Magnetic particle Imaging: Principles and clinical application

In magnetic particle imaging (MPI), superparamagnetic iron oxide nanoparticles are used as tracer materials. The nanoparticles are subjected to a sinusoidal, oscillating magnetic field, i.e. the drive field, and respond with a nonlinear change in magnetization. As a result, the acquired induction signal contains multiples of the fundamental excitation frequency which are subsequently used for determination of the particle distribution and concentration. For spatial encoding, a selection field, i.e. a magnetic gradient field, is superimposed onto the drive field such that a field-free point (FFP) is produced at a desired location within the field of view. In a simplified picture, nanoparticles located near the FFP contribute to the signal generation, whereas particles that are far from the FFP are in saturation and cannot contribute. Image reconstruction from the induction signals can be seen as the solution for the corresponding inverse problem. Herein, the current state of the art in magnetic coil design for MPI is discussed. With a new symmetrical arrangement of coils, a field-free line can be produced that promises a significantly higher sensitivity compared with the standard arrangement for an FFP. Additionally, an alternative single-sided coil assembly for applications in a sentinel lymph node biopsy scenario is presented.