Christina Debbeler

Magnetic particle imaging: current developments and future directions

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.

Imaging thermal expansion and retinal tissue changes during photocoagulation by high speed OCT

Visualizing retinal photocoagulation by real-time OCT measurements may considerably improve the understanding of thermally induced tissue changes and might enable a better reproducibility of the ocular laser treatment. High speed Doppler OCT with 860 frames per second imaged tissue changes in the fundus of enucleated porcine eyes during laser irradiation. Tissue motion, measured by Doppler OCT with nanometer resolution, was correlated with the temperature increase, which was measured non-invasively by optoacoustics. In enucleated eyes, the increase of the OCT signal near the retinal pigment epithelium (RPE) corresponded well to the macroscopically visible whitening of the tissue. At low irradiance, Doppler OCT revealed additionally a reversible thermal expansion of the retina. At higher irradiance additional movement due to irreversible tissue changes was observed. Measurements of the tissue expansion were also possible in vivo in a rabbit with submicrometer resolution when global tissue motion was compensated. Doppler OCT may be used for spatially resolved measurements of retinal temperature increases and thermally induced tissue changes. It can play an important role in understanding the mechanisms of photocoagulation and, eventually, lead to new strategies for retinal laser treatments.

Evaluation of a Cotton-Mouton relaxometer for the characterization of superparamagnetic iron oxide nanoparticles

When using superparamagnetic iron oxide nanoparticles (SPIONs) as contrast agents or tracers in biomedical applications, knowledge of the hydrodynamic diameter is crucial. The hydrodynamic diameter influences the circulation time of the particles in the blood cycle as well as the accessibility of the target structure. Common methods to determine the hydrodynamic diameter include magnetorelaxometry (MRX) or photon cross-correlation spectroscopy (PCCS). In this work, a combination of the Cotton-Mouton effect and the Brownian relaxation is used. It promises a fast and straightforward determination of the hydrodynamic diameter of SPIONs. Earlier publications already showed that the determination of the hydrodynamic diameter of SPIONs using a Cotton-Mouton relaxometer is possible. Subsequent, this work addresses the thorough investigation of the reliability of the setup. Studies show that sample temperature affects measurement results. Therefore, a calibration and temperature stabilization of the setup is mandatory. Additionally, the effect of other critical parameters as, for instance, the viscosity (which varies with temperature) or ambient light should be taken into consideration.

Concept of a rabbit-sized FFL-scanner

In the past years, different Magnetic Particle Imaging (MPI) scanners for small animals using a field free line (FFL) have been presented. In this work, a novel concept of a pre-clinical FFL MPI scanner which can accommodate rabbits is presented.

Magnetic Particle Imaging

Abstract Magnetic particle imaging (MPI) is a novel medical imaging technique, which uses magnetic fields to detect tracer material consisting of magnetic nanoparticles (MNPs). After a short motivation why medical imaging will benefit from this new technology, the basic physical principle of MPI and the development of tracer material will be described. As MPI makes specific demands on the MNPs, the theoretical and experimental analysis of MNPs for MPI will be investigated. Further, different scanner setups will be introduced, and possible image reconstruction methods will be explained. Finally, an outlook on different innovative applications will be given.

Magnetic particle imaging in vascular medicine

Abstract Magnetic particle imaging (MPI) is a new medical imaging technique that enables three-dimensional real-time imaging of a magnetic tracer material. Although it is not yet in clinical use, it is highly promising, especially for vascular and interventional imaging. The advantages of MPI are that no ionizing radiation is necessary, its high sensitivity enables the detection of very small amounts of the tracer material, and its high temporal resolution enables real-time imaging, which makes MPI suitable as an interventional imaging technique. As MPI is a tracer-based imaging technique, functional imaging is possible by attaching specific molecules to the tracer material. In the first part of this article, the basic principle of MPI will be explained and a short overview of the principles of the generation and spatial encoding of the tracer signal will be given. After this, the used tracer materials as well as their behavior in MPI will be introduced. A subsequent presentation of selected scanner topologies will show the current state of research and the limitations researchers are facing on the way from preclinical toward human-sized scanners. Furthermore, it will be briefly shown how to reconstruct an image from the tracer materials’ signal. In the last part, a variety of possible future clinical applications will be presented with an emphasis on vascular imaging, such as the use of MPI during cardiovascular interventions by visualizing the instruments. Investigations will be discussed, which show the feasibility to quantify the degree of stenosis and diagnose strokes and traumatic brain injuries as well as cerebral or gastrointestinal bleeding with MPI. As MPI is not only suitable for vascular medicine but also offers a broad range of other possible applications, a selection of those will be briefly presented at the end of the article.

Effect of key parameters on synthesis of superparamagnetic nanoparticles (SPIONs)

Abstract There are various methods to synthesize superparamagnetic nanoparticles (SPIONs) useful for MPI (magnetic particle imaging) and in therapy (Hypothermia) such as co-precipitation, hydrothermal reactions etc. In this research, the focus is to analyse the effects of crucial parameters such as effect of molecular mass of dextran and temperature of the co-precipitation. These parameters play a crucial role in the inherent magnetic properties of the resulting SPIONs. The amplitude spectrum and hysteresis curve of the SPIONs is analysed with MPS (magnetic particle spectrometer). PCCS (photon cross-correlation spectroscopy) measurements are done to analyse the size distribution of hydrodynamic diameter the resulting SPIONs.

Sensitivity study for an MPI FFL scanner

The sensitivity of a PM FFL MPI (permanent magnet field-free line magnetic particle imaging) scanner was investigated using a serial dilution. The linearity of the reconstructed signal with respect to the particle concentration has been verified. A lower detection limit of 15 μg iron in magnetite has been found.

A device for measuring the trajectory dependent magnetic particle performance for MPI

In ferrofluids, the magnetization undergoes magnetic relaxation processes, which are affected by the concentration of the fluid, the viscosity of the medium, the strength and frequencies of an external magnetic field and the structure of the magnetic core [1,2]. In many models the particles are assumed to have an uniaxial anisotropy that results in one preferred magnetization direction called the easy axis. If the particles are exposed to a magnetic field that is aligned with this easy axis, the corresponding signal response is higher compared to other excitation directions [3]. For a one dimensional excitation this alignment will be reached shortly if the particle is able to mechanically rotate and the hydrodynamic friction is low. In more dimensional excitations, such as in dynamic field free line (FFL) scanners, or in field free point (FFP) scanners, the excitation direction changes constantly [4]. If this change in direction exceeds the maximum mechanical rotation speed of the particles, they are not able to align. As a result, the particle signal will drop. In this work, we present a new device that is able to generate FFP and FFL field sequences while applying different possible offset fields.

Analyzing superparamagnetic iron oxide nanoparticles (spions) using electrical impedance spectroscopy

Conventional methods to evaluate the size of superparamagnetic iron oxide nanoparticles (SPIONs) and their coatings used in magnetic particle imaging (MPI) include photon cross-correlation spectroscopy (PCCS) [1], atomic force microscopy (AFM) [1] and transmission electron microscopy (TEM) [2]. There is however still a potential for improvement as they are expensive and only able to analyze small sample quantities. In this work, a new method using electrical impedance spectroscopy is evaluated. With this method, it is possible to analyze macroscopic samples at low costs.