Refraction losses in terahertz computed tomography

Terahertz pulsed imaging has been used to perform computed tomography of three-dimensional objects inducing high refraction losses. We were able to obtain cross-sectional images of the targets by using a computed tomographic analysis of the terahertz signal based on a multi-peak averaging procedure. Some examples are presented of objects with complex shape such as a Teflon cylinder and a dried chicken bone.     

Real-time two-dimensional terahertz tomography of moving objects

We propose real-time two-dimensional (2D) terahertz (THz) tomography for 2D cross-sectional imaging of moving objects. The proposed method does not require mechanical scanning elements for time delay and sample position, which are essential for conventional THz tomography. For this purpose, a 2D spatiotemporal THz image is acquired at a frame rate of 10 frame/s by a combination of time-to-space conversion using non-collinear 2D free-space electro-optic sampling and line focusing of a THz beam on a sample. We demonstrate the use of real-time 2D THz tomography for real-time monitoring of the paint thickness in a moving paint sample and the drying progress in a wet paint film. The proposed method provides a powerful tool for non-ionizing and non-contact probing in the fields of nondestructive testing and biomedical diagnostics.     

Real-time quadrature projection complex conjugate resolved Fourier domain optical coherence tomography

We present a novel algorithm for full-range imaging by suppression of the complex conjugate artifact in phase-shifting Fourier domain optical coherence tomography. This technique utilizes the projection of multiple phase-shifted interferograms onto an orthogonal basis set to reconstruct the complex interferogram. Full-range imaging with >30 dB suppression of the symmetric artifact is demonstrated using a 3 x 3 fiber coupler swept source OCT system, providing a depth range of 6.6mm with -8 dB roll-off in sensitivity at the depth boundaries relative to DC. Real-time display of full-range images of the anterior segment of the human eye acquired in vivo at a line rate of 6.67 kHz are presented.     

CT不完全投影数据重建算法综述

本文主要针对计算机断层成像(computed tomography,CT)不完全投影数据重建中探测器全覆盖稀疏角度重建和探测器部分覆盖截断数据重建问题,综述了其在研究方法上国内外的进展.在探测器对被检测物体可以完全覆盖的情况下,针对稀疏均匀采样和视角受限采样,探讨了离散模型迭代重建算法和压缩感知采样重建算法.在探测器对被检测物体不能完全覆盖的情况下,探讨了锥束螺旋BPF重建算法、加型迭代重建算法和锥束FDK改进算法.论文可以为CT重建领域的研究工作者提供全面的方法梳理和总结,并指出了当前研究的重点和未来研究的方向.     

Real-time in vivo imaging by high-speed spectral optical coherence tomography

An improved spectral optical coherence tomography technique is used to obtain cross-sectional ophthalmic images at an exposure time of 64 micros per A-scan. This method allows real-time images as well as static tomograms to be recorded in vivo.     

New terahertz dispersive device for single-shot spectral measurements of terahertz pulse

A new terahertz dispersive device designed for single-shot spectral measurements of broadband terahertz pulses is proposed. With two-dimensional quasi-randomly distributed element design, the device exhibits approximately the dispersive property of single-order diffraction in far field. Its far-field diffraction pattern is experimentally verified employing a continuous terahertz source centered at 2.52 THz and a pyroelectric focal-plane-array camera, which is in good agreement with the numerical result. The device provides a new approach for direct single-shot spectral measurements of broadband terahertz waves.     

一种用于大坝混凝土声层析成像用水听器线阵

声层析成像技术可以用图像的方式精确完整地反映层析面上检测体内部质量,是一种有效的无损检测手段。在大坝混凝土的安全隐患检测中发挥着越来越重要的作用,本研究旨在研制成像用的接收声波的水听器线阵,水听器线阵由12个水听器阵元组成,阵元间隔2m,水听器由直径40mm的接收型压电园管和前置放大器组成,水听器在低于20kHz的频率范围内,自由场接收电压灵敏度为-173±-3dB,在2MPa静水压力下灵敏度下降不超过2dB。制作了二例了具有清晰接收波形、一致性和可靠性好的水听器线阵,最终为客户在现场实地获得了高质量的层析图像。     

A comparative study on pre-smoothing techniques for projection data with poisson noise in computed tomography

A computer simulation was performed to compare pre-smoothing techniques for noisy projection data in reconstructed (computed tomographic) images and to find suitable pre-smoothing techniques for reducing the noise in the reconstruction. Some are applied to Poison (signal-dependent) noise, and the others to signal-independent noise obtained by the Anscombe transformation of signal-dependent noise. Selective for signal-dependent noise is found to produce superior results.     

时间-频率联合分析消除太赫兹光谱干涉

基于Fourier变换的传统太赫兹时域光谱分析技术从不同长度的太赫兹信号中产生了不一致的光谱结果,增加太赫兹时域采样长度又会造成光谱的干涉,不利于太赫兹光谱测量的研究和应用。应用小波变换方法对太赫兹波谱作时间—频率联合分析,将不同时刻的太赫兹波谱展开到二维的时间-频率平面,消除了光谱分析的不一致性和光谱干涉的影响。     

General formula for fan-beam computed tomography

In this Letter, we derive a general reconstruction formula for fan-beam computed tomography (CT) utilizing the two-dimensional Dirac function, which is useful in CT imaging for moving objects.     

T-ray computed tomography

We demonstrate a tomographic imaging modality that uses pulsed terahertz (THz) radiation to probe the optical properties of three-dimensional (3D) structures in the far-infrared. This THz-wave computed tomography (T-ray CT) system provides sectional images of objects in a manner analogous to conventional CT techniques such as x-ray CT. The transmitted amplitude and phase of broadband pulses of THz radiation are measured at multiple projection angles. The filtered backprojection algorithm is then used to reconstruct the target object, including both its 3D structure and its frequency-dependent far-infrared optical properties.     

New fast spectral analysis method for solid materials

We propose a new fast method for direct spectral analysis of solid materials based on laser ablation of the sample in deionized water and real-time transport of the aqueous suspension of nanoparticles into the inductively coupled plasma of an emission spectrometer. As a result, we have all the instrumental and methodological advantages of standard equipment, along with calibration of the spectrometer using standard aqueous solutions. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 3, pp. 281–282, May–June, 2007.     

X射线CT环形伪影去除方法

为去除X射线CT图像中的环形伪影,建立了一种投影数据预处理方法。该方法通过对各角度投影数据逐一进行分段多项式拟合的方式建立异常探元的备选校正因子集合,再根据备选校正因子的概率密度分布确定最可几校正因子。介绍了方法的物理依据、原理及其实现步骤,并分析了其在复杂条件下的适应性。结果表明:该方法对稀疏环形伪影、密集环形伪影以及伴随强噪声污染的CT图像环形伪影均可去除;与中值滤波等方法相比,可以更好地保持图像的空间分辨率。该方法可用于多材质检测对象CT图像的处理。     

Projected index computed tomography

Projected index computed tomography (PICT) is a new imaging technique that provides a computed reconstruction of the index of refraction of a sample. PICT makes use of data from standard optical coherence tomography images taken from several view angles to determine a mapping of the refractive indices of the sample. A rectilinear propagation model is assumed, so the data are understood to be related to the line integral of the refractive index in the beam paths. These data thus provide a set of angular projections of the sample. The spatial distribution of the index of the object may then be reconstructed by use of standard filtered backprojection techniques. The resultant PICT images are free of the spatial distortion that is inherent in standard optical cross-sectional images and correspond well to the manufactured dimensions of specific samples.     

Real-time, continuous-wave terahertz imaging by a pyroelectric camera

Real-time, continuous-wave terahertz (THz) imaging is demonstrated. A 1.89-THz optically-pumped farinfrared laser is used as the illumination source, and a 124 × 124 element room-temperature pyroelectric camera is adopted as the detector. With this setup, THz images through various wrapping materials are shown. The results show that this imaging system has the potential applications in real-time mail and security inspection.     

Real-time video signal transmission over a terahertz communication link

A 2.4-m communication link operating at 3.9 THz based on a terahertz quantum cascade laser and a terahertz quantum well photodetector （THz QWP） are introduced. The lumped electrical models of THz QWP for small signals are presented. A discussion of the bandwidth limit of the detecting circuit is presented. Using direct on off-keying modulation and intensity detection, the transmission of digital video signal with a data rate of 2.5 Mb/s is demonstrated. Pseudo-random binary sequences are transmitted to investigate the bit-error rate （BER） at different rates. Result shows the error free transmission when the rate is below 5 Mb/s.     

Efficient photoconductive terahertz source using line excitation

We report significant improvement in terahertz (THz) power and efficiency using photoconductive sources by use of a spatially extended line source excitation and the trap enhanced field effect that occurs in sources made on semi-insulating GaAs. The combination of high electric fields and reduced screening effects allows 10 microW of THz power to be generated with 14 mW of absorbed optical power, demonstrating nearly 0.1% optical-to-THz conversion efficiency.     

Real-time reflection imaging with terahertz camera and quantum-cascade laser

A real-time reflection imaging employing a terahertz （THz） camera as the imager and a 3.9 THz quantum- cascade laser （QCL） as the light source is demonstrated. The imaging light is collected and guided by only one off-axis parabolic mirror. The imaging distance is about 1 m. THz images of a coin and a knife are aacquired and analyzed. An actual soatial resolution with a value of about 0.33 mm is achieved.     

A simple algorithm for spectral line deconvolution

The objective of this work is to develop a numerical procedure to subtract the instrumental function from a measured spectral line profile. The measuring device (for example, a Fabry-Perot Interferometer) distorts the spectral line profile and the experimentally measured one is a convolution of this profile and the instrumental function. Restoring the spectral line profile is strongly affected by numerical instabilities and the problem has been overcome by using the Tikhonov regularization method. The approach is very simple and easy for programming and it is particularly useful for “noisy” experimental data.