An Optimized Target Field Method to Design a 3-D Selection-Field Coil for MPI System

Current selection-field coil-design methods typically use regular rectangular coils regardless of gradient linearity. This study aimed to improve the linearity and intensity of the selective field generated by four-pole coils and provide a predictable spatial resolution. The target field method is optimized and extended from single layer 2-D gradient coil design in magnetic resonance imaging (MRI) system to 3-D selection-field coil design in magnetic-particle imaging (MPI) system. The 3-D selection-field coil design was converted into multilayer 2-D coil design, and the current-density distribution of each layer was set to be the same. In this inverse method design, a specific field-free line along the vertical direction and different linear static gradients on the horizontal plane in the region of interest (ROI) were set first. The 2-D current-density solution was obtained using a Fourier series combined with the regularization approach. Stream function and 3-D printing were then utilized to convert this theoretical current density into a practical 3-D selection-field coil. A 3-D four-pole selection field coil for the MPI system was designed and constructed, and the results were presented.