Ultrasonic 3-D Imaging Using Fourier Domain Reconstruction

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The ultrasonic imaging method is investigated to accomplish high resolution and to realize good images. The method regards the imaging system as the inverse scattering problem and obtains the almost exact solution which is visualized by appropriate displaying device. Generally speaking, such a method needs to calculate multiple numerical integral. It is difficult, therefore, to obtain the image in a effectively short time. However, simply assuming Born's approximation, it can be proved that the solution can be calculated by Fourier transform and its inverse. That is, using fast Fourier transform (FFT) the solution can be very speedily calculated.The feasibility of the theory could be shown by a numerical simulation and a simple experiment in a water tank. So, the results were presented at Ultrasonic Symposium held in San Francisco in 1985 and 4th Symposium on Ultrasonic Electronics in Tokyo.After that the experimental setup was improved and holes drilled in an aluminum block were successfully visualized on the CRT display. The results were expressed at International Conference of Acoustics, Speech, and Signal Processing and at Ultrasonic workshop in Tokyo in 1986.As the secondly results the algorithm of pruned FFT and the measuring method of the scanning system were developed. The former increased efficiency of FFT. Slight vertical drift of the scanning system became significant. The measuring method had to be developed. These two results were published simultaneously in IEEE Transactions ASSP No.8 in 1986.The problems including mode-conversion of the ultrasonic waves and the leaky surface waves at the interface between metal and the outer medium has to be investigated. But they are left for the future.