Sunnybrook Health Sciences Centre, Toronto, Ontario, Canada.
Med Phys. 2012 Feb;39(2):608-22. doi: 10.1118/1.3673957.
Theoretical reasoning suggests that direct conversion digital x-ray detectors based upon photoconductive amorphous-selenium (a-Se) could attain very high values of the MTF (modulation transfer function) at spatial frequencies well beyond 20 cycles mm(-1). One of the fundamental factors affecting resolution loss, particularly at x-ray energies just above the K-edge of selenium (12.66 keV), is the K-fluorescence reabsorption mechanism, wherein energy can be deposited in the detector at locations laterally displaced from the initial x-ray interaction site. This paper compares measured MTF changes above and below the Se K-edge of a CCD based a-Se x-ray detector with theoretical expectations.
A prototype 25 μm sampling pitch (Nyquist frequency = 20 cycles mm(-1), 200 μm thick a-Se layer based x-ray detector, utilizing a specialized CCD readout device (200 × 400 area array), was used to make edge images with monochromatic x-rays above and below the K-edge of Se. A vacuum double crystal monochromator, exposed to polychromatic x-rays from a synchrotron, formed the monochromatic x-ray source. The monochromaticity of the x-rays was 99% or better. The presampling MTF was determined using the slanted edge method. The theory modeling the MTF performance of the detector includes the basic x-ray interaction physics in the a-Se layer as well as effects related to the operation of the CCD and charge trapping at a blocking layer present at the CCD/a-Se interface.
The MTF performance of the prototype a-Se CCD was reduced from the theoretical value prescribed by the basic Se x-ray interaction physics, principally by the presence of a blocking layer. Nevertheless, the K-fluorescence reduction in the MTF was observed, approximately as predicted by theory. For the CCD prototype detector, at five cycles mm(-1), there was a 14% reduction of the MTF, from a value of 0.7 below the K-edge of Se, to 0.6 just above the K-edge.
The MTF of an a-Se x-ray detector has been measured using monochromatic x-rays above and below the K-edge of selenium. The MTF is poorer above the K-edge by an amount consistent with theoretical expectations.
理论推理表明，基于光导非晶硒（a-Se）的直接转换数字 X 射线探测器在远远超过 20 周期/mm 的空间频率下可以达到非常高的调制传递函数（MTF）值。影响分辨率损失的一个基本因素，特别是在 X 射线能量略高于硒的 K 边（12.66keV）时，是 K 荧光再吸收机制，其中能量可以在探测器中沉积在与初始 X 射线相互作用位置横向偏移的位置。本文比较了基于 CCD 的 a-Se X 射线探测器在硒 K 边以上和以下的测量 MTF 变化与理论预期。
使用原型 25μm 采样间距（奈奎斯特频率=20 周期/mm，200μm 厚的 a-Se 层）的基于 a-Se 的 X 射线探测器，利用特殊的 CCD 读出装置（200×400 区域阵列），用单色 X 射线制作硒 K 边以上和以下的边缘图像。真空双晶单色仪，暴露于同步加速器的多色 X 射线，形成单色 X 射线源。X 射线的单色性为 99%或更好。使用倾斜边缘法确定预采样 MTF。用于模拟探测器 MTF 性能的理论包括 a-Se 层中的基本 X 射线相互作用物理以及与 CCD 操作相关的效应以及存在于 CCD/a-Se 界面处的阻挡层处的电荷俘获。
原型 a-Se CCD 的 MTF 性能从基本硒 X 射线相互作用物理规定的理论值降低，主要是由于存在阻挡层。尽管如此，还是观察到了 MTF 中的 K 荧光减少，大致与理论预测相符。对于 CCD 原型探测器，在 5 周期/mm 时，MTF 降低了 14%，从硒 K 边以下的 0.7 降低到 K 边以上的 0.6。
使用硒 K 边以上和以下的单色 X 射线测量了 a-Se X 射线探测器的 MTF。K 边以上的 MTF 较差，与理论预期一致。
