Contrast Agents: The Effect of Relaxation on Magnetic Particle Imaging

Introduction: Magnetic particle imaging (MPI) is a new tomographic method [1] based on the nonlinear response of super-paramagnetic iron oxide (SPIO) nanoparticles. It has promise for fast imaging in the submillimeter range with some advantages in sensitivity, contrast, and cost in comparison with MRI. A static but spatially inhomogeneous field (selection field) and homogeneous oscillating field (drive field) are applied for spatial encoding. The selection field has a very strong gradient in order to saturate nanoparticle domains outside the field-free-point (FFP). The oscillating drive fields can move the FFP around the whole field of view by using different driving frequencies in different directions. The FFP region yields the detectable signal. The average magnetization has been assumed to respond immediately to changes in the applied field. However, delays due to magnetization relaxation lead to limitations on the response time and it is the purpose of the present paper to augment previous simulations [2, 3] by taking into account relaxation time effects.