PULSED-WAVE DOPPLER

PW doppler gives information about velocity, direction and flow over time at a specific location. This location is set by the sample gate.

THE SAMPLE GATE

The PW line of sight has a gate. It is called a sample gate or sample volume. Once you put the PW line down, move this gate up or down to the exact location you want to sample. The gate is usually set at about 3-4mm. The narrower the gate, the more precise the information, because the blood cells in that narrow area are likely moving in the same direction at the same speed. 

SPECTRAL TRACE

The spectral trace is what is presented in the bottom half of the screen. The horizontal line is the baseline and sits at 0cm/s. A trace below the line is showing flow away from the transducer and a trace above the line is showing flow towards the probe. Depending on the direction of the slow, you can move the baseline up or down to capture the entire trace. 

The spectral trace of PW doppler is hollow in the middle because of the narrow sample gate. Being off axis to the flow can give a wider trace (spectral broadening). 

NYQUIST LIMIT

These are the numbers on either end of the colour spectrum. It represents the maximum velocity threshold, above which the machine will be unable to give an accurate reading on flow direction. Positive number: velocity of flow towards the transducer; negative number: velocity of flow away from the transducer.

PW doppler annotated

OPTIMAL SPECTRAL TRACE

A really good PW doppler trace should have a thin leading line, the middle of the trace should be empty. This is how you know you are on axis. It means the majority of moving blood cells are moving at the same velocity. 

Thick, shaggy line, filled in centre space: not good enough

PW doppler with optimal imaging: thin leading line, black centre space

SAMPLING ERROR

PW is read in pulses. The rate of sampling is set by the Pulse Rate Frequency (PRF). This means that the returning doppler shift frequency from the moving blood cells is read by the machine at specific intervals. 

When you adjust the scale on the PW doppler, what you are doing is increasing or decreasing the PRF. On the screen it appears as a change to the Nyquist limit. A lower PRF gives you a lower Nyquist limit, a higher PRF gives you a higher Nyquist limit. 

This reading in pulses is why PW doppler can misread the direction of flow at high velocities. If the doppler shift frequency is greater than half the pulse rate frequency: sampling error will occur potentially leading to aliasing.

The video below shows this in action. The film has a frame rate (ie PRF) of 24 frames per second. But the wheel has different numbers of squares in each circle meaning that the squares in some of the circles will move faster or slower than others: ie some circles will have a higher or lower frequency of squares passing a certain point. Given the frame rate is fixed (similar to PRF), the image on the screen appears to have some circles on the disc going forwards, backwards or staying static. 

Similarly PW doppler samples the returning frequencies at a static PRF. If the PRF is less than twice the doppler shift frequency, it may misread the direction of flow. 

Optical Illusion: The Wagon Wheel Effect (Aliasing) - Jesse Mason You Tube

ALIASING

In PW doppler this effect is called aliasing. You will see a trace presented on the screen with the tip missing. The tip will be at the top of the screen going in the opposite direction. 

The PW sample gate is at the aortic valve. There is significant AS. So the flow velocity is higher than the Nyquist limit of the PW doppler so flow is seen above the baseline (ie towards the probe)

HOW TO PREVENT ALIASING

1. Move the baseline up or down to capture the entire wave

2. Increase the scale to increase the Nyquist limit

3. Image in another plane where the sample gate can be more superficial

4. Change to a lower frequency

5. Change to CW doppler 

PW doppler optimal imaging

1. Move the baseline up or down to capture the entire wave

2. Increase the scale to increase the Nyquist limit

3. Image in another plane where the sample gate can be more superficial

4. Change to a lower frequency

5. Change to CW doppler 

READING THE SPECTRAL TRACE

The spectral trace is a reading of the velocity and direction of flow over time. The PW trace gives you enormous information about the flow. But it depends what you're are looking at. 

Vascular

PW doppler allows categorisation of the type of flow and the degree of stenosis:

In low resistance vessels, flow should occur during systole and diastole. 

PW doppler testis (low resistance flow): Arterial flow during systole and diastole is normal. A lack of diastolic flow indicates an increase in resistance ? torsoin/ oedema

High resistance vessels eg MSK, only has flow during systole. 

PW doppler of brachial artery

A lack of diastolic flow in a low resistance vessel indicates stenosis. 

Severe stenosis causes flow reversal during diastole. 

Valvular

PW of flow across valves depends on the valve being interrogated. For instance, PW of the MV gives information regarding LV diastolic dysfunction or inflow variability with respiration as a marker of tamponade. 

PW flow across the PV gives information regarding peak velocity and time to peak velocity important for differentiating acute vs chronic RV pressure overload. 

PW doppler flow across the PV

CONTINUOUS WAVE

This is where the doppler shift velocity is read continuously by the machine. So there is no Nyquist limit, Aliasing is not an issue. Use CW when determining the velocities of stenotic or regurgitant valves. It will easily read velocities of 3-4m. But because there is no sample gate, the machine reads all velocities arriving along that line. So say you are measuring the AV velocity, significant MR, may interfere with the reading. 

The spectral trace of CW doppler is filled in because there is no sample gate. 

But you still get information regarding peak velocity, time to peak velocity and the character of the flow. 

For instance, the typical high peak velocity dagger shape of flow across the AV in HOCM. 

CW doppler through the AV in HOCM