The use of two-dimensional, focused, anti-scatter-grids (ASGs) in computed tomography is one essential solution to
reduce the scatter radiation for large area detectors.
A detailed analysis of the requirements and related image quality aspects lead to the specification of the two-dimensional
focused geometry of the X-ray absorbing grids. Scatter simulations indicated trade-off conditions and provided
estimations for the expected scatter reduction performance.
Different production technologies for focused two-dimensional structures have been evaluated. The presented
technology of Tomo Lithographic Molding (Tomo<sup>TM</sup>) shows good fulfilment of the specifications. Tomo<sup>TM</sup> is a synthesis
of lithographic micromachining, precision stack lamination, molding, and casting processes with application-specific
material systems. Geometry, material properties, and scatter performance have been investigated. Different analysis
methods will be presented and results of the investigations demonstrate the performance capability of this two-dimensional
Material composition of the tungsten-polymer composite, homogeneity of wall thickness, and precision of the focusing
have the biggest influence on the X-ray behavior. Dynamic forces on the anti-scatter-grid during CT operations should
not lead to dynamic shadowing or intensity modulation on the active pixel area. Simulations of the wall deformation
have been done to estimate the maximum position deviation.
Prototype two-dimensional ASGs have been characterized and show promising results.
In Computed Tomography (CT), the image quality sensitively depends on the accuracy of the X-ray projection signal, which is acquired by a two-dimensional array of pixel cells in the detector. If the signal of X-ray photons is spread out to neighboring pixels (crosstalk), a decrease of spatial resolution may result. Moreover, streak and ring artifacts may emerge. Deploying system simulations for state-of-the-art CT detector configurations, we characterize origin and appearance of these artifacts in the reconstructed CT images for different scenarios. A uniform pixel-to-pixel crosstalk results in a loss of spatial resolution only. The Modulation Transfer Function (MTF) is attenuated, without affecting the limiting resolution, which is defined as the first zero of the MTF. Additional streak and ring artifacts appear, if the pixel-to-pixel crosstalk is non-uniform. Parallel to the system simulations we developed an analytical model. The model explains resolution loss and artifact level using the first and second derivative of the X-ray profile acquired by the detector. Simulations and analytical model are in agreement to each other. We discuss the perceptibility of ring and streak artifacts within noisy images if no crosstalk correction is applied.
A polarimeter system has been developed to investigate the reflected light of very rough non-specular surfaces. One special application is the polarimetric analysis of road surfaces to find features for the classifications relating to type and surface condition. The reflected light of such a surface varies very much depending on the inhomogeneous surface structures. The aim was to build a measurement system which is relatively insensitive to the positioning of the surface sample, is easy to calibrate and offers a high scanning rate. These demands lead us to a polarimeter setup using a combination of a four-detector-polarimeter and a retarder-polarimeter with liquid-crystal cells as retarding elements. With the help of this laboratory system it is possible to find all polarization depended features. In a further step the setup will be simplified to measure only selected values but with higher speed. The software of the system allows the complete control of the polarization generator and of both polarimeters including the calibration,the operation in different modes, the evaluation and graphical visualization of the measuring data.