Development of Ultrasonic Time-of-Flight Computed Tomography for Hard Wood with Anisotropic Acoustic Property

Abstract

The ultrasound computed tomography (CT) method presented in this paper is an improvement of conventional back projection imaging methods for wooden pillars. The sound velocity in wood changes with the direction of the ultrasound propagation path; therefore, the properties of a medium must be considered in the imaging method. We obtained the 36 time-of-flight (TOF) data of tangents on the same circumference for various diameters. We determined an irregular pathway for each diameter, if the sound velocity of the pathway is slower than 92% of the average velocity of 36 pathways. TOF data for 270 paths were measured in total by manual scanning for one CT image. Although a hole of 25 mm diameter and a slit-type (3×33 mm2) hole are not detected by the conventional back projection method, these artificial defects were detected by our method. Two slit-shaped defects were also detected by our method. We found some knots and cracks in the X-ray image in a wood specimen. These knots were detected by our method; however, it was impossible to distinguish the knots and holes from each other using the CT image.