Three-dimensional compositional mapping using multiple energy X-ray computed tomography

When a microstructure contains finer details than the X-ray computed tomography (CT) spatial resolution, the conventional techniques based on segmentation become inadequate for separating the materials composing the sample. Data-constrained modeling overcomes this problem using multiple CT data sets acquired with different X-ray beam energies. The volume fractions of the materials contained into a single voxel are then determined by solving a linear system. The ill-conditioned nature of the linear system reflects into a high sensitivity to the noise. An alternative approach to the direct solution of the linear system, based on the iterative application of the Expectation-Maximization algorithm, is here presented. Different noise conditions are investigated for a random-generated single-voxel problem and for a more complex numerical phantom

Absorption, refraction and scattering retrieval with an edge-illumination-based imaging setup

We have recently developed a new method based on edge-illumination for retrieving a three-image representation of the sample. A minimum of three intensity projections are required in order to retrieve the transmission, refraction and ultra-small-angle scattering properties of the sample. Here we show how the method can be adapted for particular cases in which some degree of a priori information about the sample might be available, limiting the number of required projections to two. Moreover, an iterative algorithm to correct for non-ideal optical elements is proposed and tested on numerical simulations, and finally validated on experimental data

Virtual edge illumination and one dimensional beam tracking for absorption, refraction, and scattering retrieval

We propose two different approaches to retrieve x-ray absorption, refraction, and scattering signals using a one dimensional scan and a high resolution detector. The first method can be easily implemented in existing procedures developed for edge illumination to retrieve absorption and refraction signals, giving comparable image quality while reducing exposure time and delivered dose. The second method tracks the variations of the beam intensity profile on the detector through a multi-Gaussian interpolation, allowing the additional retrieval of the scattering signal

Edge-illumination X-ray dark-field imaging for visualising defects in composite structures

Low velocity impact can lead to barely visible and difficult to detect damage such as fibre and matrix breakage or delaminations in composite structures. Drop-weight impact damage in a cross-ply carbon fibre laminate plate was characterized using ultrasonic C-scan measurements. This was compared to the results provided by a novel X-ray imaging technique based on the detection of phase effects, which can be implemented with conventional equipment. Three representations of the sample are provided: absorption, differential phase and dark-field. The latter is of particular interest to detect cracks and voids of dimensions that are smaller than the spatial resolution of the imaging system. The ultrasonic C-scan showed a large delamination and additional damage along the fibre directions. The damage along the fibre directions and other small scale defects were detected from the X-ray imaging. As the system is sensitive to phase effects along one direction at a time, the acquisition of an additional scan, rotating the sample 90 degrees around the beam axis, provides information in both fibre directions. These two techniques enable access to a set of complementary information, across different length scales, which can be useful in the characterization of the defects occurring in composite structures

Reverse projection retrieval in edge illumination x-ray phase contrast computed tomography

Edge illumination (EI) x-ray phase contrast computed tomography (CT) can provide three-dimensional distributions of the real and imaginary parts of the complex refractive index (n = 1 d + ib) of the sample. Phase retrieval, i.e. the separation of attenuation and refraction data from projections that contain a combination of both, is a key step in the image reconstruction process. In EI-based x-ray phase contrast CT, this is conventionally performed on the basis of two projections acquired in opposite illumination configurations (i.e. with different positions of the pre-sample mask) at each CT angle. Displacing the pre-sample mask at each projection makes the scan susceptible to motor-induced misalignment and prevents a continuous sample rotation. We present an alternative method for the retrieval of attenuation and refraction data that does not require repositioning the pre-sample mask. The method is based on the reverse projection relation published by Zhu et al. (2010) for grating interferometry-based x-ray phase contrast CT. We use this relation to derive a simplified acquisition strategy that allows acquiring data with a continuous sample rotation, which can reduce scan time when combined with a fast read-out detector. Besides discussing the theory and the necessary alignment of the experimental setup, we present tomograms obtained with reverse projection retrieval and demonstrate their agreement with those obtained with the conventional EI retrieval

X-ray phase-contrast imaging

X-ray imaging is a standard tool for the non-destructive inspection of the internal structure of samples. It finds application in a vast diversity of fields: medicine, biology, many engineering disciplines, palaeontology and earth sciences are just few examples. The fundamental principle underpinning the image formation have remained the same for over a century: the X-rays traversing the sample are subjected to different amount of absorption in different parts of the sample. By means of phase-sensitive techniques it is possible to generate contrast also in relation to the phase shifts imparted by the sample and to extend the capabilities of X-ray imaging to those details that lack enough absorption contrast to be visualised in conventional radiography. A general overview of X-ray phase contrast imaging techniques is presented in this review, along with more recent advances in this fast evolving field and some examples of applications

The first large-area, high-X-ray energy phase contrast prototype for enhanced detection of threat objects in baggage screening

X-ray imaging is the most commonly used method in baggage screening. Conventional x-ray attenuation (usually in dual-energy mode) is exploited to discriminate threat and non-threat materials: this is essentially, a method that has seen little changes in decades. Our goal is to demonstrate that x-rays can be used in a different way to achieve improved detection of weapons and explosives. Our approach involves the use of x-ray phase contrast and it a) allows much higher sensitivity in the detection of object edges and b) can be made sensitive to the sample’s microstructure. We believe that these additional channels of information, alongside conventional attenuation which would still be available, have the potential to significantly increase both sensitivity and specificity in baggage scanning. We obtained preliminary data demonstrating the above enhanced detection, and we built a scanner (currently in commissioning) to scale the concept up and test it on real baggage. In particular, while previous X-ray phase contrast imaging systems were limited in terms of both field of view (FOV) and maximum x-ray energy, this scanner overcomes both those limitations and provides FOVs up to 20 to 50 cm2 with x-ray energies up to 100 keV

Asymmetric masks for laboratory-based X-ray phase-contrast imaging with edge illumination

We report on an asymmetric mask concept that enables X-ray phase contrast imaging without requiring any movement in the system during data acquisition. The method is compatible with laboratory equipment, namely a commercial detector and a rotating anode tube. The only motion required is that of the object under investigation which is scanned through the imaging system. Two proof-of-principle optical elements were designed, fabricated and experimentally tested. Quantitative measurements on samples of known shape and composition were compared to theory with good agreement. The method is capable of measuring the attenuation, refraction and (ultra-small-angle) X-ray scattering, does not have coherence requirements and naturally adapts to all those situations in which the X-ray image is obtained by scanning a sample through the imaging system

Asymmetric masks for large field-of-view and high-energy X-ray phase contrast imaging

We report on a large field of view, laboratory-based X-ray phase-contrast imaging setup. The method is based upon the asymmetric mask design that enables the retrieval of the absorption, refraction and scattering properties of the sample without the need to move any component of the imaging system. This can be thought of as a periodic repetition of a group of three (or more) apertures arranged in such a way that each laminar beam, defined by the apertures, produces a different illumination level when analysed with a standard periodic set of apertures. The sample is scanned through the imaging system, also removing possible aliasing problems that might arise from partial sample illumination when using the edge illumination technique. This approach preserves the incoherence and achromatic properties of edge illumination, removes the problems related to aliasing and it naturally adapts to those situations in clinical, industrial and security imaging where the image is acquired by scanning the sample relative to the imaging system. These concepts were implemented for a large field-of-view set of masks (20 cm × 1.5 cm and 15 cm × 1.2 cm), designed to work with a tungsten anode X-ray source operated up to 80–100 kVp, from which preliminary experimental results are presented