Doppler Ultrasound

Question 1

What is the doppler effect?

Answer 1

Change in frequency due to relative motion of source and receiver

Question 2

What happens to the wavelengths ahead of a source?

Answer 2

The wavelengths decrease ahead and increase behind

Question 3

What is considered in order to derive the doppler equation?

Answer 3

Consider a red blood cell (a scatterer) moving towards a transducer emitting continuous wave (CW) ultrasound

What is the frequency of the reflected sound measured at the transducer

Question 4

What is the relative velocity of the wavefronts and the scatterer?

Answer 4

c_0 + v

v = scatterer velocity

Question 5

What is the sound wavelength?

Answer 5

λ = c_0 / f

Question 6

What is the frequency of the wavefronts hitting the scatterer?

Answer 6

f ‘ = (c_0 + v) f / c_0

Question 7

What is the wavelength of the reflected wave?

Answer 7

λ’’ = c_0 - v / f ‘ = (c_0 / f) c_0 - v / c_0 + v

Question 8

What is the frequency of the scatterer moving towards transducer?

Answer 8

f ‘’ = c_0 / λ ‘’ = f (c_0 + v) / (c_0 - v)

Question 9

What is the doppler frequency shift?

Answer 9

f_d = f ‘’ - f = 2 f v / c_0 - v

Question 10

What is the doppler frequency shift if reflector is moving towards transducer?

Answer 10

f_d ≈ 2 f v / c_0

Question 11

What is the doppler frequency shift if reflector is moving away from transducer?

Answer 11

f_d ≈ - 2 f v / c_0

Question 12

What is the doppler equation if scatterer moves at angle to transducer axis?

Answer 12

f_d = ± 2 f v cos θ / c_0

Question 13

What is the effect of increasing the angle of incidence on doppler shift?

Answer 13

decreases the doppler shift

Question 14

What happens if the beam is at normal incidence?

Answer 14

No frequency shift (no Doppler signal)

Question 15

In practice what is the size of the insonation angle?

Answer 15

insonation angle should be less than 60 degrees

Question 16

What is the continuous wave (CW) Doppler?

Answer 16

Continuous ultrasound wave emitted at a single frequency. Two transducers with overlapping beams required for transmit and receive

Question 17

What are CW doppler made using?

Answer 17

Made using air-backed high-Q transducers

Question 18

What is the first step in Doppler signal processing?

Answer 18

Demodulation: remove transmit frequency from received signal to give Doppler signal

Question 19

What is the second step in Doppler signal processing?

Answer 19

High pass (wall) filter: remove the (slow) tissue motion to highlight the signal from blood (low frequencies removed)

Question 20

What is the third step in Doppler signal processing?

Answer 20

Frequency estimation: estimate the Doppler shift and calculate and display flow velocity (or auditory output)

Question 21

What type of imaging produces an audio output?

Answer 21

CW doppler

Question 22

If wave is travelling towards motion of blood flow, what happens to frequency?

Answer 22

There is no frequency shift

Question 23

What does the received signal contain from CW doppler?

Answer 23

Signal at MHz frequencies with a very small Doppler shift (~Hz-kHz) encoded on it (demodulation extracts this doppler shift)

Question 24

What is a simple way to extract Doppler signal?

Answer 24

To multiply and filter the received signal

Question 25

What does the high-pass wall filter do?

Answer 25

It removes the signal from moving tissue (which produces a high amplitude) which adds to Doppler signal as well as stationary tissue and blood moving at different speeds

Question 26

What is the filter cut off?

Answer 26

between 50 Hz and 300 Hz

Question 27

What are the set backs of CW Doppler?

Answer 27

Doesn't provide depth resolution as ultrasound wave is transmitted continuously (can't see deep vessels)

Question 28

What is Pulsed Wave (PW) Doppler and what does it overcome?

Answer 28

Transducer emits short pulses at times 0 and τ (ms) The returned echoes can be localised by extracting a segment of the signal using a range gate it overcomes lack of depth resolution of CW Doppler

Question 29

What are the signals shifted by for PW Doppler and what is the moved distance of scatterer?

Answer 29

Δt c_0 Δt / 2

Question 30

What is the speed of the scatterer for PW Doppler?

Answer 30

v = c_0 Δt / 2τ

Question 31

What is the time delay and subsequent phase shift for PW Doppler?

Answer 31

Δt = 2 v τ / c_0 ΔΦ = ωΔt = 2ωvτ / c_0

Question 32

What is the rate of change of phase in angular frequency for PW Doppler?

Answer 32

ω_pw = ΔΦ/t = 2ωv / c_0

Question 33

What is the final frequency shift in PW Doppler?

Answer 33

f_pw = ω_pw/2π = 2 f v /c_0

Question 34

What is the Doppler scan line?

Answer 34

The Doppler signal from multiple range gates

Question 35

What does each PW Doppler scan-line require?

Answer 35

multiple transmit pulses which reduces frame rate significantly

Question 36

What is done to improve frame rates of PW Doppler?

Answer 36

Doppler information only calculated within a small colour box in the B-mode image

Question 37

How is the velocity in the Colour doppler image calculated?

Answer 37

From mean Doppler frequency, colour-coded for magnitude and direction

Question 38

What does the doppler image in colour box show?

Answer 38

Gives dynamic information about blood flow red = flow towards transducer (positive Doppler signal) blue = away from transducer (negative Doppler signal)

Question 39

What will show velocity changing direction?

Answer 39

Curvilinear array in transducer

Question 40

What does Power Doppler show?

Answer 40

Integral of power spectrum across frequency Not affected by aliasing, but no velocity or directional information Gives increased sensitivity for imaging small vessels

Question 41

What is Directional Power Doppler?

Answer 41

Power Doppler with directional capability of Colour Doppler

Question 42

What are the intervals sampled by a Doppler signal in PW Doppler?

Answer 42

τ = 1/ PRF PRF = pulse repetition frequency

Question 43

What happens if the PRF is too low?

Answer 43

Estimation of doppler frequency will be incorrect due to aliasing Due to insufficient number of samples per cycle of Doppler signal

Question 44

What can Spectral Doppler be used to show?

Answer 44

Flow (insonation) angle can be defined to get velocity

Question 45

What does the spectral Doppler signal (within range gate) look like when aliasing happens?

Answer 45

Positive Doppler frequency shifts beyond v_max are aliased to negative values (end of signal appears in opposite plane)

Question 46

What is Spectral Doppler used for?

Answer 46

To identify a stenosis Difference in flow conditions leads to spectral broadening shows stenosis

Question 47

What can blood velocity be measured with?

Answer 47

Doppler imaging modes