Photoresponse Linearity of a-Si:H Imaging Pixels
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PHOTORESPONSE LINEARITY OF a-Si:H IMAGING PIXELS J. YORKSTON, L.E. ANTONUK, W. HUANG, R.A. STREET' Department of Radiation Oncology, University of Michigan, Ann Arbor, MI 48109 . Xerox Palo Alto Research Center, 3333 Coyote Hill Road, Palo Alto, CA 94304 ABSTRACT Amorphous silicon imaging arrays with -3x10 5 pixels have recently been developed and x-ray images of low contrast anatomical phantoms have been demonstrated. This paper reports on the linearity of response of these a-Si:H imaging pixels as a function of reverse bias voltage. The fraction of the imaging pixel's full signal range that maintains a linear response has been found to increase with increasing voltage. INTRODUCTION In the last few years 2-dimensional hydrogenated amorphous silicon (a-Si:H) imaging arrays have been extensively studied for a wide range of imaging applications [1]. The development of these devices for medical imaging has progressed to the point where images have been demonstrated both for diagnostic x-rays (-100kVp) and radiation therapy x-rays (-2-15MV) [2,3]. The largest a-Si:H imaging arrays thus far reported have a surface area of 23cm by 25cm and a pixel pitch of 0.45mm giving a total of -290,000 pixels [4]. The total number of pixels on the imaging surface will probably increase in the future as devices are designed to meet the specific criteria of different imaging applications. For example, a device suitable for diagnostic radiology would have an -36cm x 43cm imaging surface and a pixel pitch of around 100,m, resulting in over 15 million pixels. The nature of the photoresponse of the pixels is of considerable interest for reasons of practical operation as well as for characterization of the imaging performance of the arrays. Although the photoresponse of different pixels has been shown to be very uniform over small areas [5] it is possible that for full sized devices, pixel to pixel corrections will be required before the images can be displayed. As the number of pixels increases this correction procedure could take prohibitive amounts of time. Consequently, in applications where high frame rate imaging is important eg. digital fluoroscopy, there may be a trade-off between the amount of correction possible and the achievable frame rate. If it is required that corrections be performed with only one or two parameters, this may limit the useable dynamic range of the imaging system if the system response is nonlinear. It is therefore important that the response of the pixels under different conditions be known since this would allow the operating parameters of the imaging array to be optimized. In general, it is not an absolute requirement that the response of an imaging system to a specific input signal change be linear as long as it is stable and can be characterized. An obvious example of this is x-ray film whose response to the input x-ray exposure is logarithmic and characterized by a particular film's H+D curve [6]. However, the application of linear systems theory to describe the performance of an imaging system requires
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