Real-time Imaging Applications Research Articles

Toward real-time terahertz imaging

Terahertz (THz) science and technology have greatly progressed over the past two decades to a point where the THz region of the electromagnetic spectrum is now a mature research area with many fundamental and practical applications. Furthermore, THz imaging is positioned to play a key role in many industrial applications, as THz technology is steadily shifting from university-grade instrumentation to commercial systems. In this context, the objective of this review is to discuss recent advances in THz imaging with an emphasis on the modalities that could enable real-time high-resolution imaging. To this end, we first discuss several key imaging modalities developed over the years: THz transmission, reflection, and conductivity imaging; THz pulsed imaging; THz computed tomography; and THz near-field imaging. Then, we discuss several enabling technologies for real-time THz imaging within the time-domain spectroscopy paradigm: fast optical delay lines, photoconductive antenna arrays, and electro-optic sampling with cameras. Next, we discuss the advances in THz cameras, particularly THz thermal cameras and THz field-effect transistor cameras. Finally, we overview the most recent techniques that enable fast THz imaging with single-pixel detectors: mechanical beam-steering, compressive sensing, spectral encoding, and fast Fourier optics. We believe that this critical and comprehensive review of enabling hardware, instrumentation, algorithms, and potential applications in real-time high-resolution THz imaging can serve a diverse community of fundamental and applied scientists.

Optimizing photoacoustic image reconstruction using cross-platform parallel computation

Three-dimensional (3D) image reconstruction involves the computations of an extensive amount of data that leads to tremendous processing time. Therefore, optimization is crucially needed to improve the performance and efficiency. With the widespread use of graphics processing units (GPU), parallel computing is transforming this arduous reconstruction process for numerous imaging modalities, and photoacoustic computed tomography (PACT) is not an exception. Existing works have investigated GPU-based optimization on photoacoustic microscopy (PAM) and PACT reconstruction using compute unified device architecture (CUDA) on either C++ or MATLAB only. However, our study is the first that uses cross-platform GPU computation. It maintains the simplicity of MATLAB, while improves the speed through CUDA/C++ − based MATLAB converted functions called MEXCUDA. Compared to a purely MATLAB with GPU approach, our cross-platform method improves the speed five times. Because MATLAB is widely used in PAM and PACT, this study will open up new avenues for photoacoustic image reconstruction and relevant real-time imaging applications.

Red-Emitting Fluorescent Probe for Detection of γ-Glutamyltranspeptidase and Its Application of Real-Time Imaging under Oxidative Stress in Cells and in Vivo.

γ-Glutamyltranspeptidase (GGT) plays critical roles in regulating various physiological/pathophysiological processes including the intracellular redox homeostasis. However, an effective fluorescent probe for dissecting the relationships between GGT and oxidative stress in vivo remains largely unexplored. Herein, we present a light-up fluorescent probe (DCDHF-Glu) with long wavelength emission (613 nm) for the highly sensitive and selective detection of GGT using dicyanomethylenedihydrofuran derivative as the fluorescent reporter and γ-glutamyl group as the enzyme-active trigger. DCDHF-Glu is competent to real-time image endogenous GGT in live cells and mice. In particular, DCDHF-Glu enables the direct real-time visualization of the upregulation of GGT under drug-induced oxidative stress in the HepG2 cells and the LO2 cells, as well as in vivo, vividly implying its excellent capacity in elucidation of GGT function in GGT-related biological events.

Fast and robust Block-Sparse Bayesian learning for EEG source imaging

We propose a new Sparse Bayesian Learning (SBL) algorithm that can deliver fast, block-sparse, and robust solutions to the EEG source imaging (ESI) problem in the presence of noisy measurements. Current implementations of the SBL framework are computationally expensive and typically handle fluctuations in the measurement noise using different heuristics that are unsuitable for real-time imaging applications. We address these shortcomings by decoupling the estimation of the sensor noise covariance and the sparsity profile of the sources, thereby yielding an efficient two-stage algorithm. In the first stage, we optimize a simplified non-sparse generative model to get an estimate of the sensor noise covariance and a good initialization of the group-sparsity profile of the sources. Sources obtained at this stage are equivalent to those estimated with the popular inverse method LORETA. In the second stage, we apply a fast SBL algorithm with the noise covariance fixed to the value obtained in the first stage to efficiently shrink to zero groups of sources that are irrelevant for explaining the EEG measurements. In addition, we derive an initialization to the first stage of the algorithm that is optimal in the least squares sense, which prevents delays due to suboptimal initial conditions. We validate our method on both simulated and real EEG data. Simulations show that the method is robust to measurement noise and performs well in real-time, with faster performance than two state of the art SBL solvers. On real error-related negativity EEG data, we obtain source images in agreement with the experimental literature. The method shows promise for real-time neuroimaging and brain-machine interface applications.

Design of a New Near-Infrared Ratiometric Fluorescent Nanoprobe for Real-Time Imaging of Superoxide Anions and Hydroxyl Radicals in Live Cells and in Situ Tracing of the Inflammation Process in Vivo.

The superoxide anion (O2•-) and hydroxyl radical (•OH) are important reactive oxygen species (ROS) used as biomarkers in physiological and pathological processes. ROS generation is closely related to the development of a variety of inflammatory diseases. However, the changes of ROS are difficult to ascertain with in situ tracing of the inflammation process by real-time monitoring, owing to the short half-lives of ROS and high tissue autofluorescence in vivo. Here we developed a new near-infrared (NIR) ratiometric fluorescence imaging approach by using a Förster resonance energy transfer (FRET)-based ratiometric fluorescent nanoprobe for real-time monitoring of O2•- and •OH generation and also by using in situ tracing of the inflammation process in vivo. The proposed nanoprobe was composed of PEG functionalized GQDs as the energy donor connecting to hydroIR783, serving as both the O2•-/•OH recognizing ligand and the energy acceptor. The nanoprobe not only exhibited a fast response to O2•- and •OH but also presented good biocomapatibility as well as a high photostability and signal-to-noise ratio. We have demonstrated that the proposed NIR ratiometric fluorescent nanoprobe can monitor the changes of O2•- and •OH in living RAW 264.7 cells via a drug mediating inflammation model and further realized visual monitoring of the change of O2•- and •OH in mice for in situ tracing of the inflammation process. Our design may provide a new paradigm for long-term and real-time imaging applications for in vivo tracing of the pathological process related to the inflammatory diseases.

A Side-Lobe Suppression Method Based on Coherence Factor for Terahertz Array Imaging

Terahertz (THz) arrays can be used to improve the data acquisition speed considerably in real-time imaging applications. However, the THz array imaging usually suffers from the side-lobe artifacts, which leads to a severe decline in the image quality. In this paper, a side-lobe suppression method based on coherence factor is proposed to improve the image quality. The influences of signal-to-noise ratio on the imaging results are analyzed by simulation. Furthermore, the results based on the real-world data validate the effectiveness of the proposed method, which indicates that the side-lobe is suppressed by 29 dB. This paper can benefit the development of THz imaging technique and its applications in real-time imaging realms.

On-grating graphene surface plasmons enabling spatial differentiation in the terahertz region.

We propose a graphene-on-grating nanostructure to enable second-order spatial differentiation computation in the terahertz (THz) region. The differentiation operation is based on the interference between the direct reflected field and the leakage of two excited surface plasmon polaritons counter-propagating along the graphene sheet. With the spatial coupled-mode theory, we derive that the requirement for the second-order spatial differentiation is the critical coupling condition. We numerically demonstrate such an analog computation with Gaussian beams. It shows that the spatial bandwidth of the proposed differentiator is large enough such that even when the waist radius of the Gaussian beam is as narrow as w0=0.68λ (λ is the free-space wavelength), the accuracy of the differentiator is higher than 95%. The proposed differentiator is ultra-compact, with a thickness less than 0.1λ, and useful for real-time imaging applications in THz security detections.

Demonstration of a Hyperlens-integrated Microscope and Super-resolution Imaging.

The use of super-resolution imaging to overcome the diffraction limit of conventional microscopy has attracted the interest of researchers in biology and nanotechnology. Although near-field scanning microscopy and superlenses have improved the resolution in the near-field region, far-field imaging in real-time remains a significant challenge. Recently, the hyperlens, which magnifies and converts evanescent waves into propagating waves, has emerged as a novel approach to far-field imaging. Here, we report the fabrication of a spherical hyperlens composed of alternating silver (Ag) and titanium oxide (TiO2) thin layers. Unlike a conventional cylindrical hyperlens, the spherical hyperlens allows for two-dimensional magnification. Thus, incorporation into conventional microscopy is straightforward. A new optical system integrated with the hyperlens is proposed, allowing for a sub-wavelength image to be obtained in the far-field region in real time. In this study, the fabrication and imaging setup methods are explained in detail. This work also describes the accessibility and possibility of the hyperlens, as well as practical applications of real-time imaging in living cells, which can lead to a revolution in biology and nanotechnology.

Improved Aromatic Substitution-Rearrangement-Based Ratiometric Fluorescent Cysteine-Specific Probe and Its Application of Real-Time Imaging under Oxidative Stress in Living Zebrafish.

Biothiols, including cysteine (Cys), homocysteine (Hcy), and glutathione (GSH), play a crucial role in many physiological processes. Cys production and metabolism is closely connected with Hcy and GSH; meanwhile, the dynamic antioxidant defenses network by Cys is independent of the GSH system, and Cys can serve as a more effective biomarker of oxidative stress. Hence, it is significant and urgent to develop an efficient method for specific detection of Cys over other biothiols (Hcy/GSH). However, most of the present Cys-specific fluorescent probes distinguished Cys from Hcy through response time, which would suffer from an unavoidable interference from Hcy in long-time detection. In this work, in order to improve the selectivity, we employed an improved aromatic substitution-rearrangement strategy to develop a ratiometric Cys-specific fluorescent probe (Cou-SBD-Cl) based on a new fluorescence resonance energy transfer (FRET) coumarin-sulfonyl benzoxadiazole (Cou-SBD) platform for discrimination of Hcy and GSH. Response of Cou-SBD-Cl to Cys would switch FRET on and generate a new yellow fluorescence emission with a 56.1-fold enhancement of ratio signal and a 99 nm emission shift. The desirable dual-color ratiometric imaging was achieved in living cells and normal zebrafish. In addition, probe Cou-SBD-Cl was also applied to real-time monitor Cys fluctuation in lipopolysaccharide-mediated oxidative stress in zebrafish.

Exploiting self-adaptive permutation–diffusion and DNA random encoding for secure and efficient image encryption

This paper presents a solution for secure and efficient image encryption with the help of self-adaptive permutation–diffusion and DNA random encoding. The plain image is firstly converted to DNA sequence using random encoding rules, so as to disarrange the bit distribution of the plaintext. A self-adaptive permutation–diffusion procedure is subsequently introduced for further encryption. The quantization processes of the permutation and diffusion procedures are disturbed by the intrinsic features of the plaintext, with the introduced disturbances can be automatically retrieved in the decryption end. The security of the system originates from the plaintext-related quantization of the encryption process which makes the cryptosystem secure against plaintext attack. Besides, the reusability of the random variables can dramatically promote the efficiency of the cryptosystem, which renders great potential for real-time secure image applications.

Architecture of an Ultrasound System for Continuous Real-Time High Frame Rate Imaging.

High frame rate (HFR) imaging methods based on the transmission of defocused or plane waves rather than focused beams are increasingly popular. However, the production of HFR images poses severe requirements both in the transmission and the reception sections of ultrasound scanners. In particular, major technical difficulties arise if the images must be continuously produced in real-time, i.e., without any acquisition interruption nor loss of data. This paper presents the implementation of the real-time HFR-compounded imaging application in the ULA-OP 256 research platform. The beamformer sustains an average output sample rate of 470 MSPS. This allows continuously producing coherently compounded images, each of 64 lines by 1280 depths (here corresponding to 15.7 mm width and 45 mm depth, respectively), at frame rates up to 5.3 kHz. Imaging tests addressed to evaluate the achievable speed and quality performance were conducted on phantom. Results obtained by real-time compounding frames obtained with different numbers of steering angles between +7.5° and -7.5° are presented.

Diagnosis of disk displacement using real-time MRI: Clinical report of two patients

The clinical application of real-time magnetic resonance imaging (MRI) for the diagnosis of temporomandibular joint disk displacement (DD) with and without reduction is presented. In 2 patients with presumed DD, real-time MRI at 15 frames per second was performed during the natural opening and closing of the mouth. In one patient unilateral DD with reduction and in the other patient bilateral DD without reduction were observed. In contrast with conventional static MRI, real-time MRI moving images of temporomandibular joint DD offer comprehensive information about the dynamics of all involved structures, which in turn promises more reliable diagnoses. Real-time MRI is more rapid, more reliable, more informative, and less stressful for patients with temporomandibular disorders (TMDs).

Selective area deposited n-Al0.5Ga0.5N channel field effect transistors with high solar-blind ultraviolet photo-responsivity

We report on AlGaN field effect transistors over AlN/sapphire templates with selective area grown n-Al0.5Ga0.5N channel layers for which a field-effect mobility of 55 cm2/V-sec was measured. Using a pulsed plasma enhanced chemical vapor deposition deposited 100 A thick SiO2 layer as the gate-insulator, the gate-leakage currents were reduced by three orders of magnitude. These devices with or without gate insulators are excellent solar-blind ultraviolet detectors, and they can be operated either in the photoconductive or the photo-voltaic modes. In the photo-conductive mode, gain arising from hole-trapping in the depletion region leads to steady-state photoresponsivity as high as 1.2 × 106A/W at 254 nm, which drops sharply below 290 nm. A hole-trapping limited detector response time of 34 ms, fast enough for real-time flame-detection and imaging applications, was estimated.

Real-time image haze removal using an aperture-division polarimetric camera.

Polarimetric dehazing methods have been proven to be effective in enhancing the quality of images acquired in turbid media. We report a new full-Stokes polarimetric camera, which is based on the division of aperture structure. We design a kind of automatic polarimetric dehazing algorithm and load it into the field programmable gate array (FPGA) modules of our designed polarimetric camera, achieving a real-time image haze removal with an output rate of 25 fps. We demonstrate that the image quality can be significantly improved together with a good color restoration. This technique might be attractive in a range of real-time outdoor imaging applications, such as navigation, monitoring, and remote sensing.

Disrupt toxicity testing

Dimensions of time that are relevant to molecular mechanisms are not incorporated into conventional toxicity testing methodologies. Historically, this was due to technological limitations. These limitations no longer exist. Application of real-time imaging and chemical sensor technologies presents an opportunity to overcome the challenges that have stalled essential transformation of toxicity testing methodology.

Photonics-based real-time and high-resolution ISAR imaging of non-cooperative target

Real-time and high-resolution imaging is demonstrated based on field trial detection of a non-cooperative target using a photonics-based inverse synthetic aperture radar (ISAR). By photonic generation and de-chirping of broadband linear frequency modulation signals, the radar can achieve a high range resolution thanks to the large instantaneous bandwidth (8 GHz at the K band), as well as real-time ISAR imaging using low-speed analog-to-digital conversion (25 MSa/s). A small-size unmanned aerial vehicle is employed as the non-cooperative target, and ISAR imaging is realized with a resolution far better than those achieved by the previously reported photonics-based ISARs. The capability for real-time ISAR imaging is also verified with an imaging frame rate of 25 fps. These results validate that the photonics-based radar is feasible in practical real-time and high-resolution ISAR imaging applications.

Spatially heterodyned snapshot imaging spectrometer.

Snapshot hyperspectral imaging Fourier transform (SHIFT) spectrometers are a promising technology in optical detection and target identification. For any imaging spectrometer, spatial, spectral, and temporal resolution, along with form factor, power consumption, and computational complexity are often the design considerations for a desired application. Motivated by the need for high spectral resolution systems, capable of real-time implementation, we demonstrate improvements to the spectral resolution and computation trade-space. In this paper, we discuss the implementation of spatial heterodyning, using polarization gratings, to improve the spectral resolution trade space of a SHIFT spectrometer. Additionally, we employ neural networks to reduce the computational complexity required for data reduction, as appropriate for real-time imaging applications. Ultimately, with this method we demonstrate an 87% decrease in processing steps when compared to Fourier techniques. Additionally, we show an 80% reduction in spectral reconstruction error and a 30% increase in spatial fidelity when compared to linear operator techniques.

Twin-mediated crystal growth

Abstract

Laser scanning by rotating polarization gratings.

Laser beam scanning can be realized using two independently rotating, inline polarization gratings, termed Risley gratings, in a fashion similar to Risley prisms. The analytical formulas of pointing position as well as their inverse solutions are described. On this basis, the beam scanning is investigated and the performance of scanning imaging is evaluated. It is shown that the scanning function in 1D scanning evolves from a sinusoidal to triangular scan and the duty cycle increases rapidly as the ratio of grating period to wavelength is reduced toward 2. The scan pattern in 2D scanning is determined by the ratio k of the gratings' rotatory frequency. In imaging applications, when k tends toward 1 or -1, the scan pattern becomes dense and is inclined to be spiral or rose-like, respectively, which is desirable for the purpose of enhancing spatial resolution. There is a direct trade-off between spatial resolution and frame rate. The spiral and rose scanning enable multiresolution imaging, providing a preview of the scanned area in a fraction of the overall scan time, which is extremely useful for fast, real-time imaging applications.

Contrast-enhanced continuous-terahertz-wave imaging based on superparamagnetic iron oxide nanoparticles for biomedical applications.

We present a novel contrast-enhanced continuous-terahertz-wave imaging modality based on magnetic induction heating of superparamagnetic iron oxide nanoparticles (SPIOs), which yields a highly sensitive increment in the reflection terahertz (THz) signal in SPIO solution upon exposure to an alternating magnetic field. In the differential and relative refection change focal-plane images before and after alternating magnetic field exposure, a dramatic contrast is demonstrated between water with and without SPIOs. This low-cost, simple, and stable contrast-enhanced continuous-THz-wave imaging system is suitable for miniaturization and real-time imaging application.