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 ɣ . 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.

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.

Millimeter wave (MMW) technology is expanding rapidly into security screening for dangerous items concealed under clothing. It uses safe non-ionizing radiation and penetrates clothing well. We present a new planar system at Ka band for the three-dimensional simultaneous imaging of both sides of an inspected person where the images are produced in real time by a recently proposed generalized holographic reconstruction algorithm. Low-cost linear frequency modulation (LFM) radar technology is used along with a simple but efficient method for system calibration. Experimental characterization of the spatial resolution and the sensitivity of the system prototype has been carried out. It is established that the achieved spatial resolution is 6 mm or better if the item is not obscured by clothing and it may deteriorate to 7 mm depending on the clothing hiding the item. The spatial sensitivity is confirmed to be at least 2 mm.

An efficient and more accurate millimeter-wave imaging algorithm, applied to a close-range monostatic personnel screening system, with consideration of dual path propagation loss, is presented in this paper. The algorithm is developed in accordance with a more rigorous physical model for the monostatic system. The physical model treats incident waves and scattered waves as spherical waves with a more rigorous amplitude term as per electromagnetic theory. As a result, the proposed method can achieve a better focusing effect for multiple targets in different range planes. Since the mathematical methods in classical algorithms, such as spherical wave decomposition and Weyl identity, cannot handle the corresponding mathematical model, the proposed algorithm is derived through the method of stationary phase (MSP). The algorithm has been validated by numerical simulations and laboratory experiments. Good performance in terms of computational efficiency and accuracy has been observed. The synthetic reconstruction results show that the proposed algorithm has significant advantages compared with the classical algorithms, and the reconstruction by using full-wave data generated by FEKO further verifies the validity of the proposed algorithm. Finally, the proposed algorithm performs as expected over real data acquired by our laboratory prototype.

To address the issue of incomplete acoustic processes in microwave-induced thermoacoustic imaging (MITAI) simulations, this study proposes coupling structural mechanics (SM) and pressure acoustics (PA) fields inside and outside tissues, respectively, to visualize the entire MITAI physical process. Based on this, we investigate the MITAI artifacts formation mechanism from a mechanical property perspective. This study establishes a dual-imaging target model based on COMSOL, coupling electromagnetic, heat transfer, and SM and PA modules to achieve a complete MITAI process. This method is verified by extracting the signals for image reconstruction. Furthermore, four complex biological tissue models are constructed based on the presence and location of bone, to explore the mechanical impact of bone on surrounding soft tissues. The dual-target results show that the proposed method accurately characterizes the multiphysical phenomena of microwave excitation, energy deposition, thermal expansion, and ultrasound propagation in the MITAI process. The images correctly represent both dielectric functional information and structural features of the model, while also revealing artifacts caused by the signal reflection. The image of the dual-target phantom further validates this finding. Four complex models' results show that although MITAI clearly distinguishes bones and different soft tissues, the bones degrade image quality, with greater degradation occurring in soft tissues closer to the bones. The proposed method can achieve complete MITAI process visualization, overcome incomplete acoustic simulation, and elucidate the MITAI artifacts generation mechanism from mechanical properties. This work holds considerable value for advancing MITAI mechanism research and guiding biological tissue imaging applications.

This letter proposes a millimeter-wave (mmWave) imaging system based on a multi-chip cascaded radar architecture, achieving high-resolution, large-area detection and cost-effective. By utilizing antenna-on-package (AOP) technology, multiple radar chips are flexibly integrated to form a modular array that can be arbitrarily scaled according to the imaging scenario. To address the issue of virtual aperture discontinuity caused by chip-level cascading, this letter develops a hybrid scanning strategy that combines mechanical and electronic scanning. This strategy significantly expands the effective aperture while also improves scanning efficiency. Furthermore, the corresponding Cascaded Aperture Holographic Imaging (CAHI) algorithm is proposed to accurately reconstruct high-resolution images. Finally, the CAHI algorithm is deployed on a FPGA edge computing platform, accelerating the imaging process. Experimental results demonstrate that the proposed system achieves spatial resolution better than 4 mm while significantly reducing acquisition time, compared to traditional single-radar scanning systems. This close coupling between customized hardware and algorithm provides an efficient and scalable solution for high-resolution mmWave imaging.