Ultra means above. Therefore ultrasounds are above the upper limit of human hearing - with frequencies greater than 20,000 Hz (20 kHz)! While there are industrial and military applications for ultrasound technology it is primarily used for medical imaging. Audible sound is sound that is between 20Hz and 20kHz. Infrasound is sound at frequencies below 20 Hz and Ultrasound is sound at frequencies above 20kHz. The frequency of the sound wave affects the image quality and the penetration of the sound beam.

Remember, lower frequency transducers penetrate deeper but the image quality is compromised (abdominal probe). Higher frequency transducers produce better image quality but are used for superficial imaging.
 

  

       
 

 
 
1.

The ultrasound machine sends an electrical impulse to the transducer causing crystals within the transducer to vibrate.
   
2. High frequency sound pulses (1-10 megahertz) are transmitted into the body.
   
3.

Sound waves travel into the body and hit a boundary between tissues (e.g. between soft tissue and bone, fluid and soft tissue).
   
4.

Some of the soundwaves get reflected back and others travel further until they reach another boundary and get reflected back.

   
5. The probe picks up the reflected waves and relays a message to the machine.
   
6.

The machine calculates the distance from the probe to the organ or tissue and assumes the speed of sound in soft tissue as 1540 m/s and the time of the echo's return.

   
7.

The machine displays the distances and intensities of the echoes on the screen, forming a two dimensional image.
   
 
 
  Understanding the parts and functions of an ultrasound machine is the first step in learning to produce high quality images.
 
 

Transducer probe: probe that sends and receives the sound waves
Central Processing Unit (CPU): processor that does all of the calculations
Pulse controls: changes the amplitude, frequency and duration of the pulses emitted from the transducer probe
Display: displays the image from the ultrasound data processed by the CPU
Keyboard/cursor: inputs data and takes measurements from the display
Disk storage device: stores the acquired images
Printer: prints the image from the displayed data