STimulated emission depletion (STED) magnetic particle imaging

Magnetic particle imaging (MPI) is an in vivo method for detecting magnetic nanoparticles for cell tracking and molecular target imaging. Recently developed human-sized MPI scanners allow a spatial resolution of only 5-10 mm. High-resolution MPI is required to precisely locate the magnetic nanoparticles in living organisms.

This study proposed a high-resolution imaging method by introducing the STimulated Emission Depletion (STED) fluorescence microscopy principle via a donut-shaped point spread function (PSF).

We searched for the donut focal spot by adding a direct current offset stimulation magnetic field in the direction of the excitation field and receiver coil in MPI. When the offset stimulation field is greater than the excitation field amplitude, a donut-shaped focal spot can be formed and the donut radius increases with the offset field amplitude. The formation of the donut focal spot was theoretically evaluated, and the center signal of the donut focal spot was depleted with a strict explanation. The point-source phantom was experimentally imaged on a field-free-point-based MPI scanner and exhibited a donut-shaped line profile under a large offset stimulation field. The phantom images were simulated on a field-free-line-based MPI scanner to generate donut-shaped PSFs and images.

By applying an optimized STED factor in the subtraction of the donut PSF from the regular Gaussian PSF, a much smaller PSF could be obtained for deconvolution-based image reconstruction. The reconstructed images exhibited sub-millimeter resolution, small root mean square error, high peak signal-to-noise ratio, and high structural similarity index.

The donut-shaped PSF obtained using the STED imaging method can be used to improve the MPI resolution by breaking the Langevin magnetization barrier.