First experimental evidence of the feasibility of multi-color magnetic particle imaging.

Magnetic particle imaging is a new approach to visualizing magnetic nanoparticles. It is capable of 3D real-time in vivo imaging of particles injected into the blood stream and is a candidate for medical imaging applications. To date, only one particle type has been imaged at a time, however, the ability to separate signals acquired simultaneously from different particle types or from particles in different environments would substantially increase the scope of the method. Different colors could be assigned to different signal sources to allow for visualization in a single image. Successful signal separation has been reported in spectroscopic experiments, but it was unclear how well separation would work in conjunction with spatial encoding in an imaging experiment. This work presents experimental evidence of the separability of signals from different particle types and aggregation states (fluid versus powder) using a 'multi-color' reconstruction approach. Several mechanisms are discussed that may form the basis for successful signal separation.

Quantitative "Hot Spot" Imaging of Transplanted Stem Cells using Superparamagnetic Tracers and Magnetic Particle Imaging (MPI).

Magnetic labeling of stem cells enables their non-invasive detection by magnetic resonance imaging (MRI). Practically, most MRI studies have been limited to visualization of local engraftment as other sources of endogenous hypointense contrast complicate the interpretation of systemic (whole body) cell distribution. In addition, MRI cell tracking is inherently non-quantitative in nature. We report here on the potential of magnetic particle imaging (MPI) as a novel tomographic technique for non-invasive hot spot imaging and quantification of stem cells using superparamagnetic iron oxide (SPIO) tracers. Neural and mesenchymal stem cells, representing small and larger cell bodies, were labeled with three different SPIO tracer formulations, including two preparations that have previously been used in clinical MRI cell tracking studies (Feridex® and Resovist®). Magnetic particle spectroscopy (MPS) measurements demonstrated a linear correlation between MPI signal and iron content, for both homogeneous solutions of free particles in solution and for internalized and aggregated particles in labeled cells over a wide range of concentrations. The overall MP signal ranged from 1×10-3 - 3×10-4 Am2/g Fe, which was equivalent to 2×10-14 - 1×10-15 Am2 per cell, indicating that cell numbers can be quantified with MPI analogous to the use of radiotracers in nuclear medicine or fluorine tracers in 19F MRI. When SPIO-labeled cells were transplanted in mouse brain, they could be readily detected by MPI at a detection threshold of about 5×104 cells, with MPI/MRI overlays showing an excellent agreement between the hypointense MRI areas and MPI hot spots. The calculated tissue MPI signal ratio for 100,000 vs. 50,000 implanted cells was 2.08. Hence, MPI has potential to be further developed for quantitative and easy-to-interpret, tracer-based non-invasive imaging of cells, preferably with MRI as an adjunct anatomical imaging modality.