Ultrafast electron beam X-ray computed tomography (ROFEX)
The high-performance ROFEX (ROssendorf Fast Electron beam X-ray tomography)imaging technique has been developed at Helmholtz-Zentrum Dresden-Rossendorf for the noninvasive investigation of dynamic processes. With up to 8,000 cross-sectional images per second and a spatial resolution of about 1 mm highly transient phenomena can be visualized and studied in detail. Purpose-built for the analysis of multiphase flows in elongated test sections, various research projects out of this field could already benefit from the ROFEX technology. Beyond that it can be applied to diverse other applications as for instance nondestructive testing.
Computed tomography requires radiographic projections from different angles. Contrary to conventional tomography systems, the ROFEX systems do not achieve this by mechanical rotation of the object or source-detector-compound. Instead, an electron beam is focussed towards a circular target and at the same time periodically deflected with high frequency. In this way, a focal spot moving along the target, i.e. an X-ray source rotating around the object, is generated. Synchronized with the beam deflection, a static detector ring encircling the object measures the radiation attenuated by the object with a multiple of the deflection frequency. From the data set of projections from one revolution of the electron beam a non-superimposed cross-sectional image of the attenuation distribution within the object can be reconstructed. In order to determine axial velocities of structures within the object of investigation the ROFEX systems comprise a second tomography plane, which is driven intermittently to the first one, if required. From the temporal shift between both image sequences the axial velocities of structures can be derived. Furthermore, parameters characterizing the analyzed process, such as interfacial area of a turbulent gas-liquid flow, can be extracted from the gained data from both tomography planes.
Liquid-gas flow in vertical pipes
Ultrafast X-ray computed tomography has been primarily developed for the non-invasive measurement of the phase distribution within two-phase flows in vertical pipes. For this purpose, one ROFEX scanner has been installed at the vertical test section of the experimental facility TOPFLOW, where measurements at high pressure and high temperature are possible. A traversing unit allows vertical positioning of the ROFEX scanner within a range of up to four meters. Thus, the development of both upward and downward flows within the test section can be analysed through measuring in different heights.
Gas-solid fluidized bed
In cooperation with the TU Eindhoven gas-solid fluidized beds with different solid particles have been investigated. On the one hand, general parameters such as bubble size, bubble shape and bubble velocity for the validation of simulations based on the two-fluid model could be determined by ROFEX measurements. On the other hand, the formation of diverse fragile dynamic structures could be visualized and studied in detail.
Dynamic quality control of agricultural products
In order to establish a fast online quality control for agricultural products, several studies on the capability of the ROFEX technology to identify different quality deficiencies have been performed. Especially for walnuts, chestnuts and peperoni a high potential for success has been identified. The practical implementation of the necessary online data processing is currently under intensive examination.
Current and planned ROFEX systems
The ROFEX technology is practically implemented in form of different scanners. ROFEX I is installed at the experimental facility TOPFLOW and is mainly dedicated to high pressure/high temperature experiments. It can be used for objects under investigation with diameter up to 120 mm within a vertical measurement range of about 3.5 m. ROFEX II aims at higher spatial resolution at decreased object diameters (50 mm). ROFEX III as part of the X-ray tomography lab is applicable more versatile for objects up to 195 mm diameter. At the moment, the two ROFEX scanners I and III are in operation and ROFEX II is under construction. Moreover, a system for 3-D imaging is in the conceptual phase. All systems are operated at an acceleration voltage of up to 150 kV and a maximum beam power of 10 kW.
Image reconstruction on GPUs and real-time capability
Lately, the computation intensive image reconstruction is performed in parallel on GPUs, which accelerates this step tremendously. A faster data transfer from the radiation detector to the reconstruction PC or cluster, respectively, as well as the implementation of the data processing as continuously working data pipeline (https://github.com/HZDR-FWDF/RISA) are further milestones on the way towards real-time capability of the ROFEX systems. In the near future, the latter will enable this technology to monitor processes online and to control them or associated system components in real-time. One scenario, which will benefit from the real-time capability, is the tracking of structures within the object of investigation by moving the ROFEX system. Using the example of the behaviour of single particles within a swarm, this problem is currently under investigation in the framework of a DFG funded project.
High-power ultrafast computed tomography (HECToR)
The penetration capability of X-rays and thus their applicability for imaging techniques is determined on the one hand by the energy of the X-ray photons and on the other hand by the size and material of the object under investigation. In order to analyse flows in larger pressure-resistant vessels, the high-power CT system “HECToR” (High Energy Computed Tomography Scanner Rossendorf) has been developed. The basic functionality is similar to the ROFEX systems. However, an electron beam with an electrostatic acceleration of 1 MV and a beam power of up to 100 kW is applied here. The HECToR system thus allows investigation of objects up to 400 mm in diameter. Currently, the system is in its final testing stage.
Ultrafast 3-D CT system
A first ultrafast 3-D CT system has been developed, assembled and characterized within the framework of a DFG funded project in cooperation with the University of Stuttgart. It was successful in generating multiple circular X-ray source paths on an X-ray transparent target in such a way that in combination with a single detector ring a geometrical arrangement similar to cone-beam computed tomography could be achieved. In this way, three-dimensional tomographic imaging could be performed within a spatially confined experimental area at a frame rate of 250 per second. In order to apply this principle to vertically elongated objects of investigation further technical advancements are necessary, which are subject of ongoing work.
- University of Stuttgart
- Karlsruhe Institute of Technology
- T. Frust, M. Wagner, J. Stephan, G. Juckeland, A. Bieberle
Rapid Data Processing for Ultrafast X-Ray Computed Tomography Using Scalable and Modular CUDA based Pipelines
Computer Physics Communications, akzeptiert 2017
- M. Bieberle, U. Hampel
Level-set reconstruction algorithm for ultrafast limited angle X-ray computed tomography of two-phase flows
Philosophical Transactions of the Royal Society A 373, 20140395, 2015
- F. Barthel, M. Bieberle, D. Hoppe, M. Banowski, U. Hampel
Velocity Measurement For Two-Phase Flows Based On Ultrafast X-ray Tomography
Flow Measurement and Instrumentation 46, pp. 196-203, 2015
- M. Wagner, F. Barthel, J. Zalucky, M. Bieberle, U. Hampel
Scatter analysis and correction for ultrafast X-ray tomography
Philosophical Transactions of the Royal Society A 373, 20140396, 2015
- T. Stürzel, M. Bieberle, E. Laurien, U. Hampel, F. Barthel, H.-J. Menz, H.-G. Mayer
Experimental facility for two- and three-dimensional ultrafast electron beam X-ray computed tomography
Review of Scientific Instruments 82, 023702, 2011
- F. Fischer, U. Hampel
Ultra fast electron beam X-ray computed tomography for two-phase flow measurement
Nuclear Engineering and Design 240(9), 2254-59, 2010.
- U. Hampel, M. Speck, D. Koch, H.-J. Menz, H.-G. Mayer, J. Fietz, D. Hoppe, E. Schleicher, C. Zippe, H.-M. Prasser
Ultrafast X-ray Computed Tomography with a Linearly Scanned Electron Beam Source
Flow Measurement and Instrumentation 16, pp. 65-72, 2005.
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- Patent DE102007008349 "Anordnung zur Röntgentomographie", 15.10.2009