Is high sensitivity always desirable for a grating-based differential phase contrast imaging system?
Is high sensitivity always desirable for a grating-based differential phase contrast imaging system?
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DOI：
10.1002/mp.13984
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发表时间：
2020-03
期刊：
影响因子：
3.8
通讯作者：
Chen GH
中科院分区：
文献类型：
--
作者：
Ji X;Zhang R;Li K;Chen GH
在基于光栅的X射线微分相衬（DPC）成像中，由图像对象引起的相移的测量信号幅度与所谓的系统灵敏度成比例。因此，为了实现更好的信噪比（SNR）以改善成像性能，通常认为应该通过减小分析器光栅的周期或增加相位光栅与分析器光栅之间的距离来增加系统灵敏度。这项工作的目的是从理论和实验上证明，有一个最佳的系统灵敏度，以达到最高的信噪比为给定的任务，提供了标准的相位步进采集和相位恢复方法。当系统灵敏度超过该最佳值时，信噪比降低，成像性能恶化。由于相位测量信号是一个循环变量，当系统灵敏度提高时，相位包络效应不可避免。在信号和噪声测量中都出现了相位缠绕效应。信号测量中的这种效应表现为所谓的信号统计偏差，这种效应经常影响测量的准确性。在噪声方差测量中也会出现相位包络效应，影响测量精度。在这项工作中进行了全面的理论分析，以证明相位缠绕对信号偏差和噪声方差的定量影响，从而对实际的系统SNR。信号偏差和噪声方差中的相位卷绕的联合效应产生最佳系统灵敏度以实现最高SNR。为了验证理论分析，进行了广泛的数值模拟研究和实验研究。理论分析和实验研究都表明，由于信号的周期性和相位包络效应，DPC信号的信噪比并不总是与灵敏度成正比。对于给定的折射角和曝光水平，存在使SNR最大化的最佳灵敏度因子，超过该最佳灵敏度因子，增加灵敏度将降低SNR。如果使用标准相位步进采集和相位恢复方法，则系统灵敏度的增加并不总是改善X射线DPC成像性能。
In grating-based x-ray differential phase contrast (DPC) imaging, the measured signal amplitude of the phase shift induced by an image object is proportional to the so-called system sensitivity. Therefore, to achieve a better signal-to-noise (SNR) for improved imaging performance, it is generally believed that one should increase the system sensitivity by reducing the period of the analyzer grating or increasing the distance between the phase grating and analyzer grating. The purpose of this work is to theoretically and experimentally demonstrate that there is an optimal system sensitivity to attain the highest SNR for a given task provided that the standard phase-stepping acquisition and phase retrieval methods are used. When system sensitivity goes beyond this optimal value, SNR decreases and the imaging performance deteriorates. Due to the fundamental fact that the measured phase signal is a cyclic variable, the phase wrapping effect is inevitable in DPC imaging when the system sensitivity increases. The phase wrapping effect appears in both signal and noise measurements. The effect in the signal measurement is manifested in the so-called signal statistical bias and such effect often impacts the accuracy of the measurement. The phase wrapping effect also appears in the noise variance measurement and impacts the precision of the measurement. A thorough theoretical analysis was performed in this work to demonstrate the quantitative impacts of phase wrapping on both signal bias and noise variance and thus on the actual system SNR. The joint effect of phase wrapping in both the signal bias and noise variance yields an optimal system sensitivity to achieve the highest SNR. Both extensive numerical simulation studies and experimental studies were performed to validate the theoretical analysis. Both theoretical analysis and experimental studies show that the SNR of the DPC signal is not always proportional to the sensitivity due to the cyclic nature of the signal and the phase wrapping effect. For a given refraction angle and exposure level, there exists an optimal sensitivity factor that maximizes the SNR, beyond which, increasing the sensitivity will decrease the SNR. Increase of system sensitivity does not always improve x-ray DPC imaging performance provided that the standard phase-stepping acquisition and phase retrieval methods are used.
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