| Computed tomography(CT),as an advanced nondestructive testing method,has been undergoing continuous update and improvement since its birth.With the constant decrease of the ray source focus and the limited pixel size of the detector,the micro nano CT system for detecting small millimeter level objects has appeared,and the spatial resolution is even to the micron and nanometer level.For this kind of high resolution CT system,the CT scanning time is usually long and the single CT scanning time reaches several hours to several hours.In this process,the focal point of the X ray source will drift,resulting in the decline of the image clarity and even the artifact,which seriously affects the image quality.For long CT scanning process,focus drift must be corrected.In this paper,the focus drift correction of X ray source in micro nano CT system is studied.(1)The focus drift is tested and analyzed: the focus drift is tested by the design scheme.Through the analysis of the data,the focus drift is large and the focus drift after a period of time tends to be stable;and the focus drift under different voltages is compared,and the smaller the voltage is,the smaller the focus drift is,therefore,When the workpiece is able to penetrate the workpiece,a small voltage is selected as far as possible to scan the workpiece.The microfocal ray sources commonly used in the micro nano CT system are analyzed,and the focus drift characteristics of different types of open tube and closed tube microfocal ray source are compared.The factors affecting the focus drift of the ray source are qualitatively analyzed,and the corresponding general solutions are given.After the focus drift test and analysis of a micro focal ray source,the focus drift curve in the time range is obtained by quantitative analysis of the image obtained by a long time scanning board scanning,which can be used as a reference for the follow-up focus drift correction and the comprehensive correction method of focus drift and scattering.(2)The focus drift is corrected: a scheme for focus drift correction suitable for micro nano CT system is designed.Through the correction and contrast of different workpiece,it can be found that the reconstructed image can be improved better.After the analysis of the focus drift test,it is found that in the whole focus drift process,the focus drift can reach more than ten pixel dimensions,but for the sampling of each degree,the focus is in the sub pixel level drift process on the image.In order toachieve the purpose of correcting,two kinds of correction methods for focus drift are proposed,that is,round hole setting.Position correction and cross positioning correction.After completing the correction,the validity test and the correction effect are compared under various evaluation criteria.By comparing the reconstructed image slices,it is found that the image structure becomes better and the artifact is reduced after the focus drift correction.(3)A comprehensive correction scheme is designed.Combined with the study of scattering correction in the early stage of the laboratory,a scheme of integrated focus drift and scattering correction is proposed in order to avoid the influence of scattering in the process of focus drift correction.The results are better than the other two single corrections.Two kinds of hardware correction methods,which are Beam Stop Array(BSA)and Beam Hole Array(BHA),are selected at the same time with focus drift correction.In the case of ray source tube voltage 100 kV and tube current 200 micron A,two kinds of comprehensive correction methods are adopted to the circuit board with complex structure.And ultrasonic templates and other test materials were scanned and reconstructed.Through the image quality comparison,we can obtain the quantitative results and visual results of uncorrected,single focus drift correction,single scattering correction,and integrated two correction schemes under the same workpiece.From the reconstructed image,it can be seen that the image quality after the comprehensive correction is obviously superior to several other uncorrected or single correction methods,and does not increase the additional scanning time,thus providing an effective method for the correction of focus drift and scattering signals. |
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