Department of Biomedical Engineering, University of California, Davis, CA, USA.
Med Phys. 2012 Apr;39(4):2108-18. doi: 10.1118/1.3691180.
It is important to understand signal and noise transfer in the indirect, flat-panel x-ray detector when developing and optimizing imaging systems. For optimization where simulating images is necessary, this study introduces a semiempirical model to simulate projection images with user-defined x-ray fluence interaction.
The signal and noise transfer in the indirect, flat-panel x-ray detectors is characterized by statistics consistent with energy-integration of x-ray photons. For an incident x-ray spectrum, x-ray photons are attenuated and absorbed in the x-ray scintillator to produce light photons, which are coupled to photodiodes for signal readout. The signal mean and variance are linearly related to the energy-integrated x-ray spectrum by empirically determined factors. With the known first- and second-order statistics, images can be simulated by incorporating multipixel signal statistics and the modulation transfer function of the imaging system. To estimate the semiempirical input to this model, 500 projection images (using an indirect, flat-panel x-ray detector in the breast CT system) were acquired with 50-100 kilovolt (kV) x-ray spectra filtered with 0.1-mm tin (Sn), 0.2-mm copper (Cu), 1.5-mm aluminum (Al), or 0.05-mm silver (Ag). The signal mean and variance of each detector element and the noise power spectra (NPS) were calculated and incorporated into this model for accuracy. Additionally, the modulation transfer function of the detector system was physically measured and incorporated in the image simulation steps. For validation purposes, simulated and measured projection images of air scans were compared using 40 kV∕0.1-mm Sn, 65 kV∕0.2-mm Cu, 85 kV∕1.5-mm Al, and 95 kV∕0.05-mm Ag.
The linear relationship between the measured signal statistics and the energy-integrated x-ray spectrum was confirmed and incorporated into the model. The signal mean and variance factors were linearly related to kV for each filter material (r(2) of signal mean to kV: 0.91, 0.93, 0.86, and 0.99 for 0.1-mm Sn, 0.2-mm Cu, 1.5-mm Al, and 0.05-mm Ag, respectively; r(2) of signal variance to kV: 0.99 for all four filters). The comparison of the signal and noise (mean, variance, and NPS) between the simulated and measured air scan images suggested that this model was reasonable in predicting accurate signal statistics of air scan images using absolute percent error. Overall, the model was found to be accurate in estimating signal statistics and spatial correlation between the detector elements of the images acquired with indirect, flat-panel x-ray detectors.
The semiempirical linear model of the indirect, flat-panel x-ray detectors was described and validated with images of air scans. The model was found to be a useful tool in understanding the signal and noise transfer within indirect, flat-panel x-ray detector systems.
在开发和优化成像系统时，了解间接式平板 X 射线探测器中的信号和噪声传递非常重要。对于需要模拟图像的优化，本研究引入了一种半经验模型，用于模拟具有用户定义的 X 射线剂量交互作用的投影图像。
间接式平板 X 射线探测器中的信号和噪声传递通过与 X 射线光子能量积分一致的统计特性来描述。对于入射 X 射线光谱，X 射线光子在 X 射线闪烁体中被衰减和吸收，产生光光子，这些光光子被耦合到光电二极管进行信号读出。信号均值和方差与经验确定的因子线性相关，与能量积分的 X 射线光谱相关。通过结合多像素信号统计和成像系统的调制传递函数，可以模拟图像。为了估计该模型的半经验输入，使用间接式平板 X 射线探测器在乳腺 CT 系统中获取了 500 个投影图像，使用 50-100 千伏（kV）的 X 射线光谱，用 0.1 毫米的锡（Sn）、0.2 毫米的铜（Cu）、1.5 毫米的铝（Al）或 0.05 毫米的银（Ag）进行滤波。计算每个探测器元件的信号均值和方差以及噪声功率谱（NPS），并将其纳入该模型以提高准确性。此外，还通过物理测量来确定探测器系统的调制传递函数，并将其纳入图像模拟步骤中。为了验证目的，使用 40 kV/0.1-mm Sn、65 kV/0.2-mm Cu、85 kV/1.5-mm Al 和 95 kV/0.05-mm Ag 的空气扫描模拟和测量的投影图像进行了比较。
证实了测量信号统计与能量积分 X 射线光谱之间的线性关系，并将其纳入模型中。信号均值和方差因子与每个滤波器材料的 kV 呈线性相关（0.1-mm Sn、0.2-mm Cu、1.5-mm Al 和 0.05-mm Ag 的信号均值与 kV 的 r²分别为 0.91、0.93、0.86 和 0.99；信号方差与 kV 的 r²为 0.99）。对模拟和测量的空气扫描图像中的信号和噪声（均值、方差和 NPS）进行比较表明，该模型在使用绝对百分比误差预测空气扫描图像的准确信号统计方面是合理的。总体而言，该模型在估计间接式平板 X 射线探测器获取的图像中的信号统计和探测器元件之间的空间相关性方面是准确的。
描述了间接式平板 X 射线探测器的半经验线性模型，并使用空气扫描图像进行了验证。该模型被发现是理解间接式平板 X 射线探测器系统中信号和噪声传递的有用工具。
