High-Frequency Neel Relaxation Response for Submillimeter Magnetic Particle Imaging Under Low Field Gradient

Modulating the relaxation response of magnetic nanoparticles at two distinctive frequencies can be a potential scenario to increase the spatial resolution of magnetic particle imaging (MPI), which is beneficial for biomedical research applications. Comparatively, standard MPI requires a high field gradient to acquire a sharp point-spread function to enable submillimeter imaging of magnetic nanotracers distributed within cells or small-animal models. However, this strategy may face issues in sample handling and signal processing, while decreasing the field gradient reduces the spatial resolution and causes a blurry image. To improve the MPI image quality, we introduce a high-frequency excitation field to temporally encode the Lissajous field-free-point trajectory coupled with a low-frequency magnetization response under field gradient. Unlike regular MPI, our method further decodes the moment relaxation response at narrow side-bands around the excitation frequency to obtain the spatial coordinates of magnetic nanoparticles within a field of view, instead of the magnetization harmonics induced by the driving field. Therefore, the field-free-point steering frequencies and the resulting trajectory density are critical parameters to adjust the image resolution. Furthermore, a high signal-to-noise ratio can be achieved by use of a 1-MHz excitation field even with a low amplitude. We also demonstrate two-dimensional image reconstruction of a 1-mm ring-shaped solid Resovist (R) sample with 0.05-mm thickness, as well as other circular and rod phantoms, placed in a 1.4 x 1.4 mm(2) field of view under 2 T/m symmetrical field gradient on the x-y plane and 4 T/m on the z axis. Although the spatial resolution achieved appears low to differentiate two-neighboring circular phantoms of dense liquid samples, phase shifting can be further used to resolve image distortion due to the low-frequency relaxation effects.