The Effect of Relaxation on Magnetic Particle Imaging

Magnetic particle imaging (MPI) is a new tomographic method [1] based on the nonlinear response of superparamagnetic iron oxide (SPIO) nanoparticles. It has promise for fast imaging with certain advantages in resolution, sensitivity, contrast, and cost. 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 the 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. Only the FFP region yields a detectable signal. The average magnetization has been assumed to respond immediately to changes in the applied field [1-8]. 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.