Modeling the performance of a photon counting x-ray detector for CT: Energy response and pulse pileup effects
Modeling the performance of a photon counting x-ray detector for CT: Energy response and pulse pileup effects
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DOI：
10.1118/1.3539602
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发表时间：
2011-02-01
期刊：
影响因子：
3.8
通讯作者：
Barber, William C.
中科院分区：
文献类型：
--
作者：
Taguchi, Katsuyuki;Zhang, Mengxi;Barber, William C.
用途：最近，具有能量鉴别能力的光子计数x射线探测器（PCXD）已被开发出来，可用于临床计算机断层扫描（CT）扫描仪。这些PCXD具有很大的潜力，以提高CT图像的质量，由于没有电子噪声和权重应用于计数和额外的光谱信息。然而，在临床CT中遇到高计数率的情况下，由于PCXD的有限速度，重合光子被记录为具有较高或较低能量的一个事件。这种现象被称为"脉冲堆积事件"，并导致计数损失（称为"死区时间损失"）和记录的能谱失真。尽管PCXD的性能正在得到改善，但必须开发基于探测器属性的精确模型的算法方法，以补偿这些影响。迄今为止，仅一个PCXD（型号DXMCT-1，DxRay，Inc.，Northridge，CA）已经用于临床CT研究。该研究的目的是评估DXMCT-1测量的数据与分析模型预测的能量响应、死区时间损失和脉冲堆积效应引起的扭曲记录光谱之间的一致性。使用Tc-99m（140 keV）、Co-57（122 keV）和用四个X射线管电压获得的X射线束进行能量校准（35、50、65和80 kVp）。DXMCT-1放置在距离X射线焦斑150 mm处;在管电压为80 kVp的情况下，在10至500 μ A的各种管电流值下记录计数率和光谱。使用这些测量，对于每个脉冲高度比较器，我们估计了描述光子能量-脉冲高度曲线的三个参数，探测器死区时间τ，通过a = k x I将X射线管电流I与入射计数率a相关的系数k，以及入射光谱。在单独的研究中获得所有比较器的平均脉冲形状，并在模型中用于估计失真的记录光谱。通过变异系数（COV）量化DXMCT-1测量数据与模型预测数据之间的一致性，即，结果：用解析模型计算的光子能量-脉冲高度曲线与DXMCT-1测量的曲线在COV方面的一致性在0.2%以内。测量的输出计数率和分析模型预测的输出计数率之间的COV为2.5%，死区时间损失高达60%。实验结果表明，DXMCT-1的能量响应、计数率和脉冲堆积效应与理论分析结果吻合较好。这些模型将是有用的，在开发方法，以补偿这些影响PCXD为基础的临床CT系统。(C)2011年美国医学物理学家协会。[DOI：10.1118/1.3539602]
Purpose: Recently, photon counting x-ray detectors (PCXDs) with energy discrimination capabilities have been developed for potential use in clinical computed tomography (CT) scanners. These PCXDs have great potential to improve the quality of CT images due to the absence of electronic noise and weights applied to the counts and the additional spectral information. With high count rates encountered in clinical CT, however, coincident photons are recorded as one event with a higher or lower energy due to the finite speed of the PCXD. This phenomenon is called a "pulse pileup event" and results in both a loss of counts (called "deadtime losses") and distortion of the recorded energy spectrum. Even though the performance of PCXDs is being improved, it is essential to develop algorithmic methods based on accurate models of the properties of detectors to compensate for these effects. To date, only one PCXD (model DXMCT-1, DxRay, Inc., Northridge, CA) has been used for clinical CT studies. The aim of that study was to evaluate the agreement between data measured by DXMCT-1 and those predicted by analytical models for the energy response, the deadtime losses, and the distorted recorded spectrum caused by pulse pileup effects.Methods: An energy calibration was performed using Tc-99m (140 keV), Co-57 (122 keV), and an x-ray beam obtained with four x-ray tube voltages (35, 50, 65, and 80 kVp). The DXMCT-1 was placed 150 mm from the x-ray focal spot; the count rates and the spectra were recorded at various tube current values from 10 to 500 mu A for a tube voltage of 80 kVp. Using these measurements, for each pulse height comparator we estimated three parameters describing the photon energy-pulse height curve, the detector deadtime tau, a coefficient k that relates the x-ray tube current I to an incident count rate a by a=k x I, and the incident spectrum. The mean pulse shape of all comparators was acquired in a separate study and was used in the model to estimate the distorted recorded spectrum. The agreement between data measured by the DXMCT-1 and those predicted by the models was quantified by the coefficient of variation (COV), i.e., the root mean square difference divided by the mean of the measurement.Results: Photon energy versus pulse height curves calculated with an analytical model and those measured using the DXMCT-1 were in agreement within 0.2% in terms of the COV. The COV between the output count rates measured and those predicted by analytical models was 2.5% for deadtime losses of up to 60%. The COVs between spectra measured and those predicted by the detector model were within 3.7%-7.2% with deadtime losses of 19%-46%.Conclusions: It has been demonstrated that the performance of the DXMCT-1 agreed exceptionally well with the analytical models regarding the energy response, the count rate, and the recorded spectrum with pulse pileup effects. These models will be useful in developing methods to compensate for these effects in PCXD-based clinical CT systems. (C) 2011 American Association of Physicists in Medicine. [DOI: 10.1118/1.3539602]