Doppler Ultrasound Flashcards
What is the doppler effect?
Change in frequency due to relative motion of source and receiver
What happens to the wavelengths ahead of a source?
The wavelengths decrease ahead and increase behind
What is considered in order to derive the doppler equation?
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
What is the relative velocity of the wavefronts and the scatterer?
c_0 + v
v = scatterer velocity
What is the sound wavelength?
λ = c_0 / f
What is the frequency of the wavefronts hitting the scatterer?
f ‘ = (c_0 + v) f / c_0
What is the wavelength of the reflected wave?
λ’’ = c_0 - v / f ‘ = (c_0 / f) c_0 - v / c_0 + v
What is the frequency of the scatterer moving towards transducer?
f ‘’ = c_0 / λ ‘’ = f (c_0 + v) / (c_0 - v)
What is the doppler frequency shift?
f_d = f ‘’ - f = 2 f v / c_0 - v
What is the doppler frequency shift if reflector is moving towards transducer?
f_d ≈ 2 f v / c_0
What is the doppler frequency shift if reflector is moving away from transducer?
f_d ≈ - 2 f v / c_0
What is the doppler equation if scatterer moves at angle to transducer axis?
f_d = ± 2 f v cos θ / c_0
What is the effect of increasing the angle of incidence on doppler shift?
decreases the doppler shift
What happens if the beam is at normal incidence?
No frequency shift (no Doppler signal)
In practice what is the size of the insonation angle?
insonation angle should be less than 60 degrees
What is the continuous wave (CW) Doppler?
Continuous ultrasound wave emitted at a single frequency. Two transducers with overlapping beams required for transmit and receive
What are CW doppler made using?
Made using air-backed high-Q transducers
What is the first step in Doppler signal processing?
Demodulation: remove transmit frequency from received signal to give Doppler signal
What is the second step in Doppler signal processing?
High pass (wall) filter: remove the (slow) tissue motion to highlight the signal from blood (low frequencies removed)
What is the third step in Doppler signal processing?
Frequency estimation: estimate the Doppler shift and calculate and display flow velocity (or auditory output)
What type of imaging produces an audio output?
CW doppler
If wave is travelling towards motion of blood flow, what happens to frequency?
There is no frequency shift
What does the received signal contain from CW doppler?
Signal at MHz frequencies with a very small Doppler shift (~Hz-kHz) encoded on it (demodulation removes this doppler shift)
What is a simple way to extract Doppler signal?
To multiply and filter the received signal
What does the high-pass wall filter do?
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
What is the filter cut off?
between 50 Hz and 300 Hz
What are the set backs of CW Doppler?
Doesn’t provide depth resolution as ultrasound wave is transmitted continuously (can’t see deep vessels)
What is Pulsed Wave (PW) Doppler and what does it overcome?
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
What are the signals shifted by for PW Doppler and what is the moved distance of scatterer?
Δt
c_0 Δt / 2
What is the speed of the scatterer for PW Doppler?
v = c_0 Δt / 2τ
What is the time delay and subsequent phase shift for PW Doppler?
Δt = 2 v τ / c_0
ΔΦ = ωΔt = 2ωvτ / c_0
What is the rate of change of phase in angular frequency for PW Doppler?
ω_pw = ΔΦ/t = 2ωv / c_0
What is the final frequency shift in PW Doppler?
f_pw = ω_pw/2π = 2 f v /c_0
What is the Doppler scan line?
The Doppler signal from multiple range gates
What does each PW Doppler scan-line require?
multiple transmit pulses which reduces frame rate significantly
What is done to improve frame rates of PW Doppler?
Doppler information only calculated within a small colour box in the B-mode image
How is the velocity in the Colour doppler image calculated?
From mean Doppler frequency, colour-coded for magnitude and direction
What does the doppler image in colour box show?
Gives dynamic information about blood flow
red = flow towards transducer (positive Doppler signal)
blue = away from transducer (negative Doppler signal)
What will show velocity changing direction?
Curvilinear array in transducer
What does Power Doppler show?
Integral of power spectrum across frequency
Not affected by aliasing, but no velocity or directional information
Gives increased sensitivity for imaging small vessels
What is Directional Power Doppler?
Power Doppler with directional capability of Colour Doppler
What are the intervals sampled by a Doppler signal in PW Doppler?
τ = 1/ PRF
PRF = pulse repetition frequency
What happens if the PRF is too low?
Estimation of doppler frequency will be incorrect due to aliasing
Due to insufficient number of samples per cycle of Doppler signal
What can Spectral Doppler be used to show?
Flow (insonation) angle can be defined to get velocity
What does the spectral Doppler signal (within range gate) look like when aliasing happens?
Positive Doppler frequency shifts beyond v_max are aliased to negative values (end of signal appears in opposite plane)
What is Spectral Doppler used for?
To identify a stenosis
Difference in flow conditions leads to spectral broadening shows stenosis
What can blood velocity be measured with?
Doppler imaging modes
