Large Area 2-Dimensional a-Si:H Imaging Arrays

Street, R. A.; Fujieda, Ichiro; Weisfield, Richard L.; Nelson, S.; Nylen, P.

doi:10.1557/proc-258-1145

Cited by 29 publications

(10 citation statements)

References 10 publications

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“…Thus, both the photodiode ͑through bulk effects͒ and the TFT ͑through surface effects͒ are potential sources of charge trapping; however, the former is typically identified as the dominant source. 25,46 As reported elsewhere, 10,11,14 image lag is reduced by: ͑1͒ illuminating the photodiode through the p layer; ͑2͒ operating the photodiode at a high bias voltage ͑e.g., V bias ϳϪ5 V͒; and ͑3͒ operating at signal levels far below sensor saturation.…”

Section: Referencementioning

confidence: 85%

A ghost story: Spatio-temporal response characteristics of an indirect-detection flat-panel imager

1999

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Spatial and temporal imaging characteristics of an amorphous silicon flat-panel imager (FPI) were investigated in terms relevant to the application of such devices in cone-beam computed tomography (CBCT) and other x-ray imaging modalities, including general radiography, fluoroscopy, mammography, radiotherapy portal imaging, and nondestructive testing. Specifically, issues of image lag (including the magnitude, spatial uniformity, temporal-frequency characteristics, and dependence upon exposure and frame time) and long-term image persistence ("ghosts") were investigated. As part of the basic characterization of the FPI, pixel dark signal and noise (magnitude, temporal stability, and spatial uniformity) as well as radiation response (signal size, linearity, gain, and reciprocity) were also measured. Image lag was analyzed as a function of frame time and incident exposure. First-frame lag (i.e., the relative residual signal in the first frame following readout of an exposure) was approximately 2-10%, depending upon incident exposure and was spatially nonuniform to a slight degree across the FPI; second-, third-, and fourth-frame lag were approximately 0.7%, 0.4%, and 0.3%, respectively (at 25% sensor saturation). Image lag was also analyzed in terms of the temporal-frequency-dependent transfer function derived from the radiation response, allowing a quantitative description of system components contributing to lag. Finally, the contrast of objects as a function of time following an exposure was measured in order to examine long-term image persistence ("ghosts"). Ghosts were found to persist up to 30 min or longer, depending upon the exposure and frame time. Two means of reducing the apparent contrast of ghost images were tested: (i) rapid scanning of the FPI at maximum frame rate, and (ii) flood-field exposure of the FPI; neither was entirely satisfactory. These results pose important considerations for application of FPIs in CBCT as well as other x-ray imaging modalities. For example in CBCT, the magnitude of image lag is such that significant artifacts in tomographic reconstructions may result if strategies are not adopted either to reduce or correct the lag between successive projections (e.g., rapid scanning between projections or iterative correction algorithms, respectively). Similarly, long-term image persistence may necessitate frequent recalibration of offset corrections.

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Section: Referencementioning

confidence: 85%

A ghost story: Spatio-temporal response characteristics of an indirect-detection flat-panel imager

1999

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“…The amount of trapped signal increases as the electric field strength across the photodiode decreases. 30 This occurs if V bias is decreased and/or if the incident light signal, and hence the pixel signal size, increases. In radiographic mode, this trapping has the negative consequence of essentially reducing the useful optical imaging quanta.…”

Section: The Effects Of Trapping Statesmentioning

confidence: 99%

Empirical investigation of the signal performance of a high‐resolution, indirect detection, active matrix flat‐panel imager (AMFPI) for fluoroscopic and radiographic operation

et al. 1997

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Signal properties of the first large-area, high resolution, active matrix, flat-panel imager are reported. The imager is based on an array of 1536 x 1920 pixels with a pixel-to-pixel pitch of 127 microns. Each pixel consists of a discrete amorphous silicon n-i-p photodiode coupled to an amorphous silicon thin-film transistor. The imager detects incident x rays indirectly by means of an intensifying screen placed over the array. External acquisition electronics send control signals to the array and process analog imaging signals from the pixels. Considerations for operation of the imager in both fluoroscopic and radiographic modes are detailed and empirical signal performance data are presented with an emphasis on exploring similarities and differences between the two modes. Measurements which characterize the performance of the imager were performed as a function of operational parameters in the absence or presence of illumination from a light-emitting diode or x rays. These measurements include characterization of the drift and magnitude of the pixel dark signal, the size of the pixel switching transient, the temporal behavior of pixel sampling and the implied maximum frame rate, the dependence of relative pixel efficiency and pixel response on photodiode reverse bias voltage and operational mode, the degree of linearity of pixel response, and the trapping and release of charge from metastable states in the photodiodes. In addition, X-ray sensitivity as a function of energy for a variety of phosphor screens for both fluoroscopic and radiographic operation is reported. Example images of a line-pair pattern and an anthropomorphic phantom in each mode are presented along with a radiographic image of a human hand. General and specific improvements in imager design are described and anticipated developments are discussed. This represents the first systematic investigation of the operation and properties in both radiographic and fluoroscopic modes of an imager incorporating such an array.

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“…The domain in which the above approximation is specified is described (11) Here, is an integer of the form . The geometrical meaning of will be discussed later.…”

Section: A Green's Functionmentioning

confidence: 99%

“…4). These capacitances can give rise to feedthrough charge and electronic noise, which undermine the quality of the image generated [11], [12]. From a computational standpoint, the main difficulties in the numerical extraction of capacitance include the following.…”

Section: Quasi-static Parasitic Coupling Capacitancementioning

confidence: 99%

Exponential expansion for field computation and capacitance extraction

Pham

Nathan

2001

IEEE Trans. Circuits Syst. I

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We report a new numerical technique based on the exponential expansion of the Green's function 1 for accurate and rapid computation of the three-dimensional potential field, its gradient, and the charge density distribution needed for capacitance extraction in multiple-dielectric-multiconductor systems. The exponential expansion provides a computational efficient alternative to the commonly used technique based on multipole expansion of spherical harmonics. With exponential expansion, the memory requirement is independent of the desired degree of accuracy and different forms of parallelism are available for both remotely distributed networks and closely coupled parallel systems. These features permit large-scale simulation, involving panel (element) count in the range of a few hundred thousand to several million, needed for extraction of the parasitic coupling capacitance in VLSI interconnects, including large-area amorphous silicon electronics, as well as for analysis of electrostatic interaction in micro-electro-mechanical systems (MEMS).

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Large Area 2-Dimensional a-Si:H Imaging Arrays

Cited by 29 publications

References 10 publications

A ghost story: Spatio-temporal response characteristics of an indirect-detection flat-panel imager

A ghost story: Spatio-temporal response characteristics of an indirect-detection flat-panel imager

Empirical investigation of the signal performance of a high‐resolution, indirect detection, active matrix flat‐panel imager (AMFPI) for fluoroscopic and radiographic operation

Exponential expansion for field computation and capacitance extraction

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