Field Free Point Research Articles

Trajectory Analysis Using Static Patches for Magnetic Particle Imaging

Magnetic particle imaging (MPI) is an imaging technique based on the determination of magnetic material by moving a field-free point along specified trajectories, which are used to sample the field of view. Due to technical and safety reasons, the field of view is limited in size. To enlarge the size of the field of view, trajectory patches are used, which are sampled separately and combined consecutively to an entire field of view. The aim of this paper is to analyze the effect of different trajectories combined with the patch approach. In addition, an empiric study is performed to analyze the influence of overlapped patches on each trajectory combined with cutoff as postprocessing method. As a follow-up, a new patch formation of the radial trajectory based on a phase shift between each of the patches is introduced. Finally, it can be shown that the Lissajous trajectory, which is commonly used for MPI, provides appropriate results. However, the results of overlapped patches with a circular trajectory increase spatial resolution.

Electronic field free line rotation and relaxation deconvolution in magnetic particle imaging.

It has been shown that magnetic particle imaging (MPI), an imaging method suggested in 2005, is capable of measuring the spatial distribution of magnetic nanoparticles. Since the particles can be administered as biocompatible suspensions, this method promises to perform well as a tracer-based medical imaging technique. It is capable of generating real-time images, which will be useful in interventional procedures, without utilizing any harmful radiation. To obtain a signal from the administered superparamagnetic iron oxide (SPIO) particles, a sinusoidal changing external homogeneous magnetic field is applied. To achieve spatial encoding, a gradient field is superimposed. Conventional MPI works with a spatial encoding field that features a field free point (FFP). To increase sensitivity, an improved spatial encoding field, featuring a field free line (FFL) can be used. Previous FFL scanners, featuring a 1-D excitation, could demonstrate the feasibility of the FFL-based MPI imaging process. In this work, an FFL-based MPI scanner is presented that features a 2-D excitation field and, for the first time, an electronic rotation of the spatial encoding field. Furthermore, the role of relaxation effects in MPI is starting to move to the center of interest. Nevertheless, no reconstruction schemes presented thus far include a dynamical particle model for image reconstruction. A first application of a model that accounts for relaxation effects in the reconstruction of MPI images is presented here in the form of a simplified, but well performing strategy for signal deconvolution. The results demonstrate the high impact of relaxation deconvolution on the MPI imaging process.

Axially elongated field-free point data acquisition in magnetic particle imaging.

The magnetic particle imaging (MPI) technology is a new imaging technique featuring an excellent possibility to detect iron oxide based nanoparticle accumulations in vivo. The excitation of the particles and in turn the signal generation in MPI are achieved by using oscillating magnetic fields. In order to realize a spatial encoding, a field-free point (FFP) is steered through the field of view (FOV). Such a positioning of the FFP can thereby be achieved by mechanical or electromagnetical movement. Conventionally, the data acquisition path is either a planar 2-D or a 3-D FFP trajectory. Assuming human applications, the size of the FOV sampled by such trajectories is strongly limited by heating of the body and by nerve stimulations. In this work, a new approach acquiring MPI data based on the axial elongation of a 2-D FFP trajectory is proposed. It is shown that such an elongation can be used as a data acquisition path to significantly increase the acquisition speed, with negligible loss of spatial resolution.

On the effect of finite magnetic relaxation on the magnetic particle imaging performance of magnetic nanoparticles

Magnetic particle imaging is an emerging imaging technique where contrast is provided by the non-linear response of magnetic nanoparticle tracers to an applied magnetic field consisting of a scanned field free point. Commonly the magnetic response of the nanoparticles is assumed to be instantaneous, which is at odds with experimental evidence that finite relaxation affects both the signal and resolution provided by the tracers. Here, we apply rotational Brownian dynamics simulations to obtain insight into the effect of finite relaxation and of the conditions of the magnetic field on the performance of magnetic nanoparticle tracers used for magnetic particle imaging.

Efficient gradient field generation providing a multi-dimensional arbitrary shifted field-free point for magnetic particle imaging

Magnetic Particle Imaging (MPI) is a tomographic imaging modality capable to visualize tracers using magnetic fields. A high magnetic gradient strength is mandatory, to achieve a reasonable image quality. Therefore, a power optimization of the coil configuration is essential. In order to realize a multi-dimensional efficient gradient field generator, the following improvements compared to conventionally used Maxwell coil configurations are proposed: (i) curved rectangular coils, (ii) interleaved coils, and (iii) multi-layered coils. Combining these adaptions results in total power reduction of three orders of magnitude, which is an essential step for the feasibility of building full-body human MPI scanners.

Magnetic particle imaging with a planar frequency mixing magnetic detection scanner

We present the first experimental results of our planar-Frequency Mixing Magnetic Detection (p-FMMD) technique to obtain Magnetic Particles Imaging (MPI). The p-FMMD scanner consists of two magnetic measurement heads with intermediate space for the analysis of the sample. The magnetic signal originates from the nonlinear magnetization characteristics of superparamagnetic particles as in case of the usual MPI scanner. However, the detection principle is different. Standard MPI records the higher order harmonic response of particles at a field-free point or line. By contrast, FMMD records a sum-frequency component generated from both a high and a low frequency magnetic field incident on the magnetically nonlinear particles. As compared to conventional MPI scanner, there is no limit on the lateral dimensions of the sample; just the sample height is limited to 2 mm. In addition, the technique does not require a strong magnetic field or gradient because of the mixing of the two different frequencies. In this study, we acquired an 18 mm × 18 mm image of a string sample decorated with 100 nm diameter magnetic particles, using the p-FMMD technique. The results showed that it is feasible to use this novel MPI scanner for biological analysis and medical diagnostic purposes.

Traveling Wave Magnetic Particle Imaging

Most 3-D magnetic particle imaging (MPI) scanners currently use permanent magnets to create the strong gradient field required for high resolution MPI. However, using permanent magnets limits the field of view (FOV) due to the large amount of energy required to move the field free point (FFP) from the center of the scanner. To address this issue, an alternative approach called "Traveling Wave MPI" is here presented. This approach employs a novel gradient system, the dynamic linear gradient array, to cover a large FOV while dynamically creating a strong magnetic gradient. The proposed design also enables the use of a so-called line-scanning mode, which simplifies the FFP trajectory to a linear path through the 3-D volume. This results in simplified mathematics, which facilitates the image reconstruction.

Twenty-fold acceleration of 3D projection reconstruction MPI

We experimentally demonstrate a 20-fold improvement in acquisition time in projection reconstruction (PR) magnetic particle imaging (MPI) relative to the state-of-the-art PR MPI imaging results. We achieve this acceleration in our imaging system by introducing an additional Helmholtz electromagnet pair, which creates a slow shift (focus) field. Because of magnetostimulation limits in humans, we show that scan time with three-dimensional (3D) PR MPI is theoretically within the same order of magnitude as 3D MPI with a field free point; however, PR MPI has an order of magnitude signal-to-noise ratio gain.

Improved field free line magnetic particle imaging using saddle coils

Magnetic particle imaging (MPI) is a novel tracer-based imaging method detecting the distribution of superparamagnetic iron oxide (SPIO) nanoparticles in vivo in three dimensions and in real time. Conventionally, MPI uses the signal emitted by SPIO tracer material located at a field free point (FFP). To increase the sensitivity of MPI, however, an alternative encoding scheme collecting the particle signal along a field free line (FFL) was proposed. To provide the magnetic fields needed for line imaging in MPI, a very efficient scanner setup regarding electrical power consumption is needed. At the same time, the scanner needs to provide a high magnetic field homogeneity along the FFL as well as parallel to its alignment to prevent the appearance of artifacts, using efficient radon-based reconstruction methods arising for a line encoding scheme. This work presents a dynamic FFL scanner setup for MPI that outperforms all previously presented setups in electrical power consumption as well as magnetic field quality.

Control of the temperature rise in magnetic hyperthermia with use of an external static magnetic field

Our purpose in this study was to investigate the usefulness of a method for controlling the temperature rise in magnetic hyperthermia (MH) using an external static magnetic field (SMF), and to derive an empirical equation for describing the energy dissipation of magnetic nanoparticles (MNPs) in the presence of both the alternating magnetic field (AMF) and SMF through phantom experiments.We made a device that allows for MH in the presence of an SMF with a field-free point (FFP) using a Maxwell coil pair. We measured the temperature rise of MNPs under various conditions of AMF and SMF and various distances from the FFP (d), and calculated the specific absorption rate (SAR) from the initial slope of the temperature curve.The SAR values decreased with increasing strength of SMF (Hs) and d. The extent of their decrease with d increased with an increase of the gradient of SMF (Gs). The relationships between SAR and Hs and between SAR and d could be well approximated by Rosensweig's equation in which the amplitude of AMF (Hac) is replaced by Hac2/Hac2+Hs2, except for the case when Gs was small.In conclusion, the use of an external SMF with an FFP will be effective for controlling the temperature rise in MH in order to reduce the risk of heating surrounding healthy tissues, and our empirical equation will be useful for estimating SAR in the presence of both the AMF and SMF and for designing an effective local heating system for MH.

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.

Efficient Magnetic Gradient Field Generation With Arbitrary Axial Displacement for Magnetic Particle Imaging

The magnetic particle imaging method applies a magnetic gradient field featuring a field-free point (FFP) to determine the spatial distribution of superparamagnetic nanoparticles. Such a field is generated by two opposing electromagnetic coils, carrying currents in opposite directions. To achieve a large field of view, the FFP needs to be shifted in space. This can be realized by superimposing currents flowing in the same direction in both coils. For large displacements, the power loss increases drastically. In this letter, a new coil geometry is proposed, which is considerably more efficient for generating an FFP at off-center positions. It consists of four coils in which the currents are optimized to generate the required field at minimal power loss.

Experimental generation of an arbitrarily rotated field‐free line for the use in magnetic particle imaging

The concept of a magnetic field-free line (FFL), with regard to the novel tomographic modality magnetic particle imaging (MPI), was recently introduced. Theoretical approaches predict the improvement of sensitivity of MPI by a factor of ten replacing the conventionally used field-free point (FFP) by a FFL. In this work, an experimental apparatus for generating an arbitrarily rotated and translated FFL field is described and tested. A theoretical motivation for the implemented setup is provided and the required currents are derived in dependency of the coil sensitivities. A prototype of a FFL field generator is manufactured and the fields are measured using a Hall effect sensor. An evaluation of the generated fields is performed via comparison to simulated data. To utilize the FFL concept for MPI, the setup generating the fields needs to be feasible in praxis with respect to power loss. Furthermore, rotating and translating the FFL, while keeping the setup static in space, is a crucial aspect for conveying FFL imaging to clinical applications. The implemented setup copes with both of these challenges and allows for experimental generation as well as evaluation of the required fields. The generated fields agree to within 3.5% of model predictions. This work transfers the FFL concept from theoretical considerations to the implementation of an experimental setup generating the required fields. The high agreement of the measured fields with simulated data indicates the feasibility of magnetic field generation for the implementation of FFL imaging in MPI.

A Fourier slice theorem for magnetic particle imaging using a field-free line

The magnetic particle imaging method is capable of imaging the distribution of magnetic nanoparticles in vivo. The sensitivity of the method can be significantly improved using a signal encoding scheme, which applies a field-free line (FFL) instead of the commonly used field-free point (FFP). Recent efforts have considerably improved the efficiency of the coil topology used to generate the FFL. However, until now it has not been investigated how the reconstruction of the particle distribution can be efficiently carried out when using the FFL encoding scheme. In this work, it is shown that the induced signal is linked to the Radon transform of the particle distribution. Hence, a fast algorithm for the reconstruction of the particle distribution can be derived, which first transforms the induced signal into Radon space and second applies a common reconstruction algorithm to transform the Radon data into image space.

Efficient generation of a magnetic field‐free line

Signal encoding in magnetic particle imaging (MPI) is achieved by moving a field-free point (FFP) through the region of interest. One way to increase the sensitivity of the method is to scan the region of interest with a field-free line (FFL) instead of the FFP. Recently, the first feasible FFL coil setup was introduced. The purpose of this article is to improve the efficiency of the FFL coil geometry even further. In order to reduce the electrical power loss of the setup, an additional Maxwell coil pair is introduced that is tailored to generate the static part of the FFL field. Using the proposed coil assembly, the electrical power loss for the generation of a rotating FFL is considerably reduced compared to previously known coil setups. Furthermore, the quality of the generated FFL is significantly increased. The proposed coil assembly is almost as efficient as an equivalent FFP scanner. Furthermore, the assembly cannot only be used for FFL imaging but for FFP imaging as well. Hence, the findings of this article denote an important step toward the first practical implementation of the FFL coil geometry.

Trajectory analysis for magnetic particle imaging

Recently a new imaging technique called magnetic particle imaging was proposed. The method uses the nonlinear response of magnetic nanoparticles when a time varying magnetic field is applied. Spatial encoding is achieved by moving a field-free point through an object of interest while the field strength in the vicinity of the point is high. A resolution in the submillimeter range is provided even for fast data acquisition sequences. In this paper, a simulation study is performed on different trajectories moving the field-free point through the field of view. The purpose is to provide mandatory information for the design of a magnetic particle imaging scanner. Trajectories are compared with respect to density, speed and image quality when applied in data acquisition. Since simulation of the involved physics is a time demanding task, moreover, an efficient implementation is presented utilizing caching techniques.

SU‐EE‐A4‐05: An Alternative Spatial Encoding Method for Magnetic Nanoparticle Imaging

Purpose: Magnetic particle imaging, MPI, was introduced in 2005 and promises to be sufficiently sensitive to allow molecular imaging. We introduce and explore numerically an alternative method of encoding position in magnetic nanoparticle imaging. The original MPI method localized the nanoparticle signal at the 3rd harmonic frequency using a strong magnetic field to saturate the nanoparticles outside of the field free point (FFP). We present an alternative method of encoding the signal position in which the signal at the second harmonic is recorded so signal is produced along a magnetic field gradient. Method and Materials: The signal from 20nm magnetic nanoparticles was simulated with a Langevin model using iron oxide properties. Response matrices were calculated for a linear gradient across the sample and the condition number of the matrices was used as the metric for stability of the reconstruction. Results: The conditioning of the second harmonic is significantly better conditioned than that for the third harmonic. The condition number of the response matrix used to reconstruct the spatial distribution of the signal is nearly one for ideal cases. The size of the field gradient increases for increasing number of encoded pixels but need not be as large as that required to saturate the nanoparticles outside a single voxel. For a single size nanoparticle, the response function approximates a smoothed Haar wavelet function at the smallest scale and the best conditioning occurs when the scaling of the response functions approximates the scaling of a dyadic wavelet basis. Conclusion: A linear gradient can be used to encode nanoparticle position using the second or third harmonics. The signal at the second harmonic is significantly larger than at the third harmonic frequency and the conditioning is significantly better; the combination should allow an increase in sensitivity by a factor of from 2 to 8.