Amir Mirbeik-Sabzevari

This paper presents the design, fabrication, and measurement of two ultra-wideband doubleridge open-ended waveguide antennas (OEWAs) for emerging high-resolution millimeter-wave (mm-wave) medical imaging applications. The proposed antennas can collectively provide a 95-GHz synthetic imaging bandwidth in the mm-wave regime. The antennas feature the smallest aperture sizes while providing the widest bandwidths amongst all OEWAs reported in the literature. The proposed antennas consist of coaxial to double-ridge waveguide (DRW) transitions and free-space matching sections. Two DRWs with 8:1 single-mode bandwidth in the mm-wave regime are designed. In the transition structures, coaxial flange launchers are connected to the DRWs through customized 50-Ω coaxial lines. To improve the impedance matching while meeting the fabrication requirements, an effective tuning technique is applied to adjust the input impedance of the transitions. Finally, to achieve a smooth transition and a well-matched impedance to the free space while maintaining small aperture sizes, piecewise tapered-ridge matching sections with the same aperture size as that of the designed DRWs are employed. The antennas are fabricated using the computer numerical control (CNC) milling process. Each antenna demonstrates |S ₁₁ | <; -10 dB and stable and symmetric radiation patterns across 15-40 GHz and 40-110 GHz with aperture sizes of 9.47 ×5.79 mm ² and 3.78 ×2.27 mm ² , respectively, reporting state-of-the-art results.

The goal of this paper is to develop a new skin imaging modality which addresses the current clinical need for a non-invasive imaging tool that images the skin over its depth with high resolutions while offering large histopathological-like contrasts between malignant and normal tissues. We demonstrate that by taking advantage of the intrinsic millimeter-wave dielectric contrasts between normal and malignant skin tissues, ultra-high-resolution millimeter-wave imaging (MMWI) can achieve 3-D, high-contrast images of the skin. In this paper, an imaging system with a record-wide bandwidth of 98 GHz is developed using the synthetic ultra-wideband millimeter-wave imaging approach, a new ultra-high-resolution imaging technique recently developed by the authors. The 21 non-melanoma skin cancer (NMSC) specimens are imaged and compared with histopathology for evaluation. A programmable measurement platform is designed to automatically scan the tissues across a rectangular aperture plane. Furthermore, a novel frequency-domain imaging algorithm is developed to process the recorded signals and generate an image of the cancerous tissue. The high correlations achieved between MMWI images and histological images allow for rapid and accurate delineation of NMSC tissues. The millimeter-wave reflectivity values are also found to be statistically significant higher for cancerous areas with respect to normal areas. Since MMWI does not require tissue processing or staining, it can be performed promptly, enabling diagnosis of tumors at an early stage as well as simplify the tumor removal surgery to a single-layer excision procedure.

A novel W-band (75-110 GHz) surface micromachined Vivaldi antenna fed with a new combination of coplanar waveguide and substrate-integrated waveguide is proposed. The design procedure of the feeding structure and the effects of the antenna's key structural features on its return loss and gain are studied in detail. The selected exponential taper with optimized feeding parameters results in a significantly higher gain (gain > 10 dBi) compared to similar tapered slot antennas reported in the literature over the entire bandwidth of operation. The antenna is fabricated using microfabrication processes, and high correlations between simulation and measurement results confirm its low-loss performance and symmetric and stable radiation pattern over the entire frequency range of 75-110 GHz.

This work introduces, for the first time, a millimeter-wave imaging system with a "synthetic" ultra-wide imaging bandwidth of 98 GHz to provide the ultra-high resolutions required for early-stage skin cancer detection. The proposed approach consists of splitting the required ultra-wide imaging bandwidth into four sub-bands, and assigning each sub-band to a separate imaging element, i.e. an antenna radiator. Each of the sub-band antennas transmits and receives signals only at its corresponding sub-band. The captured signals are then combined and processed to form the image of the target. For each sub-band, a Vivaldi tapered slot antenna (TSA) fed with a combination of substrate-integrated waveguide (SIW) and coplanar waveguide (CPW) is designed and micro-fabricated. Design techniques are also provided for the four similarly-shaped sub-band antennas for achieving excellent impedance matches (S11 < -10 dB) and nearly constant gains of 10 dBi over the entire 12-110-GHz bandwidth. The design procedure is validated by comparing the simulated results with measurements performed on the fabricated prototypes. Excellent agreements are obtained between simulations and measurements. Finally, the feasibility of detecting early-stage skin tumors in three dimensions is experimentally verified by employing the sub-band antennas in a synthetic ultra-wideband imaging system with a bandwidth of 98 GHz. Two separate setups, each comprising of a dispersive skin-mimicking phantom as well as two dispersive spherical tumors are constructed for imaging experiments. Lateral and axial resolutions of 200 μm are confirmed, and a successful reconstruction of the spherical tumors is achieved in both cases.

This work introduces new, stable, and broadband skin-equivalent semisolid phantoms for mimicking interactions of millimeter waves with the human skin and skin tumors. Realistic skin phantoms serve as an invaluable tool for exploring the feasibility of new technologies and improving design concepts related to millimeter-wave skin cancer detection methods. Normal and malignant skin tissues are separately mimicked by using appropriate mixtures of deionized water, oil, gelatin powder, formaldehyde, TX-150 (a gelling agent, widely referred to as "super stuff"), and detergent. The dielectric properties of the phantoms are characterized over the frequency band of 0.5-50 GHz using a precision slim-form open-ended coaxial probe in conjunction with a millimeter-wave vector network analyzer. The measured permittivity results show excellent match with ex-vivo, fresh skin (both normal and malignant) permittivities determined in our prior work over the entire frequency range. This work results in the closest match among all phantoms reported in the literature to surrogate human skin tissues. The stability of dielectric properties over time is also investigated. The phantoms demonstrate long-term stability (up to 7 months was investigated). In addition, the penetration depth of millimeter waves into normal and malignant skin phantoms is calculated. It is determined that millimeter waves penetrate the human skin deep enough (0.6 mm on average at 50 GHz) to affect the majority of the epidermis and dermis skin structures.

This letter provides the calibration and optimization steps required for obtaining wideband millimeter-wave dielectric characteristics of human biological tissues using the commercially available slim-form open-ended coaxial probe. At the first stage, the reflection coefficients measured at the probe's calibration plane are transformed to the reflection coefficients of a desired aperture plane. Then, a rational optimization problem is solved to convert the aperture-plane reflection coefficients to dielectric permittivities. The measurement protocol is verified using three reference liquids and a test liquid representing dielectric characteristics of biological tissues.

Millimeter waves have recently gained attention for the evaluation of skin lesions and the detection of skin tumors. Such evaluations heavily rely on the dielectric contrasts existing between normal and malignant skin tissues at millimeter-wave frequencies. However, current studies on the dielectric properties of normal and diseased skin tissues at these frequencies are limited and inconsistent. In this work, a comprehensive dielectric spectroscopy study is conducted for the first time to characterize the ultra-wideband dielectric properties of freshly-excised normal and malignant skin tissues obtained from skin cancer patients having undergone Mohs micrographic surgeries at Hackensack University Medical Center. Measurements are conducted using a precision slim-form open-ended coaxial probe in conjunction with a millimeter-wave vector network analyzer over the frequency range of 0.5-50 GHz. A one-pole Cole-Cole model is fitted to the complex permittivity dataset of each sample. Statistically considerable contrasts are observed between the dielectric properties of malignant and normal skin tissues over the ultra-wideband millimeter-wave frequency range considered.