Investigation On Scatter Problem In Radiographic System

One of the largest sources of uncertainty in high-energy flash x-ray radiographic image is due to the presence of systematic scatter distribution on the image, which is produced by the interactions of high-energy X-rays with dense materials. Because the scatter is different for different systems and it is not homogeneous, causing the difficulty to obtain the information of the object well and truly. So, making sure the distribution and the influencing factors of the scatter of the system is the precondition of obtaining the information accurately.In this paper, first the author analyzes the interaction of X-rays with matter and find out the cause of scatter, and analyzes the scatter law of dense object with analytic method on the radiographic system of FTO, and obtains the ratio of direct to scatter (R_DS), the influencing factor of scatter to optical path and the theoretical formula of scatter deduction. Next, a Monte-Carlo numerical simulation method is used to simulate the transport process of X-ray photons in 3m radiographic system for FTO. On one side, the effects to scatter of the parameters, including X-ray source, angle of main collimator, optical path, back-protected-cell(BPC) and the distance between recording plane and BPC, et al, are analyzed and simulated. The scatter distribution of the system and the contribution of each cell to scatter with layer analysis is simulated and the sources of scatter is found. On the other side, the author analyzes parameter optimization of the scatter deduction cell and scatter reduction cell. In addition, because BPC is the main source of scatter, experiments are designed to measure the scatter of BPC, and compare the results of experiments with MC simulation.The main results are as follows.1. When the angle of main collimator is fixed, increasing of the thickness of BPC, the scattered exposure also increases and the distribution of scatter become more unhomogeneous. For the different materials of BPC with the same penetrated optical path of X-ray photons, the higher attenuation coefficient of the material, the lower the scatter of BPC. The scatter distributions of experiment and simulation are very similar.2. Whatever the BPC is in system or not, for different objects with same size and same mass, the distributions of total scatter are approximately same.3. The impact of angle of the main collimator on scatter is very serious. The scattered exposure of system is increasing with the enlarging of the angle of main collimator, and its distribution become more unhomogeneous. Increasing the distance between BPC and the recording plane can effectively reduce the scattered exposure.4. The results of layer analysis show that the BPC is the main source of scatter in the system, the scattered percent of different layer of BPC is based on the distribution of exposure irradiating on BPC, and the scatter of object is mainly produced by the edge part of the object.5. The performance of multi-hole collimator depends on the aperture, the thickness ofcollimator and the number of holes. Either reducing the aperture or increasing the thickniss can well improve the performance of the cell.6. The added collimators include step collimator and graded collimator are effective scatter reduction cell. Their application can both reduce the scatter and remain the information of the object. Scatter reduction grid is a good scatter reduction cell, it can greatly reduce the scattered exposure of system with scatter reduction factor of 102.The paper analyzes the scatter of radiographic system using analytic method, a Monte-Carlo numerical simulation method and experimental method. The innovative points are as follows.1. The effect of the parameters of BPC on scatter is analyzed which provides useful conclusions about optimization of BPC and scatter reduction.2. It is studied how the variety of materials and size of object impact on scatter of system, and the results provided the foundation of image processing of dynamic object.3. The scatter produced by object and BPC is analyzed, it is useful for developing scatter reduction technology.4. The experiments to measure the scatter of BPC are completed and it can help to achieve the scattered exposure accurately.5. It is proved that the grid is one of the best scatter reduction cell.Its optimized design and the theoretical support of its application are provided.The numerical simulation of radiography is a very important and complicated work, and its main aim is how to make certain the scatter of a specific radiographic system. The paper analyzes the cause of scatter, the influencing factors of scatter and the methods of scatter reduction, and provides some effective methods and useful conclusions to further numerical simulative and experimental study of the flash x-ray radiography.