Relaxation-based color magnetic particle imaging for viscosity mapping

Magnetic particle imaging (MPI) uses superparamagnetic iron oxide (SPIO) nanoparticles as biomedical imaging tracers. The potential applications of MPI have recently been broadened by the introduction of "color" MPI techniques that can distinguish different nanoparticles and/or environments, e.g., by exploiting the relaxation behavior of SPIOs. One of the important applications of color MPI techniques is viscosity mapping. In this work, we show relaxation-based color MPI experiments that can distinguish the biologically relevant viscosity range of up to 5 mPa s. To find the optimal drive field parameters for viscosity, we compare color MPI results at three different frequencies. We show that frequencies around 10 kHz are well-suited for viscosity mapping using the multicore cluster Nanomag-MIP nanoparticles, providing a one-to-one mapping between the estimated relaxation time constant and viscosity.