Speckle-based Imaging Research Articles

Quantitative analysis of speckle-based X-ray dark-field imaging using numerical wave-optics simulations

The dark-field signal measures the small-angle scattering strength and provides complementary diagnostic information. This is of particular interest for lung imaging due to the pronounced small-angle scatter from the alveolar microstructure. However, most dark-field imaging techniques are relatively complex, dose-inefficient, and require sophisticated optics and highly coherent X-ray sources. Speckle-based imaging promises to overcome these limitations due to its simple and versatile setup, only requiring the addition of a random phase modulator to conventional X-ray equipment. We investigated quantitatively the influence of sample structure, setup geometry, and source energy on the dark-field signal in speckle-based X-ray imaging with wave-optics simulations for ensembles of micro-spheres. We show that the dark-field signal is accurately predicted via a model originally derived for grating interferometry when using the mean frequency of the speckle pattern power spectral density as the characteristic speckle size. The size directly reflects the correlation length of the diffuser surface and did not change with energy or propagation distance within the near-field. The dark-field signal had a distinct dependence on sample structure and setup geometry but was also affected by beam hardening-induced modifications of the visibility spectrum. This study quantitatively demonstrates the behavior of the dark-field signal in speckle-based X-ray imaging.

Translation position extracting in incoherent pattern-illumination Fourier ptychographic photography

Incoherent Pattern-illuminated Fourier ptychography (IFP) is a newly developed super-resolution method, where accurate prior knowledge of translation positions is essential for image reconstruction. To release this limitation, we propose a pre-processing algorithm capable of extracting translation positions of the structure light directly from raw images of IFP, termed translation position extracting (TPE). TPE mainly involves two steps. First, the speckle parts mixed in the acquired raw images, in which the illumination motion is encoded, are isolated by intensity averaging and division. Then the cross-correlations of the speckle dataset are computed to determine the shift positions. TPE-IFP improves the previous IFP by removal of the requirement for prior knowledge of translation positions. Its effectiveness is demonstrated by obtaining high-quality super-resolution images in absence of location information in both simulations and experiments. By further relaxing the practical conditions, the proposed TPE may accelerate the applications of IFP. What's more, as a pre-processing approach, TPE might also contribute to the estimation of pattern positions for the similar speckle-based imaging.

Comparison of X-ray speckle-based imaging deflection retrieval algorithms for the optimization of radiation dose

X-ray phase contrast imaging can provide improved or complementary information to traditional attenuation-based X-ray imaging, making the field a vast and rapidly evolving research subject. X-ray speckle-based imaging (SBI) is one phase-contrast imaging approach that has shown significant potential in providing both high sensitivity and high resolution while using a very simple experimental setup. With the aim of transferring such phase-contrast-based imaging techniques from synchrotron to laboratory X-ray sources, the issue of the deposited radiation dose still remains to be addressed. In this work, we experimentally and quantitatively compare the results from three different SBI phase retrieval algorithms using both phantoms and biological samples in order to infer the optimal configuration. The results obtained using a synchrotron beam suggest that the technique based on optical flow conservation achieves the most accurate retrieval from the lowest number of sample exposures. This constitutes an important step toward the possibility of transferring SBI into the clinic.

Ultimate resolution limits of speckle-based compressive imaging.

Compressive imaging using sparsity constraints is a very promising field of microscopy that provides a dramatic enhancement of the spatial resolution beyond the Abbe diffraction limit. Moreover, it simultaneously overcomes the Nyquist limit by reconstructing an N-pixel image from less than N single-point measurements. Here we present fundamental resolution limits of noiseless compressive imaging via sparsity constraints, speckle illumination and single-pixel detection. We addressed the experimental setup that uses randomly generated speckle patterns (in a scattering media or a multimode fiber). The optimal number of measurements, the ultimate spatial resolution limit and the surprisingly important role of discretization are demonstrated by the theoretical analysis and numerical simulations. We show that, in contrast to conventional microscopy, oversampling may decrease the resolution and reconstruction quality of compressive imaging.

X-ray phase tomography with near-field speckles for three-dimensional virtual histology

High-contrast, high-resolution imaging of biomedical specimens is indispensable for studying organ function and pathologies. Conventional histology, the gold standard for soft-tissue visualization, is limited by its anisotropic spatial resolution, elaborate sample preparation, and lack of quantitative image information. X-ray absorption or phase tomography have been identified as promising alternatives enabling non-destructive, distortion-free three-dimensional (3D) imaging. However, reaching sufficient contrast and resolution with a simple experimental procedure remains a major challenge. Here, we present a solution based on x-ray phase tomography through speckle-based imaging (SBI). We demonstrate on a mouse kidney that SBI delivers comprehensive 3D maps of hydrated, unstained soft tissue, revealing its microstructure and delivering quantitative tissue-density values at a density resolution of better than 2 m g / c m 3 and spatial resolution of better than 8 µm. We expect that SBI virtual histology will find widespread application in biomedicine and will open up new possibilities for research and histopathology.

High-precision speckle-tracking X-ray imaging with adaptive subset size choices

Speckle-tracking imaging has the advantages of simple setup and high-sensitivity to slowly varying phase gradients. Subset size choice is regarded as a trade-off problem for speckle-tracking X-ray imaging where one needs to balance the spatial resolution and accuracy, where the subset was defined as the region of interest of windowing choice for digital image correlation algorithm. An adaptive subset size choice method based on a Fourier transform for effectively detecting sample phase information without foreknowledge of the sample structure is presented in this study. The speckle-tracking phase-contrast and the form of dark-field imaging based on this method have the advantages of (i) high resolution and time saving compared to large subset choice and (ii) partially improvement the influence from experimental noises, background fluctuations, and false signals compared to small subset choice at the same time. This method has proven to be particularly robust in the experimental condition of poor signal-to-noise ratio. The proposed method may be expanded to all speckle-based imaging methods and other imaging techniques based on the subset or window matching.

Single-Shot X-Ray Speckle-Based Imaging of a Single-Material Object

We develop a means for speckle-based phase imaging of the projected thickness of a single-material object, under the assumption of illumination by spatially random time-independent x-ray speckles. These speckles are generated by passing x rays through a suitable spatially random mask. The method makes use of a single image obtained in the presence of the object, which serves to deform the illuminating speckle field relative to a reference speckle field (which only needs to be measured once) obtained in the presence of the mask and the absence of the object. The method implicitly rather than explicitly tracks speckles, and utilizes the transport-of-intensity equation to give a closed-form solution to the inverse problem of determining the complex transmission function of the object. Implementation using x-ray synchrotron data shows the method to be robust and efficient with respect to noise. Applications include x-ray phase--amplitude radiography and tomography, as well as time-dependent imaging of dynamic and radiation-sensitive samples using low-flux sources.

X-ray phase imaging with the unified modulated pattern analysis of near-field speckles at a laboratory source.

X-ray phase-contrast techniques are powerful methods for discerning features with similar densities, which are normally indistinguishable with conventional absorption contrast. While these techniques are well-established tools at large-scale synchrotron facilities, efforts have increasingly focused on implementations at laboratory sources for widespread use. X-ray speckle-based imaging is one of the phase-contrast techniques with high potential for translation to conventional x-ray systems. It yields phase-contrast, transmission, and dark-field images with high sensitivity using a relatively simple and cost-effective setup tolerant to divergent and polychromatic beams. Recently, we have introduced the unified modulated pattern analysis (UMPA) [Phys. Rev. Lett.118, 203903 (2017)PRLTAO0031-900710.1103/PhysRevLett.118.203903], which further simplifies the translation of x-ray speckle-based imaging to low-brilliance sources. Here, we present the proof-of-principle implementation of UMPA speckle-based imaging at a microfocus liquid-metal-jet x-ray laboratory source.

Influence of diffuser grain size on the speckle tracking technique.

The speckle-based X-ray imaging technique (SBT), which includes the three imaging modalities of absorption, phase contrast and dark field, is widely used in many fields. However, the influence of the grain size of the diffuser, the coherence of the X-ray source and the pixel size of the detector on the multi-mode imaging quality of SBT is still woefully unclear. In this paper, the whole SBT process is simulated and the influence of these three factors on image quality is discussed. Based on this discussion, the grain size of the diffuser for SBT applications should be limited by the pixel size of the detector and the coherence length of the X-ray source. According to analysis of the noise signal and correlation map, a suitable grain size is an indispensable condition for high-quality SBT images, because an excessively small or large grain size degrades the resolution of the imaging results and generates false signals. In addition, the power spectral density of the measured raw speckle patterns demonstrates that a smaller grain can better retain high-frequency information from an imaged sample. The simulated and experimental results verify these conclusions. The conclusions of this work will be helpful in designing suitable experimental setups for SBT applications and have the potential to promote the performance of SBT in other applications, such as X-ray optics metrology and coherence measurement.

High-energy, high-resolution, fly-scan X-ray phase tomography

High energy X-ray phase contrast tomography is tremendously beneficial to the study of thick and dense materials with poor attenuation contrast. Recently, the X-ray speckle-based imaging technique has attracted widespread interest because multimodal contrast images can now be retrieved simultaneously using an inexpensive wavefront modulator and a less stringent experimental setup. However, it is time-consuming to perform high resolution phase tomography with the conventional step-scan mode because the accumulated time overhead severely limits the speed of data acquisition for each projection. Although phase information can be extracted from a single speckle image, the spatial resolution is deteriorated due to the use of a large correlation window to track the speckle displacement. Here we report a fast data acquisition strategy utilising a fly-scan mode for near field X-ray speckle-based phase tomography. Compared to the existing step-scan scheme, the data acquisition time can be significantly reduced by more than one order of magnitude without compromising spatial resolution. Furthermore, we have extended the proposed speckle-based fly-scan phase tomography into the previously challenging high X-ray energy region (120 keV). This development opens up opportunities for a wide range of applications where exposure time and radiation dose are critical.

Computational complex optical field imaging using a designed metasurface diffuser

Various speckle-based computational imaging techniques that exploit the ability of scattering media to transfer hidden information into the speckle pattern have recently been demonstrated. Current implementations suffer from several drawbacks associated with the use of conventional scattering media (CSM), such as their time-consuming characterization, instability with time, and limited memory-effect range. Here we show that by using a random dielectric metasurface diffuser (MD) with known scattering properties, many of these issues can be addressed. We experimentally demonstrate an imaging system with the ability to retrieve complex field values using a MD and the speckle-correlation scattering matrix method. We explore the mathematical properties of the MD transmission matrix such as its correlation and singular value spectrum to expand the understanding about both MDs and the speckle-correlation scattering matrix approach. In addition to a large noise tolerance, reliable reproducibility, and robustness against misalignments, using the MD allows us to substitute the laborious experimental characterization procedure of the CSM with a simple simulation process. Moreover, dielectric MDs with identical scattering properties can easily be mass-produced, thus enabling real-world applications. Representing a bridge between metasurface optics and speckle-based computational imaging, this work paves the way to extending the potentials of diverse speckle-based computational imaging methods for various applications such as biomedical imaging, holography, and optical encryption.

Tunable X-ray speckle-based phase-contrast and dark-field imaging using the unified modulated pattern analysis approach

X-ray phase-contrast and dark-field imaging provides valuable, complementary information about the specimen under study. Among the multimodal X-ray imaging methods, X-ray grating interferometry and speckle-based imaging have drawn particular attention, which, however, in their common implementations incur certain limitations that can restrict their range of applications. Recently, the unified modulated pattern analysis (UMPA) approach was proposed to overcome these limitations and combine grating- and speckle-based imaging in a single approach. Here, we demonstrate the multimodal imaging capabilities of UMPA and highlight its tunable character regarding spatial resolution, signal sensitivity and scan time by using different reconstruction parameters.

Advances in indirect detector systems for ultra high-speed hard X-ray imaging with synchrotron light

We report on indirect X-ray detector systems for various full-field, ultra high-speed X-ray imaging methodologies, such as X-ray phase-contrast radiography, diffraction topography, grating interferometry and speckle-based imaging performed at the hard X-ray imaging beamline ID19 of the European Synchrotron—ESRF. Our work highlights the versatility of indirect X-ray detectors to multiple goals such as single synchrotron pulse isolation, multiple-frame recording up to millions frames per second, high efficiency, and high spatial resolution. Besides the technical advancements, potential applications are briefly introduced and discussed.

Synchrotron speckle-based x-ray phase-contrast imaging for mapping intra-aneurysmal blood flow without contrast agent

Intracranial aneurysms carry risk of rupture with life threatening consequences. Hemodynamic forces are considered to be the key in cerebral aneurysm rupture. Notably, the hemodynamic analysis requires precise characterization of intra-aneurysmal blood velocity field, raising the need to develop novel measurement techniques. This study aims to assess the performance of speckle-based x-ray phase contrast particle imaging velocimetry (PIV) for velocity field mapping of a pulsatile blood flow in a patient specific aneurysm model without contrast agent. Porcine blood was used in a blood circulation loop including the aneurysm model, which was imaged using the synchrotron x-ray phase contrast imaging (PCI) at an optimum phase propagation distance. X-ray images were processed to improve speckle characteristics for PIV analysis. A computational fluid dynamic (CFD) model was developed for simulation of the blood velocity field and compared to the x-ray PCI-PIV measurements. The CFD model and x-ray PCI-PIV measurements showed similar intra-aneurysmal blood flow structures although the velocity magnitudes were generally higher in CFD model. Contrary to CFD model, x-ray PCI-PIV measurements revealed formation of blood cell stagnation at the aneurysm dome at low blood flow rate, cushioning upon the blood flow shear force. Synchrotron x-ray PCI-PIV offers potential for hemodynamic analysis in patient specific cerebral aneurysm models without contrast agent.

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.

Speckle-based hyperspectral imaging combining multiple scattering and compressive sensing in nanowire mats.

Encoding of spectral information onto monochrome imaging cameras is of interest for wavelength multiplexing and hyperspectral imaging applications. Here, the complex spatiospectral response of a disordered material is used to demonstrate retrieval of a number of discrete wavelengths over a wide spectral range. Strong, diffuse light scattering in a semiconductor nanowire mat is used to achieve a highly compact spectrometer of micrometer thickness, transforming different wavelengths into distinct speckle patterns with nanometer sensitivity. Spatial multiplexing is achieved through the use of a microlens array, allowing simultaneous imaging of many speckles, ultimately limited by the size of the diffuse spot area. The performance of different information retrieval algorithms is compared. A compressive sensing algorithm exhibits efficient reconstruction capability in noisy environments and with only a few measurements.

Noise analysis of speckle-based x-ray phase-contrast imaging.

Speckle-based x-ray phase-contrast imaging has drawn increasing interest in recent years as a simple, multimodal, cost-efficient, and laboratory-source adaptable method. We investigate its noise properties to help further optimization on the method and further comparison with other phase-contrast methods. An analytical model for assessing noise in a differential phase signal is adapted from studies on the digital image correlation technique in experimental mechanics and is supported by simulations and experiments. The model indicates that the noise of the differential phase signal from speckle-based imaging has a behavior similar to that of the grating-based method.

Experimental comparison between speckle and grating-based imaging technique using synchrotron radiation X-rays.

X-ray phase contrast and dark-field imaging techniques provide important and complementary information that is inaccessible to the conventional absorption contrast imaging. Both grating-based imaging (GBI) and speckle-based imaging (SBI) are able to retrieve multi-modal images using synchrotron as well as lab-based sources. However, no systematic comparison has been made between the two techniques so far. We present an experimental comparison between GBI and SBI techniques with synchrotron radiation X-ray source. Apart from the simple experimental setup, we find SBI does not suffer from the issue of phase unwrapping, which can often be problematic for GBI. In addition, SBI is also superior to GBI since two orthogonal differential phase gradients can be simultaneously extracted by one dimensional scan. The GBI has less stringent requirements for detector pixel size and transverse coherence length when a second or third grating can be used. This study provides the reference for choosing the most suitable technique for diverse imaging applications at synchrotron facility.

From synchrotron radiation to lab source: advanced speckle-based X-ray imaging using abrasive paper.

X-ray phase and dark-field imaging techniques provide complementary and inaccessible information compared to conventional X-ray absorption or visible light imaging. However, such methods typically require sophisticated experimental apparatus or X-ray beams with specific properties. Recently, an X-ray speckle-based technique has shown great potential for X-ray phase and dark-field imaging using a simple experimental arrangement. However, it still suffers from either poor resolution or the time consuming process of collecting a large number of images. To overcome these limitations, in this report we demonstrate that absorption, dark-field, phase contrast, and two orthogonal differential phase contrast images can simultaneously be generated by scanning a piece of abrasive paper in only one direction. We propose a novel theoretical approach to quantitatively extract the above five images by utilising the remarkable properties of speckles. Importantly, the technique has been extended from a synchrotron light source to utilise a lab-based microfocus X-ray source and flat panel detector. Removing the need to raster the optics in two directions significantly reduces the acquisition time and absorbed dose, which can be of vital importance for many biological samples. This new imaging method could potentially provide a breakthrough for numerous practical imaging applications in biomedical research and materials science.

Simulations of x-ray speckle-based dark-field and phase-contrast imaging with a polychromatic beam

Following the first experimental demonstration of x-ray speckle-based multimodal imaging using a polychromatic beam [I. Zanette et al., Phys. Rev. Lett. 112(25), 253903 (2014)], we present a simulation study on the effects of a polychromatic x-ray spectrum on the performance of this technique. We observe that the contrast of the near-field speckles is only mildly influenced by the bandwidth of the energy spectrum. Moreover, using a homogeneous object with simple geometry, we characterize the beam hardening artifacts in the reconstructed transmission and refraction angle images, and we describe how the beam hardening also affects the dark-field signal provided by speckle tracking. This study is particularly important for further implementations and developments of coherent speckle-based techniques at laboratory x-ray sources.