Preliminary study on scatter quantification method for flash Multi-MeV radiography
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摘要: 高能MeV闪光照相所针对的客体通常具有极高的面密度。当X射线穿过客体时，直穿X射线的强度将被极大衰减，到达成像面的直穿信号可能被散射“噪声”所淹没，若直接对图像进行反演将严重影响照相重建精度。从散射抑制角度出发，目前主要采用网柵相机即阵列型准直孔阻挡散射，但网栅相机的应用效果受光源位置稳定性影响较大，且网栅不易加工。提出了一种可实时定量散射强度的照相方案，该方案利用狭缝准直器对散射的抑制能力不随散射强度变化而改变这一特点，对现有照相布局进行小改进，利用已知客体实验结果标定狭缝准直器对散射的抑制能力，进一步自洽确定待测客体的散射量大小。基于蒙卡方法的仿真照相实验结果表明，当采用低面密度客体标定散射抑制系数时，高面密度客体散射强度的估计值与模拟真实值偏差可小于2%。Abstract: For multi-MeV X-ray flash radiography, the areal density of object can be obtained by the primary direct X-ray. Objects of flash radiography often have very high areal densities which greatly attenuate the intensity of direct X-rays emitted by the source. At this time, the direct penetration signal that can transmit the region of interest inside the object will be smaller than that of the scattered X-ray “noise”. If the captured image is reconstructed directly without scatter correction, it will affect the accuracy of reconstruction. The main method to reduce the scatter X-ray from a physical point of view is to use an anti-scatter grid, that is, an array-type collimation hole. However, the performance may be affected by the stability of the X-ray source spot, and the manufacture of such anti-scatter grid is very difficult. This paper proposes a new imaging method that does not rely on anti-scatter grid. This method only makes small improvements on the existing imaging layout, and can easily and self-consistently determine the amount of scattering for scattering correction. A Monte Carlo simulation is given to show the performance of scatter estimation, and the relative difference between the estimated intensity of scatter and the real value (provided by the simulation) for an object is less than 2% when a known object with less areal density is applied for calibration.
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Key words:
- high-energy X-ray /
- flash photography /
- imaging plan /
- image inversion /
- scattering compensation
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图 1 闪光照相基本布局及改进方案
Figure 1. Basic layout and improvement plan (framed) of flash radiography
图 2 假设变密度FTO-Den一维密度分布
Figure 2. One-dimensional density distributions of FTO-like objects
图 3 蒙特卡罗模拟含倒准直变密度FTO-U35照相直穿散射分布
Figure 3. Primary and scatter X-ray distribution of FTO-U35 with inverted collimation
图 4 阵列屏及狭缝准直后FTO-U35照相预期强度分布
Figure 4. Expected intensity distribution of FTO-U35 at the 2D array and downstream slit
图 5 阵列屏及调整后狭缝准FTO-U35照相预期强度分布
Figure 5. Expected intensity distribution of the array screen and the adjusted slit quasi-FFTO-U35 photography
图 6 FTO-U30照相狭缝准直前后直穿及散射强度分布
Figure 6. FTO-U30 camera slit straight through and scattering intensity distribution before and after collimation
图 7 FTO-U40照相狭缝准直前后直穿及散射强度分布
Figure 7. FTO-U40 camera slit straight through and scattering intensity distribution before and after collimation
图 8 FTO-U45照相狭缝准直前后直穿及散射强度分布
Figure 8. FTO-U45 photographic slit straight-through and scattering intensity distribution before and after collimation
图 9 阵列准直器可视化横断面示意图
Figure 9. Schematic diagram of the visualized cross-section of the array collimator
表 1 采用FTO-U30客体标定R用于估计其余客体散射值
Table 1. Calibration of R by FTO-U30 object to estimate the scattering value of other objects
object name variation before and after slit
collimation (measurement)simulated
scatter valuecalculated
scatter valuescatter after
collimation (simulation)R relative
deviation/%FTO-U30(for calibration) 0.0294 0.064 — 0.0059 0.0975 — FTO-U40 0.035 0.055 0.056 — 0.0975 +1.8 FTO-U45 0.035 0.055 0.056 — 0.0975 +1.8 
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表 2 采用FTO-U45客体标定R用于估计其余客体散射值
Table 2. Calibration of R by FTO-U45 object to estimate the scattering value of other objects
object name variation before and after slit
collimation (measurement)simulated
scatter valuecalculated
scatter valuescatter after
collimation (simulation)R relative
deviation/%FTO-U45(for calibration) 0.0354 0.055 — 0.0052 0.0945 — FTO-U30 0.0294 0.062 0.046 — 0.0945 −25.6 FTO-U35 0.034 0.060 0.053 — 0.0945 −11.7
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