Glen Wade

Acoustic Echo Computer Tomography

Acoustic tomography systems described in the past were designed to provide a cross-sectional image of an object by employing transmission of a beam of ultrasonic energy. The data thus obtained were processed by a computer using a suitable algorithm. This resulted in an image based upon either the acoustic velocity or the attenuation characterisic of the object. This paper describes two novel imaging techniques which are based on the reflection of acoustic waves from the distribution of scattering centers within the object. One of the primary advantages envisioned for the proposed techniques is the use of a single collimated beam of ultrasound in the plane of the tomogram in lieu of slow mechanical scanning or multiple transducer arrays. Results are presented of a laboratory experiment which was performed to test the validity of one of the proposed concepts.

Ultrasonic Planar Scanned Tomography

Linear x-ray tomography has been used for decades as a technique that trades depth information for quantity of resolved volume. In linear focal-plane x-ray tomography, a synchronous opposing motion of x-ray source and film during exposure results in an image which has the midplane of the object in sharp focus with all other planes overlaid with various amounts of linear blurring. Nevertheless, the in-focus plane stands out well enough that x-ray diagnosticians still make extensive use of the technique.

Scanning Tomographic Acoustic Microscope: A Review

On utilise les techniques de reconstruction tomographiques pour traiter les donnees recueillies par un microscope acoustique a laser a balayage. Avantages importants par rapport aux methodes classiques: representation aisee de la sous-surface des objets complexes, lorsque la structure consideree se situe dans divers plans bien definis. On resume les principes de la propagation type aller-et-retour associee a la reconstruction des images par ce procede de microscopie acoustique. Presentation des modifications electroniques et mecaniques necessaires pour le microscope. Formules pour determiner les limites theoriques de resolution du microscope. Resultats de simulations

Holographic Image Reconstruction in Scanning Laser Acoustic Microscopy

Acoustic microscopy is an important branch of nondestructive evaluation which provides high resolution for imaging the detailed structure of a small object. When an acoustic microscope operates in the transmission mode, the micrograph is simply a shadowgraph of all the structures encountered by the paths of acoustic rays passing through the objects. Because of diffraction and overlapping, the resultant images are difficult to comprehend in the case of specimens of substantial thickness and structural complexity. To overcome this problem, it was proposed some time ago to utilize the principles of diffraction tomography and acoustic holography involving calculations of wavefield propagation. By means of computer simulation, how the scanning laser acoustic microscope (SLAM) could be modified to provide subsurface tomographic imaging was described in previously published work. Experimental results of such imaging using holographic image reconstruction are presented. The modification of SLAM and the reconstruction process are described. How to acquire accurately the complex amplitude information of the wavefield necessary for the image reconstruction is shown. Comparing the computed amplitude image thus obtained with a conventional SLAM image illustrates the power of this technique even for surface imaging. Subsurface images reconstructed by means of this approach are also presented. The results demonstrate that high-qualiry high-resolution subsurface images can be obtained from holographic data with this technique.

Computed Reconstructions from Phase-Only and Amplitude-Only Holograms

We have analyzed several types of acoustical holograms similar to and including the phase-only hologram suggested by Metherell.1 The analyses have involved three different diffraction regions: the Fraunhofer, Fresnel, and very-near-field. Each type of hologram studied is characterized by discarding some part of the information present in the wave scattered by the object and recording the portion that is left, thus allowing more efficient use of the recording medium. (For example, in the phase-only hologram the phase information in the scattered wave is retained, but the amplitude information is discarded.) The object investigated in each case was a long slit, thereby confining the analyses to one dimension.

Computer-assisted Tomographic Acoustic Microscopy for Subsurface Imaging

Computer-assisted ultrasonic tomography has received much attention in recent years. Acoustic microscopy is an important branch of non-destructive evaluation which provides high-resolution imaging of the detailed structure of an object. STAM (Scanning Tomographic Acoustic Microscope) is a system capable of producing tomographic images by scanning the source or rotating the specimen to generate a sequence of tomographic projections. This system has advantages over conventional approaches, especially for complex objects with planar structure such as integrated-circuit chips. An earlier paper provided the analysis and the reconstruction algorithms for the signal processing of planar tomographic systems including an algorithm for “back-and-forth” propagation. This paper summarizes the modifications and revisions of this latter algorithm as applied to microscopic digital imaging. Two different schemes of rotation to acquire STAM data and the corresponding reconstruction procedures are described. Simulations are presented which demonstrate the resolving capability of the STAM system.

Digital Reconstruction of Acoustic Holograms in the Space Domain with a Vector Space Approximation

We have developed a computer-assisted ultrasonic underwater imaging system. Until recently, we used back-propagation to reconstruct images. This method, accurate both in the near and far fields of the object, is an inverse filtering technique that operates in the spatial frequency domain and requires the taking of two Discrete Fourier transforms (DFT’s).

Optical Heterodyne Detection in Bragg Imaging

The technique of acoustic imaging by Bragg diffraction has been developed to the extent that it is now possible to produce images with resolution and detail comparing favorably with that attained by other acoustic visualization methods.

Ambiguity Functions for Diffraction Tomography Using Back-and- Forth Propagation

This paper presents ambiguity functions for diffraction tomography using back-and-forth propagation. Both depth and lateral ambiguities are studied. The ambiguity functions corresponding to two different schemes for data acquisition are derived. We show that both the depth and lateral ambiguities are dependent on the wavelength of the insonification, and the range of values of the incident angles employed. Projections obtained by rotating the transducer give different ambiguities in two lateral directions. Projections obtained by rotating the object give less lateral ambiguity but greater depth ambiguity.

Threshold Contrast for Three Real-Time Acoustic-Imaging Systems

Three approaches to real-time acoustic imaging are presently being worked on in various laboratories. These approaches involve the use of static-ripple diffraction, dynamic-ripple diffraction and Bragg diffraction. Experimental results indicate that good images can be obtained from each and that the systems currently appear to be quite competitive. This paper compares the ultimate potential performance of systems of each of the three types in terms of threshold contrast and sensitivity. The comparisons not only give an indication of the inherent capabilities and limitations of the systems but also of how far the present systems are from achieving ideal behavior.