Department of Physics, Ryerson University, Toronto, Ontario, Canada.
Department of Physics and Astronomy, University of Victoria, Victoria, British Columbia, Canada.
Med Phys. 2021 Nov;48(11):6781-6799. doi: 10.1002/mp.15194. Epub 2021 Sep 18.
Spectroscopic X-ray detectors (SXDs) are under development for X-ray imaging applications. Recent efforts to extend the detective quantum efficiency (DQE) to SXDs impose a barrier to experimentation and/or do not provide a task-independent measure of detector performance. The purpose of this article is to define a task-independent DQE for SXDs that can be measured using a modest extension of established DQE-metrology methods.
We defined a task-independent spectroscopic DQE and performed a simulation study to determine the relationship between the zero-frequency DQE and the ideal-observer signal-to-noise ratio (SNR) of low-frequency soft-tissue, bone, iodine, and gadolinium signals. In our simulations, we used calibrated models of the spatioenergetic response of cadmium telluride (CdTe) and cadmium-zinc-telluride (CdZnTe) SXDs. We also measured the zero-frequency DQE of a CdTe detector with two energy bins and of a CdZnTe detector with up to six energy bins for an RQA9 spectrum and compared with model predictions.
The spectroscopic DQE accounts for spectral distortions, energy-bin-dependent spatial resolution, interbin spatial noise correlations, and intrabin spatial noise correlations; it is mathematically equivalent to the squared SNR per unit fluence of the generalized least-squares estimate of the height of an X-ray impulse in a uniform noisy background. The zero-frequency DQE has a strong linear relationship with the ideal-observer SNR of low-frequency soft-tissue, bone, iodine, and gadolinium signals, and can be expressed in terms of the product of the quantum efficiency and a Swank noise factor that accounts for DQE degradation due to, for example, charge sharing (CS) and electronic noise. The spectroscopic Swank noise factor of the CdTe detector was measured to be 0.81 0.04 and 0.83 0.04 with and without anticoincidence logic for CS suppression, respectively. The spectroscopic Swank noise factor of the CdZnTe detector operated with four energy bins was measured to be 0.82 0.02 which is within 5% of the theoretical value.
The spectroscopic DQE defined here is (1) task-independent, (2) can be measured using a modest extension of existing DQE-metrology methods, and (3) is predictive of the ideal-observer SNR of soft-tissue, bone, iodine, and gadolinium signals. For CT applications, the combination of CS and electronic noise in CdZnTe spectroscopic detectors will degrade the zero-frequency DQE by 10 %-20 % depending on the electronic noise level and pixel size.
光谱 X 射线探测器（SXDs）正在开发用于 X 射线成像应用。最近，为了将探测量子效率（DQE）扩展到 SXDs，人们进行了一些努力，但这给实验带来了障碍，或者没有提供一个与任务无关的探测器性能衡量标准。本文的目的是为 SXDs 定义一个与任务无关的 DQE，该 DQE 可以使用已建立的 DQE 计量学方法的适度扩展来测量。
我们定义了一个与任务无关的光谱 DQE，并进行了模拟研究，以确定低频软组织、骨骼、碘和钆信号的零频率 DQE 与理想观察者信噪比（SNR）之间的关系。在我们的模拟中，我们使用了碲化镉（CdTe）和碲锌镉（CdZnTe）SXDs 的空间能量响应校准模型。我们还测量了具有两个能量-bin 的 CdTe 探测器和具有多达六个能量-bin 的 CdZnTe 探测器的零频率 DQE，用于 RQA9 光谱，并与模型预测进行了比较。
光谱 DQE 考虑了光谱失真、能量-bin 相关的空间分辨率、bin 间空间噪声相关性和 bin 内空间噪声相关性；它在数学上等同于在均匀噪声背景下对 X 射线脉冲高度进行广义最小二乘估计的单位剂量 SNR 的平方。零频率 DQE 与低频软组织、骨骼、碘和钆信号的理想观察者 SNR 具有很强的线性关系，可以表示为量子效率和斯旺克噪声因子的乘积，该因子考虑了由于电荷共享（CS）和电子噪声等原因导致的 DQE 降级。CdTe 探测器的光谱斯旺克噪声因子分别为 0.81 0.04 和 0.83 0.04，具有和不具有反符合逻辑以抑制 CS。CdZnTe 探测器在四个能量-bin 下工作时的光谱斯旺克噪声因子测量值为 0.82 0.02，与理论值相差在 5%以内。
本文定义的光谱 DQE 具有以下特点：（1）与任务无关；（2）可以使用现有 DQE 计量学方法的适度扩展来测量；（3）可以预测软组织、骨骼、碘和钆信号的理想观察者 SNR。对于 CT 应用，CS 和电子噪声在 CdZnTe 光谱探测器中的结合将使零频率 DQE 降低 10%-20%，具体取决于电子噪声水平和像素尺寸。
