Marlitt Erbe

Prediction of the Spatial Resolution of Magnetic Particle Imaging Using the Modulation Transfer Function of the Imaging Process

The magnetic particle imaging method allows for the quantitative determination of spatial distributions of superparamagnetic nanoparticles in vivo. Recently, it was shown that the 1-D magnetic particle imaging process can be formulated as a convolution. Analyzing the width of the convolution kernel allows for predicting the spatial resolution of the method. However, this measure does not take into account the noise of the measured data. Furthermore, it does not consider a reconstruction step, which can increase the resolution beyond the width of the convolution kernel. In this paper, the spatial resolution of magnetic particle imaging is investigated by analyzing the modulation transfer function of the imaging process. An expression for the spatial resolution is derived, which includes the noise level and which is validated in simulations and experiments.

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 generation of a magnetic field‐free line

PURPOSE 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. METHODS 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. RESULTS 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. CONCLUSIONS 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.

Generation of a static magnetic field-free line using two Maxwell coil pairs

In this paper, the generation of a magnetic field-free line (FFL) along a constant direction is investigated. It is shown that an FFL gradient field can be generated by two perpendicular Maxwell coil pairs. These coils can be used for imaging distributions of superparamagnetic nanoparticles with high sensitivity applying the magnetic particle imaging technique. To this end, the FFL has to be rapidly moved back and forth while either the object or the coil assembly rotates slowly. The field quality of the proposed coil setup is assessed by manufacturing a prototype and measuring the magnetic field using a Hall-effect sensor.

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.

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

PURPOSE 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. METHODS 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. RESULTS 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. CONCLUSIONS 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.

Comparison of commercial iron oxide-based MRI contrast agents with synthesized high-performance MPI tracers.

Abstract Magnetic particle imaging (MPI) recently emerged as a new tomographic imaging method directly visualizing the amount and location of superparamagnetic iron oxide particles (SPIOs) with high spatial resolution. To fully exploit the imaging performance of MPI, specific requirements are demanded on the SPIOs. Most important, a sufficiently high number of detectable harmonics of the receive signal spectrum is required. In this study, an assessment of commercial iron oxide-based MRI contrast agents is carried out, and the result is compared with that of a new self-synthesized high-performance MPI tracer. The decay of the harmonics is measured with a magnetic particle spectrometer (MPS). For the self-synthesized carboxymethyldextran-coated SPIO, it can be demonstrated that despite a small iron core diameter, the particle performance is as good as in Resovist, the best-performing commercial SPIO today. However, the self-synthesized particles show the lowest iron concentration compared with Resovist, Sinerem, and Endorem. As the iron dose will be an important issue in human MPI, the synthesis technique and the separation chain for self-synthesis will be pursued for further improvements. In evaluations carried out with MPS, it can be shown in this work that the quality of the self-synthesized nanoparticles outperforms the three commercial tracer materials when the decay of harmonics is normalized by the iron concentration. The results of this work emphasize the importance of producing highly uniform and monodisperse superparamagnetic particles contributing to lower application of tracer concentration, better sensitivity, or a higher spatial resolution.

Magnetic particle imaging (MPI)

Magnetic particle imaging (MPI) displays the spatial distribution and concentration of superparamagnetic iron oxides (SPIOs). It is a quantitative, tomographic imaging method with high temporal and spatial resolution and allows work with high sensitivity yet without ionizing radiation. Thus, it may be a very promising tool for medical imaging. In this review, we describe the physical and technical basics and various concepts for clinical scanners. Furthermore, clinical applications such as cardiovascular imaging, interventional procedures, imaging and therapy of malignancies as well as molecular imaging are presented.

Simulation of the magnetization dynamics of diluted ferrofluids in medical applications

Abstract Ferrofluids, which are stable, colloidal suspensions of single-domain magnetic nanoparticles, have a large impact on medical technologies like magnetic particle imaging (MPI), magnetic resonance imaging (MRI) and hyperthermia. Here, computer simulations promise to improve our understanding of the versatile magnetization dynamics of diluted ferrofluids. A detailed algorithmic introduction into the simulation of diluted ferrofluids will be presented. The algorithm is based on Langevin equations and resolves the internal and the external rotation of the magnetic moment of the nanoparticles, i.e., the Néel and Brown diffusion. The derived set of stochastic differential equations are solved by a combination of an Euler and a Heun integrator and tested with respect to Boltzmann statistics.

Magnetic Particle Imaging

Magnetic particle imaging (MPI) is a young and very promising tomographic modality capable of imaging a distribution of biocompatible superparamagnetic iron oxide (SPIO) nanoparticles within a patient with a sub-millimeter resolution in three dimensions (3D) and real-time without the need of harmful, ionizing radiation [GW05, WGR+09]. The potential held by this revolutionary new tomographic method was already demonstrated in the first, impressive invivo MPI images of a beating mouse heart presented in [WGR+09]. No existing modality is capable of providing 3D real-time images of the inside of the human body. To classify MPI within established medical imaging modalities, the crucial properties of computed tomography (CT), magnetic resonance imaging (MRI) and positron emission tomography (PET) are compared to those of MPI in Fig. 2.1.