Avoiding aliasing is every sonographer’s goal in ultrasound imaging to get accurate and reliable information on a patient. There are various ways of evading aliasing that all sonographers must know. Today, we provide in-depth info on the use of continuous wave Doppler.
What is Doppler Effect?
Doppler Effect is the variation of sound frequency due to motion. Blood flowing towards a probe causes an increase in sound frequency. The probe emits sound as it produces and detects sound waves. Its frequency reduces as blood flows away from the probe. Therefore, you can use Doppler shift to detect blood flow velocity and blood flow.
Blood approaching a probe is antegrade while that leaving it is retrograde. In patients, different variables influence the Doppler shift including frequency, insonation angle and the organ through which sound waves travel making continuous wave Doppler qualitative rather than quantitative.
How Continuous Wave Doppler reduces Aliasing?
Aliasing is associated only with pulsed Doppler, it never appears with continuous wave Doppler. A standard continuous wave Doppler has a receiver and a transmitter. The transmitter produces continuous sound waves which the receiver collects. Both elements are positioned at specific angles from one another enabling the receiver to receive reflected ultrasound waves.
The disadvantage of using a continuous wave Doppler is the range ambiguity. With this technique, velocities along the entire region of overlap between the transmit and receive beams, blend together to form the spectrum. With pulsed Doppler, velocities from blood within the sample volume are measured.
Eliminating aliasing “improves the ability to measure the maximum velocity with Doppler”.
Pulsed Doppler exhibits aliasing with high velocity flow
Continuous wave Doppler
never aliases
What are the Benefits of Continuous Wave Doppler?
- The Doppler is portable as it has two piezoelectric crystals that emit and receive waves.
- One can efficiently use a continuous Doppler as it has simple basic principles that you can learn within a short period.
- A typical Doppler doesn’t have an upper limit due to its Nyquist frequency.
