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Biomedical Ultrasound > Doppler Ultrasound > Flashcards

Doppler Ultrasound Flashcards

1
Q

What is the doppler effect?

A

Change in frequency due to relative motion of source and receiver

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2
Q

What happens to the wavelengths ahead of a source?

A

The wavelengths decrease ahead and increase behind

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3
Q

What is considered in order to derive the doppler equation?

A

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

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4
Q

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

A

c_0 + v

v = scatterer velocity

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5
Q

What is the sound wavelength?

A

λ = c_0 / f

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6
Q

What is the frequency of the wavefronts hitting the scatterer?

A

f ‘ = (c_0 + v) f / c_0

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7
Q

What is the wavelength of the reflected wave?

A

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

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8
Q

What is the frequency of the scatterer moving towards transducer?

A

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

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9
Q

What is the doppler frequency shift?

A

f_d = f ‘’ - f = 2 f v / c_0 - v

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10
Q

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

A

f_d ≈ 2 f v / c_0

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11
Q

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

A

f_d ≈ - 2 f v / c_0

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12
Q

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

A

f_d = ± 2 f v cos θ / c_0

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13
Q

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

A

decreases the doppler shift

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14
Q

What happens if the beam is at normal incidence?

A

No frequency shift (no Doppler signal)

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15
Q

In practice what is the size of the insonation angle?

A

insonation angle should be less than 60 degrees

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16
Q

What is the continuous wave (CW) Doppler?

A

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

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17
Q

What are CW doppler made using?

A

Made using air-backed high-Q transducers

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18
Q

What is the first step in Doppler signal processing?

A

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

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19
Q

What is the second step in Doppler signal processing?

A

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

20
Q

What is the third step in Doppler signal processing?

A

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

21
Q

What type of imaging produces an audio output?

A

CW doppler

22
Q

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

A

There is no frequency shift

23
Q

What does the received signal contain from CW doppler?

A

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

24
Q

What is a simple way to extract Doppler signal?

A

To multiply and filter the received signal

25
Q

What does the high-pass wall filter do?

A

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

26
Q

What is the filter cut off?

A

between 50 Hz and 300 Hz

27
Q

What are the set backs of CW Doppler?

A

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

28
Q

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

A

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

29
Q

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

A

Δt

c_0 Δt / 2

30
Q

What is the speed of the scatterer for PW Doppler?

A

v = c_0 Δt / 2τ

31
Q

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

A

Δt = 2 v τ / c_0

ΔΦ = ωΔt = 2ωvτ / c_0

32
Q

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

A

ω_pw = ΔΦ/t = 2ωv / c_0

33
Q

What is the final frequency shift in PW Doppler?

A

f_pw = ω_pw/2π = 2 f v /c_0

34
Q

What is the Doppler scan line?

A

The Doppler signal from multiple range gates

35
Q

What does each PW Doppler scan-line require?

A

multiple transmit pulses which reduces frame rate significantly

36
Q

What is done to improve frame rates of PW Doppler?

A

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

37
Q

How is the velocity in the Colour doppler image calculated?

A

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

38
Q

What does the doppler image in colour box show?

A

Gives dynamic information about blood flow

red = flow towards transducer (positive Doppler signal)

blue = away from transducer (negative Doppler signal)

39
Q

What will show velocity changing direction?

A

Curvilinear array in transducer

40
Q

What does Power Doppler show?

A

Integral of power spectrum across frequency

Not affected by aliasing, but no velocity or directional information

Gives increased sensitivity for imaging small vessels

41
Q

What is Directional Power Doppler?

A

Power Doppler with directional capability of Colour Doppler

42
Q

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

A

τ = 1/ PRF

PRF = pulse repetition frequency

43
Q

What happens if the PRF is too low?

A

Estimation of doppler frequency will be incorrect due to aliasing

Due to insufficient number of samples per cycle of Doppler signal

44
Q

What can Spectral Doppler be used to show?

A

Flow (insonation) angle can be defined to get velocity

45
Q

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

A

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

46
Q

What is Spectral Doppler used for?

A

To identify a stenosis

Difference in flow conditions leads to spectral broadening shows stenosis

47
Q

What can blood velocity be measured with?

A

Doppler imaging modes

Biomedical Ultrasound (21 decks)

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