Magnetic Particle Imaging (MPI) is a new medical imaging modality that is currently still under development. The first preclinical systems are already being sold commercially and used for animal experiments. However, it is not yet possible to scale the system up to human size at an economically justifiable cost. In contrast to much better-known imaging techniques such as magnetic resonance imaging (MRI), the MPI only measures the signals of the tracer used and reconstructs them into an image. The advantage here is that blood vessels or tumours, for example, to which the magnetic nanoparticles used as tracer material can bind via functionalisation, can be directly differentiated from bone or tissue. Due to the fast signal acquisition of several tens of volumes per second, functional relationships (beating hearts, thrombi, etc.) can also be displayed and analysed in real time. The use of magnetic signal sources also sets MPI apart from nuclear imaging procedures, in which cell-damaging radionuclides are administered as a contrast agent, the signal of which is additionally attenuated by the surrounding tissue compared to magnetic signal sources, making in-depth information more difficult to access.
MPI requires at least two magnetic field configurations that are used to assign the magnetic signals of the nanoparticles to their spatial distribution in the body: the selection field and the excitation field. The selection field, which in the simplest case is implemented as a static field, can be generated using electromagnets or neodymium magnets. The excitation field is realised with the help of current-carrying coils.
The selection field is used to select a specific area in the region to be analysed (field of view, FOV). This is done using a gradient field, which creates a field-free region in a specific area (e.g. a field-free point [FFP] or a field-free line [FFL]). In this field-free region, the particles can in principle react to further magnetic stimuli as desired. Outside the field-free region, the magnetic moments of the particles are virtually aligned and the magnetic moment of each individual particle points firmly in the direction of the gradient.
If the selection field is present, only a small part of the administered tracer can provide a magnetic response signal to a magnetic stimulus. As a rule, an extremely pure harmonic sinusoidal excitation field is used for the stimulus, which periodically drives the magnetisation of the particles within the field-free region into their saturation range. In this scenario, the harmonic response of the particles in the field-free region is similar to that of the signal generated in the MPS.
If the field-free region is now moved across the area to be analysed, a location-dependent harmonic response of the magnetic nanoparticles is obtained and the amplitude of the received signal can be used to infer the respective location-dependent concentration distribution of the magnetic nanoparticles.
A simultaneous shift of the field-free region and excitation of the tracer within it leads to a very fast variant of imaging with magnetic nanoparticles. The shift of the field-free region occurs instantaneously due to the superposition of the external excitation field, as the gradient field experiences a superposition of the field strength by the excitation field at every point in time at every location. As a result, different particles are excited at different locations depending on the time. An intelligent choice of different frequencies of the excitations in different spatial directions results in a shift
