Doppler Ultrasound Audio:
©Walter Rasmussen, R.D.C.S.
The experienced sonographer employs the Doppler ultrasound audio output as a guide to the best positioning of the transducer and cursor for obtaining the most accurate Spectral Doppler recording. The pitch of the sound will increase as blood accelerates until the peak velocity is attained. Deceleration of blood flow returns the pitch to a lower frequency and the operator will hear a rumbling sound.
The Doppler audio signal isn’t the actual sound of the heart. It is the result of the ultrasound frequency being increased or decreased due to the pressure exerted by moving red blood cells. If blood is moving away from the transducer, the frequency of the ultrasound beam is lowered (stretched) and if blood is moving toward the transducer, the ultrasound beam is compressed, thus increasing the frequency. In either case, the amount of change in frequency is proportional to the velocity of the object moving within the ultrasound beam.
This brings the question, “If blood moving away from the transducer lowers the frequency, why does it still sound higher-pitched on the ultrasound machine?” The machine is designed to invert the signal for blood flow away from the transducer so that regardless of the direction, a higher pitched sound corresponds to higher velocity on either side of the baseline.
Sonographers who are just starting in the field may experience some confusion about the importance of Doppler audio in an ultrasound exam. Some experienced sonographers turn the audio volume up very high, while others will turn it down very low. Rarely, a sonographer is observed with the audio turned-off completely.
While individual preferences vary, the sonographer should know that audio sound is very important for finding the best positioning of the Doppler cursor. While the spectral Doppler recording does show the velocity profile and an operator could perform Doppler without it, the human ear is much more sensitive to the audio output than the spectral Doppler graph, which is actually a compressed interpretation of the frequency shifts. This point could be illustrated by asking a driver of an automobile if they would prefer to drive with or without the sense of hearing? The obvious choice is to drive with the sense of hearing! And it is the same with Doppler ultrasound examinations. Spectral Doppler and Doppler audio are inseparable for a well-done ultrasound examination.