Volume 103, Issue 3, March 1998
Index of content:
- ACOUSTIC SIGNAL PROCESSING 
103(1998); http://dx.doi.org/10.1121/1.423110View Description Hide Description
In recent publications [Chew et al., IEEE Trans. Blomed. Eng. BME-9, 218–225 (1990); Borup et al., Ultrason. Imaging14, 69–85 (1992)] the inverse imaging problem has been solved by means of a two-step iterative method. In this paper, a third step is introduced for ultrasound imaging of the breast. In this step, which is based on statistical pattern recognition, classification of tissue types and a priori knowledge of the anatomy of the breast are integrated into the iterative method. Use of this material classification technique results in more rapid convergence to the inverse solution—approximately 40% fewer iterations are required—as well as greater accuracy. In addition, tumors are detected early in the reconstruction process. Results for reconstructions of a simple two-dimensional model of the human breast are presented. These reconstructions are extremely accurate when system noise and variations in tissue parameters are not too great. However, for the algorithm used, degradation of the reconstructions and divergence from the correct solution occur when system noise and variations in parameters exceed threshold values. Even in this case, however, tumors are still identified within a few iterations.
Signal processing of the echo signatures returned by submerged shells insonified by dolphin “clicks:” Active classification103(1998); http://dx.doi.org/10.1121/1.421302View Description Hide Description
A large set of dolphin-emitted acoustic pulses (“echolocation clicks”) have been examined, which were reflected from various elastic shells that were suspended, underwater, 4.5 m in front of the animal in a large test site in Kaneohe Bay, Hawaii. A carefully instrumented analog-to-digital system continuously captured the emitted clicks and also the returned, backscattered echoes (A/D conversion at 500 kHz). Using standard conditioning techniques and food reinforces, the dolphin is taught to push an underwater paddle when the “correct” target—the one he has been trained to identify—is presented to him. He communicates his consistently correct identifying choices in this manner. Many echoes returned by three types of cylindrical shells in both the time and frequency domains as well as in the joint time-frequency domain, by means of Wigner-type distributions have been examined. It will be shown exactly how specific features observable in these displays are directly related to the physical characteristics of the shells. This processing takes advantage of certain fundamental resonance principles to show which echo features contain information about the size, shape, wall thickness, and material composition of both the shell and its filler substance. In the same fashion that these resonance features give the identifying characteristics of each shell, it is believed they may also give them to the dolphin. These echo features may allow him to extract the target properties by inspection without any need for computations. It is claimed that this may be the fundamental physical explanation of the dolphin’s amazing target ID feats, upon which they base their recognition choices. This claim may be substantiated by the detailed analysis of many typical echoes returned by various shells, when they are interrogated by several dolphins. Thus far, this analysis of many echoes from many shells has only been carried out for a single dolphin.