Time-domain Imaging Research Articles

Velocity analysis with local event slopes related probability density function

Macro velocity model plays a key role in seismic imaging and inversion. The performance of traditional velocity analysis methods is degraded by multiples and amplitude-versus-offset (AVO) anomalies. Local event slopes, containing the subsurface velocity information, have been widely used to accomplish common time-domain seismic processing, imaging and velocity estimation. In this paper, we propose a method for velocity analysis with probability density function (PDF) related to local event slopes. We first estimate local event slopes with phase information in the Fourier domain. An adaptive filter is applied to improve the performance of slopes estimator in the low signal-to-noise ratio (SNR) situation. Second, the PDF is approximated with the histogram function, which is related to attributes derived from local event slopes. As a graphical representation of the data distribution, the histogram function can be computed efficiently. By locating the ray path of the first arrival on the semblance image with straight-ray segments assumption, automatic velocity picking is carried out to establish velocity model. Unlike local event slopes based velocity estimation strategies such as averaging filters and image warping, the proposed method does not make the assumption that the errors of mapped velocity values are symmetrically distributed or that the variation of amplitude along the offset is slight. Extension of the method to prestack time-domain migration velocity estimation is also given. With synthetic and field examples, we demonstrate that our method can achieve high resolution, even in the presence of multiples, strong amplitude variations and polarity reversals.

Radar‐based time‐domain head imaging using database of effective dielectric constant

In radar-based time-domain head imaging, a priori information on the effective dielectric constants from the antennas' perspective with respect to different head pixels is needed to map the significant scatterrers inside the head. A microwave-based head imaging technique, which utilises a model for the effective dielectric constant as a function of the antenna's position and imaged point inside the head, is presented. To build that model, a realistic MRI-derived numerical head surrounded by an array of wideband antennas is implemented in a full-wave electromagnetic solver. The time difference of arrival between transmission coefficients in free space and inside the head is calculated at different head pixels and used to evaluate the effective dielectric constant at those pixels. The model is validated on two different head phantoms and used to accurately detect different brain strokes.

A comparative experimental investigation on responsivity and response speed of photo-diode and photo-BJT structures integrated in a low-cost standard CMOS process

A variety of smart imaging and neuromorphic applications perform time-domain image acquisition in order to imitate biological systems and reduce the growing transmission bandwidth of the modern imaging devices. Because they operate in time-domain, they require the highest possible pixel responsivity and response speed. This work provides a comparative experimental study of different unconventional photodetecting structures with respect to these parameters. Several on-chip photodetecting device structures are designed using a low-cost standard 0.18μm CMOS process. The comparison in terms of measured quantum efficiency, light responsivity and the response speed is presented between conventional and comb-shaped N-well/P-substrate photodiodes, conventional and comb-shaped, vertical and lateral photo-bipolar-junction-transistors (photo-BJTs) and a Darlington pair of bipolar-junction phototransistors. The photodetectors are embedded in a conventional three transistor active pixel topology and measured using a customized low-cost measurement setup. The pixel quantum efficiency, responsivity and response speed are measured for each structure and the results are presented in detail. The obtained results demonstrate the benefits of using standard-CMOS-compatible BJT structures in time-domain applications. The BJT-based photodetectors show increased responsivity to green–yellow light region (500–600nm wavelength) compared to conventional N-well/P-substrate diode. The highest responsivity is achieved by a combination of lateral and vertical BJT. The fastest response is achieved by the rarely used Darlington pair configuration of BJTs, which demonstrates the potential benefit of using this structure for time-domain imaging applications. A low-cost measurement setup and the measurement methodology are described in detail to make the experiment reproducible for any other standard CMOS process.

Active Transient Sound Radiation Control from a Smart Piezocomposite Hollow Cylinder

Abstract The linear 3D piezoelasticity theory along with active damping control (ADC) strategy are applied for non-stationary vibroacoustic response suppression of a doubly fluid-loaded functionally graded piezolaminated (FGPM) composite hollow cylinder of infinite length under general time-varying excitations. The control gain parameters are identified and tuned using Genetic Algorithm (GA) with a multi-objective performance index that constrains the key elasto-acoustic system parameters and control voltage. The uncontrolled and controlled time response histories due to a pair of equal and opposite impulsive external point loads are calculated by means of Durbin’s numerical inverse Laplace transform algorithm. Numerical simulations demonstrate the superior (good) performance of the GA-optimized distributed active damping control system in effective attenuation of sound pressure transients radiated into the internal (external) acoustic space for two basic control configurations. Also, some interesting features of the transient fluid-structure interaction control problem are illustrated via proper 2D time domain images and animations of the 3D sound field. Limiting cases are considered and accuracy of the formulation is established with the aid of a commercial finite element package as well as comparisons with the current literature.

AUTOMATED TRANSIENT IDENTIFICATION IN THE DARK ENERGY SURVEY

We describe an algorithm for identifying point-source transients and moving objects on reference-subtracted optical images containing artifacts of processing and instrumentation. The algorithm makes use of the supervised machine learning technique known as Random Forest. We present results from its use in the Dark Energy Survey Supernova program (DES-SN), where it was trained using a sample of 898,963 signal and background events generated by the transient detection pipeline. After reprocessing the data collected during the first DES-SN observing season (Sep. 2013 through Feb. 2014) using the algorithm, the number of transient candidates eligible for human scanning decreased by a factor of 13.4, while only 1 percent of the artificial Type Ia supernovae (SNe) injected into search images to monitor survey efficiency were lost, most of which were very faint events. Here we characterize the algorithm's performance in detail, and we discuss how it can inform pipeline design decisions for future time-domain imaging surveys, such as the Large Synoptic Survey Telescope and the Zwicky Transient Facility. An implementation of the algorithm and the training data used in this paper are available at http://portal.nersc.gov/project/dessn/autoscan.

Inspection of panel paintings beneath gilded finishes using terahertz time-domain imaging

Scientific analysis of panel paintings could provide key information to art historians and conservators about the composition and condition of the constituent layers. Knowledge of the structure, stratigraphy, and condition of the subsurface layers of these objects is highly relevant to conservation measures, when stability problems such as delamination or internal cracking are considered. Terahertz time-domain imaging (THz-TDI) in reflection geometry is capable of highlighting interfaces between layers in a stratigraphic buildup, and thus makes structural information available in the form of subsurface 3D images and stratigraphic images (B-scans). The technique is contactless and non-invasive, and has been successfully applied to investigation of panel paintings. Regarding gilded panel paintings, the extremely high reflectivity of metals at submillimeter wavelengths generally precludes the transmission of electromagnetic waves through metallic films. Therefore, imaging of subsurface features below gilded finishes may be regarded as Very challenging. A small but non-negligible direct transmission through metal films occurs if the film thickness is of the order of the skin depth of the metal in the terahertz range, due to the skin effect. The thickness of gold leaf, a fraction of a micrometer, matches the skin depth of gold in the terahertz frequency range covered by THz-TDI devices. We therefore investigated and imaged subsurface features of panel paintings through gilded finishes with THz-TDI. Subsurface layers of three gilded panel paintings (two contemporary tempera panel replicas and one fourteenth-century icon) have been successfully imaged behind gold finishes with THz-TDI.

Dual-Modality Noninvasive Mapping of Sentinel Lymph Node by Photoacoustic and Near-Infrared Fluorescent Imaging Using Dye-Loaded Mesoporous Silica Nanoparticles.

The imaging of sentinel lymph nodes (SLNs), the first defense against primary tumor metastasis, has been considered as an important strategy for noninvasive tracking tumor metastasis in clinics. In this study, we developed an imaging contrast system based on fluorescent dye-loaded mesoporous silica nanoparticles (MSNPs) that integrate near-infrared (NIR) fluorescent and photoacoustic (PA) imaging modalities for efficient SLN mapping. By balancing the ratio of dye and nanoparticles for simultaneous optimization of dual-modality imaging (NIR and PA), the dye encapsulated MSNP platform was set up to generate both a moderate NIR emission and PA signals simultaneously. Moreover, the underlying mechanisms of the relevance between optical and PA properties were discovered. Subsequently, dual-modality imaging was achieved to visualize tumor draining SLNs up to 2 weeks in a 4T1 tumor metastatic model. Obvious differences in uptake rate and contrast between metastatic and normal SLNs were observed both in vivo and ex vivo. Based on all these imaging data, it was demonstrated that the dye-loaded MSNPs allow detection of regional lymph nodes in vivo with time-domain NIR fluorescent and PA imaging methods efficiently.

Analysis of a seventeenth-century panel painting by reflection terahertz time-domain imaging (THz-TDI): contribution of ultrafast optics to museum collections inspection

Terahertz time-domain imaging (THz-TDI) has been applied for nondestructive visualization of a hidden painting and other subsurface composition layers of a seventeenth-century panel painting belonging to the National Gallery of Denmark. Plan-type and cross-sectional scans realized by THz have been compared with images obtained by X-radiography, thus helping in a deep understanding of the strengths and limitations of this technique for art diagnostic purposes and in defining its rule among the other complementary investigation tools for nondestructive inspection of art pieces.

Targeted tumor imaging of anti-CD20-polymeric nanoparticles developed for the diagnosis of B-cell malignancies.

The expectations of nanoparticle (NP)-based targeted drug delivery systems in cancer, when compared with convectional therapeutic methods, are greater efficacy and reduced drug side effects due to specific cellular-level interactions. However, there are conflicting literature reports on enhanced tumor accumulation of targeted NPs, which is essential for translating their applications as improved drug-delivery systems and contrast agents in cancer imaging. In this study, we characterized biodegradable NPs conjugated with an anti-CD20 antibody for in vivo imaging and drug delivery onto tumor cells. NPs’ binding specificity mediated by anti-CD20 antibody was evaluated on MEC1 cells and chronic lymphocytic leukemia patients’ cells. The whole-body distribution of untargeted NPs and anti-CD20 NPs were compared by time-domain optical imaging in a localized human/mouse model of B-cell malignancy. These studies provided evidence that NPs’ functionalization by an anti-CD20 antibody improves tumor pharmacokinetic profiles in vivo after systemic administration and increases in vivo imaging of tumor mass compared to non-targeted NPs. Together, drug delivery and imaging probe represents a promising theranostics tool for targeting B-cell malignancies.

Reflection terahertz time-domain imaging for analysis of an 18th century neoclassical easel painting.

Terahertz time-domain imaging (THz-TDI) has been applied for imaging a hidden portrait and other subsurface composition layers of an 18th century (18C) easel painting by Nicolai Abildgaard, the most important 18C Danish neoclassical painter of historical and mythological subjects. For the first time, a real hidden portrait on an easel painting has been imaged by THz-TDI, with an unexpected richness of detail. THz C- and B-scans have been compared with images obtained by x-ray radiography and invasive cross-sectional imaging, leading to a deeper understanding of the strengths and limitations of this technique for art diagnostic purposes and defining its role among complementary tools for the investigation of art objects. We present a fast and effective method to separate single THz pulse reflections of interest from the entire signal across the image, adapted for uneven surfaces typically encountered in practical applications of the technique. Interfaces between layers of the painting have been successfully imaged, contributing substantially to the understanding of the structure of the painting.

Terahertz imaging modalities of ancient Egyptian mummified objects and of a naturally mummified rat.

During the last few years, terahertz (THz) imaging has been used to investigate artwork and historic artifacts. The application of THz imaging to mummy investigations is very attractive since it provides spectroscopic information over a broad frequency range and its radiation has proven to be harmless to human cells. However, compared with the current standard imaging methods in mummy imaging-X-ray and computed tomography (CT)--it remains a novel, emerging technique whose potential still needs to be fully evaluated. Here, ancient Egyptian mummified objects as well as a naturally mummified rat have been investigated by two different THz imaging systems: a broadband THz time domain imaging system and an electronic THz scanner. The obtained THz images are compared with conventional CT, X-ray, and magnetic resonance images. While the broadband THz time domain setup permits analyses of smaller samples, the electronic THz scanner allows the recording of data of thicker and larger samples at the expense of a limited spectral bandwidth. Terahertz imaging shows clear potential for mummy investigations, although currently CT imaging offers much higher spatial resolution. Furthermore, as commercial mobile THz scanners become available, THz imaging could be applied directly in museums or at excavation sites.

Terahertz spectroscopic identification of explosive and drug simulants concealed by various hiding techniques.

Terahertz time-domain spectroscopy and imaging is used to study the effects of various hiding techniques of spectral features of drug and explosive simulants in combination with different paper and textile barriers. Results show that rapid detection and identification of concealed simulants is possible in the frequency range from 1.5 to 4.0 THz by using an organic-crystal-based terahertz time-domain system and the spectral peak analysis method.

Time-Domain Wideband Adaptive Beamforming for Radar Breast Imaging

A novel wideband microwave radar imaging method is presented to detect regions of significant dielectric contrast within the breast. Clutter reduction is paramount to any radar imaging algorithm, especially with clinical patient data where the tissue composition of the breast is inhomogeneous. Time-domain data-adaptive imaging methods have been previously applied in a narrowband manner for microwave radar breast imaging when the received signal spectral content was wideband. In this study, a wideband time-domain adaptive imaging approach is presented to perform data-adaptive focusing across the spectrum to reduce clutter. An equalization filter is adapted to compensate for the propagation distortion through tissue using a calculated estimate of the average dielectric properties of the breast. The effectiveness of the proposed wideband adaptive imaging approach is evaluated in conjunction with the delay-and-sum (DAS) method using numerical, experimental, and clinical data. Target scatterers are clearly detected while clutter levels are reduced significantly, between 4 and 6 dB, when compared to the DAS technique.

A Less-Memory and High-Efficiency Autofocus Back Projection Algorithm for SAR Imaging

The back projection (BP) algorithm is an accurate time-domain imaging method for synthetic aperture radar. However, the influences of wind field and turbulence on the platform make the antenna phase centers (APCs) greatly deviate from the designed linear trajectory and degrade the BP performance. Although the inertial measurement unit can be used to measure the trajectory, the measurement error still affects the image quality to some extent. The autofocus BP algorithm under the criterion of maximum image sharpness has been proposed to compensate the motion error effectively. However, this method needs to compute and store the per-pulse back-projected values for all pixels, which results in the heavy burden for memory and time and limits its practical application. This letter demonstrates an improved way to overcome these two drawbacks without loss of focusing performance. In our new method, only minority pixels are selected for autofocus to estimate the phase error, which is used to obtain the APCs by solving a system of nonlinear equations with an optimization method. This procedure is particularly suitable for the wide application of the autofocus BP. The experimental results strongly validate the efficacy and efficiency of the improved autofocus BP algorithm.

In vivo tomographic imaging of deep-seated cancer using fluorescence lifetime contrast.

Preclinical cancer research would benefit from noninvasive imaging methods that allow tracking and visualization of early-stage metastasis in vivo. Although fluorescent proteins revolutionized intravital microscopy, two major challenges that still remain are tissue autofluorescence and hemoglobin absorption, which act to limit intravital optical techniques to large or subcutaneous tumors. Here, we use time-domain (TD) technology for the effective separation of tissue autofluorescence from extrinsic fluorophores, based on their distinct fluorescence lifetimes. In addition, we use cancer cells labeled with near infrared fluorescent proteins (iRFP) to allow deep-tissue imaging. Our results demonstrate that TD imaging allows the detection of metastasis in deep-seated organs of living mice with a more than 20-fold increase in sensitivity compared with conventional continuous wave techniques. Furthermore, the distinct fluorescence lifetimes of iRFPs enable lifetime multiplexing of three different tumors, each expressing unique iRFP labels in the same animal. Fluorescence tomographic reconstructions reveal three-dimensional distributions of iRFP720-expressing cancer cells in lungs and brain of live mice, allowing ready longitudinal monitoring of cancer cell fate with greater sensitivity than otherwise currently possible.

Seismic diffraction interpretation at Piceance Creek

We applied time-domain seismic diffraction imaging to a 3D data set from the Piceance Creek Field, Piceance Basin, northwest Colorado. The work was motivated by the need for insight into natural fracture distribution, thought to influence production. We used a novel chain of two previously developed processing steps to separate diffractions from the recorded wavefield — One step is applied to the conventional stack volume, and the other was applied to migrated dip-angle gathers. The diffractions were then imaged independently for interpretation. Comparison of seismic attributes, commonly used for fracture characterization, found that the resulting diffraction image had lateral resolution comparable to or greater than the discontinuity-type attributes and provided information complementary to azimuthal anisotropy measurements. The diffraction image from Piceance Creek had advantages over attributes in interpretation confidence because diffractions were a direct seismic response to subsurface features of intermediate size. Although these features were larger than the fractures thought to influence production, knowledge of intermediate-scale features can improve fracture prediction in the context of geologic scaling relationships or rock physics models. Qualitative interpretation of the diffraction amplitudes distinguished edge-type and line-type diffractions, indicative of fault versus channel-fill features, respectively. Even the largest faults at Piceance Creek only generated diffractions where contrasting lithologies were juxtaposed. Where there was lateral contrast, diffractions appeared to delineate small faults and channels with vertical resolution limited to the same order as the conventional seismic image.

Fast Time-Domain Imaging in Elliptical Polar Coordinate for General Bistatic VHF/UHF Ultra-Wideband SAR With Arbitrary Motion

In this paper, a fast time-domain imaging algorithm called bistatic fast factorized back projection algorithm (BFFBPA) is proposed for the general bistatic VHF/UHF ultra-wideband synthetic aperture radar. It cannot only accurately dispose the large spatial variant range cell migrations, serious range-azimuth coupling and complicated motion error, but also achieve the imaging efficiency similar to frequency-domain algorithms. It represents subimages in elliptical polar coordinate to reduce the computational load. The imaging geometry with arbitrary motion in this coordinate system is provided, and the bistatic back projection algorithm (BPA) is derived to provide a basis for the proposed BFFBPA. Considering motion errors, the more accurate sampling requirements for elliptical subimages is deduced to offer the near-optimum tradeoff between the imaging quality and efficiency, and the constraint of motion errors for acceptable sampling requirements is discussed. Based on this sampling requirement, the advantage of using elliptical subimage grids for this BFFBPA is analyzed. A phase error correction is performed to reduce the impact of phase errors caused by interpolations in the BFFBPA. The speed-up factor of this BFFBPA with respect to the bistatic BPA is derived. Simulation results and evaluations are given to prove the correctness of the theory analysis and validity of the proposed method.

Realistic modeling of ion-neutral collisions in quadrupole ion traps.

In this study, three ion-neutral collision models have been discussed and compared, including the Langevin, the hard-sphere and the mixed collision models. With the pseudo-potential approximation, analytical expressions of ion secular motions with the hard-sphere and mixed collision models have been obtained for the first time. Through numerical simulations and theoretical calculations, it is found that the mixed collision model could be used as a general description of ion-neutral collisions under different conditions. Langevin collision model is a good description of low energy collisions between small ions and neutrals, while hard-sphere collision model could be used to describe high energy collisions and/or ions with higher masses (larger physical sizes). These analytical expressions of ion motion decay profiles enable the creation of direct relationships between time-domain image currents with ion collision cross sections.

Terahertz pulse imaging in archaeology

The work presented in this paper was performed at the Oriental Institute at the University of Chicago, on objects from their permanent collection: an ancient Egyptian bird mummy and three ancient Sumerian corroded copper-alloy objects. We used a portable, fiber-coupled terahertz (THz) time-domain spectroscopic imaging system, which allowed us to measure specimens in both transmission and reflection geometry, and present time- and frequency-based image modes. The results confirm earlier evidence that THz imaging can provide complementary information to that obtainable from X-ray computed tomography (XRCT) scans of mummies, giving better visualisation of low density regions. In addition, we demonstrated that THz imaging can distinguish mineralized layers in metal artifacts.

Anatomy of the fetal membranes using optical coherence tomography: Part 1

IntroductionIn vitro studies on the structure of human fetal membranes have involved light or electron microscopy with fixation, dehydration, and staining. Recently, optical coherence tomography (OCT), an imaging technology, has provided high-resolution cross-sectional images of living biological tissues, with a penetration of 2–3 mm. We evaluated the use of this technology to examine the histologic features of human fetal membranes immediately after delivery. MethodsSamples of fetal membranes of ten patients undergoing cesarean deliveries (four uncomplicated pregnancies, four with preeclampsia, and two with chorioamnionitis) and eight patients undergoing vaginal deliveries (six uncomplicated pregnancies and two with chorioamnionitis) were collected immediately after delivery. Samples were stretched across customized disks, rinsed, and analyzed using a time-domain OCT imaging system. Following OCT scanning, the samples were placed in formalin for histologic study. The OCT images were compared to histologic images of common human fetal membrane features. ResultsWe were able to delineate the layers of the fetal membranes using bench-top time-domain OCT. The system was able to image histologic features of the fetal membranes, such as microscopic chorionic pseudocysts, ghost villi, meconium stained membranes, and chorioamnionitis. The OCT images corresponded with the histologic findings. DiscussionThis feasibility study demonstrates the potential of OCT technology for real-time assessment of human fetal membranes and may provide clinically useful information at delivery.