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
Motivation
Relaxation Simulations
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.
Previous simulations assumed immediate particle
responses. We modeled two relaxation mechanisms,
Neel and Brownian.
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.
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.
Neel Relaxation
Brownian Relaxation
Introduction
MPI uses the nonlinear response of SPIOs to
magnetic fields to generate a signal.
MPI Spectrometer
25 kHz Sine
Wave Input
Amplifier/Band Pass
Filter Network
Receive Coil
Transmit Coil
Band Stop Filter
Data Collection
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.
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.
Simulations testing the temporal applications of MPI
suggested in the Motivation section could be run.
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.
Bad Relaxation Effects
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Avenues for Future Work
Figure 3: Relaxation time vs particle size. The total relaxation
time can be found from 1/τTotal = 1/ τNeel + 1/τBrownian.
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.
POSTER TEMPLATE BY:
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.
Our simulations assumed a relaxation time dependent
only on the particle size. The time constants could be
modified to be field dependent as well.
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.
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].
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.
(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].
Current and Future Work
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.
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.
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.