Investigation of the Performance of Photon Counting Arrays Based on Polycrystalline Silicon Thin-Film Transistors

Abstract

Projection x-ray imaging is commonly employed to visualize internal human anatomy and used to produce diagnostic images. Modern projection imaging is typically performed using an active matrix, flat panel imager that is comprised of a converter layer overlying a pixelated array. The images are formed by converting x-ray photons into electrical signals, and then integrating those signals over a frame time – a method referred to as fluence integration.

Recently, imagers employing a second method for creating x-ray images – referred to as photon counting – have been developed and used to perform mammographic imaging (a form of projection imaging). Photon counting involves measuring the energy of each interacting x-ray photon and storing digital counts of the number of photons exceeding one or more energy thresholds. Because the imaging information is stored digitally, photon counting imagers are less susceptible to noise than fluence-integrating imagers – which improves image quality and/or decreases the amount of radiation required to acquire an image.

Current photon counting mammographic imagers are based on crystalline silicon and are limited in detection area. In order to produce an image, the array is moved in a scanning motion across the object of interest. A photon counting imager with larger detection area would benefit other projection imaging modalities – such as radiography (which produces, for example, chest x-ray images) or fluoroscopy (which is used for non-invasively inserting stents and other medical devices). However, techniques to increase detection area, such as tiling multiple arrays, result in increased imager complexity or cost. For this reason, our group has been exploring the possibility of creating photon counting arrays using a different semiconductor material, referred to as polycrystalline silicon (poly-Si). This material is fabricated using a thin-film process, which allows the economic manufacture of monolithic, large-area arrays and has favorable material properties for creating complex, high speed circuits.

Using poly-Si, a set of prototype arrays have been designed and fabricated. The design of the arrays consists of four components: an amplifier, a comparator, a clock generator, and a counter. Several circuit variations were created for each component, and circuit simulations were performed in order to determine energy resolution and count rate values for each variation of each component.

For the amplifier component, all circuit variations were determined to have an energy resolution of ~10% when presented with a 70 keV input x-ray photon (a typical photon energy level used in diagnostic imaging). This energy resolution value is comparable to those reported for photon counting imagers fabricated using crystalline silicon. In addition, while count rate values for the amplifier component were roughly one order of magnitude too low for radiographic and fluoroscopic applications (which require count rates on the order of 1 million counts per second per square millimeter [cps/mm2]), a hypothetical amplifier circuit variation with count rate capabilities suitable for these applications (while preserving the same ~10% energy resolution) was designed. In addition, the count rate values for the various comparator, clock generator, and counter circuit variations ranged from 100 to 3000 kcps/mm2. Finally, due to improvements in the poly-Si fabrication process (driven largely by the display industry), future photon counting arrays employing this material can have pixel pitches as small as 250 um – a size approaching that suitable for radiographic and fluoroscopic imaging.