Mandy Grüttner

Magnetic particle imaging: Introduction to imaging and hardware realization

Magnetic Particle Imaging (MPI) is a recently invented tomographic imaging method that quantitatively measures the spatial distribution of a tracer based on magnetic nanoparticles. The new modality promises a high sensitivity and high spatial as well as temporal resolution. There is a high potential of MPI to improve interventional and image-guided surgical procedures because, today, established medical imaging modalities typically excel in only one or two of these important imaging properties. MPI makes use of the non-linear magnetization characteristics of the magnetic nanoparticles. For this purpose, two magnetic fields are created and superimposed, a static selection field and an oscillatory drive field. If superparamagnetic iron-oxide nanoparticles (SPIOs) are subjected to the oscillatory magnetic field, the particles will react with a non-linear magnetization response, which can be measured with an appropriate pick-up coil arrangement. Due to the non-linearity of the particle magnetization, the received signal consists of the fundamental excitation frequency as well as of harmonics. After separation of the fundamental signal, the nanoparticle concentration can be reconstructed quantitatively based on the harmonics. The spatial coding is realized with the static selection field that produces a field-free point, which is moved through the field of view by the drive fields. This article focuses on the frequency-based image reconstruction approach and the corresponding imaging devices while alternative concepts like x-space MPI and field-free line imaging are described as well. The status quo in hardware realization is summarized in an overview of MPI scanners.

On the formulation of the image reconstruction problem in magnetic particle imaging.

Abstract In magnetic particle imaging (MPI), the spatial distribution of magnetic nanoparticles is determined by applying various static and dynamic magnetic fields. Due to the complex physical behavior of the nanoparticles, it is challenging to determine the MPI system matrix in practice. Since the first publication on MPI in 2005, different methods that rely on measurements or simulations for the determination of the MPI system matrix have been proposed. Some methods restrict the simulation to an idealized model to speed up data reconstruction by exploiting the structure of an idealized MPI system matrix. Recently, a method that processes the measurement data in x-space rather than frequency space has been proposed. In this work, we compare the different approaches for image reconstruction in MPI and show that the x-space and the frequency space reconstruction techniques are equivalent.

Analog receive signal processing for magnetic particle imaging

PURPOSE Magnetic particle imaging (MPI) applies oscillating magnetic fields to determine the distribution of magnetic nanoparticles in vivo. Using a receive coil, the change of the particle magnetization can be detected. However, the signal induced by the nanoparticles is superimposed by the direct feedthrough interference of the sinusoidal excitation field, which couples into the receive coils. As the latter is several magnitudes higher, the extraction of the particle signal from the excitation signal is a challenging task. METHODS One way to remove the interfering signal is to suppress the excitation signal by means of a band-stop filter. However, this technique removes parts of the desired particle signal, which are essential for direct reconstruction of the particle concentration. A way to recover the entire particle signal is to cancel out the excitation signal by coupling a matching cancellation signal into the receive chain. However, the suppression rates that can be achieved by signal cancellation are not as high as with the filtering method, which limits the sensitivity of this method. In order to unite the advantages of both methods, in this work the authors propose to combine the filtering and the cancellation technique. All methods were compared by measuring 10 μl Resovist, in the same field generator only switching the signal processing parts. RESULTS The reconstructed time signals of the three methods, show the advantage of the proposed combination of filtering and cancellation. The method preserves the fundamental frequency and is able to detect the tracer signal at its full bandwidth even for low concentrations. CONCLUSIONS By recovering the full particle signal the SNR can be improved and errors in the x-space reconstruction are prevented. The authors show that the combined method provides this full particle signal and makes it possible to improve image quality.

Truncation artifacts in Magnetic Particle Imaging

Figure 1 clearly shows the effect of truncation artifacts when particles are not covered by the trajectory. With the removal of edge pixels these artifacts disappear. However, some information of the image is lost depending on the size of the necessary cut-off. With the multi-resolution approach the full information is used. Instead of removing pixels they are combined to large pixels. This method results in a better quality with small cut-offs compared to the non-compensated images.

A high power driving and selection field coil for an open MPI scanner

Three Litz wires have been used in parallel in order to decrease the layer to layer voltage down to 4 kV and the coil voltage down to 8 kV. A two millimeter thick epoxy plate has been glued between each layer, in order to prevent any electrical breakdown between the layers. A special epoxy has been used to minimize the inner temperature of the coil, which reached 180°C with a dissipated power of 5.2 kW. The use of parallel Litz wire leads to currents having different phase angle in every wire which has to be corrected.

Single-sided magnetic particle imaging: magnetic field and gradient

Magnetic Particle Imaging (MPI) has been presented by Gleich and Weizenecker in 2005. Since then, a number of innovations have been introduced by many di erent research groups. In 2009, for instance, Sattel et al. presented a novel single-sided MPI scanner geometry. The major advantage of this particular scanner geometry is the unlimited measurement eld. For the imaging process in MPI, super-paramagnetic iron oxide nanoparticles (SPIONs) are applied as tracer material. The tracer is excited by sinusoidally varying magnetic elds. In this contribution, simulated magnetic elds were evaluated based on the measured eld distribution of a single-sided scanner realization. It is of particular importance to know the quality of the gradient elds, since image resolution in MPI is directly linked to the gradient strength.

Enlarging the Field of View in Magnetic Particle Imaging – A Comparison

Magnetic Particle Imaging evolves rapidly and human scanners are conceivable, already. However, the growing scanner size and therefore the increasing data within the field of view give rise to several unsolved problems. The reconstruction process, solving an inverse problem with the measured signal and the system function, is a storage consuming procedure for high resolution 3D imaging. Additionally, the size of the field of view strongly depends on the used gradient field and field amplitudes. Due to technical as well as medical limitations, such as specific absorption rates and peripheral nerve stimulation, the conventional procedures will not be sufficient to image large regions of interest. This paper compares and discusses approaches enlarging the field of view that might be used to reduce the reconstruction process and/or enlarge the field of view despite limited technical properties.

Approximated elliptical coils in magnetic particle imaging

The paper presented results of simulated and measured magnetic fields of an approximated elliptical coil. The simulation values are coincided with the measurement values. Using the relative error as difference metric, the difference between simulation and measurement is presented as well. The visual comparison between the images as well as the resulting values of the relative error show only a minimal difference in the inner zone of the coils. The gradient values for the prototype coil arrangement for x, y and z direction were also shown. With an increasing side length the gradients in x and y direction decrease, while the gradient in z direction slightly increases.

Phantom simulation based on measured gradient fields of a single-sided MPI scanner

Figure 2 shows the reconstruction result using a simulated selection field and a simulated drive field. The resolution of the reconstruction decreases with the distance to the coil assembly of the single-sided scanner. This result will be used to verify later results.

Comparison of Open Scanner Designs for Interventional Magnetic Particle Imaging.

Magnetic Particle Imaging (MPI) allows for the quantitative determination of superparamagnetic iron oxide nanoparticles in vivo and offers exciting possibilities in functional, molecular and interventional imaging. In a simulation study, different open MPI scanner designs providing a varying extent of patient access (ranging from lateral to unrestricted) were assessed regarding their applicability to interventional MPI procedures. Criteria included power loss, size and shape of the field free point trajectory as well as spatial resolution. Each scanner design yields advantages and drawbacks, thus the decision for a certain scanner geometry depends on the individual circumstances and intended applications.