Martina Bieberle

Ultrafast limited-angle-type x-ray tomography

The authors present an ultrafast electron beam x-ray computed tomography technique usable for imaging of fast processes, such as multiphase flows or moving parts in technical or biological objects. The setup consists of an electron beam unit with fast deflection capability and an ultrafast multielement x-ray detector and achieves 10000frames∕s image rate. Since full sampling of the Radon space requires an angular overlap of source path and detector which strongly decreases axial resolution, the authors devised a limited-angle-type tomography. As a demonstration they visualized the movement of particles and gas bubbles rising in a stagnant liquid.

Ultrafast three-dimensional x-ray computed tomography

X-ray computed tomography (CT) is a well established visualization technique in medicine and nondestructive testing. However, since CT scanning requires sampling of radiographic projections from different viewing angles, common CT systems with mechanically moving parts are too slow for dynamic imaging, for instance of multiphase flows or live animals. Here, we introduce an ultrafast three-dimensional x-ray CT method based on electron beam scanning, which achieves volume rates of 500 s−1. Primary experiments revealed the capability of this method to recover the structure of phase boundaries in gas-solid and gas-liquid two-phase flows, which undergo three-dimensional structural changes in the millisecond scale.

Scatter analysis and correction for ultrafast X-ray tomography

Ultrafast X-ray computed tomography (CT) is an imaging technique with high potential for the investigation of the hydrodynamics in multiphase flows. For correct determination of the phase distribution of such flows, a high accuracy of the reconstructed image data is essential. In X-ray CT, radiation scatter may cause disturbing artefacts. As the scattering is not considered in standard reconstruction algorithms, additional methods are necessary to correct the detector readings or to prevent the detection of scattered photons. In this paper, we present an analysis of the scattering background for the ultrafast X-ray CT imaging system ROFEX at the Helmholtz-Zentrum Dresden-Rossendorf and propose a correction technique based on collimation and deterministic simulation of first-order scattering.

Experimental facility for two- and three-dimensional ultrafast electron beam x-ray computed tomography

An experimental facility is described, which has been designed to perform ultrafast two-dimensional (2D) and three-dimensional (3D) electron beam computed tomographies. As a novelty, a specially designed transparent target enables tomography with no axial offset for 2D imaging and high axial resolution 3D imaging employing the cone-beam tomography principles. The imaging speed is 10 000 frames per second for planar scanning and more than 1000 frames per second for 3D imaging. The facility serves a broad spectrum of potential applications; primarily, the study of multiphase flows, but also in principle nondestructive testing or small animal imaging. In order to demonstrate the aptitude for these applications, static phantom experiments at a frame rate of 2000 frames per second were performed. Resulting spatial resolution was found to be 1.2 mm and better for a reduced temporal resolution.

Correct averaging in transmission radiography: Analysis of the inverse problem

Abstract Transmission radiometry is frequently used in industrial measurement processes as a means to assess the thickness or composition of a material. A common problem encountered in such applications is the so-called dynamic bias error, which results from averaging beam intensities over time while the material distribution changes. We recently reported on a method to overcome the associated measurement error by solving an inverse problem, which in principle restores the exact average attenuation by considering the Poisson statistics of the underlying particle or photon emission process. In this paper we present a detailed analysis of the inverse problem and its optimal regularized numerical solution. As a result we derive an optimal parameter configuration for the inverse problem.

Multiphase flow investigations with ultrafast electron beam x-ray tomography

We introduce ultrafast electron beam X-ray tomography as an imaging modality for multiphase flow studies. A dedicated electron beam tomography scanner (ROFEX) has been developed which allows cross-sectional X-ray tomography with 1 mm spatial resolution and up to 7000 cross-sectional images per second recording speed. It is applicable to flow problems in vessels with up to 120 mm diameter and moderate X-ray attenuation. The tomography system has been applied in various flow studies, including gas-liquid two-phase flow in vertical pipes and channel structures.

Level-set reconstruction algorithm for ultrafast limited-angle X-ray computed tomography of two-phase flows

Tomographic image reconstruction is based on recovering an object distribution from its projections, which have been acquired from all angular views around the object. If the angular range is limited to less than 180° of parallel projections, typical reconstruction artefacts arise when using standard algorithms. To compensate for this, specialized algorithms using a priori information about the object need to be applied. The application behind this work is ultrafast limited-angle X-ray computed tomography of two-phase flows. Here, only a binary distribution of the two phases needs to be reconstructed, which reduces the complexity of the inverse problem. To solve it, a new reconstruction algorithm (LSR) based on the level-set method is proposed. It includes one force function term accounting for matching the projection data and one incorporating a curvature-dependent smoothing of the phase boundary. The algorithm has been validated using simulated as well as measured projections of known structures, and its performance has been compared to the algebraic reconstruction technique and a binary derivative of it. The validation as well as the application of the level-set reconstruction on a dynamic two-phase flow demonstrated its applicability and its advantages over other reconstruction algorithms.

Ultrafast electron beam X-ray computed tomography for 2D and 3D two-phase flow imaging

Imaging of complex and dynamic processes such as two- or multiphase flows with high structural as well as temporal resolution has always been a challenging task. In recent years, the electron beam X-ray computed tomography technique has been developed towards a powerful imaging tool, which reaches frame rates of 8000 fps in 2D and 1000 fps in 3D. In this paper, the latest developments as well as selected applications of ultrafast electron beam X-ray CT are presented.

Study of Flow Behavior of Granular Material Inside Cylindrical Silo Using Ultrafast X-Ray Imaging Technique

Abstract This paper presents an application of an ultrafast electron beam X-ray CT scanner for investigating the gravitational flow behavior of granulates through cylindrical silo model. The CT scanner allows obtaining crosssectional images of the granular material distribution with a spatial resolution of approximately 1 mm and a time resolution of 2 kHz. In order to conduct a deep analysis of the granular flow concentration changes, two image processing algorithm steps were applied. The first step deals with preprocessing and re-centering stacks of raw images. The second step divides the preprocessed image into several concentric rings and calculates the mean value to study radial concentration changes. Independent analysis of granular concentration in each ring provides useful knowledge to study the silo discharging during mass flow and funnel flow.

A device for ultrafast three-dimensional x-ray computed tomography with a scanned electron beam

A novel tomography device has been realised which enables ultrafast three-dimensional imaging with up to 1000 volume-frames per second. This is achieved by scanning an electron beam over a specially designed annular x-ray transparent target. Its height of 37 mm is also the maximum height of the tomography volume. The maximum experiment diameter is 75 mm. The design of the setup is similar to cone beam tomography which allows image reconstruction based on Feldkamp type algorithms to be used. Various phantom experiments were performed and proved good spatial resolution of up to 1.2 mm at a volume-frame rate of 250 s−1. Eventually, the device was applied to image complex industrial two-phase flows as can be found e.g. in energy and process engineering.