Magnetic Particle Imaging (MPI) is a new medical imaging modality which relies on the visualization of magnetic nanoparticles used as a tracer material, through application of oscillating magnetic fields for detection and field gradients for localization. Magnetic tracer characteristics define the achievable image quality. The goal of this study has been to improve the characterization and understanding of magnetic nanoparticles for use in MPI, and to relate particle characteristics to image quality parameters like sensitivity and resolution. For this purpose, Resovist (a commercially available MRI contrast agent) has been studied. Two important parameters are size and relaxation times of particles. Particle size has been analysed using fractionation techniques, dynamic light scattering (DLS), and vibrating sample magnetometry (VSM). Fractionation experiments indicated a bimodal distribution of particle sizes in Resovist. The distribution of core sizes was determined by fitting a bimodal distribution to the VSM magnetization curve of Resovist, yielding two log-normal peaks with ??1?=8±1 nm, ??1?=1.9±0.1, ??2?=25±1 nm, ??2?=1.2±0.1, a volume ratio between the peaks of 1:0.4±0.1 (small to large), and ????=(3.5±0.4)x105 A·m-1. Comparison with DLS measurements provided rough estimates for the polymer shell size of 10-30 nm. A magneto-optical susceptometer has been developed to study characteristic relaxation times of particles. A magneto-optical susceptibility measurement of Resovist showed relatively good agreement with a magneto-electrical susceptibility measurement. The susceptibility curves needed to be explained by Néel relaxation times. Anisotropy values of either ????=(1.9±0.3)x10-5 J·m-2 or ????=(4.4±1.1)x103 J·m-3 were determined from the susceptibility measurements, which can be explained by surface anisotropy or magnetostatic shape anisotropy. It was not possible to determine the relevant type of anisotropy using fractions of Resovist. A model has been proposed to describe the magnetic behaviour of nanoparticles assuming a quasi-fixed orientation of particles. Simulations of spectral components of the magnetization of particles were in good agreement with measured responses of Resovist using Magnetic Particle Spectroscopy (MPS). The proposed model indicates that the optimal size of particles in Resovist is 24-28 nm for MPI applications. A tool has been developed to perform image simulations using a simple model to describe Resovist particles. MPI detection levels for Resovist were estimated to be of the order of 0.5 nmol (Fe) with a resolution of 2 mm, and 20 pmol (Fe) with a resolution of 10 mm, in combination with a temporal resolution of 30 s. With optimal particles only, sensitivities should be achievable of 50 pmol (Fe) with a resolution of 1.0 mm, and 2 pmol (Fe) with a resolution of 10 mm.
|Date of Award||31 Aug 2009|
|Supervisor||D.E. Markov (External coach), H.M.B. Boeve (External coach) & Leo J. van IJzendoorn (Supervisor 1)|