Key message Considering anisotropy in image reconstruction algorithm for ultrasound computed tomography of trees resulted in a more accurate detection of defects compared to common approaches used. Context Ultrasound computed tomography is a suitable tool for nondestructive evaluation of standing trees. Until now, to simplify the image reconstruction process, the transverse cross-section of trees has been considered as quasi-isotropic and therefore limiting the defect identification capability. Aims An approach to solve the inverse problem for tree imaging is presented, using an ultrasound-based method (travel-time computed tomography) suited to the anisotropy of wood material and validated experimentally. Methods The proposed iterative method focused on finding a polynomial approximation of the slowness in each pixel of the image depending on the angle of propagation, modifying the curved trajectories by means of a raytracing method. This method allowed a mapping of specific elastic constants using nonlinear regression. Experimental validation was performed using sections of green wood from a pine tree ( Pinus pinea L. ), with configurations that include a healthy case, a centered, and an off-centered defect. Results Images obtained using the proposed method led to a more accurate location of the defects compared to the filtered backprojection algorithm (isotropic hypothesis), considered as reference. Conclusion The performed experiments demonstrated that considering the wood anisotropy in the imaging process led to a better defect detection compared to the use of a common imaging technique.

中文翻译：

---

立树的超声计算机断层扫描：考虑木材各向异性可以更准确地检测缺陷

关键信息 考虑到树木超声计算机断层扫描图像重建算法中的各向异性，与使用的常用方法相比，可以更准确地检测缺陷。背景 超声计算机断层扫描是对立树进行无损评估的合适工具。到目前为止，为了简化图像重建过程，树木的横截面被认为是准各向同性的，因此限制了缺陷识别能力。目的 提出了一种解决树木成像逆问题的方法，该方法使用适用于木质材料各向异性并经过实验验证的基于超声的方法（行进时间计算机断层扫描）。方法 建议的迭代方法侧重于根据传播角度找到图像每个像素中慢度的多项式近似值，并通过光线跟踪方法修改弯曲轨迹。这种方法允许使用非线性回归映射特定的弹性常数。使用来自松树 ( Pinus pinea L. ) 的绿木部分进行实验验证，配置包括健康情况、居中和偏离中心的缺陷。结果 与被视为参考的滤波反投影算法（各向同性假设）相比，使用所提出的方法获得的图像导致更准确的缺陷位置。