Tonny Rubaek

Computational Validation of a 3-D Microwave Imaging System for Breast-Cancer Screening

The microwave imaging system currently being developed at the Technical University of Denmark is described and its performance tested on simulated data. The system uses an iterative Newton-based imaging algorithm for reconstructing the images in conjunction with an efficient method-of-moments solution of the associated forward scattering problem. A cylindrical multistatic antenna setup with 32 horizontally oriented antennas is used for collecting the data. It has been found that formulating the imaging algorithm in terms of the logarithm of the amplitude and the unwrapped phase of the measured signals improves its performance when compared to the more commonly used complex phasor formulation. This improvement is illustrated by imaging a simulated hemispherical breast model using both formulations. In addition to this, the importance of using the correct position and orientation of the antennas in the measurement system is shown by imaging the same breast model using a measurement setup in which the antennas are vertically oriented.

Log transformation benefits parameter estimation in microwave tomographic imaging

Microwave tomographic imaging falls under a broad category of nonlinear parameter estimation methods when a Gauss-Newton iterative reconstruction technique is used. A fundamental requirement in using these approaches is evaluating the appropriateness of the regression model. While there have been numerous investigations of regularization techniques to improve overall image quality, few, if any, studies have explored the underlying statistical properties of the model itself. The ordinary least squares (OLS) approach is used most often, but there are other options such as the weighted least squares (WLS), maximum likelihood (ML), and maximum a posteriori (MAP) that may be more appropriate. In addition, a number of variance stabilizing transformations can be applied to make the inversion intrinsically more linear. In this paper, a statistical analysis is performed of the properties of the residual errors from the reconstructed images utilizing actual measured data and it is demonstrated that the OLS algorithm with a log transformation (OLSlog) is clearly advantageous relative to the more commonly used OLS approach by itself. In addition, several high contrast imaging experiments are performed, which demonstrate that different subsets of data are emphasized in each method and may contribute to the overall image quality differences.

Prototype of microwave imaging system for breast-cancer screening

Microwave imaging for breast-cancer detection has received the attention of a large number of research groups in the last decade. In this paper, the imaging system currently being developed at the Technical university of Denmark is presented. This includes a description of the antenna system, the microwave hardware, and the imaging algorithm.

Submillimeter wave antenna With slow wave feed line

Submillimeter wave radiation, which is also referred to as terahertz radiation, has not been extensively explored until recently due to a lack of reliable components and devices in this frequency range. Current advances in technology have made it possible to explore this portion of the electromagnetic spectrum, and to create innovative imaging and sensing techniques that hold enormous potential in biomedical, metrological and security applications. Considering that realization of submillimeter wave components and antennas is still heavily constrained by problems arising from technological limitations and the necessity of having extremely miniaturized circuit elements, the design process remains quite challenging. In this paper, a design of a submillimeter wave antenna fed by a slow wave structure is described. The antenna is useful in high-power THz applications because of its ability to directly radiate energy from a vane-type interaction structure of a vacuum electron device. The parameters of the antenna are optimized while taking the technological constraints into account.

A Contrast Source Inversion Algorithm Formulated Using the Log-Phase Formulation

The contrast source inversion (CSI) algorithm was introduced for microwave imaging in 1997 and has since proven to be one of the most successful algorithms for nonlinear microwave tomography. In the CSI algorithm, the nonlinear integral equation, which must be solved to extract the constitutive electromagnetic parameters of the object under test from the microwave measurements, is represented by two linear equations, known as the data and the object equations. In this paper, the data equation in the CSI algorithm is reformulated using the so-called log-phase formulation. In this formulation, the measured data is represented by the change in the logarithm of the amplitude and the change in the unwrapped phase. This formulation has previously been applied for nonlinear tomography within the framework of a Gauss-Newton based algorithm for detection of breast cancer. Here, significant improvements have been observed compared to the more commonly used real-imaginary formulation. The modified CSI algorithm is tested on both simulated data and on a measurement of a breast. It is shown that for imaging setups with large differences in the measured signals, the new formulation of the data equation significantly improves the performance of the CSI algorithm.

Three-dimensional microwave imaging for breast-cancer detection using the log-phase formulation

The log-phase formulation is applied for the reconstruction of images from a simulation of a three-dimensional imaging system. By using this formulation, a clear improvement in the quality of the reconstructed images is achieved compared to the case in which the usual complex phasor notation is employed.

A System for THz Imaging of Low-Contrast Targets Using the Born Approximation

A THz imaging system, operating at 346 GHz and tailored for implementation of an imaging algorithm based on the Born approximation, is presented. The imaging algorithm provides focusing by compensating for the antenna footprint. This allows for using a more simple antenna system without optical focusing. Several aspects of implementing an imaging algorithm based on the Born approximation in THz imaging are discussed and key system properties are highlighted. The performance of the imaging algorithm is verified by imaging two simple dielectric targets. The results indicate that this approach provides a qualitative indication of the distribution of contrast in the samples complex permittivity and is a potential complement to existing imaging techniques.

Including antenna models in microwave imaging for breast cancer screening

Microwave imaging is emerging as a tool for screening for breast cancer, but the lack of methods for including the characteristics of the antennas of the imaging systems in the imaging algorithms limits their performance. In this paper, a method for incorporating the full antenna characteristics, in terms of the transmission-matrix representation, in a frequency-domain imaging algorithm is presented. The algorithm is tested on a simulation of the physical-anomaly tomography (PAT) scanner imaging system developed at the Technical University of Denmark and is shown to have superior performance when compared to a simple algorithm in which the antennas are assumed to be ideal dipoles.

A 340 GHz CW non-linear imaging system

A CW sub-millimetre wave imaging system is presented. The system operates around 340GHz with a 6.5% relative bandwidth and uses a non-linear imaging algorithm.