Effect of ghosting on the modulation transfer function of amorphous selenium based flat panel detectors

Chowdhury, L.; Tousignant, O.; DeCrescenzo, Giovanni; Gauthier, P.; Leboeuf, Jonathan; Rowlands, J. A.

doi:10.1117/12.650204

Cited by 3 publications

(6 citation statements)

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“…Thus, the energy dependence seen in the experimental results cannot be fully explained by x-ray interactions in the a-Se layer, namely electron transport. Other possible reasons for this discrepancy between experimental and theoretical values include Compton scattering in the Al substrate of the XLV, charge trapping in the a-Se layer, 23 and backscatter from the glass in the optical scanner. Compton scattering has been discussed and shown to cause a decrease in MTF, albeit independent of x-ray energy.…”

Section: Discussionmentioning

confidence: 99%

“…Charge trapping, or incomplete charge collection, is known to decrease the sensitivity of a direct-conversion detector and is referred to as ghosting. However, charge trapping has also been shown to cause a decrease in MTF for a negatively biased detector, 23 such as the XLV. Due to the relatively small thickness of the a-Se layer ͑150 m͒, this effect is expected to be quite small in our prototype.…”

Section: Discussionmentioning

confidence: 99%

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The x‐ray light valve: A low‐cost, digital radiographic imaging system—Spatial resolution

2008

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An x-ray light valve ͑XLV͒ coupled with an optical scanner has the potential to meet the need for a low-cost, high quality digital imaging system for general radiography. The XLV/scanner concept combines three well-established, and hence, low-cost technologies: An amorphous selenium ͑a-Se͒ layer as an x-ray-to-charge transducer, a liquid crystal ͑LC͒ cell as an analog display, and an optical scanner for image digitization. The XLV consists of an a-Se layer and LC cell in a sandwich structure which produces an optical image in the LC layer upon x-ray exposure. The XLV/scanner system consists of an XLV in combination with an optical scanner for image readout. Here, the effect of each component on the spatial resolution of an XLV/scanner system is investigated. A theoretical model of spatial resolution of an XLV is presented based on calculations of the modulation transfer function ͑MTF͒ for a-Se and a LC cell. From these component MTFs, the theoretical MTF of the XLV is derived. The model was validated by experiments on a prototype XLV/scanner system. The MTF of the scanner alone was obtained by scanning an optical test target and the MTF of the XLV/scanner system was measured using x rays. From the measured MTF of the scanner, the theoretical MTF of the XLV/scanner system was established and compared with the experimental results. Good general agreement exists between experimental and theoretical results in the frequency range of interest for general radiography, although the theoretical curves slightly overstate the measured MTFs. The experimental MTF of the XLV was compared with the MTF of two clinical systems and was shown to have the capability to exceed the resolution of flat-panel detectors. From this, the authors can conclude that the XLV has an adequate resolution for general radiography. The XLV/scanner also has the potential to eliminate aliasing while maintaining a MTF that exceeds that of a flat-panel imager.

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Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

The x‐ray light valve: A low‐cost, digital radiographic imaging system—Spatial resolution

2008

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“…Note that the MTFs of stages ͑i͒ and ͑iii͒ are independent of dose and the combined MTF of these two stages can be considered as the dose independent MTF, i.e., MTF 0 . Since the charge collection is considered as an independent stage in the cascaded system model, the total MTF, i.e., MTF T can be expressed as 4 MTF T ͑f͒ = MTF 0 ͑f͒ ϫ MTF CC ͑f͒, ͑15͒…”

Section: Iib Mtf Componentsmentioning

confidence: 99%

“…The effect of the recombination of a drifting carrier with an oppositely charged trapped carrier on MTF CC is the same as that of a drifting carrier becoming trapped in the bulk. 4 The difference between the calculated trapped carrier distribution after a probe pulse and that arising from the previous ghosting pulse is the actual trapped carrier distribution for that probe pulse and is used to calculate the MTF CC .…”

Section: Iic Theoretical Model For Mtf CCmentioning

confidence: 99%

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Effect of repeated x‐ray exposure on the resolution of amorphous selenium based x‐ray imagers

et al. 2010

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Purpose: A numerical model and the experimental methods to study the x-ray exposure dependent change in the modulation transfer function ͑MTF͒ of amorphous selenium ͑a-Se͒ based active matrix flat panel imagers ͑AMFPIs͒ are described. The physical mechanisms responsible for the x-ray exposure dependent change in MTF are also investigated. Methods: A numerical model for describing the x-ray exposure dependent MTF of a-Se based AMFPIs has been developed. The x-ray sensitivity and MTF of an a-Se AMFPI have been measured as a function of exposure. The instantaneous electric field and free and trapped carrier distributions in the photoconductor layer are obtained by numerically solving the Poisson's equation, continuity equations, and trapping rate equations using the backward Euler finite difference method. From the trapped carrier distributions, a method for calculating the MTF due to incomplete charge collection is proposed. Results: The model developed in this work and the experimental data show a reasonably good agreement. The model is able to simultaneously predict the dependence of the sensitivity and MTF on accumulated exposure at different applied fields and bias polarities, with the same charge transport parameters that are typical of the particular a-Se photoconductive layer that is used in these AMFPIs. Under negative bias, the MTF actually improves with the accumulated x-ray exposure while the sensitivity decreases. The MTF enhancement with exposure decreases with increasing applied field. Conclusions:The most prevalent processes that control the MTF under negative bias are the recombination of drifting holes with previously trapped electrons ͑electrons remain in deep traps due to their long release times compared with the time scale of the experiments͒ and the deep trapping of drifting holes and electrons.

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Direct‐conversion flat‐panel imager with avalanche gain: Feasibility investigation for HARP‐AMFPI

Wronski

Rowlands

2008

Medical Physics

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The authors are investigating the concept of a direct-conversion flat-panel imager with avalanche gain for low-dose x-ray imaging. It consists of an amorphous selenium (a-Se) photoconductor partitioned into a thick drift region for x-ray-to-charge conversion and a relatively thin region called high-gain avalanche rushing photoconductor (HARP) in which the charge undergoes avalanche multiplication. An active matrix of thin film transistors is used to read out the electronic image. The authors call the proposed imager HARP active matrix flat panel imager (HARP-AMFPI). The key advantages of HARP-AMFPI are its high spatial resolution, owing to the direct-conversion a-Se layer, and its programmable avalanche gain, which can be enabled during low dose fluoroscopy to overcome electronic noise and disabled during high dose radiography to prevent saturation of the detector elements. This article investigates key design considerations for HARP-AMFPI. The effects of electronic noise on the imaging performance of HARP-AMFPI were modeled theoretically and system parameters were optimized for radiography and fluoroscopy. The following imager properties were determined as a function of avalanche gain: (1) the spatial frequency dependent detective quantum efficiency; (2) fill factor; (3) dynamic range and linearity; and (4) gain nonuniformities resulting from electric field strength nonuniformities. The authors results showed that avalanche gains of 5 and 20 enable x-ray quantum noise limited performance throughout the entire exposure range in radiography and fluoroscopy, respectively. It was shown that HARP-AMFPI can provide the required gain while maintaining a 100% effective fill factor and a piecewise dynamic range over five orders of magnitude (10(-7)-10(-2) R/frame). The authors have also shown that imaging performance is not significantly affected by the following: electric field strength nonuniformities, avalanche noise for x-ray energies above 1 keV and direct interaction of x rays in the gain region. Thus, HARP-AMFPI is a promising flat-panel imager structure that enables high-resolution fully quantum noise limited x-ray imaging over a wide exposure range.

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Effect of ghosting on the modulation transfer function of amorphous selenium based flat panel detectors

Cited by 3 publications

References 1 publication

The x‐ray light valve: A low‐cost, digital radiographic imaging system—Spatial resolution

The x‐ray light valve: A low‐cost, digital radiographic imaging system—Spatial resolution

Effect of repeated x‐ray exposure on the resolution of amorphous selenium based x‐ray imagers

Direct‐conversion flat‐panel imager with avalanche gain: Feasibility investigation for HARP‐AMFPI

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