arXiv subjects

Emmanuel Brun

Publications and source records attributed to Emmanuel Brun.

10 recordsLinked to original sources

arXiv2025

Membrane stepping optimization in Modulation Based Imaging

Modulation-based imaging (MoBI) is an X-ray phase-contrast technique that uses an intensity modulator (or membrane) in the beam. Although MoBI can be performed in a single shot, multiple exposures are typically needed to improve the quality of the result. The membrane is typically moved using a regular stepping pattern for convenience; however, the impact of the membrane movement scheme on image quality has not been fully investigated yet. In this work, we explore optimized movement strategies aiming at improving MoBI performance. An experimental study tested optimization schemes based on global and local standard deviation metrics, and compared them with regular and random stepping motions. The results demonstrated superior contrast-to-noise ratio and reduced angular sensitivity in the optimized approaches compared to conventional stepping. These results were consistent across different membrane types, with honeycomb membranes showing the highest compatibility with the optimization procedure. Noise power spectrum analysis further validated the advantages of the optimized motion strategies. Overall, the results demonstrate that an optimized membrane movement can significantly improve MoBI image quality without increasing experimental complexity.

physics.optics↗

arXiv2025

On the optimisation of the geometric pattern for structured illumination based X-ray phase contrast and dark field imaging: A simulation study and its experimental validation

Phase-contrast and dark-field imaging are relatively new X-ray imaging modalities that provide additional information to conventional attenuation-based imaging. However, this new information comes at the price of a more complex acquisition scheme and optical components. Among the different techniques available, such as Grating Interferometry or Edge Illumination, modulation-based and more generally single-mask/grid imaging techniques simplify these new procedures to obtain phase and dark-field images by shifting the experimental complexity to the numerical post-processing side. This family of techniques involves inserting a membrane into the X-ray beam that locally modulating the intensity to create a pattern on the detector which serves as a reference. However, the topological nature of the mask used seems to determine the quality of the reconstructed phase and dark-field images. We present in this article an in-depth study of the impact of the membrane parameters used in a single mask imaging approach. A spiral topology seems to be an optimum both in terms of resolution and contrast-to-noise ratio compared to random and regular patterns.

physics.optics↗

arXiv2024

Review of speckle-tracking algorithms for x-ray phase contrast imaging: low dose applications

X-ray speckles have been used for a wide variety of experiments, ranging from imaging (and tomography), wavefront sensing, spatial coherence measurements all the way to x-ray photon correlation spectroscopy (XPCS) and ptychography. In the near-field regime, x-ray speckle-grains preserve shape and size under free-space propagation for a static random modulation of the illumination, which permits using them as wavefront markers. The introduction of an object in the modulated field will lead to a displacement of the speckles due to refraction. Retrieving the local displacements enables access to the gradient of the phase-shift induced by the sample. The numerical process to retrieve the phase information is not trivial and numerous algorithms have been developed in the past decade with various advantages and limitations. This review focuses on near-field x-ray speckle phase imaging in the differential mode as described previously, introducing the existing algorithms with their specifications and comparing their performances under various experimental conditions.

physics.med-ph↗

arXiv2023

Dark-field and directional dark-field on low coherence X-ray sources with random mask modulations: validation with SAXS anisotropy measurements

Phase Contrast Imaging (PCI), Dark-Field (DF) and Directional Dark-Field (DDF) imaging are recent X-ray imaging modalities that have demonstrated their interest by providing access to information and contrasts different from those provided by conventional absorption X-ray imaging. However, access to these two types of images is currently limited because the acquisitions require the use of coherent sources such as synchrotron radiation or complicated optical setups to exploit the coherence requirements. This work demonstrates the possibility of efficiently performing phase contrast, dark-field and directional dark-field imaging on a low-coherence laboratory system equipped with a conventional X-ray tube, using a simple, fast and robust single-mask technique. The transfer to a low spatial coherence laboratory system was made possible by using random modulation based imaging (MoBI) and extending the low coherence system algorithm to retrieve dark-field and directional dark-field.

eess.IV↗

arXiv2021

Directional dark-field implicit x-ray speckle tracking using an anisotropic-diffusion Fokker-Planck equation

When a macroscopic-sized non-crystalline sample is illuminated using coherent x-ray radiation, a bifurcation of photon energy flow may occur. The coarse-grained complex refractive index of the sample may be considered to attenuate and refract the incident coherent beam, leading to a coherent component of the transmitted beam. Spatially-unresolved sample microstructure, associated with the fine-grained components of the complex refractive index, introduces a diffuse component to the transmitted beam. This diffuse photon-scattering channel may be viewed in terms of position-dependent fans of ultra-small-angle x-ray scatter. These position-dependent fans, at the exit surface of the object, may under certain circumstances be approximated as having a locally-elliptical shape. By using an anisotropic-diffusion Fokker-Planck approach to model this bifurcated x-ray energy flow, we show how all three components (attenuation, refraction and locally-elliptical diffuse scatter) may be recovered. This is done via x-ray speckle tracking, in which the sample is illuminated with spatially-random x-ray fields generated by coherent illumination of a spatially-random membrane. The theory is developed, and then successfully applied to experimental x-ray data.

physics.med-ph↗

arXiv2019

X-ray Multimodal Intrinsic-Speckle-Tracking

We develop X-ray Multi-modal Intrinsic-Speckle-Tracking (MIST), a form of X-ray speckle-tracking that is able to recover both the position-dependent phase shift and the position-dependent small-angle X-ray scattering (SAXS) signal of a phase object. MIST is based on combining a Fokker-Planck description of paraxial X-ray optics, with an optical-flow formalism for X-ray speckle-tracking. Only two images need to be taken in the presence of the sample, corresponding to two different transverse positions of the speckle-generating membrane, in order to recover both the refractive and local-SAXS properties of the sample. Like the optical-flow X-ray phase-retrieval method which it generalises, the MIST method implicitly rather than explicitly tracks both the transverse motion and the diffusion of speckles that is induced by the presence of a sample. Application to X-ray synchrotron data shows the method to be efficient, rapid and stable.

eess.IV↗

arXiv2019

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.

physics.med-ph↗

arXiv2018

Single-Image Geometric-Flow X-Ray Speckle Tracking

We develop a speckle-tracking method for x-ray phase-contrast imaging, based on the concept of geometric flow. This flow is a conserved current associated with deformation of illuminating x-ray speckles induced by passage through a sample. The method provides a rapid, efficient, and accurate algorithm for quantitative phase imaging. It is highly photon efficient and able to image dynamic objects, since a single radiograph of the sample is sufficient for the phase recovery. We experimentally quantify the resolution and contrast of the approach with both two-dimensional and three-dimensional phase-imaging applications using x-ray synchrotron radiation. Finally, we discuss adaptations of the method to imaging with compact x-ray sources that have a large source size and significant spectral bandwidth.

physics.med-ph↗

arXiv2013

PyHST2: an hybrid distributed code for high speed tomographic reconstruction with iterative reconstruction and a priori knowledge capabilities

We present the PyHST2 code which is in service at ESRF for phase-contrast and absorption tomography. This code has been engineered to sustain the high data flow typical of the third generation synchrotron facilities (10 terabytes per experiment) by adopting a distributed and pipelined architecture. The code implements, beside a default filtered backprojection reconstruction, iterative reconstruction techniques with a-priori knowledge. These latter are used to improve the reconstruction quality or in order to reduce the required data volume and reach a given quality goal. The implemented a-priori knowledge techniques are based on the total variation penalisation and a new recently found convex functional which is based on overlapping patches. We give details of the different methods and their implementations while the code is distributed under free license. We provide methods for estimating, in the absence of ground-truth data, the optimal parameters values for a-priori techniques.

math.NA↗

arXiv2013

A Convex Functional for Image Denoising based on Patches with Constrained Overlaps and its vectorial application to Low Dose Differential Phase Tomography

We solve the image denoising problem with a dictionary learning technique by writing a convex functional of a new form. This functional contains beside the usual sparsity inducing term and fidelity term, a new term which induces similarity between overlapping patches in the overlap regions. The functional depends on two free regularization parameters: a coefficient multiplying the sparsity-inducing $L_{1}$ norm of the patch basis functions coefficients, and a coefficient multiplying the $L_{2}$ norm of the differences between patches in the overlapping regions. The solution is found by applying the iterative proximal gradient descent method with FISTA acceleration. In the case of tomography reconstruction we calculate the gradient by applying projection of the solution and its error backprojection at each iterative step. We study the quality of the solution, as a function of the regularization parameters and noise, on synthetic datas for which the solution is a-priori known. We apply the method on experimental data in the case of Differential Phase Tomography. For this case we use an original approach which consists in using vectorial patches, each patch having two components: one per each gradient component. The resulting algorithm, implemented in the ESRF tomography reconstruction code PyHST, results to be robust, efficient, and well adapted to strongly reduce the required dose and the number of projections in medical tomography.

math.NA↗