Ultrasonic Imaging
Ultrasonic imaging is a technique widely used to reconstruct the properties of the materials under inspection from global wave propagation data. Acoustic tomograms provide an excellent noninvasive means of obtaining information about inhomogeneous media a
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5.1 Introduction Ultrasonic imaging is a technique widely used to reconstruct the properties of the materials under inspection from global wave propagation data. Acoustic tomograms provide an excellent noninvasive means of obtaining information about inhomogeneous media as noted by many investigators (Kak 1979; Herman 1980; Greenleaf 1981; Crawford and Kak 1982; Devaney 1982, 1986; Dean 1983; Denis et al. 1986; Tomikawa et al. 1986; Dumoulin and de Belleval 1987; Falls et al. 1989; Frick et al. 1997; Douglas Mast 1999). As for X-ray computed tomography, ultrasonic tomography refers to the cross-sectional imaging of an object from either transmission or reflection data collected by illuminating the sample from different directions. Ultrasonic tomographic reconstruction techniques can be classified as: - techniques based on the projection-slice theorem (filtered back-projection and direct Fourier transform), which are fast, but restricted to projection data that are sets of straight rays. - techniques based on iteration procedures (algebraic reconstruction techniques and simultaneous iterative reconstruction techniques) that are relatively slow, but may be used with complex sampling geometries and a bending ray path. Different types of waves can be used for ultrasonic imaging of solids (Schomberg 1982; Schechter et al. 1994, 1996; Sulivan et al. 1996). The most common are the longitudinal waves but shear waves and surface waves like Rayleigh or Lamb waves can also be used (Hutchins et al. 1993; Pei et al. 1995; McKeon and Hinders 1999). First, a precise analysis of the wave type launched into the sample must be made to understand the propagation phenomena in an inhomogeneous solid wood specimen, with structural defects. Second, the possible mode conversion must be studied and avoided, because of the strongly induced degradation of ultrasonic images by such phenomena. The resolution of the ultrasonic imaging techniques is very much limited by the wave length and by the size of the transducers. However, unlike X-rays, ultrasonic rays do not travel in a straight line through heterogeneous and anisotropic materials. For this reason, the computational requirements of this technique are much more important than for X-ray computed tomography and an increase in speed and accuracy of the reconstruction algorithms is necessary. V. Bucur, Nondestructive Characterization and Imaging of Wood © Springer-Verlag Berlin Heidelberg 2003
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Ultrasonic Imaging
Ultrasonic tomography applied to wood is an important challenge for wood scientists because of the natural variability, the anisotropy and the inhomogeneity of this material. Ultrasonic imaging techniques applied to wood must be able to distinguish between the natural structure of the material and its pathological features. Ultrasonic velocities and attenuation in different anisotropic directions, the reflective properties of wood surfaces and the back scatter of ultrasonic waves from the inhomogeneities must all be considered. Proper signal processing methods must be chosen
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