Point spread function acquisition of light field camera in 3D dose verification system for radiotherapy
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摘要: 基于闪烁光场成像的放疗新型3D剂量测量中，需利用点扩散函数将每一个二维平面的真实闪烁光数据从重叠图像中提取出来。利用刀口法，结合光场数字重聚焦以及聚焦测距法，实验获取光场相机不同重聚焦位置处的点扩散函数，并给出棋盘格标定板在参数α=0.6时的重聚焦面聚焦在α=0.7位置处的高斯离焦点扩散函数表达式，研究光场相机点扩散函数的规律，通过测量几个必要值便可由函数拟合得到所有点扩散函数，减少光学分层成像中因分层数量增加引起的标定工作量。将结果引入图像处理，通过反卷积运算可获得每一个二维重聚焦面的真实闪烁光数据，有助于3D剂量的实时精确测量。Abstract: In advanced 3D dose verification system for radiotherapy based on scintillation light field imaging, it is necessary to use the point spread function to extract the real scintillation light data of each 2D plane from the overlapping images. The point spread functions of the light field camera at different refocusing positions are measured by knife-edge method combining the digital refocusing of light field and the focussing ranging method. And when the refocusing plane refocused at α=0.6, the function expression of Gaussian defocus for checkerboard calibration plate at α=0.7 was given. The law of light field camera point spread function was studied. All point spread functions could be obtained through function fitting by measuring five necessary values and the workload of point spread function calibration caused by the increase of the number of layers in optical layered imaging was reduced by using this law. Introducing the research results into image processing could obtain the real scintillation data of each 2D refocusing surface through deconvolution operation, which would be helpful for the real-time and accurate measurement of 3D dose.
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图 1 聚焦测距法进行重聚焦位置标定
Figure 1. Calibration of refocusing position by focussing ranging method
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图 5 棋盘格标定板上黑白跳变区域
Figure 5. Black and white transition area on checkerboard calibration plate
表 1 灰度值的阶跃函数拟合得到的σ值
Table 1 σ values obtained by step function fitting of gray value
σ（fitting values） σ（average） h（1，1） 1.7575 1.7972 1.6650 1.9002 1.5879 1.7416 h（1，2） 2.1958 1.8310 2.0035 1.7968 1.9951 1.9644 h（1，3） 2.6024 2.7266 2.8954 3.1100 2.4014 2.7472 h（2，1） 3.4686 3.4609 3.3747 3.8571 3.7960 3.5915 h（2，2） 1.6504 1.7625 1.6619 1.6045 1.5275 1.6414 h（2，3） 1.8200 2.2526 1.8180 2.0692 1.8146 1.9549 h（3，1） 4.2815 4.5886 4.6952 4.1621 4.5598 4.4574 h（3，2） 2.6816 2.6859 2.2923 2.6932 2.6208 2.5948 h（3，3） 1.5570 1.7985 1.6752 1.4891 1.7167 1.6473 
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表 2 高斯离焦模型参数σ值
Table 2 σ values of Gaussian defocus model
0.7 0.8 0.9 1.0 1.1 σ1 σ2 平均 σ1 σ2 平均 σ1 σ2 平均 σ1 σ2 平均 σ1 σ2 平均 1.4020 1.4115 1.4068 1.6408 1.6442 1.6425 2.1934 2.1877 2.1906 2.5380 2.5401 2.5391 3.1135 3.1274 3.1205 1.6165 1.6261 1.6213 1.2169 1.2257 1.2213 1.6113 1.5805 1.5959 1.9534 1.9963 1.9749 2.5159 2.4230 2.4695 1.9852 1.9778 1.9815 1.5853 1.5832 1.5843 1.2048 1.2143 1.2048 1.5614 1.5533 1.5574 2.0782 2.0156 2.0469 2.3196 2.2931 2.3064 1.5148 1.4900 1.5024 1.2079 1.1860 1.1970 1.1073 1.0559 1.0816 1.4853 1.4905 1.4879 2.7577 2.8124 2.7851 1.8811 1.8180 1.8496 1.4382 1.4196 1.4289 1.2034 1.2104 1.2069 1.0063 0.9887 0.9975
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