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Initial Empirical Investigation of Photon Counting Circuits Based On Polycrystalline Silicon TFTs

AK Liang*, M Koniczek, Y El-Mohri, Q Zhao, LE Antonuk, University of Michigan, Ann Arbor, MI


(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: Photon counting detectors offer several advantages over conventional, fluence-integrating x-ray imagers. Such detectors, generally based on crystalline silicon, have been employed for mammography. To accelerate adoption of this technology to other imaging modalities such as fluoroscopy, breast CT, and cone-beam CT, we are investigating the creation of photon counting detectors based on polycrystalline silicon (poly-Si) – a semiconductor material well-suited for the economic manufacture of monolithic, large-area devices. A first set of prototype pixels and pixel circuit components have been developed and initial empirical measurements have been performed to determine whether they provide sufficient count rate performance for these target applications.

Methods: Each prototype pixel consists of four circuit components: an amplifier, comparator, clock generator, and counter. Each component was also fabricated as a separate circuit, and test pulses input to those individual circuits allowed determination of count rate and other component-specific performance metrics such as gain and hysteresis.

Results: The count rate of the amplifier component is ~10 kcps/pixel, which would be insufficient for resolving the input flux of the target applications (estimated to be up to 62.5 kcps/pixel assuming a pixel pitch of 250 µm). However, the count rates of the remaining 3 components are approximately an order of magnitude higher and would be sufficient.

Conclusion: Empirical measurements of individual components of the first poly-Si photon counting prototypes have been performed. The results indicate that, while the amplifier component would limit the count rate of the overall imager, the remaining three components demonstrate count rate capability sufficient to resolve the input x-ray flux of the target applications of fluoroscopy, breast CT, and cone-beam CT. A new amplifier circuit providing higher count rate has been identified through simulation and future prototypes incorporating this design are expected to provide sufficient count rate for those applications.

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