Solid-State Devices and Nanotechnology, Department of Electrical and Computer Engineering, University of Michigan, 1301 Beal Avenue Ann Arbor, MI 48109-2122, United States of America.
Phys Med Biol. 2019 Jan 10;64(2):025012. doi: 10.1088/1361-6560/aaf5f6.
Medical imaging systems like full field digital mammography (FFDM) and digital breast tomosynthesis (DBT) commonly use amorphous selenium (a-Se) based passive pixel sensor (PPS) direct conversion x-ray detectors. On one hand, direct conversion detectors inherently offer better resolution characteristics in terms of a higher modulation transfer function (MTF), in comparison to the indirect CsI:Tl PPS x-ray imager. On the other hand, especially at lower doses, this superior performance of the direct imager is seldom retained in its detective quantum efficiency (DQE) curves. It is well known that a-Se PPS x-ray imagers suffer from high additive electronic noise originating from the from the amorphous silicon (a-Si) thin film transistor (TFT) array that is being used in the current back-plane technology. This degrades the noise power spectrum (NPS) and subsequently the overall DQE. To address this deficiency, we propose to replace the PPS back-plane by active pixel sensor (APS) back-plane technology, which has the potential to reduce the back-plane electronic noise by amplifying the input signal, especially at low doses. The proposed APS is based on amorphous In-Ga-Zn-O (a-IGZO) TFT technology, which can offer high mobility (5-20 cm V s), low leakage current (<10 A) and low flicker noise (Hooge's parameter α ~ 1.5 [Formula: see text] 10), leading to better imager noise performance. To test our hypothesis, we used linear cascaded systems analysis to model the imaging performance (MTF, NPS and DQE) of the PPS and APS a-Se direct imagers. This model was first validated using experimentally measured data obtained for a 85 µm pixel pitch a-Se/a-Si TFT PPS imager. Using this model, we analyzed the noise performance of the direct a-Se and indirect CsI:Tl x-ray a-IGZO APS imagers at different dose and electronic noise levels. Obtained results clearly showed that lowering back-plane electronic noise can significantly improve the performance of the a-Se/a-IGZO TFT APS imager. Our simulated results showed that a higher DQE at lower radiation doses (maximum DQE of 0.6 can be achieved at an exposure level of 1 µGy) can be achieved with the a-Se detector, thereby making this combination a promising candidate for low dose applications like DBT.
医学成像系统，如全视野数字化乳腺摄影（FFDM）和数字乳腺断层合成（DBT），通常使用基于非晶硒（a-Se）的无源像素传感器（PPS）直接转换 X 射线探测器。一方面，与间接 CsI:Tl PPS X 射线成像仪相比，直接转换探测器在调制传递函数（MTF）方面固有地提供更好的分辨率特性。另一方面，特别是在较低剂量下，直接成像仪的这种优越性能很少在其探测量子效率（DQE）曲线中保留。众所周知，a-Se PPS X 射线成像仪受到源自当前背极板技术中使用的非晶硅（a-Si）薄膜晶体管（TFT）阵列的高附加电子噪声的影响。这会降低噪声功率谱（NPS），并随后降低整体 DQE。为了解决这个缺陷，我们建议用有源像素传感器（APS）背极板技术取代 PPS 背极板，该技术有可能通过放大输入信号来降低背极板电子噪声，特别是在低剂量下。所提出的 APS 基于非晶态 In-Ga-Zn-O（a-IGZO）TFT 技术，该技术可以提供高迁移率（5-20 cm V s）、低漏电流（<10 A）和低闪烁噪声（Hooge 参数α~1.5 [公式：见正文] 10），从而改善成像器的噪声性能。为了验证我们的假设，我们使用线性级联系统分析来模拟 PPS 和 APS a-Se 直接成像仪的成像性能（MTF、NPS 和 DQE）。该模型首先使用针对 85 µm 像素间距的 a-Se/a-Si TFT PPS 成像仪获得的实验测量数据进行验证。使用该模型，我们分析了不同剂量和电子噪声水平下直接 a-Se 和间接 CsI:Tl X 射线 a-IGZO APS 成像仪的噪声性能。得到的结果清楚地表明，降低背极板电子噪声可以显著提高 a-Se/a-IGZO TFT APS 成像仪的性能。我们的模拟结果表明，在较低辐射剂量下（在 1 µGy 的曝光水平下可以达到最高 DQE 为 0.6），a-Se 探测器可以实现更高的 DQE，从而使这种组合成为低剂量应用（如 DBT）的有前途的候选者。
