206 research outputs found

    THz and mm-Wave Sensing of Corneal Tissue Water Content: Electromagnetic Modeling and Analysis.

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    Terahertz (THz) spectral properties of human cornea are explored as a function of central corneal thickness (CCT) and corneal water content, and the clinical utility of THz-based corneal water content sensing is discussed. Three candidate corneal tissue water content (CTWC) perturbations, based on corneal physiology, are investigated that affect the axial water distribution and total thickness. The THz frequency reflectivity properties of the three CTWC perturbations were simulated and explored with varying system center frequency and bandwidths (Q-factors). The modeling showed that at effective optical path lengths on the order of a wavelength the cornea presents a lossy etalon bordered by air at the anterior and the aqueous humor at the posterior. The simulated standing wave peak-to-valley ratio is pronounced at lower frequencies and its effect on acquired data can be modulated by adjusting the bandwidth of the sensing system. These observations are supported with experimental spectroscopic data. The results suggest that a priori knowledge of corneal thickness can be utilized for accurate assessments of corneal tissue water content. The physiologic variation of corneal thickness with respect to the wavelengths spanned by the THz band is extremely limited compared to all other structures in the body making CTWC sensing unique amongst all proposed applications of THz medical imaging

    Terahertz time-domain spectroscopy for non-invasive assessment of water content in biological samples

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    We apply terahertz time-domain spectroscopy for the quantitative non-invasive assessment of the water content in biological samples, such as Carpinus caroliniana tree leaves and pork muscles. The developed experimental terahertz time-domain spectroscopy system operates both in transmission and reflection modes. The Landau-Looyenga-Lifshitz-based model is used for the calculation of the water concentration within the samples. The results of the water concentration measurements are compared with the results of the gravimetric measurements. The obtained results show that the water content in biological samples can be measured non-invasively, with a high accuracy, utilizing terahertz waves in transmission and reflection modes

    Optical feedback effects on terahertz quantum cascade lasers: modelling and applications

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    Terahertz (THz) quantum cascade lasers (QCLs) are compact sources of radiation in the 1–5 THz range with significant potential for applications in sensing and imaging. Laser feedback interferometry (LFI) with THz QCLs is a technique utilizing the sensitivity of the QCL to the radiation reflected back into the laser cavity from an external target. We will discuss modelling techniques and explore the applications of LFI in biological tissue imaging and will show that the confocal nature of the QCL in LFI systems, with their innate capacity for depth sectioning, makes them suitable for skin diagnostics with the well-known advantages of more conventional confocal microscopes. A demonstration of discrimination of neoplasia from healthy tissue using a THz, LFI-based system in the context of melanoma is presented using a transgenic mouse model. © (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only

    Experimental and Analysis of Electromagnetic Characterization of Biological and Non-Biological Materials in Microwave, Millimeter-wave, and Terahertz Frequency Bands

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    The goal of this research is to characterize the electromagnetic properties of biological and non-biological materials at terahertz (THz), millimeter-wave, and microwave frequency bands. The biological specimens are measured using the THz imaging and spectroscopy system, whereas the non-biological materials are measured using the microwave and millimeter-wave free-space system. These facilities are located in the Engineering Research Center at the University of Arkansas. The THz imaging system (TPS 3000) uses a Ti-Sapphire laser directed on the photoconductive antennas to generate a THz time domain pulse. Upon using the Fourier Transform, the spectrum of the pulsed THz signal includes frequencies from 0.1 THz to 4 THz. On the other hand, the free space system uses a PNA network analyzer to produce a signal at frequencies ranging from 10 MHz to 110 GHz. For the biological specimens, the research focused on imaging the freshly excised breast tumors to detect the cancer on the margins using THz radiation. The tumor margin assessment depends on the THz contrast between cancer, collagen, and fat tissues in the tumor. Three models of breast tumors are investigated in this research: humans, mice (xenograft and transgenic), and Sprague Dawley rats. The results showed good differentiation between the cancerous and non-cancerous tissues in all three models. In addition, an excellent distinction was observed between cancer, collagen, and fat in the formalin-fixed paraffin-embedded (FFPE) block tissue with ~ 90-95% correlation with the pathology images. Furthermore, the FFPE ductal carcinoma in situ (DCIS) tumors are investigated, also using the THz imaging. The THz images of the DCIS samples are compared with those of the FFPE invasive ductal carcinoma (IDC) specimens. The results demonstrated that THz electric field reflection from the IDC were higher than that from the collagen, DCIS, and then the fat tissue region. Furthermore, a pilot study is conducted to investigate the effect of optical clearance (e.g., glycerol solution) on THz images of freshly excised tumors. The results showed that the glycerol reduced the absorption coefficients of pre-treated tissues compared with those of untreated tissues. For the non-biological materials, the research focuses on characterizing highly conductive non-magnetic radar absorbing materials (RAM) for the automotive industry. The ingredients of material components in the RAM samples are unrevealed under a non-disclosure agreement (NDA). The material characterization involves the extraction of the complex relative permittivity utilizing the transmission measurement data obtained at the K-band (18 GHz to 26.5 GHz) and the W-band (75 GHz to 110 GHz). The measurements are obtained using the free-space conical horn antenna system. A transmission line based extraction model is implemented, and the results are validated with the experimental measurements of the S-parameters. The maximum error reported between the measured and the calculated S-parameters was less than 1 dB. In conclusion, the THz imaging of breast cancer tumors presents a potential margin assessment of other solid tumors, and the microwave, millimeter-wave, and THz spectroscopy of materials demonstrate a potential application in the fifth and sixth generations of wireless communications

    Terahertz reflectometry of burn wounds in a rat model

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    We present sub-millimeter wave reflectometry of an experimental rat skin burn model obtained by the Terahertz Time-Domain Spectroscopy (THz-TDS) technique. Full thickness burns, as confirmed by histology, were created on rats (n = 4) euthanized immediately prior to the experiments. Statistical analysis shows that the burned tissue exhibits higher reflectivity compared to normal skin over a frequency range between 0.5 and 0.7 THz (p < 0.05), likely due to post-burn formation of interstitial edema. Furthermore, we demonstrate that a double Debye dielectric relaxation model can be used to explain the terahertz response of both normal and less severely burned rat skin. Finally, our data suggest that the degree of conformation between the experimental burn measurements and the model for normal skin can potentially be used to infer the extent of burn severity

    Development of correction algorithm for pulsed terahertz computed tomography (THz-CT)

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    For last couple of decades, there has been a considerable improvement in Terahertz (THz) science, technology, and imaging. In particular, the technique of 3-D computed tomography has been adapted to the THz range. However, it has been widely recognized that a fundamental limitation to THz computed tomography imaging is the refractive effects of the sample under study. The finite refractive index of materials in the THz range can severally refract THz beams which probe the internal structure of a sample during the acquisition of tomography data. Refractive effects lead to anomalously high local absorption coefficients in the reconstructed image near the material’s boundaries. Three refractive effects are identified: (a) Fresnel reflection power losses at the boundaries, (b) an increase in path length of the probing THz radiation, and (c) steering of the THz beam by the sample such that the emerging THz radiation is no longer collected by the THz detector. In addition, the finite size of the THz beam dominates the measured THz transmission when the edges of the sample are probed using THz tomography. These boundary phenomena can dominate in the reconstructed THz-CT images making it difficult to distinguish any hidden finer structural defect(s) inside the material. In this dissertation, an algorithm has been developed to remove these refractive and finite beam size effects from THz-CT reconstructed images. The algorithm is successfully implemented on cylindrical shaped objects. A longer term goal of the research is to study the internal structure of natural cork wine stoppers by pulsed Terahertz tomography (THz-CT). It has previously been shown that THz imaging can detect the internal structure of natural cork. Moreover, the internal structure of natural cork stoppers dominates the diffusion of gasses and liquids through the cork. By using THz computed tomography, one can recreate a 3D image of the sample’s internal structure which could then be used to predict non-destructively the diffusion properties of the cork. However, refractive and boundary effects which arise in the THz tomographic image masks the presence of the cork’s internal structure. Applying the correction algorithms which are developed in this dissertation to natural cork stoppers suppresses the refractive and boundary anomalies enabling better visualization of the cork’s internal structure

    THz: Research Frontiers for New Sources, Imaging and Other Advanced Technologies

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    The THz region of the electromagnetic spectrum is a frontier research area involving application of many disciplines, from outdoor to indoor communications, security, drug detection, biometrics, food quality control, agriculture, medicine, semiconductors, and air pollution. THz research is highly demanding in term of sources with high power and time resolution, detectors, and new spectrometer systems. Many open questions still exist regarding working at THz frequencies; many materials exhibit unusual or exotic properties in the THz domain, and researchers need new methodologies to exploit these opportunities. This book contains original papers dealing with emerging applications, new devices, sources and detectors, and materials with advanced properties for applications in biomedicine, cultural heritage, technology, and space
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