Research On Correction Method For Cupping Artifacts In The Image Of Industrial CT

In the X-ray industrial CT system, the cupping artifact caused by the beamhardening not only affects the quality of the image but also reduces the identificationability of the image and the precision of the density measurement. Therefore, it isespecially important to do the pre-processing operation on the data to correct thecupping artifact in the X-ray industrial CT system.The present thesis introduces the linearized correction algorithm based onpolynomial fitting and its realization detailly. The hardening correction model is alsodesigned. Then this algorithm is used to correct the simulation CT images and actual CTimages respectively. Experimental results show that the algorithm can eliminate thecupping artifacts in CT image and improve the uniformity of the image effectively.However, the algorithm also brings some problems such as the salt-and-pepper noise ofthe corrected image is greater than the original image and the edge artifacts becomeworse. To solve the above problems and obtain better effect, the causes of the problemsare analyzed theoretically. When X-ray interacts with matter, the phenomena of beamhardening and scattering appear. These phenomena together with the crosstalk causedby the scattering in the detector have impacts on the quality of CT images. Therefore, toobtain better correction effect, the influence of scattering and crosstalk must beconsidered simultaneously in the correction of the beam hardening problem.To improve the weakness of using only the linearized correction algorithm, acorrection method based on the combination of software and hardware is proposed. Alead foil sheet is placed between the object to be measured and the collimator to absorbthe scattering ray. Then the linearized correction algorithm is used to correct the beamhardening. To ensure that using one single correction model, better correction effect canbe obtained under different energy, series of hardening curves are obtained by scanningthe hardening correction model with several commonly used X-ray energy. Thecorrection is done by the hardening curves to the CT image obtained under thecorresponding energy. Experiment results show that this correction method eliminatescupping artifacts and reduces the noise at the uniform area of the workpiecesignificantly. Moreover, the uniformity of the image is improved effectively, and boththe detail and edge of the corrected CT image are kept well.To solve the problem of the edge artifact in the CT image, theoretical analysis is done to find the cause of the edge artifact one of which is the crosstalk produced by thescattering of X-ray in the near detector. Mathematic model of the crosstalk in thechannel of the detector is presented. In the following, a correction method based onreducing the coefficient of the crosstalk is proposed. By doing experiments on the mainfactors that influence the crosstalk ratio, the relationship of the crosstalk ratio with theenergy of the incident X-ray and the thickness of the object to be measured is obtained.Furthermore, the relation function between the projection data and crosstalk ratio isobtained. Then the projection data can be corrected by the relation function. Correctionexperiments on the actual CT images show that this method can reduce the edge artifactsignificantly.