Magnetic particle imaging (MPI) is a novel imaging method that was first proposed by Gleich and Weizenecker in 2005. Applying static and dynamic magnetic fields, MPI exploits the unique characteristics of superparamagnetic iron oxide nanoparticles (SPIONs). The SPIONs’ response allows a three-dimensional visualization of their distribution in space with a superb contrast, a very high temporal and good spatial resolution. Essentially, it is the SPIONs’ superparamagnetic characteristics, the fact that they are magnetically saturable, and the harmonic composition of the SPIONs’ response that make MPI possible at all. As SPIONs are the essential element of MPI, the development of customized nanoparticles is pursued with the greatest effort by many groups. Their objective is the creation of a SPION or a conglomerate of particles that will feature a much higher MPI performance than nanoparticles currently available commercially. A particle’s MPI performance and suitability is characterized by parameters such as the strength of its MPI signal, its biocompatibility, or its pharmacokinetics. Some of the most important adjuster bolts to tune them are the particles’ iron core and hydrodynamic diameter, their anisotropy, the composition of the particles’ suspension, and their coating. As a three-dimensional, real-time imaging modality that is free of ionizing radiation, MPI appears ideally suited for applications such as vascular imaging and interventions as well as cellular and targeted imaging. A number of different theories and technical approaches on the way to the actual implementation of the basic concept of MPI have been seen in the last few years. Research groups around the world are working on different scanner geometries, from closed bore systems to single-sided scanners, and use reconstruction methods that are either based on actual calibration measurements or on theoretical models. This review aims at giving an overview of current developments and future directions in MPI about a decade after its first appearance.

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In 2005, B. Gleich and J. Weizenecker initially presented the promising new tracer based medical imaging modality Magnetic Particle Imaging (MPI). It uses the nonlinear magnetization behavior of particles consisting of an iron oxide core coated with dextrane (super paramagnetic iron oxide nanoparticles, SPIONs). MPI has the potential to perform real-time imaging in the sub millimeter-range without the use of harmful radiation. To acquire a particle signal from the tracer, an alternating homogenous magnetic field (drive field) is applied. Due to the nonlinearity of the particle magnetization, the magnetic field is distorted and higher harmonics are generated that indicate a particle concentration within the field of view (FOV). To gain the spatial distribution, another magnetic field that exhibits a high gradient is applied simultaneously. This second field is called selection field. Basically, there are two different types of selection fields. The first approach uses a constant magnetic gradient field containing a field-free point (FFP). Only SPIONs within the close vicinity of the field-free point contribute to the particle signal because all other particles within the FOV are magnetically saturated. As the FFP is moved by the drive field through the FOV a spatial distribution can be obtained. The second approach uses a rotatable field-free line (FFL) that is additionally translated by the drive field to obtain one dimensional projections for various angles. The advantage of the FFL approach is a higher signal quality at equal size and gradient strength while a drawback is a higher power consumption and a more complex coil topology to generate, rotate and translate the FFL. In this work, the currently world's largest FFL MPI Scanner is investigated. Single components of the generated magnetic field are measured precisely to accomplish an accurate simulation of a translating and rotating fieldfree line.

Abstract:In 2005, B. Gleich and J. Weizenecker initially presented the tracer based medical imaging modality Magnetic Particle Imaging (MPI). It uses the nonlinear magnetization behavior of super paramagnetic iron oxide nanoparticles (SPIONs). MPI has the potential to perform real-time imaging in the sub millimeter-range without the use of harmful radiation. To acquire a particle signal from the tracer, an alternating homogenous magnetic field (drive field) is applied. Due to the nonlinearity of the particle magnetization, the magnetic field is distorted and higher harmonics are generated that indicate a particle concentration within the field of view (FOV). For the spatial distribution, another magnetic field that exhibits a high gradient (selection field) is applied simultaneously. Basically, there are two different types of selection fields containing either a fieldfree point (FFP) or a field-free line (FFL). Because of magnetic saturation, only SPIONs within the close vicinity of the FFP or FFL contribute to the particle signal. As the FFP is moved by the drive field through the FOV a spatial distribution of the SPIONs can be obtained. In the other encoding concept, the FFL rotates and is additionally translated by the drive field to obtain one dimensional projections for various angles. In this work, the currently world's largest FFL MPI Scanner is investigated. Single components of the generated magnetic field are measured precisely to accomplish an accurate simulation of a translating and rotating FFL.