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 study, 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.
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 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.
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