Noise Analysis of Image Sensor Arrays for Large-Area Biomedical Imaging
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Noise Analysis of Image Sensor Arrays for Large-Area Biomedical Imaging Jackson Lai1, Denis Striakhilev2, Yuri Vygranenko2, Gregory Heiler1, Arokia Nathan3, and Timothy Tredwell1 1 Carestream Health Inc., Rochester, NY, 14615 2 Department of Electrical and Computer Engineering, University of Waterloo, Waterloo, N2L 3G1, Canada 3 London Centre for Nanotechnology, London, United Kingdom ABSTRACT Large-area digital imaging made possible by amorphous silicon thin-film transistor (a-Si TFT) technology, coupled with a-Si photosensors, provides an excellent readout platform to form an integrated medical image capture system. Major development challenges evolve around the optimization of pixel architecture for detector fill factor and manufacturability, all the while suppressing that noise stems from pixel array and external electronics. This work discusses the behavior and modeling of system noise that arises from imaging array operations. An active pixel sensor (APS) design with on-pixel amplification was studied. Our evaluation demonstrates that a 17 × 17 inch array with 150 ×150 µm pixels can achieve system noise as low as 1000 electrons through proper design and optimization. INTRODUCTION Large-area, digital-image sensors are revolutionizing medical imaging by enabling electronic storage capability, immediate feedback, and possibilities to support previously unachievable applications related to computer-aided image processing. Hydrogenated amorphous silicon thinfilm transistor (a-Si:H TFT) technology, frequently used in liquid crystal display applications, is extended to perform backplane readout for large-area detector systems. Technological attributes of a-Si:H, such as uniformity over a large area, compatibility with various substrate materials, and research maturity, have provided an excellent development platform for high performance, low noise, and fully integrated digital detector systems. The signal-to-noise ratio (SNR) of the readout backplane is one the key performance evaluation attributes. Biomedical imaging applications place an even more stringent requirement on SNR compared to other imaging systems because of safety standards in the medical arena. A typical imaging array implemented with a-Si:H technology has one TFT in the pixel level acting as a row access switch. However, the design has no inherited signal amplification, making it prone to electronic noise and unsuitable for low-dosage applications such as fluoroscopy [1-2]. Active pixel sensors (APSs) have been investigated to alleviate some design concerns. In a conventional APS design, each pixel contains a read, a reset, and an amplifier TFT [2], and the composite source-follower amplifier setup provides the signal gain to overcome external and pixel noise. The design involves higher circuit complexity that potentially leads to higher noise, hence it is critical to investigate the noise performance of the design.
This work presents a systematic noise analysis of an APS pixel array using a-Si:H technology. First, the most significant
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