Weak Absorption Contrast Research Articles

Virtual differential phase-contrast and dark-field imaging of x-ray absorption images via deep learning.

Weak absorption contrast in biological tissues has hindered x-ray computed tomography from accessing biological structures. Recently, grating-based imaging has emerged as a promising solution to biological low-contrast imaging, providing complementary and previously unavailable structural information of the specimen. Although it has been successfully applied to work with conventional x-ray sources, grating-based imaging is time-consuming and requires a sophisticated experimental setup. In this work, we demonstrate that a deep convolutional neural network trained with a generative adversarial network can directly convert x-ray absorption images into differential phase-contrast and dark-field images that are comparable to those obtained at both a synchrotron beamline and a laboratory facility. By smearing back all of the virtual projections, high-quality tomographic images of biological test specimens deliver the differential phase-contrast- and dark-field-like contrast and quantitative information, broadening the horizon of x-ray image contrast generation.

Grating based X-ray phase contrast CT imaging with iterative reconstruction algorithm

Grating based X-ray imaging technology is a coherent imaging technique that bears tremendous potential in three-dimensional tomographic imaging of weak absorption contrast specimens. Three kinds of contrast information including absorption, phase and scattering can be retrieved separately based on a single set of raw projections. However, the grating based X-ray imaging with the conventional phase-retrieval method using the conventional phase-stepping approach and filtered back projection (FBP) reconstruction algorithm require large amounts of raw data, so that long exposure time and large amounts of radiation dose is accepted by the sample. According to the traditional grating based X-ray imaging system, we propose a low dose, fast, multi-contrast CT reconstruction approach based on the iterative reconstruction algorithm that optimizes dose efficiency but does not share the main limitations of other reported methods. Prior to reconstruction, firstly, the projections are acquired with the phase stepping approach and multi-contrast projections are retrieved from the raw data by conventional retrieval algorithm. Then the rotational variable differential phase projections are converted to rotational invariable projections by means of decomposing the differential phase projections into the rotational invariable projections in two mutually perpendicular derivative directions via the transformation of coordinates. Finally, the absorption, phase and scattering information are reconstructed with the iterative reconstruction algorithm and the phase is retrieved based on the fast Fourier transform (FFT). We validated and assessed the phase reconstruction approach with a numerical simulation on a phase Shepp-Logan phantom. The experiment was performed at the X-ray imaging and biomedical application beam line (BL-13W) in the Shanghai Synchrotron Radiation Facility (SSRF) where 20 keV X-ray from a Si(111) monochromator is emitted. The X-ray interferometer was positioned at 34 m from the Wiggler source. The images were recorded with a scintillator/lens-coupled CCD camera with 2048 pixel2048 pixel resolution and an effective pixel size of 9 m. The numerical tests and the experimental results demonstrate that, for the small radiation dose deposited in the sample, the iterative reconstruction algorithm provides phase reconstructions of better quality and higher signal to noise ratio than the conventional FBP reconstruction algorithm, and also provides the multi-contrast 3D images, including absorption image, phase image and scattering image. This development is of particular interest for applications where the samples need inspecting under low dose and high speed conditions, and will play an important role in the nondestructive and quantitative imaging in the industry, biomedical and medical diagnosis fields.

X-ray analyzer-based phase-contrast computed laminography.

X-ray analyzer-based phase-contrast imaging is combined with computed laminography for imaging regions of interest in laterally extended flat specimens of weak absorption contrast. The optics discussed here consist of an asymmetrically cut collimator crystal and a symmetrically cut analyzer crystal arranged in a nondispersive (+, -) diffraction geometry. A generalized algorithm is given for calculating multi-contrast (absorption, refraction and phase contrast) images of a sample. Basic formulae are also presented for laminographic reconstruction. The feasibility of the method discussed was verified at the vertical wiggler beamline BL-14B of the Photon Factory. At a wavelength of 0.0733 nm, phase-contrast sectional images of plastic beads were successfully obtained. Owing to strong circular artifacts caused by a sample holder, the field of view was limited to about 6 mm in diameter.

Trimodal low-dose X-ray tomography

X-ray grating interferometry is a coherent imaging technique that bears tremendous potential for three-dimensional tomographic imaging of soft biological tissue and other specimens whose details exhibit very weak absorption contrast. It is intrinsically trimodal, delivering phase contrast, absorption contrast, and scattering ("dark-field") contrast. Recently reported acquisition strategies for grating-interferometric phase tomography constitute a major improvement of dose efficiency and speed. In particular, some of these techniques eliminate the need for scanning of one of the gratings ("phase stepping"). This advantage, however, comes at the cost of other limitations. These can be a loss in spatial resolution, or the inability to fully separate the three imaging modalities. In the present paper we report a data acquisition and processing method that optimizes dose efficiency but does not share the main limitations of other recently reported methods. Although our method still relies on phase stepping, it effectively uses only down to a single detector frame per projection angle and yields images corresponding to all three contrast modalities. In particular, this means that dark-field imaging remains accessible. The method is also compliant with data acquisition over an angular range of only 180° and with a continuous rotation of the specimen.

Characterization of Metal Matrix Composites by Synchrotron Refraction Computed Topography

X-ray computed tomography is an important tool for evaluating the three dimensional microstructure of modern materials non-destructively. To resolve material structures in the micrometre range and below high brilliance synchrotron radiation has to be taken. But materials of low absorption or mixed phases show a weak absorption contrast at there interfaces. A Contrast enhancement can be achieved by exploiting the refraction of X-rays at interfaces. This technique was developed and applied at the NDT department of the Federal Institute for Materials Research and Testing (BAM) during the last decade. It meets the actual demand for improved non-destructive characterisation of high performance composites, ceramics and other low density materials and components. The technique is based on Ultra Small Angle Scattering (USAXS) by micro structural elements causing phase related effects like refraction and total reflection at a few minutes of arc as the refractive index of X-rays is nearly unity. The extraordinary refraction contrast of inner surfaces is far beyond absorption effects and hence especially useful for materials of low absorption or mixed phases, showing similar X-ray absorption properties. Crack orientation and fibre-matrix debonding in plastics, polymers, ceramics and metal-matrix-composites after cyclic loading and hydro thermal aging can be visualized. By combining the refraction technique with the computed tomography technique the three dimensional imaging of the micro structure of the materials is obtained. In most cases the investigated inner surface and interface structures correlate to mechanical properties. Recent results with a sub-micrometer resolution will be presented.

High-throughput, high-resolution X-ray phase contrast tomographic microscopy for visualisation of soft tissue

The use of conventional absorption based X-ray microtomography can become limited for samples showing only very weak absorption contrast. However, a wide range of samples studied in biology and materials science can produce significant phase shifts of the X-ray beam, and thus the use of the phase signal can provide substantially increased contrast and therefore new and otherwise inaccessible information. The application of two approaches for high-throughput, high-resolution X-ray phase contrast tomography, both available on the TOMCAT beamline of the SLS, is illustrated. Differential Phase Contrast (DPC) imaging uses a grating interferometer and a phase-stepping technique. It has been integrated into the beamline environment on TOMCAT in terms of the fast acquisition and reconstruction of data and the availability to scan samples within an aqueous environment. The second phase contrast approach is a modified transfer of intensity approach that can yield the 3D distribution of the phase (refractive index) of a weakly absorbing object from a single tomographic dataset. These methods are being used for the evaluation of cell integrity in 3D, with the specific aim of following and analyzing progressive cell degeneration to increase knowledge of the mechanistic events of neurodegenerative disorders such as Parkinson's disease.

Phase-contrast and holographic computed laminography

In-line phase contrast is combined with laminography to image in three dimensions regions of interest in laterally extended flat specimens of weak absorption contrast. The principle of the method and a theoretical description of the imaging process are outlined. The present instrumental implementation enables reconstructing nondestructively the internal structure at different lateral specimen positions with micron resolution. The feasibility and application potential are demonstrated for both phase-contrast and holographic (i.e., using phase retrieval) laminography by the three-dimensional imaging of fuel-cell diffusion layers.

Benchtop phase-contrast X-ray imaging

Clinical radiography has traditionally been based on contrast obtained from absorption when X-rays pass through the body. The contrast obtained from traditional radiography can be rather poor, particularly when it comes to soft tissue. A wide range of media of interest in materials science, biology and medicine exhibit very weak absorption contrast, but they nevertheless produce significant phase shifts with X-rays. The use of phase information for imaging purposes is therefore an attractive prospect. Some of the X-ray phase-contrast imaging methods require highly monochromatic plane wave radiation and sophisticated X-ray optics. However, the propagation-based phase-contrast imaging method adapted in this paper is a relatively simple method to implement, essentially requiring only a microfocal X-ray tube and electronic detection. In this paper, we present imaging results obtained from two different benchtop X-ray sources employing the free space propagation method. X-ray phase-contrast imaging provides higher contrast in many samples, including biological tissues that have negligible absorption contrast.

Phase-and-amplitude computer tomography

A tomographic technique is proposed for reconstruction under specified conditions of the three-dimensional distribution of complex refractive index in a sample from a single projection image per view angle, where the images display both absorption contrast and propagation-induced phase contrast. The algorithm achieves high numerical stability as a consequence of the complementary nature of the absorption and phase contrast transfer functions. The method is pertinent to biomedical imaging and nondestructive testing of samples exhibiting weak absorption contrast. © 2006 American Institute of Physics. DOI: 10.1063/1.2226794 X-ray computer tomography CT is a well-established technique for three-dimensional 3D imaging of internal structure of samples. 1,2 Conventional CT imaging is based on the differential attenuation of transmitted x rays by constituents of the sample. This contrast mechanism is effective for distinguishing between elemental components with significant differences in atomic number or density, e.g., between flesh and bones in the case of medical CT. However, the difference in x-ray attenuation by different types of soft tissues e.g., healthy and malignant ones is typically rather weak, which results in poor image contrast, hampering diagnostics. It has been suggested that x-ray phase contrast can be utilized for improvement of the contrast in transmission images of noncrystalline samples consisting predominantly of light chemical elements. 3‐5 Subsequently, phase-contrast x-ray CT PCT has been implemented in several forms, including x-ray interferometry, 6,7 , analyzer-based 8,9 and propagation-based phase contrast, 10‐12 and others. The sub