the center of the coil is about 15 mT with an input current of 1 A. Figure 7: Receive coil. The coil is 1.45 cm long and had exactly enough coils to fill this length. It is made with 30 gauge wire. Signal is generated at a point where there is no field, the field free point (FFP). The FFP is moved around by several pairs of coils, as shown. Acknowledgments/References • Special thanks to: • Zhen Yao, CWRU Dept. of Physics • Yong Wu, CWRU Dept. of Physics • Lisa Bauer, CWRU Dept. of Physics • Dr. Mark Griswold, UH Dept. of Radiology • Matthew Riffe, UH Dept. of Radiology • [1] B. Gleich and J. Weizenecker. “Tomographic Imaging Using the Nonlinear Response of Magnetic Particles.” Nature,435, pp. 1214-7, 2005. Bad Relaxation Effects Low Spatial Resolution Figure 4: Simulation Results. Blue indicates low SPIO concentration; red indicates large SPIO concentration. Relaxation causes blurring for large particles; poor spatial resolution causes blurring for small particles. Figure 2: MPI Apparatus. The drive field coils have currents running in the same direction; the selection field coils have currents running in the opposite directions [1].