X-space MPI relaxometry: methods and initial data

Abstract

Magnetic Particle Imaging (MPI) is a new imaging modality promising high sensitivity and high-resolution imaging of ultra-small superparamagnetic iron oxide (USPIO) nanoparticles.1 A new mathematical theory for MPI based on x-space was recently developed that indicates the spatial resolution of MPI improves with the cube of the USPIO iron core diameter.2 A system that can accurately measure the USPIO point spread function and relaxation time constants would enable MPI researchers to decouple magnetic particle development and imaging system development. This system would enable magnetic nanoparticle manufacturers and MPI researchers to measure the intrinsic spatial resolution of the USPIOs to be used in the imaging system without an imager. Therefore, we have developed a magnetic nanoparticle relaxometer that estimates nanoparticle diameter and relaxation constant using a modified form of the x-space theory of MPI. Fitting of the measured signal to the theoretical signal uses nonnegative least squares with an optimal Tikhonov regularization fitting scheme. The technique estimates magnetic nanoparticle diameter, relaxation time constants from the nanoparticle signal. Our measurements have excellent sensitivity and change little with independent, repeated measurements. While more experiments are necessary, our data lends the first experimental evidence to support the cubic dependence of spatial resolution on magnetic nanoparticle diameter.