Magnetic Particle Imaging – Challenges and Promises of a New Modality

In this keynote, a review on recent results of a novel imaging modality based on magnetic nanoparticles is given. This imaging concept, called magnetic particle imaging (MPI), falls into the category of functional imaging and, hence, the magnetic nanoparticles may serve as tracers of metabolic processes. Since the particles of choice consist of superparamagnetic iron oxide (SPIO) cores coated with biopolymers, imaging of the metabolism may be possible without any radioactive agents. These particles are subjected to an oscillating magnetic field, a so-called drive field, and, subsequently, react with a nonlinear re-magnetization. This behavior is in accordance with Langevin's theory of paramagnetism and can be detected with appropriate receive coils. Due to the nonlinearity, the induced signal in the receive coils contains harmonics of the fundamental frequency of the drive field. These harmonics can be used to determine the nanoparticle concentration. For spatial encoding an additional magnetic gradient field, a so-called selection field, is superimposed onto the drive field such that a field-free point is established within the volume of interest. Only particles located at the field-free point contribute to the desired signal in the receive coils. Particles outside are saturated and do not further show any re-magnetization dynamics upon the excitation by the drive field. It has been shown by simulation and imaging that MPI is able to provide sub-millimeter resolution and, by simulation, a high sensitivity. Thereby, real-time applications are feasible, which has been demonstrated recently with a tracer applied in clinically accepted concentration. However, today, there are quite interesting challenges within the practical set-up of a scanning device and also in the design of new MPI nanoparticles.