Enhanced Detection of Magnetic Nanoparticles Using a Novel Microwave Ferromagnetic Resonance Imaging System.

Objective: An aluminium faceted chamber designed for 3D microwave imaging (MWI) of the breast has been integrated into an electromagnet in order to carry out signal acquisition experiments for an inverse scattering-based ferromagnetic resonance imaging (FRI) system, or magnetic contrast-enhanced MWI system. METHODS: For this proof of concept, the chamber has been equipped with four wire monopole antennas, and low-contrast oil-based targets have been tested with varying concentrations of iron oxide magnetic nanoparticles (MNP) to serve as ferromagnetic contrast agents. The electromagnet is capable of sustaining a static polarizing magnetic field (PMF) greater than 0.2 Tesla (2000 Gauss) across the imaging chamber to modulate the MNPs' ferromagnetic response, effectively changing the targets' magnetic permeability. Differential scattered field data are then collected through the application and withdrawal of this PMF. RESULTS: This study has successfully characterized a particular narrow band of frequencies within the asymmetric faceted chamber that demonstrate significant differential responses corresponding to the weak magnetic signal physically isolated from the MNPs, tested on different sizes and positions of targets containing various concentrations of MNPs. CONCLUSION: Similar to ferromagnetic resonance (FMR) spectroscopy, in which detection of FMR phenomena is best achieved at probing frequencies coinciding with the structural resonant frequency of a metallic cavity, these resonant frequencies of interest yield a high level of sensitivity to MNP permeability changes and are suitable for imaging within the chamber. SIGNIFICANCE: These represent the first experimental results of a full-scale FRI system capable of detecting and eventually imaging MNPs at biologically relevant concentrations.

Novel Stopping Criteria for Optimization-Based Microwave Breast Imaging Algorithms.

A discontinuous Galerkin formulation of the Contrast Source Inversion algorithm (DGM-CSI) for microwave breast imaging employing a frequency-cycling reconstruction technique has been modified here to include a set of automated stopping criteria that determine a suitable time to shift imaging frequencies and to globally terminate the reconstruction. Recent studies have explored the use of tissue-dependent geometrical mapping of the well-reconstructed real part to its imaginary part as initial guesses during consecutive frequency hops. This practice was shown to improve resulting 2D images of the dielectric properties of synthetic breast models, but a fixed number of iterations was used to halt DGM-CSI inversions arbitrarily. Herein, a new set of stopping conditions is introduced based on an intelligent statistical analysis of a window of past iterations of data error using the two-sample Kolmogorov-Smirnov (K-S) test. This non-parametric goodness-of-fit test establishes a pattern in the data error distribution, indicating an appropriate time to shift frequencies, or terminate the algorithm. The proposed stopping criteria are shown to improve the efficiency of DGM-CSI while yielding images of equivalent quality to assigning an often liberally overestimated number of iterations per reconstruction.

A wideband microwave tomography system with a novel frequency selection procedure.

In this paper, we describe a 2-D wideband microwave imaging system intended for biomedical imaging. The system is capable of collecting data from 3 to 6 GHz, with 24 coresident antenna elements connected to a vector network analyzer via a 2 x 24 port matrix switch. As one of the major sources of error in the data collection process is a result of the strongly coupling 24 coresident antennas, we provide a novel method to avoid the frequencies where the coupling is large enough to prevent successful imaging. Through the use of two different nonlinear reconstruction schemes, which are an enhanced version of the distorted born iterative method and the multiplicative regularized contrast source inversion method, we show imaging results from dielectric phantoms in free space. The early inversion results show that with the frequency selection procedure applied, the system is capable of quantitatively reconstructing dielectric objects, and show that the use of the wideband data improves the inversion results over single-frequency data.