太赫兹成像技术在肿瘤检测中的应用

太赫兹(THz)波是频率位于0.1 THz^10 THz的电磁波。因其具有非电离性,以及可与多数生物分子产生共振响应等特性,在生物医学领域有着巨大应用潜力,尤其在肿瘤检测方面。太赫兹成像技术作为生物医学领域一种新的成像技术,吸引国内外多个研究小组对其开展深入研究。本文列举分析了多种太赫兹成像技术在肿瘤检测的应用,其中可分为太赫兹扫描成像、太赫兹层析成像、太赫兹全息成像以及太赫兹近场成像,介绍了这些成像方式的基本原理以及国内外研究现状,最后对太赫兹成像技术在生物领域的未来做出展望。     

Terahertz near-field nanoscopy of mobile carriers in single semiconductor nanodevices

We introduce ultraresolving terahertz (THz) near-field microscopy based on THz scattering at atomic force microscope tips. Nanoscale resolution is achieved by THz field confinement at the very tip apex to within 30 nm, which is in good agreement with full electro-dynamic calculations. Imaging semiconductor transistors, we provide first evidence of 40 nm (lambda/3000) spatial resolution at 2.54 THz (wavelength lambda=118 microm) and demonstrate the simultaneous THz recognition of materials and mobile carriers in a single nanodevice. Fundamentally important, we find that the mobile carrier contrast can be directly related to near-field excitation of THz-plasmons in the doped semiconductor regions. This opens the door to quantitative studies of local carrier concentration and mobility at the nanometer scale. The THz near-field response is extraordinary sensitive, providing contrast from less than 100 mobile electrons in the probed volume. Future improvements could allow for THz characterization of even single electrons or biomolecules.     

基于时延优化的三维成像声呐视野展宽方法研究

针对平面阵,研究并探讨了一种近场聚焦波束形成中时延参数的优化方法,以扩大三维成像声呐的有效视野范围。该方法是在方位角和俯仰角重新定义时,将精确的近场时延表达式按泰勒公式展开,取前三项,并对每一项进行加权。然后,通过求三个加权系数的偏导,再令其等于0,得出最优加权系数,此时优化的时延表达式与精确的近场时延表达式的误差最小,即优化的时延表达式更接近于精确的时延表达式,能够有效地用于扩大成像声呐的有效视野。最后,通过计算机仿真验证了该方法的有效性。     

基于太赫兹量子阱光电探测器的成像技术研究进展

太赫兹(THz)波对非极性材料有较好的穿透性,对生物医学组织无电离效应,因而非常适合无损检测、生物医学成像等应用。THz量子阱光电探测器(THz QWPs)具有响应速度快、响应率高、噪声等效功率低、体积小的特点。相较于其他探测器,THz QWPs作为成像系统接收器时,系统具有成像分辨率高、成像速度快、成像信噪比高、结构紧凑等优势。本文综述了基于THz QWPs的成像研究进展,并对成像系统核心指标的影响因素进行了分析和总结。采用更稳定的装置固定THz QWPs,提升器件响应速度、探测灵敏度、阵列规模,可以有效提升系统各项核心性能。     

Terahertz polarization real-time imaging based on balanced electro-optic detection

A terahertz (THz) polarization real-time imaging system that can effectively reduce experimental time consumption for acquiring a sample's polarization information is achieved. An alternative THz polarization measurement method is proposed. In this method, a <110> zinc-blende crystal is used as the sensor, and the probe polarization is adjusted to detect THz electric fields on the two orthogonal polarization components. The relative sensitivity of the imaging system to the THz polarization angle is estimated to be less than 0.5°. To illustrate the ability of the system, two samples are designed and measured by using the system. From their THz polarization real-time images, each region of these samples can be precisely presented. Experimental results clearly show the special influences of different materials on the THz polarization. This work effectively extends the information content obtained by THz real-time imaging and improves the feasibility of the imaging technique.     

Towards terahertz near-field microscopy

We have detected sub-wavelength spot sizes in the near-field of a metal tip, which is illuminated with terahertz (THz) pulses. The THz near-field is detected by using electro-optic detection in a (100) oriented GaP crystal. Contrary to conventional electro-optic detection, which uses (110) oriented detection crystals, (100) crystals only allow the detection of THz light, polarized perpendicular to our crystal surface. This component is strongly localized near the apex of the tip, which has sub-wavelength dimensions. The detection process is blind to the incident THz radiation, which is polarized parallel to the crystal surface. As a result, a sub-wavelength THz spot size with an intensity full-width half maximum (FWHM) diameter of lambda/200 is observed.     

Development of an MKIDs-Based THz Superconducting Imaging Array (TeSIA) at 0.85 THz

Dome A, the highest point of the cold and dry Antarctic ice sheet, offers the best access to atmospheric windows at THz/FIR wavelengths on Earth. China is planning to build a 5-m THz telescope (DATE5) there. To achieve its scientific goals associated with large sky surveys, we are developing a THz superconducting imaging array (TeSIA) at 0.85 THz (350-μm window) with a pixel number of 32?×?32 and targeting background-limited sensitivity. In this paper, detailed system design and performance of the TeSIA based on aluminum MKIDs are presented.     

THz wavefront manipulation based on metal waveguides

In this paper, two waveguiding structures for arbitrary wavefront manipulation in the terahertz spectral region were proposed, designed and characterized. The first structure consists of parallel stack copper plates forming an array of parallel-plate waveguides (PPWGs). The second structure is three-dimensional metal rectangular waveguides array. The phase delay of the input wave after passing through the waveguide array is mainly determined by the effective index of the waveguides. Therefore, the waveguide array can be engineered using different core width distribution to generate any desired light beam. Examples, working at the frequency of 0.3 THz show that good focusing phenomenon with different focus lengths and spot sizes were observed, as well as arbitrarily tilted propagation of incident plane waves. The structure introduces a new method to perform wavefront manipulation, and can be utilized in many important applications in terahertz imaging and communication systems.     

Real-time terahertz scanning imaging by use of a pyroelectric array camera and image denoising

A high-resolution large-area terahertz (THz) scanning imaging system is demonstrated based on a 124×124 pyroelectric array camera and a CO(2) pumped continuous-wave THz laser. By applying a scanning mechanism to the real-time imaging setup, images of large-area targets were accomplished. Self-made resolution charts were employed to test the resolution. In order to improve the image quality, the noise in the images was studied and modeled, and then the performance of several denoising methods was compared with real-time THz original images. The experimental results show that, with the help of anisotropic diffusion, noise can be effectively suppressed, and the results are visually pleasant even when there is great attenuation. Those results greatly confirm application potentials of THz imaging using pyroelectric cameras in the field of concealed object detection.     

Phase-aberration correction using signals from point reflectors and diffuse scatterers: measurements

A method for phase-aberration correction of phased-array images is tested using a model of near-field velocity inhomogeneities. A set of grooved room-temperature vulcanizing plates was constructed to simulate near-field aberrations encountered in clinical ultrasound imaging. As expected, large image distortion was experienced when grooved plates producing significant aberrations were placed near the surface of the array. An iterative aberration correction procedure based on cross-correlation measures between neighboring elements in a phased array, using signals reflected from diffuse scatterers, significantly reduced the effects of these aberrations, producing images nearly identical to those generated in the absence of aberrations. The results suggest that a practical phase-aberration correction system can be constructed for medical ultrasound imaging and possibly all coherent imaging systems by using a sampled aperture.     

Microwave antenna imaging,diagnostics, and phaseless reconstructions

Microwave antenna imaging techniques are a practical and popular method for antenna diagnostic analysis. Phase retrieval methods, however, are just beginning to emerge as an alternative microwave antenna measurements technique when phase cannot be directly measured. This article focuses on recent advances in microwave antenna imaging, diagnostic techniques, and phase retrieval methods for bi-polar planar near-field antenna measurements. An overview of the bi-polar planar near-field technique is included. The application of optimal sampling interpolation, holographic imaging and diagnostics, and iterative Fourier phase retrieval for the bi-polar planar near-field modality is explored in detail. Experimental results for a waveguide-fed slot array antenna are presented to illustrate these methods. © 1997 John Wiley & Sons, Inc. Int J Imaging Syst Technol, 8, 396–406, 1997     

Interaction between metal nanotip and substrate under terahertz wave radiation

In this paper, the interaction between a gold nanotip and a silver substrate under the terahertz (THz) wave radiation is simulated by a finite difference time domain method and theoretically analyzed from the quasi-static charges model. Results from simulations and theoretical analysis show that with the increase in the distance between the nanotip and the substrate, the maximum electric field intensity on the substrate prominently decreases. With the increase in radius of the nanotip, diameter of the focused THz spot almost linearly increases and the intensity of THz spot also has a slight increase. These results are in agreement with the physical principles of the tip-enhanced near-field optical microscopy (TENOM), which indicates the TENOM technique is also suitable for the THz frequency range.     

成像声呐中聚焦波束形成技术研究及实现

介绍了聚焦波束形成的基本原理,分析了一种基于半圆阵的相位补偿方法。通过Matlab仿真得出聚焦波束形成的波束图,相比远场方法,波束宽度减小,旁瓣得到抑制。设计了一种基于FPGA的数字聚焦波束形成器的实时处理结构,使用8组加权系数即可完成成像声呐近场范围内分辨力的改进。通过乒乓操作和并行结构提高处理速度,实时产生72个波束。实验结果表明,所设计的聚焦波束形成器使某型成像声呐近场分辨力得到了提高。     

Design, simulation and experimental study of near-field beam forming techniques using conformal waveguide arrays

Microwave hyperthermia is rapidly evolving as a fourth modality in the fight against cancer, along with surgery, radiation and chemotherapy. This form of cancer treatment utilises a narrow microwave beam to heat the tumour volume to a temperature of ˜42°C; however, with minimal energy delivery to neighbouring healthy tissue, which is one of the main challenges in hyperthermia technology. Potentially, this application can be achieved by using a phased array of apertures or dipoles to generate and control the beam focus within the near-field treatment region. This paper describes another approach to near-field beam forming by using of a conformal waveguide array, operating in the K band (18-26 GHz). The array comprises a central movable element that acts as the focusing element, and surrounding fixed directing elements. The focusing element gives conformal property to the array and serves two purposes: firstly to obtain a sharp focus at a prescribed near-field location, and secondly the added flexibility to move the beam around the tumour. Several simulations and measurements have been performed on linear and planar configurations, which demonstrate the ability of the array to achieve beam widths as small as ˜4 mm, with a maximum beam movement range of ˜15 mm.     

太赫兹脉冲焦平面成像技术的发展与应用

作为太赫兹技术中的重要组成部分,太赫兹脉冲焦平面成像一经问世就引起了行业内的广泛关注,人们引入了各种方法去提升此成像技术的测量性能,同时也尝试将此成像技术应用于不同的工业和基础研究领域。本文综述了近年来人们对太赫兹脉冲焦平面成像的技术改良和应用研究,包括提升成像系统的空间分辨率、信噪比、信息获取能力,以及将此成像技术应用于光谱识别检测、超表面器件功能验证、太赫兹特殊光束测量、太赫兹表面波观测等,希望该综述能够推动太赫兹脉冲焦平面成像的进一步技术革新和应用拓展。     

Optimization of the Digital Near-Field Beamforming for Underwater 3-D Sonar Imaging System

This paper proposes a novel approximation for the delays of digital near-field beamforming based on a planar array. An optimized algorithm for the underwater 3-D sonar imaging system based on the novel approximation will also be described. The algorithm is applied to both an evenly spaced planar array and an unevenly spaced planar array. A data path for the optimized algorithm will be illustrated. Comparisons of the memory and the computational requirements between the optimized algorithm and the direct method (DM) beamforming will be presented. Comparisons of the proposed approximation and the Fresnel approximation will also be demonstrated. This paper illustrates that the proposed approximation enlarges the validity region of the system's view scene as compared with the Fresnel approximation. The optimized algorithm has the advantage of reducing the memory requirements for parameter storage comparing with DM beamforming.      

Phase aberration correction using near-field signal redundancy. I. Principles [Ultrasound medical imaging

The signal redundancy principle in the near field is analyzed quantitatively. It is found that common midpoint signals are not identical (or redundant) for echoes coming from arbitrary target distributions in the near field. A dynamic near-field correction is proposed to reduce the difference between common midpoint signals for echoes coming from the region of interest. When phase aberrations are present, it is shown that the dynamic correction can generally be done assuming no phase aberration, and the relative time-shift between common midpoint signals can be used to measure phase-aberration profiles. A phase-aberration correction algorithm based on that principle is proposed. In this algorithm, after common midpoint signals are collected they are dynamically corrected for near-field effects and cross-correlated with one another. In a related way, the phase errors are measured from peak positions of these cross-correlation functions. The phase-aberration profile across the array is derived from these measurements. The relationship between the errors in the derived phase aberration profile and the errors in the measured relative time-shift between common midpoint signals is derived. A method for treating the situation of different transmission and reception phase-aberration profiles is also proposed. This algorithm works for general target distributions, iteration is not required, and it can be used in other near-field, pulse-echo, imaging systems.     

Adaptive imaging on a diagnostic ultrasound scanner at quasi real-time rates

Constructing an ultrasonic imaging system capable of compensating for phase errors in real-time is a significant challenge in adaptive imaging. We present a versatile adaptive imaging system capable of updating arrival time profiles at frame rates of approximately 2 frames per second (fps) with 1-D arrays and up to 0.81 fps for 1.75-D arrays, depending on the desired near-field phase correction algorithm. A novel feature included in this system is the ability to update the aberration profile at multiple beam locations for 1-D arrays. The features of this real-time adaptive imaging system are illustrated in tissue-mimicking phantoms with physical near-field phase screens and evaluated in clinical breast tissue with a 1.75-D array. The contrast-to-noise ratio (CNR) of anechoic cysts was shown to improve dramatically in the tissue-mimicking phantoms. In breast tissue, the width of point-like targets showed significant improvement: a reduction of 26.2% on average. Brightness of these targets, however, marginally decreased by 3.9%. For larger structures such as cysts, little improvement in features and CNR were observed, which is likely a result of the system assuming an infinite isoplanatic patch size for the 1.75-D arrays. The necessary requirements for constructing a real-time adaptive imaging system are also discussed.     

Three-dimensional terahertz wave imaging

Pulsed terahertz (THz) wave sensing and imaging is a coherent measurement technology. Like radar, based on the phase and amplitude of the THz pulse at each frequency, THz waves provide temporal and spectroscopic information that allows us to develop various three-dimensional (3D) terahertz tomographic imaging modalities. The 3D THz tomographic imaging methods we investigated include THz time-of-flight tomography, THz computed tomography (CT) and THz binary lens tomography. THz time-of-flight uses the THz pulses as a probe beam to temporally mark the target, and then constructs a 3D image of the target using the THz waves scattered by the target. THz CT is based on geometrical optics and inspired from X-ray CT. THz binary lens tomography uses the frequency-dependent focal-length property of binary lenses to obtain tomographic images of an object. Three-dimensional THz imaging has potential in such applications as non-destructive inspection. The interaction between a coherent THz pulse and an object provides rich information about the object under study; therefore, 3D THz imaging can be used to inspect or characterize dielectric and semiconductor objects. For example, 3D THz imaging has been used to detect and identify the defects inside a Space Shuttle insulation tile.     

Ultrasensitive Kilo-Pixel Imaging Array of Photon Noise-Limited Kinetic Inductance Detectors Over an Octave of Bandwidth for THz Astronomy

We present the development of a background-limited kilo-pixel imaging array of ultrawide bandwidth kinetic inductance detectors (KIDs) suitable for space-based THz astronomy applications. The array consists of 989 KIDs, in which the radiation is coupled to each KID via a leaky lens antenna, covering the frequency range between 1.4 and 2.8 THz. The single pixel performance is fully characterised using a representative small array in terms of sensitivity, optical efficiency, beam pattern and frequency response, matching very well its expected performance. The kilo-pixel array is characterised electrically, finding a yield larger than 90% and an averaged noise-equivalent power lower than 3 \(\times \) 10\(^{-19}\) W/Hz\(^{1/2}\). The interaction between the kilo-pixel array and cosmic rays is studied, with an expected dead time lower than 0.6% when operated in an L2 or a similar far-Earth orbit.