Ultrasound is sound that has a frequency above the audible range of man that is greater than 20 kHz. In imaging the usual range of frequencies used is between 2 MHz and 10 MHz.

• 超声是一种声波,在医学影象中,超声仪使用2MHz-10MHz间的声波,此一频率已远远超过人耳所能听到的20khz。