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.

A shielded planar array of active slot antennas is proposed for applications in the microwave imaging of the breast. The frequency band is from 3 GHz to 8 GHz, where reflection loss better than 10 dB and inter-element mutual coupling less than –20 dB are achieved. The center-to-center element spacing is 12× 12 mm. A low-noise amplifier (LNA) is integrated with each antenna element for use in a receiving configuration. A ground plane within the multi-layer design separates the antenna array from the electronics array while providing shielding from the back and improving the power coupling into the tissue. The proposed structure resolves an outstanding problem in the design of large shielded antenna arrays for tissue imaging, namely, the interconnects between the array elements and the electronic circuits, which are separated by multiple dielectric layers and a grounded plane. The critical development is the new low-reflection triple-wire vertical transition that carries the signal from the antenna layer to the electronics layer while traversing through multiple layers of different permittivities and through openings in the ground plane. The design is verified by simulations and measurements. The passive and active designs are compared, illustrating the full utilization of the LNA gain. The performance of the proposed active array is also examined in imaging experiments with a compressed-breast phantom.

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.

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.

A novel image-reconstruction method is proposed for the processing of data acquired at random spatial positions. The images are reconstructed and updated in real time concurrently with the measurements to produce an evolving image, the quality of which is continuously improving and converging as the number of data points increases with the stream of additional measurements. It is shown that the images converge to those obtained with data acquired on a uniformly sampled surface, where the sampling density satisfies the Nyquist limit. The image reconstruction employs a new formulation of the method of scattered power mapping (SPM), which first maps the data into a three-dimensional (3D) preliminary image of the target on a uniform spatial grid, followed by fast Fourier space image deconvolution that provides the high-quality 3D image.

We present a novel real-time image reconstruction method processing linear frequency modulated (LFM) signals. The method exploits the principles of the Fourier-space scattered power mapping (F-SPM). We show that F-SPM, originally developed for frequency-domain signals, can be easily modified to process time-domain data with the same reconstruction speed and image quality. To facilitate validation, we have developed an in-house time-domain radar simulator, which generates synthetic LFM data much faster than full-wave time-domain simulations, which are prohibitively slow. The new image-reconstruction method is validated through synthetic data generated by the radar simulator as well as experimental data acquired with off-the-shelf millimeter-wave (77 to 81 GHz) LFM radar. Comparisons in terms of reconstruction speed and accuracy are carried out with the method of microwave holography, which is deemed the fastest image-reconstruction method for LFM radar.