Development of a novel MPI scanner based on a field free line

The purposes of the presented project are the design, realization, and investigation of a real-time capable Magnetic Particle Imaging (MPI) scanner with an innovative topology, where for the first time rotating permanent magnets are used for construction. Of great relevance are the minimized power loss and the achievable size of the field of view. Essential criteria for a clinically successful tomographic imaging device are a high spatial resolution, a high sensitivity, the real-time capability and a minimal risk for the patient. Considering a combination of these criteria, Magnetic Particle Imaging (MPI) is setting new standards. The modality is based on the visualization of nanoparticles with a core that consists of super-paramagnetic iron oxide (SPIO).The most important physical characteristics of the SPIOs are the non-linear magnetization curve and the saturation behavior. The non-linearity of the particle magnetization enables signal generation, by causing the particles to emit a non-sinusoidal electromagnetic signal, when subjected to a sinusoidally oscillating magnetic field (drive field). The higher harmonics of this signal can then be measured. An additional magnetic gradient field (selection field) is used for spatial encoding. This field is generated in a way that all particles outside a distinct point, where the resulting magnetic flux density vanishes (FFP: field-free point), are in magnetic saturation. This way, spatial encoding is achieved, since only particles inside the FFP generate a signal.The sensitivity of the technique can be greatly improved by using a field-free line (FFL) instead of an FFP. Sufficient information for a CT-like spatial reconstruction of the particle distribution is acquired, when the FFL is slowly rotated and quickly shifted over the field of view.So far, the realization of real-time capable FFL-scanners failed worldwide, due to the high power losses in the selection field generating electromagnetic coils. The key point for the proposed research project is the development of the first dynamical FFL scanner that generates the FFL-selection-field with permanent magnets. For the geometry of the magnets an innovative scanner topology will be utilized.The reduction of the number of coils will reduce the coupling of the individual signal paths, which typically leads to a reduction of the magnetic field quality and to a very high energy demand. Additionally, no coils, power sources and analogue filters are needed for the generation of the selection field. However, the main advantage will be the reduction of power losses to below 0.1 % of a comparable scanner with electromagnetic selection field coils. Additionally, due to the mechanical FFL rotation, the influence of the time dependency of the selection field on the imaging process can be analyzed.

Im Rahmen des Projektes wurde ein FFL‐MPI‐Scanner entworfen und realisiert, der eine Rotation der FFL durch eine mechanische Drehung der Magnete zur Erzeugung der FFL ermöglicht. Das Design erlaubt es, für eine 2D‐Bildgebung jeweils lediglich eine Anregungs‐ und Empfangsspule zu verwenden und reduziert so die Komplexität des MPI‐Systems. Zur Erreichung der Ziel wurden zunächst umfangreiche Simulationen durchgeführt, auf Basis derer die einzelnen Komponenten des Scanners konstruiert wurden. Anschließend wurden diese in ein Gesamtsystem integriert. Anhand verschiedener Bildgebungsexperimenten konnte die Funktionalität des Systems gezeigt werden. Die erreichbare örtliche Auflösung liegt bei etwa 1 mm bei gleichzeitiger Echtzeitbildgebung.