MRI meets MPI: Towards Bimodal Scanners

Magnetic Particle Imaging (MPI) is a non-invasive imaging modality that can directly detect superparamagnetic nanoparticles. It has firstly been published 10 years back in nature in 2005 [1]. Magnetic nanoparticles are being detected by their non-linear magnetization response to externally applied fields. The tracers employed are the same kind of nanoparticles used in MRI as contrast agents but MPI promises by far more sensitive detection thresholds and does not suffer from ambiguity due to the indirect detection done by MRI. Furthermore, MPI offers a high temporal resolution on the order of milliseconds along with a sub-millimeter spatial resolution. On the other hand, MPI cannot acquire any (anatomical) background information and therefore, like other tracer-based techniques such as Positron Emission Tomography (PET), has to rely on additional imaging techniques for this purpose.

Current MPI scanner types can be grouped in three basic categories: the originally proposed setup using strong permanent or resistive magnets to generate a main gradient field with a field-free point (FFP) and additional resistive magnets for moving it. Secondly, instead of a FFP an entire field-free line (FFL) can be employed for projection imaging [2]. Lastly, a setup using an array of resistive magnets to generate a traveling wave with a moving FFP has been proposed [3].

First in-vivo measurements were presented in 2009 using the original MPI scanner and co-registering the MPI data with anatomical information obtained by MRI [4]. In this study the 3D-flow of tracers through the murine heart could be observed at sub-millimeter resolution and about 20 ms temporal resolution. The first results using a MRI-MPI hybrid scanner followed in 2014 demonstrating the successful combination of low field MRI with traveling wave MPI using a shared receive coil [5]. Here, a phantom containing tracer material was in-situ examined with both modalities and the resulting images were overlaid without the need for an elaborate co-registration based on the known system parameters for both scanners.

Current development of combined scanners aims at even higher integration of MRI and MPI using more shared components for the generation of homogeneous magnetic fields required in MRI and strong gradient fields required in MPI [6][7]. Along with the research on whole body MPI scanners this will enable MPI to become an alternative to current nuclear medical modalities.