27:T48c,

This paper describes an efficient computer method for generating three-dimensional (3-D) ultrasonic speckle patterns for sector scans of an annular array with dynamic focusing in both transmit and receive. Assuming random scatterers in an attenuating medium, the system synthesizes the waveform for each scan line using echo data received at all annular elements of the transducer when short pulses are transmitted by one annulus after another. The amount of echo data needed to synthesize one waveform is tremendously reduced by reducing the 3-D distributions of scatterers to line distributions and by representing scatterers on a small line segment by a single equivalent scatterer taking into account the two-way travel time differences. Furthermore, by a judicious 3-D arrangement of the scatterer lines and scan lines, it is possible to synthesize the waveform for a new scan line with negligible computation overhead. The waveforms of the scan lines are detected to obtain a B-mode image or speckle patterns. The patterns obtained on several differently-oriented image planes showed a good statistical agreement with experimentally obtained patterns.

28:T548,

Limited diffraction beams, such as Durnin's J ₀ Bessel beam, are a class of nonspreading solutions to the isotropic/homogeneous scalar wave equation. These beams can be approximately produced with finite aperture and energy over a deep depth of field. In this paper, we report the application of a broadband J ₀ Bessel beam to nondestructive evaluation (NDE) of materials. Pulse-echo images of a stainless steel block phantom were obtained with both the Jo Bessel beam and a conventional focused Gaussian beam. Results show that uniformly high resolutions were obtained with the J ₀ Bessel beam over a large distance. In addition, the lateral resolution of the J ₀ Bessel beam is almost independent of the speed of sound of the materials inspected. In contrast, the lateral resolution of images obtained with the conventional focused Gaussian beam changes dramatically with the distance and the focal length of the beam in water is greatly reduced by the steel block. Therefore, limited diffraction beams could be useful for nondestructive evaluation of materials of different speeds of sound. Restoration of pulse-echo images obtained with these beams could be simplified.

29:T5b3,