Simulation study of full-field X-ray fluorescence imaging with a pinhole camera
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摘要: 为解决全场X射线荧光(XRF)成像中针孔形状和尺寸的选取问题，采用Geant4软件，模拟了6种不同类型针孔和4种不同的针孔孔径，分析了这些参数对点扩散函数和调制传递函数的影响；模拟了不同能量X射线荧光平面源的成像过程，并用均值滤波和Richardson迭代法对图像进行处理，分析了图像处理的效果。模拟结果表明：对于能量小于20 keV的荧光X射线，双锥孔结合直孔模型的点扩散函数尖锐性和等晕性更好，调制传递函数的截止频率更大，空间分辨更好，更适合做全场XRF成像的针孔；均值滤波结合Richardson迭代法的图像处理算法对全场XRF图像处理的效果较好。Abstract: To solve the problem of selecting shape and size of the pinhole in the full-field X-ray fluorescence (XRF) imaging, the Geant4 Monte Carlo simulation software was used to simulate pinholes of 6 different types and 4 different diameters. The effects of the parameters on the spatial resolution which referred to the point spread function and the modulation transfer function were analyzed. The imaging process of different energy fluorescence X-ray plane sources is simulated, and the performance of image processing was analyzed by mean filter and the Richardson iteration method. The simulation results show that the pinhole model of the double conical-hole combined with the straight-hole has better sharpness and isoplanatism of the point spread function, a higher cut-off frequency of modulation transmission function, and better spatial resolution for X-ray of the energy less than 20 keV, which meant it is more suitable for full-field XRF imaging; the algorithm of mean filtering combined with the Richardson iteration performs better in full-field XRF image processing.
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Key words:
- full-field XRF /
- Geant4 simulation /
- spatial resolution /
- image processing
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图 2 不同材料在1～30 keV范围内的平均自由程[13]
Figure 2. The mean free path of different materials in the range of 1-30 keV
图 5 孔径为50 μm, 100 μm, 200 μm和400 μm时不同孔型的PSF
Figure 5. PSF with different aperture types at the pinhole diameter of 50 μm, 100 μm, 200 μm and 400 μm
图 6 孔径为50 μm, 100 μm, 200 μm和400 μm时不同孔型的MTF
Figure 6. MTF with different aperture types at the pinhole diameter of 50 μm, 100 μm, 200 μm and 400 μm
图 7 条纹状荧光面源和不同能量的PSF
Figure 7. Images of the original plane source and the PSF of different energy
图 10 图像处理后不同特征X射线组成的图案
Figure 10. Pattern composed of different characteristic X-rays after image processing
表 1 孔径为50 μm, 100 μm, 200 μm和400 μm时不同孔型下PSF的尖锐性和等晕性
Table 1. Sharpness and isoplanatism of PSF with different pinhole mask structures at the pinhole diameter of 50 μm, 100 μm, 200 μm and 400 μm
diameter/μm pinhole mask structures left sharpness/μm right sharpness/μm isoplanatism/μm 50 (a) straight hole model 65 65 98 (b) double cone hole model 65 65 64 (c) double cone and straight hole model 52 52 64 (d) right cone hole model 65 65 66 (e) left cone hole model 65 65 66 (f) penumbra hole model 52 52 66 100 (a) straight hole model 65 65 200 (b) double cone hole model 78 78 100 (c) double cone and straight hole model 65 65 97 (d) right cone hole model 78 78 99 (e) left cone hole model 78 78 99 (f) penumbra hole model 65 65 97 200 (a) straight hole model 78 78 399 (b) double cone hole model 78 78 205 (c) double cone and straight hole model 65 65 198 (d) right cone hole model 78 78 206 (e) left cone hole model 78 78 205 (f) penumbra hole model 65 65 198 (a) straight hole model 78 78 800 (b) double cone hole model 104 104 411 400 (c) double cone and straight hole model 78 78 399 (d) right cone hole model 104 104 411 (e) left cone hole model 104 104 411 (f) penumbra hole model 78 78 398 
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表 2 孔径为50 μm, 100 μm, 200 μm和400 μm时不同孔型下MTF的截止频率
Table 2. Cut-off frequency (lp/mm) of MTF with different pinhole mask structures at the pinhole diameter of 50 μm, 100 μm, 200 μm and 400 μm
pinhole mask structure diameter/μm 50 100 200 400 (a) straight hole model 6.9 3.7 2.0 1.0 (b) double cone hole model 10.0 6.7 3.6 1.9 (c) double cone and straight hole model 10.1 6.9 3.7 2.0 (d) right cone hole model 10.1 6.8 3.6 1.9 (e) left cone hole model 10.1 6.8 3.6 1.9 (f) penumbra hole model 10.2 6.9 3.7 2.0
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表 3 图像处理的效果
Table 3. The performance of image processing
energy/keV statistical error/% original SNR/dB processed SNR/dB iterations 6.4 1.2 18.7 26.2 2 8.0 0.8 18.2 26.9 4 9.8 0.7 20.2 27.2 4 12.6 1.2 18.8 26.8 4
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