Department of Medical Physics, University of Wisconsin-Madison, 1111 Highland Avenue, Madison, WI 53705, United States of America.
Department of Radiology, University of Wisconsin-Madison, 600 Highland Avenue, Madison, WI 53792, United States of America.
Phys Med Biol. 2021 Apr 14;66(8). doi: 10.1088/1361-6560/abf1fe.
This work reports an edge enhancing effect experimentally observed in cadmium telluride (CdTe)-based photon counting detector (PCD) systems operated under the charge summing (CS) mode and irradiated by high-flux x-rays. Experimental measurements of the edge spread functions (ESFs) of a PCD system (100m pixel size, 88 ns deadtime) were performed at different input flux levels from 4.5 × 10count per second (cps) mmto 1.5 × 10cps mmfor the single pixel mode (SP) and the CS mode. A theoretical model that incorporates the impacts of inter-pixel communications and the arbitration process involved in the CS mode was developed to help explain the physical origin of the observed edge enhancing effect. Compared with the monotonically increasing ESF of the SP mode, the ESF of the CS mode measured at high-flux levels shows a peak at an intermediate location (50m from the edge). The peak became more pronounced with increasing flux levels. The theoretically calculated ESFs agreed well with experimental results with relative errors less than 5% at all flux levels and tested. These results indicate that the anomalous edge enhancing effect is jointly caused by the pileup effect and the CS circuit that introduces negative correlations between adjacent pixels. When the input flux is high enough to deliver photons to multiple adjacent pixels within the same deadtime period, the CS mode may treat the coincident x-rays as shared charges and thus introduce count losses in addition to the well-known pileup count loss. When a high contrast object partially blocks certain pixels from x-rays, the adjacent unblocked pixels have an increased probability of registering counts as a result of the negative correlation. This leads to a peak on the ESF at a pixel-to-edge distance half of the pixel pitch.
本工作报道了在采用电荷求和（CS）模式工作的碲化镉（CdTe）基光子计数探测器（PCD）系统中观察到的一种边缘增强效应，该系统受到高通量 X 射线照射。在不同的输入通量水平下，对 PCD 系统（100m 像素尺寸，88ns 死时间）的边缘扩展函数（ESF）进行了实验测量，输入通量水平范围为单像素模式（SP）和 CS 模式下的 4.5×10cpsmm 至 1.5×10cpsmm。开发了一个理论模型，该模型考虑了像素间通信的影响以及 CS 模式中涉及的仲裁过程，以帮助解释观察到的边缘增强效应的物理起源。与 SP 模式单调增加的 ESF 相比，在高通量水平下测量的 CS 模式的 ESF 在中间位置（距边缘 50m 处）出现峰值。随着通量水平的增加，峰值变得更加明显。理论计算的 ESF 与实验结果吻合良好，在所有测试的通量水平下相对误差均小于 5%。这些结果表明，异常的边缘增强效应是由堆积效应和 CS 电路共同引起的，CS 电路在相邻像素之间引入了负相关。当输入通量足够高，以至于在同一死时间内将光子传递到多个相邻像素时，CS 模式可能会将同时到达的 X 射线视为共享电荷，从而除了众所周知的堆积计数损失之外还会导致计数损失。当高对比度物体部分阻挡某些像素接收 X 射线时，由于负相关，相邻未被阻挡的像素注册计数的概率增加。这导致 ESF 在像素到边缘距离为像素间距的一半处出现峰值。
