title: X-ray imaging performance of scintillator-filled silicon pore arrays.
abstract: 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.
authors.affiliation: Philips Research Europe, Weisshausstr 2, 52080 Aachen, Germany. matthias.simon@philips.com
authors.forename: Matthias
authors.initials: M
authors.lastname: Simon
authors.forename: Klaus Jürgen
authors.initials: KJ
authors.lastname: Engel
authors.forename: Bernd
authors.initials: B
authors.lastname: Menser
authors.forename: Xavier
authors.initials: X
authors.lastname: Badel
authors.forename: Jan
authors.initials: J
authors.lastname: Linnros
mesh_terms: D016014:Linear Models; D008670:Metals / Q000737:chemistry; D055095:Optics and Photonics; D010087:Oxides / Q000737:chemistry; D019047:Phantoms, Imaging; D011859:Radiography / Q000379:methods*; D012666:Semiconductors; D012680:Sensitivity and Specificity; D012825:Silicon / Q000737:chemistry*
chemical_list: D008670:Metals; D010087:Oxides; D012825:Silicon