MPI scanner an their image reconstruction: Using specifically designed magnetic fields, MPI scanners scan their field of view (FOV) within a short acquisition time of several milliseconds. MPI scanners are able to make magnetic material inside their field of view (FOV) visible. In contrast to MRI of CT tomographic devices, MPI is a tracer-based technology, which means, that only magnetic material is visible. It is similar to PET scanners, which are exclusively sensitive to radioactive substances.
As MPI tracer, superparamagnetic iron oxide nanoparticles (SPIONs) with a diameter of about 25 nm are used. The basic idea behind the signal generation in MPI is based on the nonlinear magnetization response to magnetic fields. A SPION with a size of about 25 nm comes with a specific feature: it can be seen as a tiny little magnet (it is energetically useful to generate a single Weiss area within the particle). A given ensemble of SPIONs (about 10^12 particles or more) show no magnetization in the absense of an external magnetic field since their directions are statistcally distributed. Exposing the SPIONs to an increasing external magnetic field causes an increase of the SPION magnetization since all particles start to align along the magnetic field lines. When all particles are aligned, no more magnetization can be reached (saturation magnetization). This behavior between external magnetic field and SPION magnetization can be described by the Lagevin function.
However, MPI scanners are using specific timy-varying magnetic fields to selectivly drive the SPIONs inside the FOV into saturation to encode their distribution step-by-step.
This ability of MPI scanners can be investigated by their system functions, which describe the encoding patterns. In the shown case of an MPI scanner after Gleich&Weizenecker, these system functions follow a checkerboard pattern with different square sizes and numbers. The combination of multiple patterns generate the final image.
A system matrix can be seen as a transfer function, which describe how to convert a acquired signal within an MPI scanner to a final image. The system matrix consists of multiple system functions.
With the MFS software, the encoding ability of MPI scanners can be investigated in a fast way, which allows the initial proof of novel approaches and ideas.