Implementation and evaluation of the ultrasonic TOF tomography for the NDT of concrete structures

In contrast to X-rays, ultrasound propagates along a curved path due to spatial variations in the refraction index of the medium. Thus, for ultrasonic TOF tomography, the propagation path of the ultrasound must be known to correctly reconstruct the slice image. In this paper, we propose a new path determination algorithm, which is essentially a numerical solution of the eikonal equation viewed as a boundary value problem. Due to the curved propagation path of ultrasound, the image reconstruction algorithm takes the algebraic approach, for instance, the ART or the SART. Note that the image reconstruction step requires the propagation path and the paths can be determined only if the image is known. Thus, an iterative approach is taken to solve this apparent dilemma. First, the slice image is initially reconstructed assuming straight propagation paths. Then, the paths are computed based on the recently reconstructed image (using our path determination algorithm) which is then used to update the reconstructed image. This process of image reconstruction and path determination repeats until convergence. This approach is tested with simulation data as well as TOF data from a real concrete structure using a mechanical scanner.