Spectral Doppler : What It Is And How’s It Used In Ultrasound!

The use of Doppler is the backbone of what we do in the world of ultrasound. There are two major types of Doppler that we use in our line of work.  One is spectral Doppler and the other is Color Doppler. And if you’ve spent any time around an echo lab or ultrasound lab, then you’ve definitely heard about these two types of Doppler.

In this article, we’ll be talking specifically about spectral Doppler.  I’ll explain what it is and how we use it in echocardiography.

What Is Spectral Doppler?

Spectral Doppler is unique in the sense that it displays blood flow measurements graphically, as opposed to just visually like with color Doppler or just audibly.  

With spectral Doppler, not only is the velocity of blood flow displayed on a graph, but it also shows the velocity of blood flow spread over time.  This graphing of the blood flow plays a very important role in echocardiography as well as other modalities of ultrasound.

How Does Spectral Doppler Work?     

So you might be wondering how exactly spectral Doppler works. Here’s a quick and easy run down of the way spectral Doppler works, including some of the physics behind it.

High frequency sound waves are sent into the body via a ultrasound transducer. Some of these sound waves reflect off of the red blood cells and travel back to the transducer where the frequencies are processed.

As you know, red blood cells are constantly moving all at different speeds and directions. And you can imagine the “messy signal” of soundwaves, or frequencies, that make it back to the transducer.

Enter Spectral Analysis.  Spectral analysis is the process that breaks down the incredibly complex signal that’s been received and separates all of them out and puts them back together in a nice and neat graph.  Displayed on this graph are all the individual velocities that make up the reflected Doppler signal.

The most common method of spectral analysis is the  Fast Fourier Transform, or FFT. Autocorrelation is another, however it is used to analyze Doppler signals for color Doppler.

Kinds Of Spectral Doppler Ultrasound

In ultrasound, there are two types of spectral Doppler that we use.  They are Pulsed Wave Doppler (PW Doppler) and Continuous Wave Doppler (CW Doppler). As I mentioned, both PW and CW Doppler are types of spectral Doppler.  And each of them serves very important roles of their own.

Pulsed Wave Doppler (PW)

PW Doppler is a type of spectral Doppler that gives us, the user, an option to measure blood flow velocities at a very specific location, or in a given sample.  The specific area in which the velocity of the blood flow is measured is called the “sample volume” or “range gate”.

With pulsed wave Doppler, a pulsed signal is emitted into the body via a transducer to a specific depth, where you want to measure the blood flow velocity.  The depth of this sample volume or range gate is determined by the sonographer.

The downside to PW Doppler is that it has a maximum range of velocities it can measure.  This is due to the fact that the tranducer waits to hear back from the pulse of sound it sent before sending another pulse of sound. Otherwise aliasing will occur and the accuracy of the measurements are lost.

Continuous Wave Doppler (CW)

CW Doppler is the other kind of spectral Doppler.  CW Doppler plays a completely different role than PW Doppler.  

Where PW measures the velocity at a very specific point, CW Doppler measures velocities along the entire path of the ultrasound beam.  

With CW Doppler, the sonographer is able to direct the ultrasound beam to any location. But what’s displayed on the screen, are all of the velocities along that individual beam.  

While PW waits to hear back from the pulse of sound it sent before sending another, in CW Doppler, the pulses of sound are constantly, or continuously, being sent and received. Because of this constant sending and receiving, we are able to measure very high frequencies, such as aortic stenosis or mitral stenosis.

The downside, however, is that the sonographer is given all the velocities along the ultrasound beam, making it impossible to measure the velocity at a given location.

Conclusion

In short, this is the simple definition of spectral Doppler.  Remember, pulsed wave Doppler is excellent for measuring in specific locations (LVOT velocity) and continuous wave Doppler is great for accurately measuring those high maximum velocities (peak AV velocity).