Ultrasonic propagation can often be usefully described in terms of rays. As these rays are in general curved, transmission tomography is more complicated for ultrasound than for x-rays. It is shown here that in most media there are “forbidden” regions which are not intersected by any “minimum propagation time” rays. It is concluded that it is seldom feasible to obtain better images than those reconstructed on the assumption that the ultrasonic rays travel along straight lines.
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