Indirect flat-panel detector with avalanche gain

Zhao, Wei; Hunt, David E.; Tanioka, Kenkichi; Rowlands, J. A.

doi:10.1117/12.536863

Cited by 17 publications

(14 citation statements)

References 22 publications

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“…Probable applications include especially medical imaging for pediatric patients, fluoroscopy, and tomosynthesis that must capture many images at low dose level. A gain of 10 should be sufficient to extend the usable range of DQE to a captured dose of only 0.1 µR [19,8]. …”

Section: Resultsmentioning

confidence: 99%

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Selenium detector with a grid for selenium charge gain

Lee

2005

Medical Imaging 2005: Physics of Medical Imaging

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Amorphous selenium direct-conversion x-ray detectors have been used successfully for full field digital mammography (FFDM) and digital radiography (DR). Such detectors characteristically exhibit high spatial resolution and conversion efficiency that is a function of the applied electric field [1]. At an electric field of 10 volts per micron, about 50 electron volts of photon energy are required to generate an electron-hole pair in a typical amorphous selenium x-ray conversion layer. At FFDM and DR imaging x-ray energies each absorbed photon can generate only about 250 to 1000 electronhole pairs. Each absorbed x-ray photon is only contributing 4x10 -17 to 1.6x10 -16 coulombs of imaging charge. On the noise side, detectors operating at room temperature have a basic thermal (kTC) noise of 300 to 600 electrons per pixel from the image charge storage capacitor. Electronic noise from the front-end charge amplifier is also amplified by one plus the ratio of the TFT data line capacitance and the feedback capacitance of the charge amplifier. Medical imaging applications must therefore employ low noise thin film transistor (TFT) arrays, low data line capacitance and low noise charge integration amplifiers to achieve high signal-to-noise ratio (SNR) and detective quantum efficiency (DQE).To achieve quantum-noise limited imaging results with the lowest practical x-ray exposure dose, it is desirable to include an additional low-noise gain stage in the x-ray conversion layer. This is particularly important for the application of dynamic imaging or for tomosynthesis where x-ray dose per frame is very limited. A new structure for an amorphous selenium detector that employs an internal biased gain grid to cause avalanche-gain within the x-ray conversion layer is being proposed. A signal charge amplification of at least 10X can be achieved without introducing excessive noise. Quantum-limited image detection should then be attainable for even very low exposures.

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Section: Resultsmentioning

confidence: 99%

“…Recently, Zhao [8] proposed and demonstrated an indirect conversion detector employing a CsI phosphor layer for x-ray capture and conversion to visible photons. Also included was a thin layer of photoconductive selenium, operated at high electric field to simultaneously create and amplify resulting image charge by avalanche gain.…”

Section: Introductionmentioning

confidence: 99%

Selenium detector with a grid for selenium charge gain

Lee

2005

Medical Imaging 2005: Physics of Medical Imaging

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“…through avalanche gain, the HR CsI layers can be used to boost the DQE at high spatial frequencies. 21 This is particularly important for high resolution x-ray imaging applications, e.g., mammography.…”

Section: Dqe Modelmentioning

confidence: 99%

“…We are investigating the feasibility of a new indirect flat-panel detector with avalanche optical sensors. 21 The avalanche gain could compensate for the gain loss in HR CsI, so that it can be used for high resolution x-ray imaging applications, e.g., mammography, in order to boost the DQE at high spatial frequencies. Figure 5 (left) is the measured MTF of different CsI samples.…”

Section: Conversion Gain and Swank Factormentioning

confidence: 99%

Inherent imaging performance of cesium iodide scintillators

Zhao

Ristić²,

Rowlands³

2004

SPIE Proceedings

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Columnar structured cesium iodide (CsI) scintillators doped with Thallium (Tl) have been used extensively for indirect x-ray imaging detectors. The purpose of this paper is to develop a methodology for systematic investigation of the inherent imaging performance of CsI as a function of thickness and design type. The results will facilitate the optimization of CsI design for different x-ray imaging applications, and allow validation of physical models developed for the light channeling process in columnar CsI layers. CsI samples of different types and thicknesses were obtained from the same manufacturer. They were optimized either for light output (HL) or image resolution (HR), and the thickness ranged between 150 and 600 microns. During experimental measurements, the CsI samples were placed in direct contact with a high resolution CMOS optical sensor with a pixel pitch of 48 microns. The modulation transfer function (MTF), noise power spectrum (NPS) and detective quantum efficiency (DQE) of the detector with different CsI configurations were measured experimentally. The aperture function of the CMOS sensor was determined separately, which allows estimation of the MTF of CsI alone. We also measured the pulse height distribution of the light output from both the HL and HR CsI at different x-ray energies, from which the x-ray quantum efficiency, Swank factor and xray conversion gain were determined. Our results showed that the MTF at 5 cycles/mm for the HR type is 50 % higher than for the HL, however at a cost of ~ 36 % reduction in light output. The Swank factor below K-edges of Cs and I is 0.91 and 0.93 for the HR and HL types, respectively, which means that their DQE(0) are essentially identical for the same thickness. The presampling MTF decreases as a function of thickness, and the rate of decrease drops as the thickness increases. This indicates that the light channeling process in CsI improves the MTF of thicker layers more significantly than it does for the thinner ones.

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“…1 Shown in Figure 1 is a side view of the proposed detector. The photons emitted from the CsI are absorbed by the HARP layer and generate electron-hole pairs near the top interface (light entrance side).…”

Section: Introductionmentioning

confidence: 99%

Indirect flat-panel detector with avalanche gain: design and operation of the avalanche photoconductor

Zhao

Reznik

et al. 2005

Medical Imaging 2005: Physics of Medical Imaging

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An indirect flat-panel imager (FPI) with avalanche gain is being investigated for low-dose x-ray imaging. It is made by optically coupling a structured x-ray scintillator CsI(Tl) to an amorphous selenium (a-Se) avalanche photoconductor called HARP. The final electronic image can be read out using either an array of thin film transistors (TFT) or field emitters (FE). The advantage of the proposed detector is its programmable gain, which can be turned on during low dose fluoroscopy to overcome electronic noise, and turned off during high dose radiography to avoid pixel saturation. This paper investigates the important design considerations for HARP such as avalanche gain, which depends on both the thickness d Se and the applied electric field E Se . To determine the optimal design parameter and operational conditions for HARP, we measured the E Se dependence of both avalanche gain and optical quantum efficiency of an 8 µm HARP layer. The results were applied to a physical model of HARP as well as a linear cascaded model of the FPI to determine the following x-ray imaging properties in both the avalanche and non-avalanche modes as a function of E Se : (1) total gain (which is the product of avalanche gain and optical quantum efficiency); (2) linearity; (3) dynamic range; and (4) gain non-uniformity resulting from thickness non-uniformity. Our results showed that a HARP layer thickness of 8 µm can provide adequate avalanche gain and sufficient dynamic range for x-ray imaging applications to permit quantum limited operation over the range of exposures needed for radiography and fluoroscopy.

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Indirect flat-panel detector with avalanche gain

Cited by 17 publications

References 22 publications

Selenium detector with a grid for selenium charge gain

Selenium detector with a grid for selenium charge gain

Inherent imaging performance of cesium iodide scintillators

Indirect flat-panel detector with avalanche gain: design and operation of the avalanche photoconductor

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