Transverse MNP Signal-Based Isotropic Imaging for Magnetic Particle Imaging

Magnetic particle imaging (MPI) is a tracer-based imaging modality known for its high sensitivity and temporal resolution. It makes the quantitative visualization of the concentration and spatial location of magnetic nanoparticles (MNPs) possible. The $x$ -space method is widely used in MPI for high-speed image reconstruction. In this method, the received signal is mapped directly onto the corresponding spatial position. However, the anisotropic 2-D point spread function (PSF) in the $x$ -space method introduces artifacts and blur in the reconstructed images. In this study, an isotropic MPI method, transverse MNP signal-based isotropic imaging (TSI), was developed. It makes isotropic imaging possible in 2-D unidirectional Cartesian MPI. Unlike conventional methods that rely on MNP signals registered from the same direction as the excitation, TSI only utilizes MNP signals received perpendicular to the excitation direction. In TSI, the velocity of the time domain signal is first normalized to obtain a "raw-image." The raw-image is then integrated, differentiated, and added to generate an isotropic MPI image. Because all the operations in TSI are analytical, it preserves the real-time imaging characteristics of the $x$ -space while isotropic imaging is performed. The PSF of TSI was calculated through analytical derivation and the resolution isotropy was validated through simulation and imaging experiments on an in-house-built MPI system. The simulation and experimental results demonstrate that, by improving the isotropy, TSI achieves an approximately 2.5- $\times $ increase in resolution in the nonexcitation direction compared with the conventional $x$ -space method. The proposed TSI algorithm does not increase hardware complexity and requires only half the scanning time of the previously proposed multichannel method. In addition, the transverse MNP signal used in TSI is geometrically decoupled from the excitation magnetic field, eliminating the need for a special compensation design for the receiving coils. This shows promise for future MPI designs with flexible surface coils.