A Field Cancellation Signal Extraction Method for Magnetic Particle Imaging

Nowadays, magnetic particle imaging (MPI) signal detection and excitation happens at the same time. This concept, however, leads to a strong coupling of the drive (excitation) field (DF) with the receive chain. As the induced DF signal is several orders of magnitude higher, special measures have to be taken to suppress this signal portion within the receive signal to keep the required dynamic range of the subsequent analog-to-digital conversion in a technically feasible range. For frequency space MPI, high-order bandstop-filters have been successfully used to remove the DF signals, which unfortunately removes the fundamental harmonic components of the signal of the magnetic nanoparticles (MNP) as well. According to the Langevin theory, the fundamental harmonic component has a large signal contribution and is important for direct reconstruction of the particle concentration. To separate the fundamental harmonic component of the MNP from the induced DF signal, different concepts have been proposed using signal cancellation based on additional DF signals, also in combination with additional filtering. In this paper, we propose a field cancellation (FC) concept in which a receive coil (RC) consists of a series connection of a primary coil in combination with an additional cancellation coil. The geometry of the primary coil was chosen to be sensitive for the MNP signal while the cancellation coil was chosen to minimize the overall inductive coupling of the FC-RC with the DF. Sensitivity plots and mutual coupling coefficients were calculated using a thin-wire approximation. A prototype FC-RC was manufactured and the effectiveness of the reduction of the mutual inductive coupling (d) was tested in an existing mouse MPI scanner. The difference between the simulations (d_s = 70 dB) and the measurements (d_m = 55 dB) indicated the feasibility as well as the need for further investigations.