Time-domain Imaging Research Articles

Non-invasive Florentine Renaissance Panel Painting Replica Structures Investigation by Using Terahertz Time-Domain Imaging (THz-TDI) Technique

The potentials of the Terahertz Time-Domain Imaging (THz-TDI) technique for a non-invasive inspection of panel paintings have been considered in detail. The THz-TD data acquired on a replica of a panel painting made in imitation of Italian Renaissance panel paintings were processed in order to provide insights as to the limits and potentials of the technique in detecting different kinds of underdrawings and paint layers. Constituent layers, construction techniques, and anomalies were identified and localized by interpreting the extracted THz dielectric stratigraphy.

Velocity-independent estimation of kinematic attributes in vertical transverse isotropy media using local slopes and predictive painting

Agood seismic velocity model is required for many routine seismic imaging techniques. Velocity model building from seismic data is often labor intensive and time consuming. The process becomes more complicated by taking nonhyperbolic traveltime estimations into account. An alternative to the conventional time-domain imaging algorithms is to use techniques based on the local event slopes, which contain sufficient information about the traveltime moveout for velocity estimation and characterization of the subsurface geologic structures. Given the local slopes, there is no need for a prior knowledge of a velocity model. That is why the term “velocity independent” is commonly used for such techniques. We improved upon and simplified the previous versions of velocity-independent nonhyperbolic approximations for horizontally layered vertical transverse isotropy (VTI) media by removing one order of differentiation with respect to offset from the imaging kinematic attributes. These kinematic attributes are derived in terms of the local event slopes and zero-offset two-way traveltime (TWTT). We proposed the use of predictive painting, which keeps all the attributes curvature independent, to estimate the zero-offset TWTT. The theoretical contents and performance of the proposed approach were evaluated on synthetic and field data examples. We also studied the accuracy of moveout attributes for shifted hyperbola, rational, three-parameter, and acceleration approximations on a synthetic example. Our results show that regardless of the approximation types, NMO velocity estimate has higher accuracy than the nonhyperbolicity attribute. Computational time and accuracy of the inversion of kinematic attributes in VTI media using our approach were compared with routine/conventional multiparameter semblance inversion and with the previous velocity-independent inversion techniques.

Enhanced detection of early photons in time-domain optical imaging by running in the “dead-time” regime

Optical tomography can yield anatomical and molecular information about biological tissue. However, its spatial resolution is poor in thick samples owing to high scatter. Early photon approaches, where photon arrival times are measured with time-resolved detectors, provide one means of improving spatial resolution through selection of photons that travel a straighter path. Here, a novel approach to significantly enhance detection of early photons in time-correlated single photon counting with avalanche photodiodes has been discussed. Results suggest that the early photon detection rate can be increased by about 10 orders of magnitude by running the detector in a dead-time regime.

Sweep distortion removal from terahertz images via blind demodulation

Inspection and imaging through scattering layers is a decades-old problem in optics. Unlike x-ray and ultrasonic imaging techniques, time-domain spectroscopy methods can provide detailed chemical and structural information of subsurfaces along with their depth. Although high-resolution time-of-flight measurement in time-domain spectroscopy provides 3D information, unfortunately it also induces an unwanted sensitivity to misalignments of the system and distortion of the layers themselves. Such high sensitivity to alignment and sample surface is a well known problem in time-domain and interferometric imaging, and is a major concern when the alignment error is comparable to the pulse wavelength. Here, we propose and implement an algorithmic framework based on low-rank matrix recovery and alternating minimization to remove such unwanted distortions from time-domain images. The method allows for recovery of the original sample texture in spite of the presence of temporal-spatial distortions. We address a blind-demodulation problem where, based on several observations of the sample texture modulated by undesired sweep distortions, the two classes of signals are separated with minimal damage to the main features. The performance of the method is examined in both synthetic and real data in the case of a terahertz time-domain system, and the successful reconstructions are demonstrated. The proposed general scheme can be implemented to advance inspection and imaging applications in THz and other time-resolved spectral imaging modalities.

Gigahertz time-domain spectroscopy and imaging for non-destructive materials research and evaluation.

By using optical sampling with repetition frequency modulation of pump/probe laser pulses on photoconductive emitter/detector antennas, the high-speed time/frequency domain gigahertz imaging is reported due to the absence of opto-mechanical delay line in this optical scheme. The clear contrast for a 3-cm wide metal plate, which was placed behind a 5-cm thick concrete block, was observed with a 1 × 1 mm image pixilation. On average, it took only ~0.75 s per pixel/waveform acquisition/assignment with a 675 ps time-domain window. This could become a valuable non-destructive evaluation technique in gigahertz spectral range with all benefits of time-domain spectroscopy.

Fabrication of gradient-refractive-index lenses for terahertz applications by three-dimensional printing

In this article we present the design, fabrication, and characterization of gradient-refractive-index lenses for terahertz (THz) applications. The fabrication was performed by three-dimensional printing. These new low-cost lenses were tested using THz time-domain spectroscopy and imaging in order to measure their optical properties. The results show a focusing capacity within the diffraction limit for frequencies below 700 GHz.

Optimal estimator for tomographic fluorescence lifetime multiplexing.

We use the model resolution matrix to analytically derive an optimal Bayesian estimator for multiparameter inverse problems that simultaneously minimizes inter-parameter cross talk and the total reconstruction error. Application of this estimator to time-domain diffuse fluorescence imaging shows that the optimal estimator for lifetime multiplexing is identical to a previously developed asymptotic time-domain (ATD) approach, except for the inclusion of a diagonal regularization term containing decay amplitude uncertainties. We show that, while the optimal estimator and ATD provide zero cross talk, the optimal estimator provides lower reconstruction error, while ATD results in superior relative quantitation. The framework presented here is generally applicable to other multiplexing problems where the simultaneous and accurate relative quantitation of multiple parameters is of interest.

Development of a wavefront sensor for terahertz pulses.

Wavefront characterization of terahertz pulses is essential to optimize far-field intensity distribution of time-domain (imaging) spectrometers or increase the peak power of intense terahertz sources. In this paper, we report on the wavefront measurement of terahertz pulses using a Hartmann sensor associated with a 2D electro-optic imaging system composed of a ZnTe crystal and a CMOS camera. We quantitatively determined the deformations of planar and converging spherical wavefronts using the modal Zernike reconstruction least-squares method. Associated with deformable mirrors, the sensor will also open the route to terahertz adaptive optics.

Characterization of a time-resolved non-contact scanning diffuse optical imaging system exploiting fast-gated single-photon avalanche diode detection.

We present a system for non-contact time-resolved diffuse reflectance imaging, based on small source-detector distance and high dynamic range measurements utilizing a fast-gated single-photon avalanche diode. The system is suitable for imaging of diffusive media without any contact with the sample and with a spatial resolution of about 1 cm at 1 cm depth. In order to objectively assess its performances, we adopted two standardized protocols developed for time-domain brain imagers. The related tests included the recording of the instrument response function of the setup and the responsivity of its detection system. Moreover, by using liquid turbid phantoms with absorbing inclusions, depth-dependent contrast and contrast-to-noise ratio as well as lateral spatial resolution were measured. To illustrate the potentialities of the novel approach, the characteristics of the non-contact system are discussed and compared to those of a fiber-based brain imager.

Image enhancement with Gaussian filtering in time‐domain microwave imaging system for breast cancer detection

A time-domain microwave imaging system with improved image quality is presented for early detection of breast cancer. The time-domain system has the advantage of short scan time compared with the frequency-domain system but suffers from low signal-to-noise ratio (SNR) due to high attenuation of the RF signal within the breast tissue. Signal averaging of repeated measurements can improve the SNR, but this method also smears the signal peaks due to the signal-averaged jitter, which adversely affects the reconstructed image quality. An image enhancement method using Gaussian bandpass filtering (BPF) is proposed for a time-domain imaging system. The results from the proposed method were compared with those from the deconvolution method, which is typically used to remove the effects of signal-averaged timing jitter in short pulse measurements. Simulation results with a 3D hemispherical homogeneous breast model containing tumour cells revealed improvement of the reconstructed image quality with the proposed method in the presence of timing jitter. The signal-to-mean ratio was increased from 6.46 to 14.15 with BPF, whereas deconvolution resulted in an increase to 9.36. Simulation results with a non-homogeneous realistic breast model also showed image enhancement with the proposed method.

THz pulsed time-domain imaging of an oil canvas painting: a case study of a painting by Pablo Picasso

The terahertz pulsed time-domain imaging technique and near-infrared observation were applied to investigate an oil painting on canvas by Pablo Picasso. The multilayer structure is clearly observed in cross-sectional image by terahertz pulsed time-domain imaging, and particular Cubism style lines were revealed under newly painted area by near-infrared image.

GPU acceleration of time-domain fluorescence lifetime imaging.

Fluorescence lifetime imaging microscopy (FLIM) plays a significant role in biological sciences, chemistry, and medical research. We propose a Graphic Processing Units (GPUs) based FLIM analysis tool suitable for high-speed and flexible time-domain FLIM applications. With a large number of parallel processors, GPUs can significantly speed up lifetime calculations compared to CPU - OpenMP (parallel computing with multiple CPU cores) based analysis. We demonstrate how to implement and optimize FLIM algorith ms on GPUs for both iterative and non-iterative FLIM analysis algorithms. The implemented algorithms have been tested on both synthesized and experimental FLIM data. The results show that at the same precision the GPU analysis can be up to 24-fold faster than its CPU - OpenMP counterpart. This means that even for high precision but time-consuming iterative FLIM algorithms, GPUs enable fast or even real-time analysis.

テラヘルツ時間領域分光法とイメージングへの応用

テラヘルツ時間領域分光法とイメージングへの応用

A Fast Time-Delay Calculation Method in Through-Wall-Radar Detection Scenario

In TWR (Through Wall Radar) signal processing procedure, time delay estimation is one of the key steps in target localization and high resolution imaging. In time domain imaging procedure such as back projection imaging algorithm, round trip propagation time delay at the path of “transmitter-target-receiver” needs to be calculated for each pixel in imaging region. In typical TWR scenario, transmitter and receiver are at one side and targets at the other side of a wall. Based on two-dimensional searching algorithm or solving two variables equation of four times, traditional time delay calculation algorithms are complex and time consuming, and cannot be used to real-time imaging procedure. In this paper, a new fast time-delay (FTD) algorithm is presented. Because of that incident angle at one side equals to refracting angle at the other side, an equation of lateral distance through the wall can be established. By solving this equation, the lateral distance can be obtained and total propagation time delay can be calculated subsequently. Through processing simulation data, the result shows that new algorithm can be applied effectively to real-time time-delay calculation in TWR signal processing.

Nondestructive Inspection of Thin Basalt Fiber Reinforced Composites Using Combined Terahertz Imaging and Infrared Thermography

The inspection of thin basalt fiber reinforced composite materials was carried out using two nondestructive methods: terahertz time domain imaging and infrared thermography. In order to combine the information about the defects arising in examined materials the inspection results were parametrized. In order to acquire more information content, new approximation based features are proposed. Then, a knowledge extraction based multivariate analysis of preselected features’ vector was carried out. Finally, in order to integrate features distributions of representing different dynamic level of information, a multiresolution wavelet based data fusion algorithm was applied. The results are presented and discussed.

Enhancement of time-domain acoustic imaging based on generalized cross-correlation and spatial weighting

In this paper, an alternative formulation of the time-domain beamforming is proposed using the generalized cross-correlation of measured signals. This formulation uses spatial weighting functions adapted to microphone positions and imaging points. The proposed approach is demonstrated for acoustic source localization using a microphone array, both theoretically and experimentally. An increase in accuracy of acoustic imaging results is shown for both narrow and broadband sources, while a factor of reduction up to 20 in the computation time can be achieved, allowing real-time or volumetric source localization over very large grids.

Application of the Laplace transform in time-domain optical spectroscopy and imaging.

Application of the Laplace transform in time-domain optical spectroscopy and imaging.

Association Between Trabecular Meshwork Anteroposterior Length and Anterior Chamber Angle Width

To investigate the association between trabecular meshwork anteroposterior length and anterior chamber angle width in nonglaucomatous subjects. Prospective, cross-sectional study. Time-domain and spectral-domain anterior segment optical coherence tomography images for 561 eyes from 366 nonglaucomatous subjects were analyzed to determine trabecular meshwork anteroposterior length and 3 anterior chamber angle width parameters measured at different distances from the scleral spur: angle opening distance at 250μm (AOD250), 500μm (AOD500), and 750μm (AOD750) from the scleral spur; trabecular-iris space area at 500μm (TISA500) and 750μm (TISA750) from the scleral spur; angle recess area at 750μm (ARA750) from the scleral spur. Univariable and multivariable linear mixed-effect regression models, the latter adjusting for age, sex, ethnicity, axial length, and the use of both eyes in the same subject, were used to evaluate the association between trabecular meshwork anteroposterior length and anterior chamber angle width parameters. Mean trabecular meshwork anteroposterior length was 824.86 ± 181.77μm. Univariate regression analyses showed a significant positive association between trabecular meshwork anteroposterior length and all anterior chamber angle width parameters: AOD250 (P < .001; β= 335.13), AOD500 (P < .001; β= 271.84), AOD750 (P< .001; β= 202.56), TISA500 (P < .001; β= 780.78), TISA750 (P < .001; β= 449.17), ARA750 (P < .001; β= 381.39). Multivariate regression analyses showed a significant positive association between trabecular meshwork anteroposterior length and all anterior chamber angle width parameters: AOD250 (P= .005; β= 294.02), AOD500 (P= .036; β= 172.94), AOD750 (P= .049; β= 125.58), TISA500 (P= .004; β= 611.51), TISA750 (P= .011; β= 333.96), ARA750 (P < .0001; β= 309.12). Wider anterior chamber angle is associated with greater trabecular meshwork anteroposterior length.

NMR Relaxometry and Imaging to Study Water Dynamics during Soaking and Blanching of Soybean

Abstract In the present study, soaking and blanching soybean were studied by time-domain nuclear magnetic resonance (TD-NMR) relaxometry and magnetic resonance imaging (MRI). Four proton fractions with different mobilities were observed in dried, soaked and blanched soybean samples, which reflects macromolecule, oil and water in different microenvironment of soybean. The proton distribution of soybean was significantly changed during soaking process. According to the result of MRI, water may initially penetrated into soybean from its radicle and micropyle. In terms of water distribution, the optimum soaking time for soybean was appeared to be 8 h. Blanching also significantly changed the proton distribution of soybean. During blanching, all four proton fractions became more mobile, and the accumulation of extracellular water was observed due to the partial destruction of cell/membrane integrity. The heat-induced variations of cotyledon tissues structure mainly occurred in the first 30 min, which implied blanching process should last at least for 30 min.

Wall Painting Investigation by Means of Non-invasive Terahertz Time-Domain Imaging (THz-TDI): Inspection of Subsurface Structures Buried in Historical Plasters

Characterization of subsurface features of wall paintings is important in conservation and technical art history as well as in building archaeology and architecture fields. In this study, an area of the apsidal wall painting of Nebbelunde Church (Rodby, Denmark) has been investigated by means of terahertz time-domain imaging (THz-TDI). Subsurface structures have been detected at different depths inside the lime-based plaster of the wall painting until approximately 1 cm from the surface. The surface morphology of the buried structures has been 3D imaged in detail, providing a substantial contribution in their characterization.