Grating-based X-ray Phase-contrast Imaging Research Articles

Fabrication of absorption gratings with X-ray lithography for X-ray phase contrast imaging

Grating-based X-ray phase contrast imaging is promising especially in the medical area. Two or three gratings are involved in grating-based X-ray phase contrast imaging in which the absorption grating of high-aspect-ratio is the most important device and the fabrication process is a great challenge. The material with large atomic number Z is used to fabricate the absorption grating for excellent absorption of X-ray, and Au is usually used. The fabrication process, which involves X-ray lithography, development and gold electroplating, is described in this paper. The absorption gratings with 4 [Formula: see text]m period and about 100 [Formula: see text]m height are fabricated and the high-aspect-ratio is 50.

Analysis and correction of bias induced by phase stepping jitter in grating-based X-ray phase-contrast imaging.

Grating-based X-ray phase-contrast (gbPC) is an X-ray phase-contrast imaging method involving optical gratings that typically employs the Talbot self-imaging effect. X-ray phase contrast is known to provide significant benefits for biomedical imaging. To investigate these benefits for gbPC, a high-sensitivity gbPC micro-CT setup for small biological samples has been constructed. A gbPC projection measurement simultaneously retrieves the transmittance, differential-phase and dark-field modalities of a sample. Phase stepping, the most common gbPC acquisition technique, involves several acquisitions at different lateral positions of one of the gratings. The three modalities can then be retrieved by least-squares- or FFT-based methods. Unfortunately, increasing differential-phase sensitivity also leads to an increased magnitude of artifacts introduced during retrieval of the modalities from the phase-stepping data, which limits image quality. Most importantly, processing of phase-stepping data with incorrect stepping positions (i.e., spatial sampling jitter) can introduce artifacts to the modalities. Using data from the high-sensitivity gbPC setup, as well as simulations, we show that an artifact is introduced by the jitter which is correlated with the phase of the stepping curve. We present a theoretical explanation for this correlation by introducing small deviations to an equidistant sampling of a stepping curve and approximating the effect on the calculation of the three gbPC modalities with a first-order Taylor approximation. Finally, we present an algorithm for the detection and removal of these artifacts that exploits these correlations. We show that this algorithm is able to eliminate these artifacts without degrading true image information.

Non-Destructive Testing of Archaeological Findings by Grating-Based X-Ray Phase-Contrast and Dark-Field Imaging

The analysis of archaeological findings reveals the remaining secrets of human history. However, it is a challenging task to investigate and simultaneously preserve the unique remains. Available non-destructive examination methods are limited and often insufficient. Thus, we considered X-ray grating interferometry as a non-destructive and advanced X-ray imaging method to retrieve more information about archaeological findings. In addition to the conventional attenuation image, the differential phase and the dark-field image are obtained. We studied the potential of the scattering-sensitive dark-field and the phase-shift sensitive differential phase image to analyse archaeological findings. Hereby, the focus lies on organic remnants. Usually, the organic materials have vanished due to decomposition processes, but the structures are often preserved by mineralisation and penetration of corrosion products. We proved that the combination of the attenuation and the dark-field image in particular, enables a separation of structural properties for fabric remnants. Furthermore, we achieved promising results for the reconstruction of sub-pixel sized fibre orientations of woven fabric remnants by employing the directional dark-field imaging method. We conclude from our results that a further application of X-ray dark-field imaging on wet organic findings and on the distinction of different types of organic remnants at archaeological findings is promising.

Towards sub-micrometer high aspect ratio X-ray gratings by atomic layer deposition of iridium

X-ray grating interferometry is an excellent technique for X-ray phase contrast imaging and X-ray wavefront sensing with applications in materials science, biology and medical diagnosis. Among other requirements, the method depends on the availability of highly X-ray absorbing metallic gratings. Here, we report on the fabrication and characterization of high aspect ratio iridium gratings with a period of 1 μm and a depth of 30 μm combining deep reactive ion etching of silicon and atomic layer deposition of iridium. The implementation of such structures can greatly enhance the sensitivity of grating-based X-ray phase contrast imaging and thus, expand further its broad range of applications.

Implementation of a Talbot-Lau interferometer in a clinical-like c-arm setup: A feasibility study

X-ray grating-based phase-contrast imaging has raised interest regarding a variety of potential clinical applications, whereas the method is feasible using a medical x-ray tube. Yet, the transition towards a clinical setup remains challenging due to the requirement of mechanical robustness of the interferometer and high demands applying to medical equipment in clinical use. We demonstrate the successful implementation of a Talbot-Lau interferometer in an interventional c-arm setup. The consequence of vibrations induced by the rotating anode of the tube is discussed and the prototype is shown to provide a visibility of 21.4% at a tube voltage of 60 kV despite the vibrations. Regarding clinical application, the prototype is mainly set back due to the limited size of the field of view covering an area of 17 mm × 46 mm. A c-arm offers the possibility to change the optical axis according to the requirements of the medical examination. We provide a method to correct for artifacts that result from the angulation of the c-arm. Finally, the images of a series of measurements with the c-arm in different angulated positions are shown. Thereby, it is sufficient to perform a single reference measurement in parking position that is valid for the complete series despite angulation.

Measurement and simulative proof concerning the visibility loss in x-ray Talbot-Lau Moiré imaging

X-ray grating-based phase-contrast imaging is a very promising imaging technique for medical imaging and non-destructive testing. Simultaneously to the attenuation properties additional information about the phase-shift and the scattering properties of an object can be gained. Nevertheless, to obtain this information multiple shots per image have to be taken in the so-called phase-stepping process. Thus, it often is a time consuming imaging procedure. In contrast, Moiré imaging with a Talbot-Lau interferometer is a very fast single-shot imaging method. To obtain the Moiré pattern which encodes the object information the interferometer has to be slightly detuned. This leads to a loss in visibility and therefore to a loss in image quality compared to the more time consuming phase-stepping method. Thus, it is important for the Moiré imaging method to evaluate the mechanisms of visibility loss. In this publication we present measurements and simulations investigating the visibility loss due to detuning the Talbot-Lau interferometer. Thus, a deeper understanding of Moiré imaging can be gained.

Signal-to-noise ratio comparison of angular signal radiography and phase stepping method**Project supported by the National Research and Development Project for Key Scientific Instruments (Grant No. CZBZDYZ20140002), the National Natural Science Foundation of China (Grant Nos. 11535015, 11305173, and 11375225), the project supported by Institute of High Energy Physics, Chinese Academy of Sciences (Grant No. Y4545320Y2), and the Fundamental Research Funds

Grating-based x-ray phase contrast imaging has the potential to be applied in future medical applications as it is compatible with both laboratory and synchrotron source. However, information retrieval methods are important because acquisition speed, scanning mode, image quality, and radiation dose depend on them. Phase-stepping (PS) is a widely used method to retrieve information, while angular signal radiography (ASR) is a newly established method. In this manuscript, signal-to-noise ratios (SNRs) of ASR are compared with that of PS. Numerical experiments are performed to validate theoretical results. SNRs comparison shows that for refraction and scattering images ASR has higher SNR than PS method, while for absorption image both methods have same SNR. Therefore, our conclusions would have guideline in future preclinical and clinical applications.

Single-shot grating-based x-ray differential phase contrast imaging with a modified analyzer grating**Project supported by the National Natural Science Foundation of China (Grant Nos. 11275204, 11475175, and 11405175), the China Postdoctoral Science Foundation (Grant No. 2017M612097), and the Fundamental Research Funds for the Central Universities (Grant No. WK2310000065)

X-ray grating interferometer has attracted widely attention in the past years due to its capability in achieving x-ray phase contrast imaging with low brilliance source. However, the widely used phase stepping information extraction method reduces system stability and prolongs data acquisition time by several times compared with conventional x-ray absorption-based imaging. The mechanical stepping can be avoided by using a staggered grating, but at the cost of low vertical spatial resolution. In this paper, employing a modified staggered grating and the angular signal radiography, we proposed a single-shot grating-based x-ray differential phase contrast imaging with decent vertical spatial resolution. The theoretical framework was deduced and proved by numerical experiments. Absorption, phase, and scattering computed tomography can be performed without phase stepping. Therefore, we believe this fast and highly stable imaging method with decent resolution would be widely applied in x-ray grating-based phase contrast imaging.

Low-dose and fast grating-based x-ray phase-contrast imaging

X-ray phase-contrast imaging has experienced rapid development over the last few decades, and in this technology, the phase modulation strategy of phase-stepping is used most widely to measure the sample's phase signal. However, because of its discontinuous nature, phase-stepping has the defects of worse mechanical stability and high exposure dose, which greatly hinder its wide application in dynamic phase measurement and potential clinical applications. In this manuscript, we demonstrate preliminary research on the use of integrating-bucket phase modulation method to retrieve the phase information in grating-based X-ray phase-contrast imaging. Experimental results showed that our proposed method can be well employed to extract the differential phase-contrast image, compared with the current mostly used phase-stepping strategy, advantage of integrating-bucket phase modulation technique is that fast measurement and low dose are promising.

Optimization of grating duty cycle in non-interferometric grating-based X-ray phase contrast imaging.

Grating-based X-ray phase contrast imaging technology is one of the most potential imaging methods in real applications. It can be classified into two categories: interferometry and non-interferometric imaging. The non-interferometric grating-based X-ray phase contrast imaging (NIGPCI) instrument has a great advantage in the forthcoming commercial applications for the flexible system design and the use of large periodic gratings. The performance of the NIGPCI instrument depends on its angular sensitivity to a great extent. Therefore, good angular sensitivity is mandatory in order to obtain high quality phase-contrast images. Several parameters, such as the X-ray spectrum, the inter-grating distances, and the parameters of the three gratings, influence the angular sensitivity of the imaging system. However, the quantitative relationship between the angular sensitivity and grating duty cycle is unclear. Therefore, this paper is devoted to revealing their internal relation by theoretical deduction and emulation of the imaging process with the theories of linear system and Fourier optics. Furthermore, a quantitative analysis method to optimize the duty cycles of gratings is proposed and its applicability to a general NIGPCI system is verified.

Effect of current density on the deposit stress in gold electroplating

As the key components of grating-based X-ray phase contrast imaging, absorption gratings are essential to be fabricated. In fact, the internal stress is one of the critical issues for the application of the electroplated gold deposit as the absorption metal for absorption gratings. It is common that high internal stress levels can cause the deposit cracking, blistering and peeling away from the substrate material. This study investigates the effect of current density on the internal stress by the bent strip method. The surface morphologies of gold deposits are examined using scanning electron microscopy (SEM) and the crystal structure of the electroplated deposit is analyzed by X-ray diffraction (XRD). The change of current density reverses the internal stress of the deposits from compressive to tensile. The value of deposit stress can be near zero by optimizing the current density.

Linear information retrieval method in X-ray grating-based phase contrast imaging and its interchangeability with tomographic reconstruction

In X-ray grating-based phase contrast imaging, information retrieval is necessary for quantitative research, especially for phase tomography. However, numerous and repetitive processes have to be performed for tomographic reconstruction. In this paper, we report a novel information retrieval method, which enables retrieving phase and absorption information by means of a linear combination of two mutually conjugate images. Thanks to the distributive law of the multiplication as well as the commutative law and associative law of the addition, the information retrieval can be performed after tomographic reconstruction, thus simplifying the information retrieval procedure dramatically. The theoretical model of this method is established in both parallel beam geometry for Talbot interferometer and fan beam geometry for Talbot-Lau interferometer. Numerical experiments are also performed to confirm the feasibility and validity of the proposed method. In addition, we discuss its possibility in cone beam geometry and its advantages compared with other methods. Moreover, this method can also be employed in other differential phase contrast imaging methods, such as diffraction enhanced imaging, non-interferometric imaging, and edge illumination.

An improved acquisition scheme for the reverse projection method

In recent years, efforts have been made on the information extraction method in x-ray grating-based phase contrast imaging. Among them, the reverse projection method proposed by P.P. Zhu in 2010 features as a fast, low-dose method to extract information without stepping the gratings. It enables to extend the phase contrast imaging system to the vivo studies. However, it has strict requirement on the system stability and the source stability. In this manuscript, an improved acquisition scheme has been proposed using a staggered grating making the reverse projection method more efficient with a lower requirement of the system and source stability.

Comparison of laboratory grating-based and speckle-tracking x-ray phase-contrast imaging

Phase-contrast imaging with x-rays is a developing field for imaging weakly absorbing materials. In this work, two phase-contrast imaging methods, grating- and speckle-based imaging, that measure the derivative of the phase shift, have been implemented with a laboratory source and compared experimentally. It was found that for the same dose conditions, the speckle-tracking differential phase-contrast images have considerably higher contrast-to-noise ratio than the grating-based images, but at the cost of lower resolution. Grating-based imaging performs better in terms of resolution, but would require longer exposure times, mainly due to absorption in the grating interferometer.

Noise analysis of grating-based x-ray differential phase-contrast imaging with angular signal radiography**Project supported by the National Natural Science Foundation of China (Grant Nos. 11535015, 11305173, and 11375225).

X-ray phase-contrast imaging is one of the novel techniques, and has potential to enhance image quality and provide the details of inner structures nondestructively. In this work, we investigate quantitatively signal-to-noise ratio (SNR) of grating-based x-ray phase contrast imaging (GBPCI) system by employing angular signal radiography (ASR). Moreover, photon statistics and mechanical error that is a major source of noise are investigated in detail. Results show the dependence of SNR on the system parameters and the effects on the extracted absorption, refraction and scattering images. Our conclusions can be used to optimize the system design for upcoming practical applications in the areas such as material science and biomedical imaging.

Research on dual energy grating based X-ray phase contrast imaging

There exist some problems in a grating-based X-ray differential phase contrast imaging system, such as complex imaging system, low imaging efficiency and high requirements for step precision. The phase information extraction method of imaging system has been developed into an existing two-stepping phase shift method from the original phase stepping method, which improves the imaging efficiency and reduces the imaging radiation dose and imaging time. However, the method of two-stepping phase shift still needs to move the grating, and the requirement for accuracy of the step position is also very high. According to the problems mentioned above, in this paper we propose a dual energy multi-line X-ray source and a dual energy analysis grating. The dual energy multi-line X-ray source can emit two different levels of X-ray structure light, which can replace the X-ray source and source grating. The dual energy analysis grating is composed of two different types of scintillator materials, which are in staggered distribution. One is scintillator material that can transform high energy X-ray into visible light, and the other one can convert low energy X-ray into visible light. The dual energy analysis grating can replace traditional analysis grating and the conversion screen of X-ray CCD detector. By using the dual energy multi-line X-ray source and dual energy analysis grating in grating-based X-ray differential phase contrast imaging system, a dual energy grating-based X-ray phase contrast imaging system is proposed in this paper. In addition, in this paper we show the structure and imaging principle of the imaging system. The imaging system can achieve high and low energy X-ray imaging without moving grating. Two levels of X-ray imaging are equivalent to the analysis grating displacement π phase, which is in line with the traditional two-stepping method of two image phase shift requirements. Therefore, after the normalization processing of the two kinds of energies, the phase information can be extracted by the traditional two-stepping phase shift method. In order to validate the correctnesses of the imaging principle of the proposed imaging system and extraction method of phase information, the imaging system is simulated. The simulation is performed on the assumption that an X-ray beam passes through a polymethyl methacrylate sphere as a phase specimen, and the method is adopted by using the proposed dual energy X-ray about left and right lumbar imaging to extract phase information. The simulation result shows that the imaging system can realize normal imaging, and the first-order derivative distribution of the sphere phase extracted by the dual energy X-ray method is consistent with the experimental result.

铋光栅X射线相衬成像条纹对比度的定量计算

铋光栅X射线相衬成像条纹对比度的定量计算

Optimisation of image reconstruction for phase-contrast x-ray Talbot–Lau imaging with regard to mechanical robustness

X-ray grating-based phase-contrast imaging opens new opportunities, inter alia, in medical imaging and non-destructive testing. Because, information about the attenuation properties and about the refractive properties of an object are gained simultaneously.Talbot–Lau imaging requires the knowledge of a reference or free-field image. The long-term stability of a Talbot–Lau interferometer is related to the time span of the validity of a measured reference image. It would be desirable to keep the validity of the reference image for a day or longer to improve feasibility of Talbot–Lau imaging.However, for example thermal and other long-term external influences result in drifting effects of the phase images. Therefore, phases are shifting over time and the reference image is not valid for long-term measurements. Thus, artifacts occur in differential phase-contrast images. We developed an algorithm to determine the differential phase-contrast image with the help of just one calibration image, which is valid for a long time-period. With the help of this algorithm, called phase-plane-fit method, it is possible to save measurement-time, as it is not necessary to take a reference image for each measurement.Additionally, transferring the interferometer technique from laboratory setups to conventional imaging systems the necessary rigidity of the system is difficult to achieve. Therefore, short-term effects like vibrations or distortions of the system lead to imperfections within the phase-stepping procedure. Consequently, artifacts occur in all three image modalities (differential phase-contrast image, attenuation image and dark-field image) of Talbot–Lau imaging. This is a problem with regard to the intended use of phase-contrast imaging for example in clinical routine or non-destructive testing. In this publication an algorithm of Vargas et al is applied and complemented to correct inaccurate phase-step positions with the help of a principal component analysis (PCA). Thus, it is possible to calculate the artifact free images. Subsequently, the whole algorithm is called PCA minimization algorithm.

Correcting Intensity Drift in X-ray Grating-based Phase Contrast Imaging

X-ray phase-contrast imaging has become an attractive technique because it can deliver additional information on weakly absorbing materials. Grating-based phase contrast imaging with conventional x-ray source is a breakthrough in x-ray phase contrast imaging because it provides attenuation, refraction and scattering information simultaneously. Therefore, it has potential to be applied in medical and industrial applications. However, in actual experiments, we found that the photon intensity drift of the x-ray source would influence the final images, especially the refraction images. After analyzing the phase-stepping curve, we proposed a correction method to fix the problem due to the effect of intensity drift. The proposed correction method is successfully applied to grating-based phase-contrast imaging setup having un-stable x-ray source. The experimental results show that our method could solve this problem.

Quantitative analysis of fringe visibility in grating-based x-ray phase-contrast imaging.

The newly developed x-ray differential phase-contrast imaging technique has attracted increasing research interest. In this study, we quantitatively analyze the fringe visibility obtained in differential phase-contrast imaging. Numerical results of the visibility for polychromatic x rays with different structure heights of absorption gratings are shown and discussed. Furthermore, the fringe visibility of the nonabsorption grating approach is calculated, and based on the results, we conclude that this approach can potentially be applied for higher x-ray photon energies. These analytic results will be useful for designing a differential phase-contrast imaging system for different applications.