Gary A. Ybarra

Active microwave imaging. I. 2-D forward and inverse scattering methods

Active microwave imaging (MWI) for the detection of breast tumors is an emerging technique to complement existing X-ray mammography. The potential advantages of MWI arise mainly from the high contrast of electrical properties between tumors and normal breast tissue. However, this high contrast also increases the difficulty of forming an accurate image because of increased multiple scattering. To address this issue, we develop fast forward methods based on the combination of the extended Born approximation, conjugate- and biconjugate-gradient methods, and the fast Fourier transform. We propose two nonlinear MWI algorithms to improve the resolution for the high-contrast media encountered in microwave breast-tumor detection. Numerical results show that our algorithms can accurately model and invert for the high-contrast media in breast tissue. The outcome of the inversion algorithms is a high-resolution digital image containing the physical properties of the tissue and potential tumors.

Active Microwave Imaging II: 3-D System Prototype and Image Reconstruction From Experimental Data

A 3-D microwave imaging system prototype and an inverse scattering algorithm are developed to demonstrate the feasibility of 3-D microwave imaging for medical applications such as breast cancer detection with measured data. In this experimental prototype, the transmitting and receiving antennas are placed in a rectangular tub containing a fluid. The microwave scattering data are acquired by mechanically scanning a single transmit antenna and a single receive antenna, thus avoiding the mutual coupling that occurs when an array is used. Careful design and construction of the system has yielded accurate measurements of scattered fields so that even the weak scattered signals at S21 = -90 dB (or 30 dB below the background fields) can be measured accurately. Measurements are performed in the frequency domain at several discrete frequencies. The collected 3-D experimental data in fluid are processed by a 3-D nonlinear inverse scattering algorithm to unravel the complicated multiple scattering effects and produce high-resolution 3-D digital images of the dielectric constant and conductivity of the imaging domain. Dielectric objects as small as 5 mm in size have been imaged effectively at 1.74 GHz.

Microwave breast imaging: 3-D forward scattering simulation

Active microwave imaging (MWI) is emerging as a promising technique for the detection of biomedical anomalies such as breast cancer because of the high electrical contrasts between malignant tumors and normal tissue. Previously, we have developed fast two-dimensional forward and inverse scattering algorithms for MWI systems. In this paper, we report the full three-dimensional (3-D) forward scattering simulation in order to account for 3-D effects and to provide a fast solver in future 3-D nonlinear inverse scattering methods. The 3-D fast forward method is based on the stabilized biconjugate-gradient fast Fourier transform (BCGS-FFT) algorithm. The method has been validated for various MWI measurement scenarios. Using this fast simulation method, we demonstrate the importance of accounting for 3-D effects in MWI, and we compare numerical results with the measurements from an experimental prototype.

Microwave Imaging in Layered Media: 3-D Image Reconstruction From Experimental Data

A prototype microwave imaging system for imaging 3-D targets in layered media is developed to validate the capability of microwave imaging with experimental data and with 3-D nonlinear inverse scattering algorithms. In this experimental prototype, the transmitting and receiving antennas are placed in a rectangular tub containing a fluid. Two plastic slabs are placed in parallel in the fluid to form a five-layer medium. The microwave scattering data are acquired by mechanically scanning a single transmitting antenna and a single receiving antenna, thus avoiding the mutual coupling that occurs when an array is used. The collected 3-D experimental data in the fluid are processed by full 3-D nonlinear inverse scattering algorithms to unravel the complicated multiple scattering effects and produce 3-D digital images of the dielectric constant and conductivity of the imaging domain. The image reconstruction is focused on the position and dimensions of the unknown scatterers. Different dielectric and metallic objects have been imaged effectively at 1.64 GHz.

Optimal signal processing of frequency-stepped CW radar data

An optimal signal processing algorithm is derived for estimating the time delay and amplitude of each scatterer reflection using a frequency-stepped CW system. The channel is assumed to be composed of abrupt changes in the reflection coefficient profile. The optimization technique is intended to maximize the target range resolution achievable from any set of frequency-stepped CW radar measurements made in such an environment. The algorithm is composed of an iterative two-step procedure. First, the amplitudes of the echoes are optimized by solving an overdetermined least squares set of equations. Then, a nonlinear objective function is scanned in an organized fashion to find its global minimum. The result is a set of echo strengths and time delay estimates. Although this paper addresses the specific problem of resolving the time delay between the first two echoes, the derivation is general in the number of echoes. Performance of the optimization approach is illustrated using measured data obtained from an HP-8510 network analyzer. It is demonstrated that the optimization approach offers a significant resolution enhancement over the standard processing approach that employs an IFFT. Degradation in the performance of the algorithm due to suboptimal model order selection and the effects of additive white Gaussian noise are addressed. >

Fundamentals of ECE: A Rigorous, Integrated Introduction to Electrical and Computer Engineering

The Electrical and Computer Engineering (ECE) Department at Duke University, Durham, NC, is undergoing extensive curriculum revisions that incorporate novel content, organization, and teaching methods. The cornerstone of the new curriculum is a theme-based introductory course, fundamentals of ECE. To introduce students to the major areas of ECE in their first year of study, this course is organized around three concepts: 1) how to interface with the physical world; 2) how to transmit energy and information; and 3) how to extract, interpret, and analyze information. To provide insight and motivation, the course is designed to introduce multiple areas of ECE, emphasizing how they are interrelated and how they contribute to the design and functioning of real-world applications. Also, the course must engage its students, many of whom are evaluating ECE as a prospective major and career. To achieve these goals, the course adopts a unifying theme, tightly couples lecture and laboratory exercises, and includes a laboratory experience that emphasizes design, integration, and real applications. The interactive classroom content and laboratory exercises are developed iteratively so that each course component supports the other, rather than one being dominant and driving the other. As the context focus of the laboratory, a robotic platform enables the exploration of a broad range of ECE concepts, both independently and integrated into an entire system. For their final design project, students form small groups, which in turn combine into larger teams, to create robots that work together to overcome realistic challenges. This paper describes the curricular objectives and key course elements that guide course development, the resulting content and structure of the course, and the assessment data that indicate successful achievement of the curricular goals.

Transcending the traditional: Using tablet PCs to enhance engineering and computer science instruction

Traditional instructional methods present many obstacles to effective teaching and learning in engineering and computer science courses. These include a reliance on text-based or static mediums to convey equation- and graphics-heavy concepts, a disconnect between theoretical lecture presentations and applied laboratory or homework exercises, and a difficulty in promoting collaborative activities that more accurately reflect an engineering approach to problem solving. Additionally, technical courses can suffer, like any other course, when students are not actively engaged in the learning and when instructors cannot gauge student understanding. This project has explored the utility of Tablet PCs for overcoming these challenges within a sample of courses in engineering and computer science. There were three primary questions: which knowledge domains benefit from the use of Tablet PCs; whether observed benefits are derived from Tablet PC-specific activities; and what problems limit the effectiveness of Tablet PCs in educational settings? The evaluation of assessment data using regression approaches demonstrated that Tablet-PC-specific activities had a consistent, meaningful, and positive impact upon engineering and computer science courses.

A tapered microstrip patch antenna array for use in breast cancer screening via 3D active microwave imaging

Given the measured performance of the antennas in the imaging array and the modeled scattered field data of small tumors within the human breast model, along with the known parameters associated with the other system components (e.g. losses through RF switching system and sensitivity of the measurement device), detection and screening of tumors with the clinical microwave imaging array that has been developed is certainly feasible. Since successful inversions of phantoms from previous experimental data with 3D imaging systems have already demonstrated the capability of the inversion algorithm developed at Duke, what remains is to finish implementing improvements in the hardware system (transition to MEMS based RF switching system) and to construct the Greens function from the completed forward model of the 3D prototype clinical system. From there, optimizations based on phantom imaging experiments will be performed, ultimately leading to clinical trials of the Duke 3D microwave breast imaging system.

A 3D EIT system for breast cancer imaging

Electrical impedance tomography (EIT) is a developing and promising imaging modality for early detection of breast cancer. An EIT system utilizes an array of electrodes to apply currents to an imaging domain and measures the resulting voltages on the periphery. The measurement results are then input to a reconstruction algorithm to produce an image of the impedance distribution inside the domain. In this work, a full 3D EIT system has been developed and the system design, measurement strategy and reconstruction algorithm are presented. Several sets of experimental data are collected and phantom tumor images are reconstructed from these data sets

Techtronics: hands-on exploration of technology in everyday life

Techtronics is an after-school science enrichment program partnering the Pratt School of Engineering at Duke University and Rogers-Herr Middle School in Durham, North Carolina. This program, funded through a three-year grant from the Burroughs Wellcome Fund, introduces students in grades 6-8 to four branches of engineering: civil, electrical and computer, biomedical, and mechanical through hands-on project building. Students design and build bridges, robots, heart monitors, and solar energy systems. A group of 20 middle school students is divided into five teams of four students. An engineering undergraduate crew leader is paired with each team as a project facilitator. An engineering graduate student coordinator leads the entire group. This paper presents a detailed description of the Techtronics program including student background, coordinator and crew leaders, project content, and the outcomes and assessment process.