关键词: X射线, CT, 木材, 正弦图, 旋转中心

Abstract: [Objective] A computed tomography(CT) imaging system for wood based on equidistant fan-shaped X-Ray beam was developed and a method of correcting the rotation center based on the sinogram was presented in this paper. Applied the CT imaging system in wood,the reconstructed images of wood could be used to facilitate universities and scientific research institution, and to provide technical support for the structural morphology research.[Method] Firstly, the image reconstruction principle of the CT system was analyzed to verify that there was a functional relationships between the projection address of any point in the scanning fault and the rotation angle. The trajectory of projection addresses was a sinusoidal curve associated with the rotation angles during scanning process.Secondly, the difference between the center of symmetry of the projection address and the midpoint of the detector was calculated. In the process of image reconstruction, the offset of rotation center was compensated to obtain the correct projection position of any point.Thirdly, a Chinese fir with the diameter of 205 mm and the height of 400 mm was selected as the sample, and the offset of rotation center was corrected by 8, 9, 10, 11 and 12 detecting pointsrespectively. Five reconstructed images were obtained by scanning the fault 1 of the sample, and these images were analyzed quantitatively. Finally, the fault 2 of the sample was scanned to reconstruct two images before and after the correction of the rotation center, and the images were evaluated and analyzed.[Result] The 5 reconstructed images of fault 1 could all reflect the internal structural characteristics of wood. As the corrected values getting closer to the offset of rotation center, the PSNR and SSIM values increased, and the RE values were decreased. It showed that the reconstructed images were more similar to the original image. In order to obtain the best reconstructed image, the corrected value of the rotation center must be equal to the offset. The two reconstructed images of fault 2 were quite different. For example, quite a lot of ring artifacts of the reconstructed image before correction brought many difficulties in distinguishing the growth rings. The edge marks diffused to the periphery, and the false similar crack structures appeared in the reconstructed image. In the corrected image, there weren't ring artifacts or peripheral edge marks. Growth rings in the heartwood could be seen clearly, and the reconstructed image was perfect.The experimental result show that the image at the corrected rotation center was much better than the pre-correction image. The method of correcting the rotation center through the sinogram has a good application effect in the wood CT tomography system.[Conclusion] The method for correcting rotation center was proposed in this paper. It effectively solved the problem on random migration of the rotation center, suppressed the ring artifacts, and improved the quality of reconstructed images. After the rotation center is corrected, the reconstructed images can show internal structural characteristics suchascracks, growth rings, heartwood and sapwood much more clearly.The CT imaging system meets the requirements of universities and scientific research institutions for detecting and analyzing the internal structural characteristics of wood.

Key words: X-ray, computed tomography (CT), wood, sinogram, rotation center