关键词: X射线, 计算机断层扫描, 立木成像

Abstract: [Objective] A computed tomography (CT) imaging system based on fan-shaped X-ray beam was developed for high-resolution tomographic imaging of standing wood. Many subjects such as nuclear physics, computer science and signal processing etc. were integrated in the CT imaging system. The CT technology was applied in standing wood, the reconstruction image of standing wood could be used to facilitate researchers accurately understanding internal structure and defect characteristics of wood, which would provide technical support for the wood science research in China.[Method] CT imaging for standing wood based on the X-ray absorption extent might be different in various wood structures. So, by measuring the X-ray attenuation coefficients within wood, solving the two-dimensional distribution matrix of coefficients on standing wood's cross section, transforming the matrix into a gray-scale image, and the tomographic imaging for standing wood could be realized. The CT imaging system includes an X-ray source, an X-ray detector, an electric rotational stage and a computer, the computer controls the CT system as a central processing unit. During standing wood's scanning, the X-ray source and the X-ray detector were stationary, while the standing wood placed in the center of electric rotational stage. The fan-shaped beam was emitting from X-ray source to the standing wood and the X-ray detector captured the penetrated X-ray intensity when the standing wood was at different angles of rotation. The computer gave detector an order to collect data via USB, and converted the X-ray intensity into digital signal, then transferred to the computer and saved as database files. In order to meet the requirements of computer tomographic imaging reconstruction, the data needs to be pre-processed, including removing detector's dark field, compensating detector's abnormal responded channels, correcting detector's inconsistent response, and calculating the line integral projection value of standing wood's cross section. Based on the fan-shaped X-ray beam and equidistant line array detector, the filtered back projection reconstruction algorithm was used in the standing wood CT system.[Result] The performance of the CT system for standing wood was assessed, the space resolution of the CT system is 0.4 mm, and the maximum diameter of the detected standing wood is 29.5 cm. The scanning experiments were carried out on a block of Thuja, Pinus and plywood at 360 degrees, respectively. The experiment-1 verified that the standing woods' projection data at various angles were effective and the according CT reconstruction images were reasonable. The experiment-2 demonstrated that the CT reconstruction images were correct by comparing the pictures and the reconstruction images of the standing woods' cross sections.[Conclusion] The reconstruction images clearly revealed the structural features of standing woods' cross sections, such as shape, crack, growth ring and pith, etc. The quality of the reconstruction images meet requirements for detecting standing woods' internal structure.

Key words: X-ray, computed tomography, standing wood imaging