Modification of the scanning laser acoustic microscope for holographic and tomographic imaging

Int J Imaging Syst Tech

1991

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We present the recent developments of the Scanning Tomographic Acoustic Microscope (STAM). The STAM was proposed as a method to achieve 3D imaging capability for the Scanning Laser Acoustic Microscope (SLAM). With the addition of a quadrature receiver, the complex scattered wave field can now be detected, and consequently the STAM is capable of subsurface holographic and tomographic imaging. The resolution improvement of the STAM can be attributed directly to the detection of the phase information and the image reconstruction technique. The STAM is sensitive to phase errors in the tomographic projections. In particular, the quadrature phase error and the initial phase error in the complex projections are critical to the tomographic reconstruction process. For multiple-angle tomography, high-precision projection registration and alignment become necessary. By obtaining solutions to these implementation problems, we have succeeded in obtaining images superior to the original SLAM images. In addition, quantitative ultrasonic imaging is possible with the STAM, and a method is presented to image the velocity parameter of simple specimens. With these capabilities, the STAM may become a useful tool for high-resolution subsurface nondestructive evaluation.

Section: Introductionmentioning

confidence: 99%

Recent advances in scanning tomographic acoustic microscopy

Int J Imaging Syst Tech

1991

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“…The transmitted wavefield is detected at the plane z d , , where a coverslip is placed in contact with the coupling medium. The coverslip is .a plastic block with a gold-plated surface, which is dynamically beam scanned over the mirrored surface picks up the wavefield information, which is then detected using a quadrature receiver [4]. The dual outputs of the quadrature receiver are digitized for computer processing.…”

Section: Introductionmentioning

confidence: 99%

Multiple-frequency and multiple-angle tomography with the scanning tomographic acoustic microscope

IEEE Symposium on Ultrasonics

In this paper we present recent developments in multiple-frequency and multiple-angle tomography for the Scanning Tomographic Acoustic Microscope (STAM). The STAM extends the capabilities of the Scanning Laser Acoustic Microscope (SLAM) by using tomographic processing to recover the depth information. Recently, high quality tomographic reconstructions were obtained by addressing the problems of projection phase errors and projection misalignment. In addition to discussing these implementation problems for the STAM, we present reconstructions using experimental data for both multiple-frequency and multiple-angle tomography. These results demonstrate the feasibility and reconstruction improvement of the STAM.

Tomographic reconstruction of multiple‐layer specimens with the scanning tomographic acoustic microscope

Int J Imaging Syst Tech

1990

Self Cite

In this article we report the recent results of tomographic image reconstructions of multiple-layer specimens at 100 MHz with the Scanning Tomographic Acoustic Microscope (STAM) system. The experiment utilizes 12 uniformly spaced projections and the results show significant improvement over the holographic images.Acoustic microscopy has been a very important approach to high-resolution nondestructive evaluation of microscopic structures. The Scanning Laser Acoustic Microscope (SLAM) is one of the most widely used systems among the acoustic microscopes currently available. Based on the operating format, SLAM is a transmission-mode system with plane-wave illumination, The data acquisition is performed by a scanned laser beam that picks up the dynamic surface profile of the resultant acoustic waveform. The SLAM system has several fundamental advantages. It does not require focusing of the illumination beam. There is no mechanical movement of the specimen. The laser read-out approach provides a high-speed data acquisition capability. However, the SLAM remains limited to an orthographic imaging device because of the lack of subsurface imaging capability. This seriously limits the applications of the SLAM systems for specimens with certain thickness. Figure 1 illustrates the structure of the two-layer test specimen. Figure 2 is the orthographic image produced by a conventional SLAM system.With the addition of a quadrature receiver, the SLAM system is then capable of holographic subsurface imaging because of the detection of both the amplitude and phase of the dynamic wavefield [1]. For this experiment, the illumination acoustic wave is operated at 100MHz. The quadrature receiver is operated at 32.4 MHz after first down-converting the frequency from 100 MHz. The holographic image reconstruction is performed by the backward propagation algorithm [2]. Figure 3 shows the holographic images of the two layers. The holographic images demonstrate some improvement over the conventional SLAM image in terms of better separation of layers. However, leakage between layers still exists.The objective to upgrade the capability of the SLAM system to the tomographic level is to improve the depth resolution over the holographic reconstruction. Tomographic microscopy can be achieved by extending the spatial-frequen- cy content of the image information with a wider coverage of the observation angle. The experiment included in this article utilized 12 projections. These projections are evenly spaced with the spacing of 30 degrees to form a complete perspective coverage. The tomographic reconstruction is the superposition of the holographic images reconstructed from the 12 projections by backward propagation. The tomographic superposition process also requires an extensive sequence of preprocessing procedures, including data-acquisition-phase error removal, resampling for uniform data sample spacing, estimation of the initial phase of projections, and displacement estimation for accurate array alignment [3,4]. Figure 4 shows the tomograph...