Zhao Bo, Zhao Wei
Department of Radiology, State University of New York at Stony Brook, L-4 Health Science Center, Stony Brook, New York 11794-8460, USA.
Med Phys. 2005 Jan;32(1):128-36. doi: 10.1118/1.1827791.
We investigated temporal performance of amorphous selenium (a-Se) detectors specifically designed for mammographic imaging. Our goal is to quantify the inherent lag and ghosting of a-Se photoconductor as a function of imaging conditions. Two small area electroded a-Se samples, one positively and the other negatively biased on the entrance side of x rays, were used in the experiments. The study of lag and ghosting was performed by delivering a number of raw exposures as experienced in screening mammography to the samples at different electric field strength E(Se) while measuring the current through the a-Se sample. Ghosting at different operational conditions was quantified as the percentage x-ray sensitivity (x-ray generated photocurrent measured from the sample) reduction compared to before irradiation. Lag was determined by measuring the residual current of a-Se at a given time after the end of each x-ray exposure. Both lag and ghosting were measured as a function of E(Se) and cumulative exposure. The values of E(Se) used in our experiments ranged from 1 to 20 V/microm. It was found that ghosting increases with exposure and decreases with E(Se) for both samples because of the dominant effect of recombination between trapped electrons and x-ray generated holes. Lag on the other hand has different dependence on E(Se) and cumulative exposure. At E(Se) < or = 10 V/microm, the first frame lag for both samples changed slowly with cumulative exposure, with a range of 0.2%-1.7% for the positively biased sample and 0.5%-8% for the negatively biased sample. Overall the positively biased sample has better temporal performance than the negatively biased sample due to the lower density of trapped electrons. The impact of time interval between exposures on the temporal performance was also investigated. Recovery of ghosting with longer time interval was observed, which was attributed to the neutralization of trapped electrons by injected holes through dark current.
我们研究了专门为乳腺成像设计的非晶硒（a-Se）探测器的时间性能。我们的目标是量化a-Se光电导体的固有滞后和残影，作为成像条件的函数。实验中使用了两个小面积电极化的a-Se样本，一个在X射线入射侧带正电，另一个带负电。通过在不同电场强度E(Se)下，向样本施加一系列乳腺筛查中所经历的原始曝光量，同时测量通过a-Se样本的电流，来进行滞后和残影的研究。将不同操作条件下的残影量化为与辐照前相比X射线灵敏度（从样本测量的X射线产生的光电流）降低的百分比。通过测量每次X射线曝光结束后给定时间a-Se的残余电流来确定滞后。滞后和残影均作为E(Se)和累积曝光量的函数进行测量。我们实验中使用的E(Se)值范围为1至20 V/μm。结果发现，由于俘获电子与X射线产生的空穴之间复合的主导作用，两个样本的残影都随曝光量增加而增加，随E(Se)减小。另一方面，滞后对E(Se)和累积曝光量有不同的依赖性。在E(Se)≤10 V/μm时，两个样本的第一帧滞后随累积曝光量变化缓慢，正偏置样本的范围为0.2% - 1.7%，负偏置样本的范围为0.5% - 8%。总体而言，由于俘获电子密度较低，正偏置样本的时间性能优于负偏置样本。还研究了曝光之间的时间间隔对时间性能的影响。观察到随着时间间隔延长，残影会恢复，这归因于通过暗电流注入的空穴对俘获电子的中和作用。
