Majid Ostadrahimi

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 × 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.

Analysis of Incident Field Modeling and Incident/Scattered Field Calibration Techniques in Microwave Tomography

Imaging with microwave tomography systems requires both the incident field within the imaging domain as well as calibration factors that convert the collected data to corresponding data in the numerical model used for inversion. The numerical model makes various simplifying assumptions, e.g., 2-D versus 3-D wave propagation, which the calibration coefficients are meant to take into account. For an air-based microwave tomography system, we study two types of calibration techniques-incident and scattered field calibration-combined with two different incident field models: a 2-D line-source and an incident field from full-wave 3-D simulation of the tomography system. Although the 2-D line-source approximation does not accurately model incident field in our system, the use of scattered field calibration with the 2-D line-source provides similar or better images to incident and scattered field calibration with an accurate incident field. Thus, if scattered field calibration is used, a simple (but inaccurate) incident field is acceptable for our microwave tomography system. While not strictly generalizable, we expect our methodology to be applicable to most other microwave tomography systems.

A Novel Microwave Tomography System Based on the Scattering Probe Technique

In this paper, we introduce a novel microwave tomography system, which utilizes 24 double-layered Vivaldi antennas, each of which is equipped with a diode-loaded printed-wire probe. By biasing the probes diodes, the impedance of the probe is modified, allowing an indirect measurement of the electric field at the probes locations. Each printed-wire probe is loaded with five equally spaced p-i-n diodes, in series. We show that electric field data collected in this way within the proposed tomography system can be used to reconstruct the dielectric properties of an object of interest. Reconstructions for various objects are shown. Although the results are still preliminary, sufficient experimentation has been done to delineate the advantages of such an indirect method of collecting scattered-field data for tomographic imaging purposes.

Enhancement of Gauss–Newton Inversion Method for Biological Tissue Imaging

The multiplicatively regularized Gauss-Newton inversion (GNI) algorithm is enhanced and utilized to obtain complex permittivity profiles of biological objects-of-interest. The microwave scattering data is acquired using a microwave tomography system comprised of 24 co-resident antennas immersed into a saltwater matching fluid. Two types of biological targets are imaged: ex vivo bovine legs and in vivo human forearms. Four different forms of the GNI algorithm are implemented: a blind inversion, a balanced inversion, a shape-and-location inversion, and a novel balanced shape-and-location inversion. The latter three techniques incorporate typical permittivity values of biological tissues as prior information to enhance the reconstructions. In those images obtained using the balanced shape-and-location reconstruction algorithm, the various parts of the tissue being imaged are clearly distinguishable. The reconstructed permittivity values in the bovine leg images agree with those obtained via direct measurement using a dielectric probe. The reconstructed images of the human forearms qualitatively agree with magnetic resonance imaging images, as well as with the expected dielectric values obtained from the literature.

Investigating a double layer Vivaldi antenna design for fixed array field measurement

Vivaldi antenna is widely known as a broadband antenna. In this paper, we investigate a modified Vivaldi antenna with improved cross polarisation working in the ultra-wideband (UWB) frequency range (3.1-10.6 GHz) to be used as multiple probes for microwave tomography system. Our study includes investigation of radiation characteristics of the antenna, antenna design steps, fabrication sensitivity effects on the antenna performance and proposing and implementing a twenty-four antenna element system for fast data acquisition, including a novel method for frequency selection in microwave tomography applications. We also studied the fidelity parameter of the antennas inside the twenty-four element setup. The mutual coupling of adjacent elements, in spite of close proximity, is less than -17dB and fidelity variations for the antennas located in front of transmitter are less than 10%.

Reflection Coefficient Measurement for House Flooring Materials at 57-64 GHz

In this paper, the reflection characteristics of the house flooring construction materials at MilliMeter-Wave (MMW) frequencies are studied. Since this investigation is performed for North American housing, the 57-64 GHz frequency band is considered. In this study, three common flooring materials: hardwood, vinyl and carpet (cut- and loop-pile) are chosen. The reflection characteristics of the aforementioned materials are also measured when the supporting materials, i.e. plywood and underpad, exist. A Continues Wave (CW) transmit-receive measurement setup with measuring possibility of entire range of 57-64 GHz is employed. The reflection coefficient for all available incident angle and transmission loss for face-to-face antenna direction are measured for the chosen flooring materials. The relative permittivity associated to these materials based on the Fresnels reflection formula are found. Moreover, the results reveal the frequency dependence of the flooring materials within 57-64 GHz.

A modified double layer Tapered Slot Antenna with improved cross polarization

A method for improving polarization purity of the Tapered Slot Antenna (TSA) is investigated. The conventional TSA antenna and the proposed new design are investigated. Simulation and measurement results show a significant improvement in the cross polarization level of the proposed geometry, within a wide frequency bandwidth.

A 3-D Dual-Polarized Near-Field Microwave Imaging System

A novel 3-D dual-polarized microwave imaging system based on the modulated scattering technique (MST) is presented. The system collects the magnitude and phase of the scattered field using 120 MST probes and 12 transmitter/collector antennas distributed around an object-of-interest in the near-field region. The 12 antennas form a middle circumferential layer while the printed MST probes are arranged on three circumferential layers including the middle layer. The antennas are linearly polarized double-layer Vivaldi antennas, each fixed inside its own cylindrical conducting cavity and slanted with respect to the vertical axis of the imaging chamber. The MST probes are etched on both sides of a thin substrate and loaded with five evenly distributed p-i-n diodes along their length. These are positioned vertically and horizontally so that the z- and φ-components of the electric field is measured. Field data are collected using MST, calibrated, and then inverted using a multiplicatively regularized finite-element contrast source inversion algorithm. The system performance is evaluated by collecting and inverting data from different 3-D targets.

A Multiprobe-Per-Collector Modulated Scatterer Technique for Microwave Tomography

Scattering probes with collector antennas can be utilized for microwave tomography (MWT) applications based on the modulated scatterer technique. Using this technique, we previously demonstrated a novel tomography system that utilizes a single printed-wire probe in front of each collector of a multicollector MWT system. Each collector is implemented as a multilayer Vivaldi antenna. In this letter, the number of collector antennas is reduced while maintaining the number of probes. This results in a nonuniform distribution of probes with respect to the collectors and requires special calibration techniques to infer the scattered-field at the probe location. The advantages of using such a configuration for MWT are investigated. Image reconstructions for a number of targets using data collected from this system are shown and compared to results obtained from data collected using a standard MWT system that uses only the Vivaldi antennas. It is shown that the new configuration successfully extracts useful data at the locations of the probes, resulting in good tomographic constructions.

A heterogeneous breast phantom for microwave breast imaging

Recently there has been significant attention given to imaging biological tissues using microwave radiation. In order to verify microwave imaging algorithms, realistic body models are needed to measure and simulate the penetration of microwave energy into the tissue and to reconstruct the image. We have created a phantom which has dielectric properties that are close to the properties of the real breast tissue. The phantom includes materials that accurately simulate the dielectric properties of skin, fat, gland and tumor tissues while providing good contrast of conductivity. The phantom is fabricated from materials that are widely available and is easy to make. In addition the elasticity of the materials enables the phantom to be shaped into two dimensional (2D) or three dimensional (3D) forms.