A cascaded model of spectral distortions due to spectral response effects and pulse pileup effects in a photon-counting x-ray detector for CT
A cascaded model of spectral distortions due to spectral response effects and pulse pileup effects in a photon-counting x-ray detector for CT
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DOI：
10.1118/1.4866890
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发表时间：
2014-04-01
期刊：
影响因子：
3.8
通讯作者：
Taguchi, Katsuyuki
中科院分区：
文献类型：
--
作者：
Cammin, Jochen;Xu, Jennifer;Taguchi, Katsuyuki
目的：能量鉴别、光子计数探测器（PCD）是一种新兴的计算机断层扫描（CT）技术，对临床CT具有各种潜在益处。由于光子与探测器的相互作用以及多个重合光子的相互作用，PCD测量的光子能量可能会失真。这些影响导致记录的X射线光谱失真，这可能导致重建的CT图像中的伪影和组织识别的不准确性。基于模型的补偿技术有可能解释失真效应。这种方法只需要少量的参数，适用于广泛的光谱和计数率，但它需要一个准确的模型中发生的PCD的光谱失真。本研究的目的是开发这些光谱失真的模型，并使用PCD（型号DXMCT-1; DxRay，Inc.，Northridge，CA）和各种X射线光谱在很宽的计数率范围内。方法：作者假设光谱失真的复杂现象可以建模为：（1）将它们分离为计数率独立因子，我们称之为光谱响应效应（SRE），以及计数率依赖因子，我们称之为脉冲堆积效应（PPE），（2）为SRE和PPE开发单独的模型，以及（3）将SRE和PPE模型级联成组合的SRE+PPE模型，其描述低计数率和高计数率下的PCD畸变。SRE模型描述了记录光谱的概率分布，在给定入射光子能量的情况下，具有光峰值和连续拖尾。模型参数从三种放射性同位素的校准测量中获得，然后对其他能量进行线性插值。所使用的PPE模型是在作者以前的工作[K. Taguchi等人，“Modeling the performance of a photon counting x-ray detector for CT：Energy response and pulse pileup effects，”Med.Phys.38（2），1089-1102（2011）]。针对使用不同衰减器实现的各种水平的死区时间损失比（DLR）和入射光谱形状，根据加权变异系数（COVW），评估通过级联SRE+PPE模型计算的X射线光谱与测量光谱之间的一致性，即，结果：在低计数率下，当DLR <10%时，DXMCT-1测量的畸变谱与SRE计算的畸变谱一致，COVW小于4%。在较高的计数率下，测量的X射线谱与级联SRE+PPE模型计算的X射线谱一致;以PMMA为衰减剂，在DLR为22%时，COVW为5.6%，在DLR高达55%时，COVW为6.7%。SRE模型预测了不同类型和厚度衰减器的低计数率下的失真记录光谱。该研究还验证了PCD中复杂的光谱畸变可以通过级联计数率无关的SRE和计数率相关的PPE来充分建模的假设。(C)2014年作者。所有文章内容，除非另有说明，是根据知识共享署名3.0未移植许可证许可。
Purpose: Energy discriminating, photon-counting detectors (PCDs) are an emerging technology for computed tomography (CT) with various potential benefits for clinical CT. The photon energies measured by PCDs can be distorted due to the interactions of a photon with the detector and the interaction of multiple coincident photons. These effects result in distorted recorded x-ray spectra which may lead to artifacts in reconstructed CT images and inaccuracies in tissue identification. Modelbased compensation techniques have the potential to account for the distortion effects. This approach requires only a small number of parameters and is applicable to a wide range of spectra and count rates, but it needs an accurate model of the spectral distortions occurring in PCDs. The purpose of this study was to develop a model of those spectral distortions and to evaluate the model using a PCD (model DXMCT-1; DxRay, Inc., Northridge, CA) and various x-ray spectra in a wide range of count rates.Methods: The authors hypothesize that the complex phenomena of spectral distortions can be modeled by: (1) separating them into count-rate independent factors that we call the spectral response effects (SRE), and count-rate dependent factors that we call the pulse pileup effects (PPE), (2) developing separate models for SRE and PPE, and (3) cascading the SRE and PPE models into a combined SRE+PPE model that describes PCD distortions at both low and high count rates. The SRE model describes the probability distribution of the recorded spectrum, with a photo peak and a continuum tail, given the incident photon energy. Model parameters were obtained from calibration measurements with three radioisotopes and then interpolated linearly for other energies. The PPE model used was developed in the authors' previous work [K. Taguchi et al., "Modeling the performance of a photon counting x-ray detector for CT: Energy response and pulse pileup effects," Med. Phys. 38(2), 1089-1102 (2011)]. The agreement between the x-ray spectra calculated by the cascaded SRE+PPE model and the measured spectra was evaluated for various levels of deadtime loss ratios (DLR) and incident spectral shapes, realized using different attenuators, in terms of the weighted coefficient of variation (COVW), i.e., the root mean square difference weighted by the statistical errors of the data and divided by the mean.Results: At low count rates, when DLR < 10%, the distorted spectra measured by the DXMCT-1 were in agreement with those calculated by SRE only, with COVW's less than 4%. At higher count rates, the measured spectra were also in agreement with the ones calculated by the cascaded SRE+PPE model; with PMMA as attenuator, COVW was 5.6% at a DLR of 22% and as small as 6.7% for a DLR as high as 55%.Conclusions: The x-ray spectra calculated by the proposed model agreed with the measured spectra over a wide range of count rates and spectral shapes. The SRE model predicted the distorted, recorded spectra with low count rates over various types and thicknesses of attenuators. The study also validated the hypothesis that the complex spectral distortions in a PCD can be adequately modeled by cascading the count-rate independent SRE and the count-rate dependent PPE. (C) 2014 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.