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Range migration (or range focusing) techniques are widely used in optical, acoustic, and microwave real-time image reconstruction methods. They have been successfully applied to far-field 3-D imaging where they rely on plane-wave assumptions, which ignore the data amplitude variation over the acquisition aperture. The accuracy of the plane-wave assumption, however, quickly degrades in close-range imaging, where amplitude variations are significant and where the range to the target is on the order of the range sampling step. Here, we present a range-focusing method of improved accuracy, which is applicable to both far-zone and close-range monostatic radar. It refocuses the system point-spread function (PSF) to any range location, taking into account both magnitude and phase changes. The approach can be applied with any Fourier-based imaging algorithm utilizing the Lippmann–Schwinger equation as the underlying scattering model. Here, it is validated through examples based on simulated and measured data where the images are reconstructed with quantitative microwave holography (QMH). QMH employs measured PSFs to achieve quantitative imaging in real-time. The proposed range-migration method is applicable with measured PSFs, too, leading to reduced system-calibration effort and the ability to focus an image at any desired range. [ABSTRACT FROM AUTHOR]

The Fourier-space scattered-power mapping (F-SPM) is proposed as a computationally efficient alternative to the original real-space SPM method for real-time quantitative image reconstruction with an emphasis on close-range and near-field applications. Similar to SPM, F-SPM can employ either the Born or the Rytov data approximations in a linear scattering model, resulting in two distinct algorithms. However, to exploit the complementarity of the two approximations, a strategy is proposed for their combined use in a single inversion process. The combined Born–Rytov F-SPM method consistently improves the image quality in comparison with the images generated when using either approximation separately. The improvement is most significant when the limitations of either the Born or the Rytov approximations are violated. In the cases where neither or both of these limitations are violated, the images are of comparable quality to those generated by the standalone algorithms. The proposed Born–Rytov F-SPM algorithm is verified and compared to the standalone Born-based F-SPM and Rytov-based F-SPM in examples utilizing simulated and measured data. The gain in computational speed compared to the original real-space SPM is also demonstrated. [ABSTRACT FROM AUTHOR]

When I received Francesca’s invitation to contribute to the “Women in Engineering” column in IEEE Antennas and Propagation Magazine, my first thought was that I was not cut out for nontechnical writing. Inspiration and excitement come easy when you report “to the world” that you found a solution to an important problem. But talking about my professional experience…well, that, I am afraid, could be somewhat boring. [ABSTRACT FROM AUTHOR]

General theory of holographic inversion with linear frequency modulation (GHI-LFM) has been proposed recently, providing the framework for further advances in the holographic methods for image reconstruction with millimeter-wave linear frequency modulation (LFM) radar. Here, accelerated holographic imaging (AHI)-LFM, an AHI method for LFM radar that exploits range stacking, is proposed. The method is developed and validated for the purpose of whole-body security screening. The image quality and the computational efficiency are compared with the original GHI-LFM algorithm. It is demonstrated that the AHI-LFM algorithm improves the reconstruction speed by at least an order of magnitude while preserving the same image quality for the image sizes arising in practical whole-body security screening. [ABSTRACT FROM AUTHOR]

Microwave and millimeter-wave measurements acquire total-field responses from measurements, yet imaging algorithms instead require the data in the form of scattered-field responses. Two approaches exist for the extraction of the scattered-field data from the total-field responses, namely, the Born and the Rytov data approximations. It is well known that, depending on the target’s size, contrast, and structural complexity, one approximation can achieve an improved accuracy over the other. Yet, the Rytov approximation is rarely used in microwave and millimeter-wave imaging, likely due to phase-unwrapping problems occurring in the case of strongly heterogeneous electrically large targets. Here, we propose a method to utilize the Born and the Rytov approximations simultaneously in a single inversion step for real-time imaging with quantitative microwave holography (QMH). We show through examples with simulated and experimental data that in near-field imaging scenarios, including the imaging of a breast-tissue phantom, there are significant benefits in employing the new method. [ABSTRACT FROM AUTHOR]

We present a general theory of the holographic image reconstruction with linear frequency modulation (LFM) radars. For the first time, the system limitations in terms of the object extent and distance are derived and explicitly related to the LFM radar frequency-modulation slope $\gamma $. The holographic inversion formula is improved to account for the spherical spread of the scattered wave. The theory and the generalized holographic inversion algorithm are validated by synthetic benchmark data as well as experimental data from an in-house LFM-radar prototype operating at 29.9-GHz central frequency and bandwidth of 5.8 GHz. Experiments confirm that the lateral spatial resolution is about 5 mm. For optimal performance, the system is calibrated using a simple but effective calibration approach based on a measurement with a metallic plate. Experiments, with a volunteer carrying metallic and nonmetallic objects, demonstrate very good performance in realistic scenarios. [ABSTRACT FROM AUTHOR]

An ultrawideband (UWB) active slot antenna for tissue sensing arrays is developed and measured to operate from 3 to 8 GHz. The sensing element reported here is to be used in the imaging of the breast. It integrates a low-noise amplifier (LNA) with a printed-slot antenna to achieve gain enhancement of about 20 dB. The first prototype integrates the biasing of the LNA on the same board as the antenna. The second prototype uses an external bias tee providing dc power to the LNA through the coaxial connector of the active antenna. Both prototypes achieve the expected gain enhancement in comparison with the passive antenna element. [ABSTRACT FROM AUTHOR]

A new direct-inversion method, scattered-power mapping (SPM), for fast quantitative microwave imaging is introduced. It builds on a recently proposed inversion strategy that demonstrated quantitative direct reconstruction with experimentally acquired system point-spread functions. In comparison with this initial work, SPM features a drastic improvement in the computational time along with reduced number of calibration measurements. Moreover, SPM is versatile allowing a forward model of scattering cast either as a linearized Born model or as a Rytov model. SPM is intended as a tool to solve weak-scattering problems or as a linear-inversion module within nonlinear iterative reconstruction. [ABSTRACT FROM AUTHOR]

We propose a wideband theory for adjoint variable sensitivity analysis of problems with non-dispersive anisotropic materials. The method is developed based on the transmission line modeling (TLM) technique. The anisotropic material property can be the full tensors of permittivity, permeability, electrical conductivity, magnetic resistivity, magnetoelectric coupling, or electromagnetic coupling. The tensors are assumed constant and may contain non-diagonal elements. Our algorithm estimates the gradient of the desired response with respect to all the designable parameters using at most one extra simulation, regardless of the number of parameters. The theory has been implemented in an algorithm for 2-D and 3-D structures. Our estimated sensitivities match well the computationally expensive central finite difference approximations. [ABSTRACT FROM PUBLISHER]

A rigorous frequency-domain forward model of scattering is derived that relates any desired complex-valued scalar response to the vectors of the electromagnetic field. The model is valid in any reciprocal medium. The S-parameter forward model is derived as a special case. The forward model for the open-circuit voltage at the terminals of a receiving antenna is also given. The proposed model is useful in microwave imaging since it eliminates the need to employ inadequate approximations of: 1) Green’s dyadic function and 2) the relation between the field in the antenna vicinity and its response. As a validation, the magnitude and phase distributions of fields are mapped using a small scattering probe in reflection-coefficient measurements. The maps contain both the magnitude and the phase distributions. The application of the proposed model in imaging is also demonstrated. [ABSTRACT FROM PUBLISHER]