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The x-ray time of flight method for investigation of ghosting in amorphous selenium-based flat panel medical x-ray imagers.

机译:X射线飞行时间方法,用于研究基于非晶硒的平板医疗X射线成像仪中的重影。

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Amorphous selenium (a-Se) based real-time flat-panel imagers (FPIs) are finding their way into the digital radiology department because they offer the practical advantages of digital x-ray imaging combined with an image quality that equals or outperforms that of conventional systems. The temporal imaging characteristics of FPIs can be affected by ghosting (i.e., radiation-induced changes of sensitivity) when the dose to the detector is high (e.g., portal imaging and mammography) or the images are acquired at a high frame rate (e.g., fluoroscopy). In this paper, the x-ray time-of-flight (TOF) method is introduced as a tool for the investigation of ghosting in a-Se photoconductor layers. The method consists of irradiating layers of a-Se with short x-ray pulses. From the current generated in the a-Se layer, ghosting is quantified and the ghosting parameters (charge carrier generation rate and carrier lifetimes and mobilities) are assessed. The x-ray TOF method is novel in that (1) x-ray sensitivity (S) and ghosting parameters can be measured simultaneously, (2) the transport of both holes and electrons can be isolated, and (3) the method is applicable to the practical a-Se layer structure with blocking contacts used in FPIs. The x-ray TOF method was applied to an analysis of ghosting in a-Se photoconductor layers under portal imaging conditions, i.e., 1 mm thick a-Se layers, biased at 5 V/ microm, were irradiated using a 6 MV LINAC x-ray beam to a total dose (ghosting dose) of 30 Gy. The initial sensitivity (S0) of the a-Se layers was 63 +/- 2 nC cm(-2) cGy(-1). It was found that S decreases to 30% of S0 after a ghosting dose of 5 Gy and to 21% after 30 Gy at which point no further change in S occurs. At an x-ray intensity of 22 Gy/s (instantaneous dose rate during a LINAC x-ray pulse), the charge carrier generation rate was 1.25 +/- 0.1 x 10(22) ehp m(-3) s(-1) and, to a first approximation, independent of the ghosting dose. However, both hole and electron transport showed a strong dependence on the ghosting dose: hole transport decreased by 61%, electron transport by up to approximately 80%. Therefore, degradation of both hole and electron transport due to the recombination of mobile charge carriers with trapped carriers (of opposite polarity) were identified as the main cause of ghosting in this study.
机译:基于非晶硒(a-Se)的实时平板成像器(FPI)正在进入数字放射学部门,因为它们具有数字X射线成像的实用优势,并且其图像质量等于或优于常规系统。当检测器的剂量较高(例如,门静脉成像和乳房X线照相)或以高帧频获取图像(例如,FPI)时,FPI的时间成像特性会受到重影(即辐射引起的灵敏度变化)的影响。透视)。本文介绍了X射线飞行时间(TOF)方法,作为研究a-Se光电导体层中重影的工具。该方法包括用短X射线脉冲照射a-Se层。根据在a-Se层中产生的电流,可以对重影进行量化,并评估重影参数(载流子生成速率,载流子寿命和迁移率)。 X射线TOF方法的新颖之处在于:(1)可以同时测量X射线灵敏度(S)和重影参数;(2)可以分离空穴和电子的传输;(3)该方法适用FPI中使用的具有阻塞触点的实用a-Se层结构。 X射线TOF方法应用于在门户成像条件下a-Se光电导体层中的重影分析,即,使用6 MV LINAC x-射线辐照1 V厚的a-Se层,偏置5 V / microm。射线束的总剂量(重影剂量)为30 Gy。 a-Se层的初始灵敏度(S0)为63 +/- 2 nC cm(-2)cGy(-1)。已经发现,在5Gy的重影剂量之后,S降低到SO的30%,而在30Gy之后降低到21%,此时S没有进一步变化。在22 Gy / s的x射线强度(LINAC x射线脉冲期间的瞬时剂量率)下,载流子生成速率为1.25 +/- 0.1 x 10(22)ehp m(-3)s(-1 ),并且,至第一次近似,与重影剂量无关。然而,空穴和电子传输都显示出对重影剂量的强烈依赖性:空穴传输下降了61%,电子传输下降了约80%。因此,在本研究中,由于移动载流子与捕获的载流子(极性相反)的复合而导致的空穴和电子传输的退化被认为是造成重影的主要原因。

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