In the case of medical scans and non-destructive testing, experts use relatively low strength ultrasound waves. Much stronger waves are used for removing unwanted deposits like painful stones.
Let us make it clearer. If we have problems caused by stones in kidney, gall bladder, or liver, powerful ultrasound waves are fired from outside our body in order to make the stone vibrate, and disintegrate. For this purpose, doctors now use what is known as an ultrasonic gun, which causes relatively lesser pain. The gun forces out short bursts of high frequency vibrations that cause the stones to resonate. These then smash the stones into pieces.
A similar technique is used to destroy cancerous tumours and damaged regions of the brain called lesions. Here, ultrasound energy is applied to heat in order to destroy diseased tissues.