Schmidt Taly Gilat, Zimmerman Kevin C, Sidky Emil Y
Department of Biomedical Engineering, Marquette University, Milwaukee, WI 53233, USA.
Phys Med Biol. 2015 Feb 21;60(4):1583-600. doi: 10.1088/0031-9155/60/4/1583. Epub 2015 Jan 23.
Photon-counting x-ray detectors with pulse-height analysis provide spectral information that may improve material decomposition and contrast-to-noise ratio (CNR) in CT images. The number of energy measurements that can be acquired simultaneously on a detector pixel is equal to the number of comparator channels. Some spectral CT designs have a limited number of comparator channels, due to the complexity of readout electronics. The spectral information could be extended by changing the comparator threshold levels over time, sub pixels, or view angle. However, acquiring more energy measurements than comparator channels increases the noise and/or dose, due to differences in noise correlations across energy measurements and decreased dose utilisation. This study experimentally quantified the effects of acquiring more energy measurements than comparator channels using a bench-top spectral CT system. An analytical and simulation study modeling an ideal detector investigated whether there was a net benefit for material decomposition or optimal energy weighting when acquiring more energy measurements than comparator channels. Experimental results demonstrated that in a two-threshold acquisition, acquiring the high-energy measurement independently from the low-energy measurement increased noise standard deviation in material-decomposition basis images by factors of 1.5-1.7 due to changes in covariance between energy measurements. CNR in energy-weighted images decreased by factors of 0.92-0.71. Noise standard deviation increased by an additional factor of [Formula: see text] due to reduced dose utilisation. The results demonstrated no benefit for two-material decomposition noise or energy-weighted CNR when acquiring more energy measurements than comparator channels. Understanding the noise penalty of acquiring more energy measurements than comparator channels is important for designing spectral detectors and for designing experiments and interpreting data from prototype systems with a limited number of comparator channels.
具有脉冲高度分析功能的光子计数X射线探测器可提供光谱信息，这可能会改善CT图像中的物质分解和对比度噪声比（CNR）。在探测器像素上可同时获取的能量测量数量等于比较器通道的数量。由于读出电子设备的复杂性，一些光谱CT设计的比较器通道数量有限。通过随时间、子像素或视角改变比较器阈值水平，可以扩展光谱信息。然而，由于能量测量之间的噪声相关性差异和剂量利用率降低，获取比比较器通道更多的能量测量会增加噪声和/或剂量。本研究使用台式光谱CT系统通过实验量化了获取比比较器通道更多的能量测量的影响。一项对理想探测器进行建模的分析和模拟研究调查了在获取比比较器通道更多的能量测量时，物质分解或最佳能量加权是否有净益处。实验结果表明，在双阈值采集中，由于能量测量之间协方差的变化，独立于低能量测量获取高能量测量会使物质分解基础图像中的噪声标准差增加1.5至1.7倍。能量加权图像中的CNR降低了0.92至0.71倍。由于剂量利用率降低，噪声标准差又增加了[公式：见原文]倍。结果表明，在获取比比较器通道更多的能量测量时，对双物质分解噪声或能量加权CNR没有益处。了解获取比比较器通道更多的能量测量所带来的噪声代价，对于设计光谱探测器、设计实验以及解释来自比较器通道数量有限的原型系统的数据非常重要。
