Small magnetic devices have been steered in arbitrary direction and with variable force using a preclinical demonstrator system for magnetic particle imaging (MPI). Fast localization due to the high imaging rate of over 40 volumes/s and strong forces due to the high field gradient of more than 1 T/m render an MPI system, a good platform for image-guided steering of magnetic devices. In this paper, these capabilities are demonstrated in phantom experiments, where a closed feedback loop has been realized to exert translational forces in horizontal and vertical direction on a magnetic device moving in a viscous medium. The MPI system allows for the controlled application of those forces by combining variable homogeneous fields with strong field gradients.
In Magnetic Particle Imaging (MPI), the data acquisition can be performed by steering a field-free point (FFP) through the field of view (FOV). When using drive fields to achieve the FFP movement, a fast spatial encoding of the FOV is enabled. Due to safety limits, in terms of patient heating and nerve stimulation, the amplitude of the applied currents needs to be small, which results in a small FOV. In order to enlarge the FOV, focus fields featuring a high amplitude and a low frequency can be used. In this contribution, an approach based on a linear focus field shift, which enlarges the FOV in axial direction, is proposed. Such a shift results in an elongation of a planar two-dimensional trajectory that features a controlled sub-sampling of the FOV.I Results show that a focus field based trajectory elongation in axial direction is a suitable data acquisition for MPI. In addition to this, the use of self-intersection points as trajectory specific axial distance measure has been proposed.
In recent years, a thermotherapy based on magnetic fluid nanomaterials can be used not only to generate heat, but also to deliver and release drugs in the process of treatment, even if the tissue tumors is deep. Hence, the unique approach of magnetic fluid nanomaterials hyperthermia is becoming a promising treatment. However, the spread of nanomaterials to normal tissues around the tumor is inevitable. That is to say, it is impossible that the normal tissue round tumor is injured. Therefore, accurately controlling the heating boundary is a crucial factor. Several methods have been proposed to distinguish the normal tissue from the tumor in the process of thermotherapy. Results show that the algorithm based on PSO is capable of calculating of a zero static magnetic field strength in the process of thermotherapy.
In conventional MPI scanners the magnetic nano particle concentration distribution is acquired by moving a field free point (FFP) through the FOV [1]. This method allows the sampling of a signal from a small point-like area per time. To gain a higher SNR, it was suggested to increase the simultaneously acquired volume by introducing the principal of a field free line (FFL) to MPI [2]. We propose a new field generator geometry which is capable of creating an FFL with a simpler coil geometry than the existing approaches.
Magnetic nanoparticles (NPs) are increasingly important in many biomedical applications, such as drug delivery, hyperthermia, and magnetic resonance imaging (MRI) contrast enhancement. To build the most effective magnetic nanoparticle systems for various biomedical applications, characteristics of particle, including size, surface chemistry, magnetic properties, and toxicity have to be fully investigated. In this work, comparison of some magnetic multicomponent nanoparticles for bio-medical applications is discussed. In this investigation, multi-component ferrite nanoparticles were prepared by the hydrothermal synthesis, sol-gel, and solid state methods. In addition, x-ray powder diffractometry (XRD), scanning electron microscopy (SEM), and Quantum Design Physical Properties System (PPMS) were used to characterize the structural, morphological and magnetic properties of the nanoparticles. The size and crystal structure of the nanoparticles were characterized by using XRD results. The magnetic properties of the samples were performed for each sample at ± 1.5 T by PPMS.
Magnetic particle imaging (MPI), as introduced by Gleich and Weizenecker, is based on utilizing the nonlinear magnetization response M for detection of superparamagnetic iron oxide nanoparticles (MNP). The excited M contains not only the fundamental excitation frequency ω0 but also its harmonics when applying an ac excitation magnetic field. To improve the detection sensitivity for the magnetization M of MNP and the imaging technique based on the detection of a second harmonic response, a 2D imaging system consisting of four magnetic fields, dc bias field Hdc, ac modulation field Hac, gradient field of Z-axis and gradient field of X-axis have been constructed. The advantage of the detection of a second harmonic response is that no large ac modulation field Hac is required. This study demonstrated a MNP detection method using a second harmonic of magnetic response. The position of the 10-microlitre MNP sample was clearly indicated in 2D imaging.
In magnetic particle imaging (MPI) [1], image artifacts and blurring appear on a reconstructed image such that magnetization signals generated from magnetic nanoparticles (MNPs), which exist at the boundary of the field free point, are also detected. In order to overcome these problems, we propose a new reconstruction method using neural networks [2]. This proposed method can estimate MNP distribution based on a data set of input (system-functions) and desired-output (MNP-location) pairs used as the teaching data for a neural network. By employing neural networks, we expect to suppress image blurring by learning a sufficient number of data sets to indicate a relationship between image blurring and the corresponding MNP location. We perform numerical experiments to confirm the effectiveness of this method.
First magnetic field measurements look promising and show excellent agreement with simulations. Power loss optimization realizes FFL imaging at moderate cooling effort. The gain of sensitivity is directly going to improve image quality. The next step is the complete scanner assembling.
Previous work has explored the effects of nanoparticle relaxation from the system matrix and x-space reconstruction points of view [1-8]. Here we demonstrate how relaxation can be used to “colorize” an image based on relaxation mechanisms using x-space reconstruction [9] and a pixel-wise linear model.
Real-time MPI [1] has the potential to serve as a noninvasive alternative to X-ray angiography. In order to achieve real time MPI, we must (a.) generate vector-drive field waveforms at a location governed in real time by the physician, (b.) acquire the MPI image data in real time, (c.) reconstruct the MPI images in real time. We have designed our MPI data acquisition and control (DAQ) to enable all these steps in real time.
An alternative encoding scheme using a field free line (FFL) promises a ten-fold higher sensitivity and faster reconstruction algorithms for magnetic particle imaging (MPI) [1]. Yet, different scanner designs for two-dimensional imaging have been introduced [2,3]. This paper introduces a simulation study that combines both approaches which relies on permanent magnets and includes a rotational device as proposed in [4]. Therefore, this work uses a rotationally symmetric design as a basis and suggests a potential assembly. This assembly is realized and used initially to capture one-dimensional images which are shown below.
The noninvasive nanoscale thermometer with high measurement speed will become a very promising research. The speed of the temperature estimation using the model of AC magnetization spectrum is limited by the frequency of AC applied magnetic field [1-3]. You may need MHz magnetic field to get a faster temperature measuring or even higher frequency, and it would require a huge power to support the magnetic field and maintain the strength. So it is very difficult to guarantee the stability of the magnetic field. Instead of using AC magnetic field, DC magnetic field has a wider room for improvement of measurement speed.
Magnetic particle imaging (MPI) is a new medical imaging technique capable of recovering the distribution of superparamagnetic particles from their measured induced signals [1]. In literature there are two main MPI reconstruction techniques: measurement-based (MB) and x-space (XS). In the first approach the unknown magnetic particles concentration is reconstructed in the harmonic-space using a System Function (SF), describing the relation between particle positions and the signal response [2, 3]. The second approach requires the knowledge of the field free point (FFP) exact position and velocity at all time steps during the scanning process [4, 5]. The x-space method is based on the assumption of ideal magnetic field shapes used for spatial encoding (selection field), and for signal excitation (focus-drive field). The realization of human size devices with an open geometry requires specific calibration procedures related to the methods used in the reconstruction phase. One of the advantages of open bore scanners would be an easier open access to the patient, especially in interventional scenarios with simultaneous and real-time scanning processes. In this case of geometry configurations with larger FOV, the exact velocity gridding for x-space MPI could be difficult to achieve during the whole scanning process. Hence, our proposal is an innovative technique named hybrid x-space (HXS) resulting from the combination of the measurement-based and the classical x-space approach, reducing and optimizing the calibration time by a compressive sensing technique using circulant matrices.
Effects of number of magnet in Halbach magnet system for producing homogeneous magnetic field It has been demonstrated that the field homogeneity of the Halbach magnet is high enough to be used in various versions of portable devices such as NMR relaxometer, and MRI. This study discussed effects of the number of magnets used in the Halbach magnet system, and compared their initial simulation outcomes. Two kinds of magnets settlements were modeled and investigated within a multidimensional framework. The simulation outcomes show that the settlements of permanent magnets play important role on the magnetic flux density and homogeneity at the center of the magnet design. The maximum magnetic flux density was obtained at the centre, as expected.
Magnetic Particle Imaging (MPI) has gained a variety of potential applications since its first introduction in 2005 [1]. It offers safe tracers with applications in angiography, stem cell tracking, and cancer imaging [2]. The response of superparamagnetic iron oxide (SPIO) nanoparticles strongly affects the quality of the MPI images, which is a unique and not-before-seen feature for a medical imaging system [3]. In theory, as the diameter of the nanoparticle increases, the resolution also increases [1-4]. In reality, however, larger nanoparticles start to lag behind the rapidly changing drive field [5-6]. This effect, called “relaxation”, causes the resolution and the signal-to-noise ratio (SNR) of the MPI images to degrade. Therefore, characterization of the nanoparticle response has an essential role in determining and ultimately improving the quality of the MPI systems. Previously, measurements of the point spread function and relaxation effects were performed using an MPI relaxometer with a gradiometer receive coil, where two solenoids were wound in series [7]. In this work, two different coil topologies are compared: one with a two-section gradiometer design as in [7], and the other with a three-section gradiometer design. We analyze the robustness and efficiency of each setup in terms of the homogeneity of the drive field, the sensitivity profile of the receiver coil, the mutual inductance of the two coils, and power requirements.
The drive field suppression and the particle signal amplification are key components in Magnetic Particle Imaging (MPI) [1]. However, for high signal-to-noise ratio (SNR) the noise contribution of the receive chain has to be minimized as well. Main noise sources are the receive coil, the coils in the filter stage and the semiconductor elements in the pre-amplifier. The voltage induced by the drive field is about six orders of magnitude higher then the particle signal [2], and must be suppressed by at least 80 dB to keep the pre-amplifier in its linear region. Usually this problem is adressed by applying a notch filter [1] to remove the drive field signal as well as the particle signal at the excitation frequency prior to amplifying the signal. However, this removes the strongest particle signal as well, and therefore induces problems to the reconstruction which have to be addressed, e.g. DC drifts in x-space MPI [3].
Magnetic Particle Spectroscopy (MPS) is a powerful tool for the characterization of MPI tracers in terms of their harmonic signal. However, there is no established procedure to derive important imaging parameters from MPS spectra, such as the achievable spatial resolution or the minimum iron concentration in a voxel. In order to provide this information for a specific tracer material, acquiring MPS spectra at different offset fields is suggested. This paper shows that a MPS device, equipped with an offset field generator, can easily be extended from 0D spectroscopy to 1D imaging via sequential measurements using different offset fields for every position in the field of view. Doing so, MPS can be used to compare tracers concerning their 1D imaging characteristics.
Cancer has become one of the leading causes of death among all disease in the past decades [1]. Thermotherapy (also called hyperthermia) is a new type of cancer treatment in which body tissue is exposed to high temperatures, kills cancer cells, and without significant side-effects. It is a promising approach after surgery, radiotherapy, and chemotherapy [2]. There are several methods to produce heat for cancer treatment, such as microwave, ultrasound, radio-frequency, laser, and electromagnetic [3][4][5]. In recent years, a thermotherapy based on magnetic fluid nanomaterials can be used not only to generate heat, but also to deliver and release drugs in the process of treatment [6]. Hence, magnetic fluid nanomaterials hyperthermia is becoming a promising treatment approach attribute to its unique advantage. According to theory of Brownian relaxation, Neel relaxation, and the experimental result in [7], the specific absorption rate (SAR) is reduced drastically with the increasing of static magnetic field strength which is perpendicular to the alternating field.
Magnetic particle imaging (MPI) is a quantitative imaging technique that allows to determine the spatial distribution of magnetic nanoparticles. In the present work, a model-based system matrix was generated and applied for the first time to the reconstruction of experimental 1D MPI data. The building elements of the simulated system matrix consist of properly altered Chebyshev polynomials. This special structure closely resembles the experimentally measured system matrix and is a main key towards an efficient and memory saving reconstruction in MPI.
A young tomographic imaging method is growing up: since the first publication of magnetic particle imaging (MPI) different hardware designs and scanner concepts have been shown, furthermore multiple studies on optimizing magnetic tracer materials have been published. The next step on the way to the clinical routine is the preclinical testing phase. Being a radiation-free imaging method MPI provides a very fast and sensitive scanner concept similar to PET or SPECT, but also relies on the anatomical background information obtained by MRI or CT. Functionalizing magnetic tracer materials might offer new insights into the pathogenesis of diseases and complex immune processes. In this paper, first results of pre-clinical tests are presented to investigate the suitability of MPI for disease models. Localizing antibody-labeled T cells in the organism using targeted SPIONs demonstrates the suitability of the MPI method to visualize immune processes in a murine GVHD model. With these results the way is paved for investigating further disease models.
