Determination of size and shape of small inclusions from sound-field information

The resolution of scanning ultrasonic systems is limited to the wavelength used and the focus size. Common methods for determination of object sizes close to or even smaller than wavelength require knowledge of the object geometry and use absolute amplitude information only. We present new approaches for determining shape and size of small objects by additional evaluation of sound field effects like the directivity of reflected waves. First measurements show that determining size and shape of objects smaller than the size of the sound field is possible with geometric considerations on maximum amplitude positions of different reflection modes. Practical usable relative evaluation criteria for time changes in dependence of displacement can be found for ball sizes down to 1/5 wavelength.