Frequency is the number of cycles per second. A cycle is composed of one compression and one rarefaction. Frequency describes how many times in one second the acoustic variables pressure, density, and particle motion change.
Frequency is described as the number of times per second that the particles in a medium oscillate back and forth as a sound wave propagates through a medium.
The frequency of sound waves used in diagnostic imaging range from 1 MHz to 10 MHz.
Frequency is determined by the sound source and cannot be changed by the sonographer. In order to change the frequency the sonographer has to select a different transducer.
Modern ultrasound transducers contain multiple piezoelectric crystals. The frequency is changed by selecting the different piezoelectric crystal in a transducer.
The unit of frequency is Hertz (Hz). Frequency is measured in hertz (Hz), kilohertz (kHz), and megahertz (MHz).
One cycle per second = 1 Hertz (Hz)
One thousand cycles per second = 1 kilohertz (KHz)
One million cycles per second = 1 megahertz (MHz)
Example: A 5 MHz transducer operates at 5,000,000 cycles per second.
The ultrasound transducers are referred to by the operating, resonant or main frequency.
The frequency of ultrasound wave is determined by two factors.
|Transducer Frequencies||Imaging Use|
|2.5 MHz||Adult Echo, Deep Abdomen, OB/Gyn|
|3.5 MHz||Adult Echo, General Abdomen, OB/Gyn|
|5.0 MHz||Pediatric Echo, Vascular, Breast, Gyn|
|7.5 MHz||Pediatric Echo, Breast, Thyroid|
|10.0 MHz||Pediatric Echo, Breast, Thyroid, Superficial Veins, Superficial masses|
Higher frequency transducers provide better axial resolution but cannot obtain images at greater depths. The higher frequency ultrasound waves attenuate rapidly.
The lower frequency transducers can obtain images at greater depths but have lower axial resolution and therefore lower quality images.