Imaging
The potential for ultrasonic imaging of objects, with a 3 GHZ sound wave producing resolution comparable to an optical image, was recognized by Sokolov in 1939 but techniques of the time produced relatively low-contrast images with poor sensitivity. Ultrasonic imaging uses frequencies of 2 megahertz and higher; the shorter wavelength allows resolution of small internal details in structures and tissues. The power density is generally less than 1 watt per square centimetre, to avoid heating and cavitation effects in the object under examination. High and ultra high ultrasound waves are used in acoustic microscopy, with frequencies up to 4 gigahertz. Ultrasonic imaging applications include industrial non-destructive testing, quality control and medical uses.
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