PW and CW spectral DOPPLER

Echocardiography Textbook


Spectral Doppler Information:

  • Flow velocity.
  • Flow direction.
  • Signal time.
  • Signal intensity.



The flow velocity is plotted on the y-axis. When there is no flow, only the baseline is visible on the display.

The direction of flow is also shown on the y-axis. Flows directed towards the transducer are located on top of the baseline, flows that are directed away from the transducer are located at the bottom of the baseline.

The Signal intensity is shown as the brightness of the stream. A brighter Spectrum corresponds to higher intensity (many red cells are moving in this area) and vice versa.

The signal Time is plotted on the x-axis.


When examining from a Pulsed Wave Doppler (PW), the same piezoelectric element sends and receives a signal. With a continuous wave Doppler (CW), one piezoelectric element sends a signal, and the other acts as a receiver.

The place where the signal is analyzed is called the Sample Volume.



When switching to the PW or CW mode, the Sample volume will appear on the screen. It can be moved to any position where we want to study the characteristics of the flow.

The point where the Sample volume is located is called the Baseline.

The superiority of PW over CW is the depth resolution that CW does not have. In PW mode, it is possible to study the flow characteristics at the Sample Volume point at different depths.


Nyquist limit

The main disadvantage of pulse wave (PW) doppler versus continuous wave (CW) is the existence of the Nyquist limit.

The maximum speed that can be learned in PW mode is called the Nyquist limit. If velocity is higher than this limit spectrum will be distorted.

Nyquist limit = PRF/2

PRF - Pulse repetition frequency



PW doppler examination. Velocity is higher than Nyquist limit and spectrum is distorted



The same flow by CW doppler


In CW mode, it is possible to examine the flow at any speed. The main disadvantage of CW Doppler is that it has no depth resolution.


Spectral Doppler (PW, CW) Examinations




How to Simulate PW mode?

How to Simulate CW mode?