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X-ray imaging performance of scintillator-filled silicon pore arrays
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2008 (English)In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 35, no 3, 968-981 p.Article in journal (Refereed) Published
Abstract [en]

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 mu m. A very high aspect ratio was achieved with wall thicknesses of 4-7 mu m and pore depths of about 400 mu m. 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.

Place, publisher, year, edition, pages
2008. Vol. 35, no 3, 968-981 p.
Keyword [en]
x-ray imaging, pore array, scintillator, optical scatter, Monte Carlo, simulation, detective quantum efficiency, modulation transfer function, high spatial resolution, flat detector, CsI, monte-carlo-simulation, flat-panel imagers, digital mammography, active-matrix, detector, noise, fluoroscopy
URN: urn:nbn:se:kth:diva-17347DOI: 10.1118/1.2839441ISI: 000253542400016ScopusID: 2-s2.0-40049089355OAI: diva2:335391
QC 20100525Available from: 2010-08-05 Created: 2010-08-05Bibliographically approved

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Linnros, Jan
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Microelectronics and Information Technology, IMIT
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