1 - Acoustic Microscopy

High resolution ultrasound images (90% of Rayleigh limit at all depths) were obtained by computer analysis of digitized (at 10 and 20 points per microsecond, 8 bits per point) signals detected with a 32 element lead zirconium titanate 3.0 MHz array. Every sixth element was used as a transmitter and pulsed 31 times while the signals from the remaining 31 elements were addressed sequentially by an analog switch and their signals digitized and recorded. Each pixel in the cross-sectional image was produced by calculating the inner product of a specific window function and the ensemble of digitized signals. Thus the array is mathematically focused optimally for each pixel in the image. The acquisition and storage of all received signals allows subsequent approximate calculation of spatial distributions of such parameters as reflection, index of refraction, attenuation, etc. The extension of these techniques through the use of a fast analog and digital computer interface to obtain realtime and stop-action imaging is treated. Examples of images produced by these algorithms, the time requirements of various algorithms as determined by the computation speeds of present and anticipated digital and analog processing hardware is presented. Supported in part by NIH research grants HT-4-2904, RR-7, and HL-04664. Also supported in part by a contract from the Office of Naval Research to E. M. Eyring, with whom S. A. Johnson was a part-time research associate.

Digital Computer Simulation Study of a Real-Time Collection, Post-Processing Synthetic Focusing Ultrasound Cardiac Camera

This paper briefly reviews some aspects of holography and its application to acoustic imaging with an emphasis on concept. Included age discussions of the identity of holography, the principle of optical holography, holography applied to the acoustic domain and holography by linear detection, the applications of optical and acoustic holography, the concept and front-end embodiment of acoustic imaging systems that can be considered holographic, and the limitations of acoustic holography. This paper is intended to be tutorial and to provide discussions on relevant topics not covered by other papers in this issue on the subject of holography.

Holography and its application to acoustic imaging

A 1.1 GHz acoustic microscope is being developed with an anticipated resolution capability of 5 μm. The microscope uses a unique scanning method which employs the photoconductive effect to locally switch a CdS piezoelectric transducer. A focused laser beam scanned across the CdS transducer sequentially deactivates resolution elements and generates the video signal. To form the image, the video signal intensity modulates a CRT display which is scanned synchronously with the laser beam.

A 1.1 GHz Scanned Acoustic Microscope

In this paper we will present a technique which allows us to record high-resolution acoustic images of biological specimens. The image is detected in the extremely near field of the object and the imaging, itself, is accomplished in a thin liquid layer containing 1 µm polystyrene spheres. The response of the spheres to the “radiation pressure” of the acoustic fields traversing the liquid layer permits the spheres to condense into a pattern which produces the acoustic image. The force on an individual sphere as used in our device resulting from the radiation pressure can be calculated and by means of this calculation it is easy to optimize the magnitude and show that it is linearly proportional to the acoustic intensity — an effect necessary for our system of imaging.

High-Resolution Acoustic Imaging by Contact Printing

This paper reports a new technique for the visualization of ultrasonic interference in solids. The technique utilizes small (≈ 1 μm) polystyrene spheres suspended in a very thin liquid film formed on the surface of the solid. The nonlinear response of this film to acoustic waves is such that the spheres reveal the interference pattern present at the surface. There are two important advantages to this technique. The first is the simplicity of the technique, no elaborate equipment is required. The second is the easing of restrictions on ultimate resolution by our ability to visualize ultrasonic interference in solids.

J. A. Cunningham

Papers

Acoustic Interference in Solids and Holographic Imaging