Topic 11: Doppler Ultrasound

Question 1

What is a wall filter?

Answer 1

The high pass filter used to removes frequencies in the CW doppler recieved signal that originate from the motion of the tissue surrounding the blood

Question 2

Explain range gating

Answer 2

range gating is the localisation of doppler signals coming from a particular depth by selecting a particular time range. This is only done in Pulsed-Wave as opposed to Continuous-Wave doppler.

Question 3

Explain aliasing

Answer 3

The frequency at which the pulses are emitted in PW doppler is called the pulse repetition frequency. If the PRF is too low, doppler shift will not be estimated correctly due to aliasing.

Question 4

Describe the advantages and disadvantages of CW doppler

Answer 4

CW - cons:
1 - separate transmit and recieve transducers are required
2 - the spatial resolution is limited to the overlap of the two transducers

CW - pros:
1 - continuous monitoring
2 - no aliasing
3 - cheaper

PW - pros:
1 - can localise a doppler signal coming from a certain depth
2 - forms an image

PW - cons:
Aliasing and not continuous

Question 5

What is the color doppler?

Answer 5

Color doppler: An image of the flow velocity is calculated from a series of PW doppler scan lines. Mean doppler frequency is calculated and color coded for magnitude and direction.
It is displayed in a ‘color box’ overlaid on a greyscale B-mode image.

Question 6

Would the spectral width of the doppler signal increase or decrease immeadiately downstream of a stenosis? Why?

Answer 6

Increase because there are a larger range of velocities.

Question 7

If M-mode imaging already images motion why are the doppler modes necessary?

Answer 7

M-mode imaging images time, the brightness, an axial line in the image, whereas the brightness mode images in space. They have different applications. M-mode imaging is only 1-dimensional.

Question 8

What is spectral doppler?

Answer 8

Spectral doppler: The spectral doppler signal displayed for a single point (gate) over time.

Question 9

What is power doppler?

Answer 9

Power doppler: The integral of the power spectrum across frequency is shown. The images are no longer affected by aliasing but no longer contain speed or directional information.

Question 10

Directional power doppler

Answer 10

As power doppler but the direction of the flow is color coded.

Question 11

Types of doppler imaging in practice?

Answer 11

color doppler
power doppler
directional power doppler
spectral doppler