Two real-time reconstruction algorithms, quantitative microwave holography and scattered-power mapping, have been shown to be successful in the imaging of compressed tissue of relatively small thicknesses such as 1 cm and 2 cm. In both cases, planar data acquisition of frequency-swept transmission coefficients has been employed. Despite the fact that these algorithms are based on a linear forward model of scattering, they have been capable of providing quantitative estimates of the tissue permittivity due to the experimentally derived kernel of the scattering integral. Here, we demonstrate similar performance with a thicker (about 5 cm) compressed-breast phantom. This thickness is greater or comparable to the median thickness employed in mammography, depending on the view (craniocaudal or mediolateral oblique). The two methods are described in a common mathematical framework for the first time. The importance of the system calibration and the choice of a host medium are discussed through experiments. A new method for focusing onto suspect regions is demonstrated. The limitations of real-time imaging are highlighted along with an outlook to improving the image resolution and suppressing artifacts without sacrificing the reconstruction speed.

A picosecond pulse generator is designed to generate a stable differentiated Gaussian (monocycle) waveform. The design approach to increasing the center frequency, bandwidth and peak-to-peak voltage, as compared to previously reported ultra-wideband (UWB) generators, is described. The 280 ps wide pulse achieves a 1:10 fractional bandwidth (FBW) ratio extending from 500 MHz to beyond 5 GHz at the -10 dB level. A measurement procedure is proposed for evaluating the jitter and noise performance of UWB pulse generators, and it is applied to the fabricated prototype. The procedure exploits jitter and noise definitions from high-speed digital electronics, which are adapted here for the jitter and noise evaluation of UWB pulse generators at microwave frequencies. The problems in obtaining the absolute and relative jitter of a UWB generator are discussed along with proposed solutions. The impact of the input trigger on the pulse stability is demonstrated through the dramatic improvement achieved by the integration of a jitter cleaner in the generator’s circuit.

A parallelized field-programmable gate array (FPGA) architecture is proposed to realize an ultra-fast, compact, and low-cost dual-channel ultra-wideband (UWB) pulsed-radar system. This approach resolves the main shortcoming of current FPGA-based radars, namely their low processing throughput, which leads to a significant loss of data provided by the radar receiver. The architecture is integrated with an in-house UWB pulsed radar operating at a sampling rate of 20 gigasamples per second (GSa/s). It is demonstrated that the FPGA data-processing speed matches that of the radar output, thus eliminating data loss. The radar system achieves a remarkable speed of over 9000 waveforms per second on each channel. The proposed architecture is scalable to accommodate higher sampling rates and various waveform periods. It is also multi-functional since the FPGA controls and synchronizes two transmitters and a dual-channel receiver, performs signal reconstruction on both channels simultaneously, and carries out user-defined averaging, trace windowing, and interference suppression for improving the receiver’s signal-to-noise ratio. We also investigate the throughput rate while offloading radar data onto an external device through an Ethernet link. Since the radar data rate significantly exceeds the Ethernet link capacity, we show how the FPGA-based averaging and windowing functions are leveraged to reduce the amount of offloaded data while fully utilizing the radar output.

A transmitting (Tx) array comprising 16×16 shielded slot-antenna elements realized on printed circuit board (PCB) technology is proposed. The array is designed for an electronically switched microwave compressed-breast imager operating in the frequency range from 3 GHz to 8 GHz. The element area is only 12×12 mm2 with the mutual coupling being below −20 dB and the element return loss being better than 10 dB. The seamless interconnect between the high-density array and the radio-frequency (RF) feed network becomes possible due to the innovative use of low-cost high-speed vertical board-to-board connectors (common in computer technology) instead of coaxial connectors. The array can be easily reconfigured to operate in switched or simultaneous transmission regimes by pairing it with the respective RF feed-network PCBs inserted in the vertical connectors. Here, we present two such networks: (i) for simultaneous transmission from all array elements (power-splitting feed network) and (ii) for transmission from one antenna at a time (RF-switched feed network). The performance of the array and the RF feed networks is verified through simulations and measurements. The benefits of a tissue-matched buffer layer between the array and the compressed breast are demonstrated to render the antenna impedance match insensitive to the dielectric properties of the breast tissue and to enhance the uniformity of the incident-field magnitude distribution. We also investigate the specific absorption rate (SAR) distribution in the breast tissue under conditions of simultaneous or single-element illumination, and provide means to compute the input power levels that ensure compliance with safety regulations.

An equivalent-time (ET) sampling ultra-wideband (UWB) dual-channel receiver is proposed, which is controlled by a field-programmable gate array (FPGA). It has a programmable repetition period and ET sampling rate (up to 20 gigasamples per second). The architecture employs a programmable delay chip (PDC) to achieve ultra-high speed of over 8 900 traces per second for a typical 1 μs repetition period. Compared with previously reported high-speed (PDC-based) receivers, it offers superior time-sampling accuracy. The design incorporates a custom dual-channel radio frequency (RF) front-end with a low-jitter clock source, critical in achieving time-sampling stability. Importantly, a simple yet effective method is proposed to correct the systematic timebase distortions due to the PDC, whose delay inaccuracies are the main signal-degradation factor in ET receivers realizing picosecond sampling intervals. The realized low-cost system operates as a high-speed oscilloscope with a 10-dB receiver bandwidth of 6 GHz and with accuracy comparable to that of bench-top high-speed oscilloscopes. Performance metrics and measurement procedures are proposed to evaluate and compare time-sampling receivers. These are applied to the proposed receiver, including tests as part of a compact pulsed radar. Its performance is compared with two high-speed bench-top oscilloscopes as well as previously reported ET receiver prototypes.

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.