Klaus Juergen Engel

X‐ray imaging performance of scintillator‐filled silicon pore arrays

The need for fine detail visibility in various applications such as dental imaging, mammography, but also neurology and cardiology, is the driver for intensive efforts in the development of new x-ray detectors. The spatial resolution of current scintillator layers is limited by optical diffusion. This limitation can be overcome by a pixelation, which prevents optical photons from crossing the interface between two neighboring pixels. In this work, an array of pores was etched in a silicon wafer with a pixel pitch of 50 microm. A very high aspect ratio was achieved with wall thicknesses of 4-7 microm and pore depths of about 400 microm. Subsequently, the pores were filled with Tl-doped cesium iodide (CsI:Tl) as a scintillator in a special process, which includes powder melting and solidification of the CsI. From the sample geometry and x-ray absorption measurement the pore fill grade was determined to be 75%. The scintillator-filled samples have a circular active area of 16 mm diameter. They are coupled with an optical sensor binned to the same pixel pitch in order to measure the x-ray imaging performance. The x-ray sensitivity, i.e., the light output per absorbed x-ray dose, is found to be only 2.5%-4.5% of a commercial CsI-layer of similar thickness, thus very low. The efficiency of the pores to transport the generated light to the photodiode is estimated to be in the best case 6.5%. The modulation transfer function is 40% at 4 lp/mm and 10%-20% at 8 lp/mm. It is limited most likely by the optical gap between scintillator and sensor and by K-escape quanta. The detective quantum efficiency (DQE) is determined at different beam qualities and dose settings. The maximum DQE(0) is 0.28, while the x-ray absorption with the given thickness and fill factor is 0.57. High Swank noise is suspected to be the reason, mainly caused by optical scatter inside the CsI-filled pores. The results are compared to Monte Carlo simulations of the photon transport inside the pore array structure. In addition, some x-ray images of technical and anatomical phantoms are shown. This work shows that scintillator-filled pore arrays can provide x-ray imaging with high spatial resolution, but are not suitable in their current state for most of the applications in medical imaging, where increasing the x-ray doses cannot be tolerated.

Two-dimensional anti-scatter grids for computed tomography detectors

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 (TomoTM) shows good fulfilment of the specifications. TomoTM 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 grid technology. 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.