Imaging magnetic nanoparticles using the signal's frequency spectrum

Abstract

Current methods of magnetic particle imaging generate a signal by cyclically saturating nanoparticles creating measurable harmonics in the induced magnetization. The sensitivity promises to be competitive with SPECT so molecular imaging is possible. The signal was localized by saturating the nanoparticles outside a voxel using a strong static magnetic field and sweeping the voxel across the sample to form an image. However, in applications where enough nanoparticles are present, signal can be detected at several higher harmonic frequencies and we show that the distribution of signal between those frequencies contributes localizing information. We tested one-dimensional implementations but the methods can be generalized to three dimensions. Spatial encoding was accomplished by using multiple drive frequencies that varied spatially. Two drive coils tuned to different drive frequencies and mounted on the same axis were used to explore the method. The response was measured from a single sample of iron oxide nanoparticles at eight positions along that axis to estimate response function for the reconstruction. Then two identical samples were placed at pairs of locations to test the method. The sample positions were reconstructed from the measured spectrum of the signal generated. The number of independent parameters is limited but four independent parameters can be achieved with very good conditioning and eight independent parameters can be achieved with reasonable conditioning.