Phase Contrast Imaging System Research Articles

Design of a Talbot phase-contrast microscopy imaging system with a digital detector for laser-driven X-ray backlighter sources

Laser-driven shock waves in matter propagate with multiple kilometers per second and therefore require sources like a laser-driven backlighter, which emit the X-rays within picoseconds, to be able to capture sharp images. The small spatial extent of shocks in low-density materials pose challenges on the imaging setup. In this work, we present a design process for a single-shot X-ray phase-contrast imaging system geared towards these objects, consisting of a two-grating Talbot interferometer and a digital X-ray detector. This imaging system is optimized with respect to the detectable refraction angle of the X-rays induced by an object, which implies a high phase sensitivity. Therefore, an optimization parameter is defined that considers experimental constraints such as the limited number of photons, the required magnification, the size and spectrum of the X-ray source, and the visibility of the moiré fringes. In this way, a large parameter space is sampled and a suitable imaging system is chosen. During a campaign at the PHELIX high-power laser facility a static test sample was imaged which is used to benchmark the optimization process and the imaging system under real conditions. The results show good agreement with the predicted performance, which demonstrates the reliability of the presented design process. Likewise, the process can be adapted to other types of laser experiments or X-ray sources and is not limited to the application presented here.

Transient cavity-cavity strong coupling at terahertz frequency on LiNbO3 chips.

Terahertz (THz) microcavities have garnered considerable attention for their ability to localize and confine THz waves, allowing for strong coupling to remarkably enhance the light-matter interaction. These properties hold great promise for advancing THz science and technology, particularly for high-speed integrated THz chips where transient interaction between THz waves and matter is critical. However, experimental study of these transient time-domain processes requires high temporal and spatial resolution since these processes, such as THz strong coupling, occur in several picoseconds and microns. Thus, most literature studies rarely cover temporal and spatial processes at the same time. In this work, we thoroughly investigate the transient cavity-cavity strong-coupling phenomena at THz frequency and find a Rabi-like oscillation in the microcavities, manifested by direct observation of a periodic energy exchange process via a phase-contrast time-resolved imaging system. Our explanation, based on the Jaynes-Cummings model, provides theoretical insight into this transient strong-coupling process. This work provides an opportunity to deeply understand the transient strong-coupling process between THz microcavities, which sheds light on the potential of THz microcavities for high-speed THz sensor and THz chip design.

Bottom-up Gold Filling of Trenches in Curved Wafers

A Bi3+ -stimulated Au electrodeposition process in slightly alkaline Na3AuSO32+Na2SO3 electrolytes has been previously demonstrated for void-free extreme bottom-up filling of high aspect ratio trenches in gratings that are key to advanced X-ray imaging technologies. Effective use of the full area of the gratings with conventional X-ray sources requires they have a finite radius of curvature to align the high aspect ratio Au-filled trenches with divergent X-rays. With that in mind, this work demonstrates bottom-up Au filling in gratings that are attached to curved holders. Contactless mapping of the Au-filled gratings captures residual curvature that is retained after their release from the curved holders (i.e., intrinsic curvature). X-ray diffraction captures the elastic strains in the Si underlying the Au-filled trenches of the grating. Complementary measurements on the surfaces of unpatterned Si wafers mounted on curved holders capture the actual curvatures and strains imposed in the mounted gratings. The gratings, either intrinsically curved or re-bent, provide high visibility across the entire field of view of an X-ray phase contrast imaging system with elastic strains substantially below those in planar gratings bent to the same radius.

Dynamic flyer in barrel imaging via high intensity short-pulse laser.

The thin flyer is a small-scale flying object, which is well known as the core functional element of the initiator. Understanding how flyers perform has been a long-standing issue in detonator science. However, it remains a significant challenge to explore how the flyer is formed and functions in the barrel of the initiator via tabletop devices. In this study, we present dynamic and unprecedented images of flyer in barrel via high intensity short-pulse laser. Advanced radiography, coupled with a high-intensity picosecond laser X-ray source, has enabled the provision of state-of-the-art radiographs in a single-shot experiment for observing micron-scale flyer formation in a hollow cylinder in nanoseconds. The flyer was clearly visible in the barrel and was accelerated and restricted differently from that without the barrel. This first implementation of a tabletop X-ray source provided a new approach for capturing dynamic photographs of small-scale flying objects, which were previously reported to be accessible only via an X-ray phase-contrast imaging system at the advanced photon source. These efforts have led to a significant improvement of radiographic capability and a greater understanding of the mechanisms of "burst" of exploding foil initiators for this application.

Fabrication of small-period neutron absorption grating by pressurized particle filling method

Neutron absorption gratings play a crucial role in neutron phase contrast imaging systems, where the fabrication of large-size and small-period absorption gratings that meet imaging requirements presents a significant challenge. The pressurized particle filling method has been successfully applied to fabricate large-size absorption gratings. Here, we investigated the feasibility of the pressurized particle filling method for fabricating small-period gratings and proposed an optimized pressurized particle filling method. The grating surface was covered with a uniform particle layer and then pressurized, utilizing the adaptive deformation of the particle layer to achieve uniform particle filling. Neutron absorption gratings with an area of 60 × 60 mm2 and periods of 8 and 4 µm were fabricated through this method. The particle filling rate and fabrication efficiency were successfully improved. In addition, the evaluation of the particle filling uniformity method by analyzing the proportion of particles on the grating surface was proposed. The better uniformity of small-period neutron absorption gratings indicated that the optimized pressurized particle filling method can achieve relatively uniform particle filling.

Intelligent Phase Contrast Meta-Microscope System.

Phase contrast imaging techniques enable the visualization of disparities in the refractive index among various materials. However, these techniques usually come with a cost: the need for bulky, inflexible, and complicated configurations. Here, we propose and experimentally demonstrate an ultracompact meta-microscope, a novel imaging platform designed to accomplish both optical and digital phase contrast imaging. The optical phase contrast imaging system is composed of a pair of metalenses and an intermediate spiral phase metasurface located at the Fourier plane. The performance of the system in generating edge-enhanced images is validated by imaging a variety of human cells, including lung cell lines BEAS-2B, CLY1, and H1299 and other types. Additionally, we integrate the ResNet deep learning model into the meta-microscope to transform bright-field images into edge-enhanced images with high contrast accuracy. This technology promises to aid in the development of innovative miniature optical systems for biomedical and clinical applications.

Phase retrieval for X-ray differential phase contrast radiography with knowledge transfer learning from virtual differential absorption model

Grating-based X-ray phase contrast radiography and computed tomography (CT) are promising modalities for future medical applications. However, the ill-posed phase retrieval problem in X-ray phase contrast imaging has hindered its use for quantitative analysis in biomedical imaging. Deep learning has been proved as an effective tool for image retrieval. However, in practical grating-based X-ray phase contrast imaging system, acquiring the ground truth of phase to form image pairs is challenging, which poses a great obstacle for using deep leaning methods. Transfer learning is widely used to address the problem with knowledge inheritance from similar tasks. In the present research, we propose a virtual differential absorption model and generate a training dataset with differential absorption images and absorption images. The knowledge learned from the training is transferred to phase retrieval with transfer learning techniques. Numerical simulations and experiments both demonstrate its feasibility. Image quality of retrieved phase radiograph and phase CT slices is improved when compared with representative phase retrieval methods. We conclude that this method is helpful in both X-ray 2D and 3D imaging and may find its applications in X-ray phase contrast radiography and X-ray phase CT.

Design considerations for an ultrahigh-bandwidth Phase Contrast Imaging system applied to fusion grade devices

The PCI diagnostic is an internal reference interferometer that creates an image of absolutely calibrated electron density fluctuations integrated along the line of sight of the probing light beam. While conventional PCI diagnostics installed on fusion experiments worldwide employ light of wavelength equal to 10.59 μm, the same system using light at 1.55 μm wavelength would extend the spectral response in wave-number and frequency by factors of seven and over one hundred, respectively, thereby potentially providing quantitative measurements of the internal structure of density perturbations induced by either turbulent or radio-frequency waves, simultaneously covering ion to electron gyro-radius scales up to the GHz frequency region. Based on a previously developed 1.55 μm PCI prototype system, constraints to the design for such a diagnostic in fusion grade devices are presented and compared to those faced with the conventional method.

Influence of barrel diameter on exploding foil initiators

AbstractExploding foil initiators (EFIs) offer several safety and timing advantages over other means of initiating high explosives. In order to increase the knowledge about the barrel diameter on sensitivity of HNS‐IV, several samples with different barrel diameter were investigated. Mean threshold voltage, and flyer velocity were measured and compared. Meanwhile, we present the flyer images with restricted condition of EFIs during flight by X‐ray phase contrast imaging system at the 45 TW laser facility at China Academy of Engineering Physics. The barrel diameter has a related to mean threshold voltage of EFIs. The lowest mean threshold voltage is 2830 V with 0.4 mm barrel diameter. The mean threshold voltage is increased by about 54 % as the barrel diameter decreases from 0.4 mm to 0.2 mm. But the decreased barrel diameter significantly increased the flyer velocity. The dynamic flyer image gives the evidence to prove that the impacting width of flyer is the key parameter of EFIs. The impacting width of flyer is related to barrel diameter. Meanwhile, the curvature of the flyer is related to barrel diameter and exploding foil width. This fundamental understanding has led to increased knowledge on the mechanism of EFIs and has allowed us to improve our predictive capability through modeling.

Differential phase contrast imaging system based four quadrants liquid crystal device

Differential phase contrast imaging system based four quadrants liquid crystal device

An ultrahigh-bandwidth Phase Contrast Imaging system for fusion plasmas

A novel Phase Contrast Imaging system that uses probing light in the near infrared region has been developed to image electron density fluctuations in fusion plasmas. As compared to standard systems operating in the mid infra-red region, the spectral response of the system is extended in wave-number and frequency response by 7 and 100 times, respectively. The internal structure of turbulence and radio-frequency waves is therefore accessible across an unprecedented wavelength and frequency range, extending into the electron gyro-radius scale and the GHz frequency region.

Characterization of the solid deuterium layer in the inertial confinement fusion cryogenic target without the requirement of cryocooler deactivation

For the preparation of cryogenic fuel layer in inertial confinement fusion experiments, the vibration amplitude of cryogenic target must be reduced to the order of 1 μm or less to ensure the fuel layer to be distinguished by the X-ray phase contrast imaging system. An efficient vibration-reduction system with the Gifford-McMahon cryocooler mounted at the horizontal direction is designed, which is based on a two-stage floating thermal shielding and flexible connection. The obtained lowest temperature of the sample mount is less than 10 K, the temperature stability is within ±1 mK@18 K, the cooling power is greater than 500 mW, and the corresponding vibration amplitude is less than 1 μm. With this proposed method, the X-ray phase contrast imaging of cryogenic deuterium layer can be performed successively without requirement of cryocooler deactivation. Finally, the achieved resolution of the X-ray phase contrast imaging is about 1.5 μm while the contrast of solid-gas interface is larger than 5%, which is verified by the in-situ characterization of solidification of deuterium in the cryogenic target.

Signal Retrieval from Non-Sinusoidal Intensity Modulations in X-ray and Neutron Interferometry Using Piecewise-Defined Polynomial Function.

Grating-based phase-contrast and dark-field imaging systems create intensity modulations that are usually modeled with sinusoidal functions to extract transmission, differential-phase shift, and scatter information. Under certain system-related conditions, the modulations become non-sinusoidal and cause artifacts in conventional processing. To account for that, we introduce a piecewise-defined periodic polynomial function that resembles the physical signal formation process, modeling convolutions of binary periodic functions. Additionally, we extend the model with an iterative expectation-maximization algorithm that can account for imprecise grating positions during phase-stepping. We show that this approach can process a higher variety of simulated and experimentally acquired data, avoiding most artifacts.

Optimized Deep Reactive-Ion Etching of Nanostructured Black Silicon for High-Contrast Optical Alignment Marks

High-contrast alignment marks are promising optical elements for the high-precision alignment of X-ray gratings in phase-contrast X-ray imaging systems. These marks contain micrometer-scale bands of optically absorptive and reflective areas that determine their relative degree of optical contrast. Nanostructured black silicon (n-BSi) enabling the broadband, quasi-omnidirectional absorption of light, is a promising material to define these absorptive areas. In this work, we have developed an optimized deep reactive-ion etching (DRIE) process to fabricate n-BSi for these alignment marks. The DRIE process was optimized by systematically changing the polymer deposition time, T, while keeping all other process conditions constant. As T varied from 0.6 to 2.5 s, three distinct etching regimes were observed: (1) smooth etching, (2) n-BSi, and (3) etching stop. During the n-BSi regime (T = 1.6 to 2.1 s), varying T altered the n-BSi morphology, resulting in a broad spectrum of nanostructures: nanopillars to nanopores. Our investigation of the process–structure–property relationships among the process etching, morphology evolution, and reflectance of n-BSi revealed that morphologies with an increased height, aspect ratio, and degree of tapering and a lower base spacing were most effective at suppressing reflection for λ = 500–800 nm. Our results demonstrated the lowest reflectance of the absorptive areas, Rabs = 0.01 (∼1%) and Rrel_abs = 0.03 (relative with respect to Si) at λ = 600 nm for T = 1.9 s (100 cycles), thus giving the highest contrast ratio, ϕ = 0.9. To increase the device throughput, we varied the cycle number from 5 to 100 times at T = 1.9 s to study its effect on growth, morphology, and reflectance of n-BSi. Our results demonstrate that Rabs, Rrel_abs, and ϕ plateaued at 50 cycles, cutting in half our total fabrication time, t. In addition, our alignment marks demonstrated rotational and translational precision alignment capabilities. Lastly, we present two design principles for T and the cycle number to fabricate the optimal n-BSi morphology for the targeted application using any DRIE tool.

Factors Affecting the Spatial Resolution in 2D Grating–Based X-Ray Phase Contrast Imaging

X-ray phase contrast imaging is a promising technique in X-ray biological microscopy, as it improves the contrast of images for materials with low electron density compared to traditional X-ray imaging. The spatial resolution is an important parameter to evaluate the image quality. In this paper, simulation of factors which may affect the spatial resolution in a typical 2D grating–based phase contrast imaging system is conducted. This simulation is based on scalar diffraction theory and the operator theory of imaging. Absorption, differential phase contrast, and dark-field images are retrieved via the Fourier transform method. Furthermore, the limitation of the grating-to-detector distance in the spatial harmonic method is discussed in detail.

Angular sensitivity of an x-ray differential phase contrast imaging system with real and virtual source images.

In this work, a novel, to the best of our knowledge, approach based on an x-ray thin lens imaging theory is proposed to predict the angular sensitivity responses of dual-phase-grating differential phase contrast (DPC) interferometers. Experimental validations have been performed to demonstrate the high accuracy of theoretical predictions using two different setups: one with real source images and the other with virtual source images. This new sensitivity calculation method is helpful to optimize the DPC imaging performance of a dual-phase-grating system.

Enhancing the X-Ray Differential Phase Contrast Image Quality With Deep Learning Technique.

The purpose of this work is to investigate the feasibility of using deep convolutional neural network (CNN) to improve the image quality of a grating-based X-ray differential phase contrast imaging (XPCI) system. In this work, a novel deep CNN based phase signal extraction and image noise suppression algorithm (named as XP-NET) is developed. The numerical phase phantom, the ex vivo biological specimen and the ACR breast phantom are evaluated via the numerical simulations and experimental studies, separately. Moreover, images are also evaluated under different low radiation levels to verify its dose reduction capability. Compared with the conventional analytical method, the novel XP-NET algorithm is able to reduce the bias of large DPC signals and hence increasing the DPC signal accuracy by more than 15%. Additionally, the XP-NET is able to reduce DPC image noise by about 50% for low dose DPC imaging tasks. This proposed novel end-to-end supervised XP-NET has a great potential to improve the DPC signal accuracy, reduce image noise, and preserve object details. We demonstrate that the deep CNN technique provides a promising approach to improve the grating-based XPCI performance and its dose efficiency in future biomedical applications.

Proposal of a compact grating-based phase contrast imaging system using an x-ray polycapillary lens

As the grating-based phase contrast imaging has drawn much attention in various scientific fields, there is an increasing demand for a compact imaging system with high efficiency in practical applications. In this paper, a compact imaging configuration by replacing the source grating in a conventional imaging system with a robust, easily-fabricated, and cost-effective x-ray polycapillary lens is proposed and its feasibility is examined using a full-scale Monte Carlo simulation. A typical x-ray spectrum for diagnostic mammography is adopted in the simulation. Under this spectrum, a new polycapillary lens is designed and optimized to meet the requirement of the imaging system. The phantom consists of polyethylene, cortical bone, and adipose tissue spheres of different sizes. An accurate phase image of the phantom is successfully reconstructed using an eight-step phase-stepping method. Excellent agreement of the phase shift and attenuation of the phantom is obtained between the simulation and the theoretical prediction. Compared with a conventional imaging system using a source grating at the same parameters, the imaging efficiency of our proposed system is improved 1.7 times, while its background visibility is reduced from 44.4% to 12.4%. Our results suggest the possibility of developing such a bench-top system for flexible phase contrast imaging applications.

Method of single exposure phase contrast imaging without analyser grating

X-ray differential phase contrast imaging technology can reveal the weakly absorbing fine structure which cannot be observed by the conventional X-ray absorption contrast imaging. Hence, it has potential applications in many fields such as medical science, and material science. But in the traditional X-ray grating differential phase contrast imaging system, the analyser grating is used as a spatial filter, and needs to be phase stepped, then multiple exposures are required for information extraction. Thus this leads to high-dose radiation exposure and low efficiency of X-ray utilization. All these disadvantages limit its application in various disciplines. In order to cope with the above issues, a new algorithm based on single-shot grating differential phase contrast imaging system without analyzer grating is proposed. In such a system the absorption, refraction and scattering information can be obtained from one projective image of the sample acquired by a high-resolution detector. The reliability of this new algorithm is confirmed by numerical simulation. Compared with the phase stepping algorithm, the proposed algorithm can provide very accurate and reliable information extraction results without requiring the grating period to match with detector pixel size. It facilitates the reduction of the radiation damage to sample. We emphasize that this method is highly compatible with the future X-ray phase contrast imaging clinical applications.

Investigation of mode activity in NBI-heated experiments of Wendelstein 7-X

The 2018 operation phase (OP 1.2b) of the stellarator Wendelstein 7-X (W7-X) included, for the first time, neutral beam injection (NBI) to heat the plasma. Since the injection geometry at W7-X is not parallel, this generates both passing and trapped fast particles. During longer phases of NBI injection, with the primary purpose to study the heating efficiency of this system, Alfvén eigenmodes (AEs) were observed by a number of diagnostics, including the phase contrast imaging (PCI) system, the magnetic pick-up coils (Mirnov coils), and the soft x-ray multi-camera tomography system (XMCTS).Alfvén eigenmodes are of great interest for future fusion reactors as it has been shown that the resonant interaction of fast ions with self-excited AEs can lead to enhanced transport of fast ions and potentially to energy losses. This is especially true for so-called gap-modes, Alfvén eigenmodes with frequencies in gaps of the continuous spectrum, since they lack continuum damping. These modes are commonly known to be excited by fast ions, but other destabilizing mechanisms, e.g. the electron-pressure gradient are also possible.In this article we present a first analysis of the experimentally observed frequencies from the theoretical side. The calculation of shear Alfvén wave continua for selected cases and the assignment of observed frequencies to the gaps of the continuous spectra are presented. Using the ideal-MHD code CKA (Könies A. 2007 10th IAEA TM on Energetic Particles in Magnetic Confinement System), we find gap modes that match the experimental measurements in terms of the observed frequencies. We emphasize the crucial roles played by the coupling of sound and Alfvén waves as well as of the Doppler shift arising as a consequence of the radial electric field in W7-X.We employ the perturbative gyrokinetic code CKA-EUTERPE (Feh´er 2013 Simulation of the interaction between Alfv´en waves and fast particles), using a slowing-down distribution function for the fast ions as calculated by the Monte-Carlo particle following code ASCOT (Hirvijoki et al 2014 Comput. Phys. Commun. 185 1310–21) to assess the fast-ion drive. We find that the fast-ion drive is insufficient to overcome the background-plasma damping. The fact that unstable modes were observed experimentally may point to problems with the modelling or indicate the existence of other destabilizing mechanisms, e.g. associated with the electron-pressure gradient (Windisch et al 2017 Plasma Phys. Control. Fusion 59 105002) that sensitively depend on the profiles of the background plasma.