Doppler Ultrasound Flashcards
State the Doppler Equation.
f_d=[2*f_t*v*cos(theta)]/c f_d=f_r-f_t f_d = Doppler shift frequency f_t = transmitted frequency f_r = received frequency v = velocity of target c = speed of sound in medium theta = angle between beam and target motion
Briefly explain how a CW-Doppler probe measures velocity.
- 1 element continuously emits, 1 continuously receives.
- Emitted and received signals are mixed together and high passed filtered to remove the signal from the ultrasound.
- Remaining frequencies arise from Doppler shift caused by moving targets.
- velocity is calculated using Doppler equation, 1540 SoS, and user defined angle correction.
Explain how quadrature demodulation gives directional information from the Doppler signal.
- Quadrature signal created by phase shifting transmitted signal by pi/2.
- Quadrature and in-phase signal both separately mixed with received signal to create imaginary and real components of signal respectively.
- After filtering the relative phase on the real to imaginary signals will depend on whether the target tissue velocity is positive or negative.
Explain how directional information can be sent to audio speakers in CW-Doppler.
- Forward flow can be sent by mixing the imaginary part with the pi/2 phase shifted real part.
- Reverse flow can be sent by mixing the real part with the pi/2 phase shifted imaginary part.
Explain what the “Wall Thump” filter is, and its purpose in CW-Doppler.
- Scattering coefficients of wall tissues are 10-100 times larger than those of RBCs.
- Wall motion is much slower, and so has lower Doppler frequency.
- High-pass filtering the Doppler signal (once demodulated) removes the low velocity motion of the walls, leaving only the blood signal.
How is a Doppler spectrogram formed from the Doppler signal?
- Fast-Fourier transform of the signal will give the frequency peaks of the signal.
- The frequency of the peak will give the velocity of the target, and the height of the peak (Power density) will give the number of targets at that velocity.
- Pixels at that velocity for that time will be made brighter proportional to their power density.
What are the main differences between PW and CW Doppler?
- CW Doppler has long sensitive region that is a function of the probe geometry and, generally not, user definable.
- CW-Doppler is more sensitive than PW and doesn’t suffer from aliasing.
- PW-Doppler has a sensitive region that is definable for both depth and length.
What are the two methods for assessing velocity using PW-Doppler?
- Phase-Domain
- Time-Domain
Explain how phase domain systems measure velocity in PW-Doppler.
- Similar to CW demodulation, but the short duration of the received wave compared to the reference signal means that the frequency cannot be accurately measured.
- The amplitude of the mixed signal will vary as the phase of the received wave varies. This varying amplitude varies at the Doppler frequency, and can be measured,
Why does “Downshifting” in PW-Doppler mean that the real Doppler effect is not measured?
- Attenuation of ultrasound in tissue increases with frequency.
- Higher frequencies are attenuated faster, and so the pulses are downshifted as they propagate, which is greater than the Doppler shift.
How does a time-domain system measure velocity?
- RF data is interrogated and the time difference between the same echo allows the determination of the distance traveled.
- Time between the the pulses (1/PRF=PRI) is known and so the velocity can be calculated.
How would simultaneous Doppler be performed for a small depth and low PRF?
- B-mode pulse sent out after each Doppler pulse.
How would simultaneous Doppler be performed for a high PRF?
- A few B-mode pulses after a large group of Doppler pulses.
How would simultaneous Doppler be performed for a large depth and low PRF?
- Gaps in Doppler trace filled by interpolation.
Explain what is meant by intrinsic spectral broadening.
- Finite beam width will focus on the target at different angles.
- Difference in actual angle and angle correct will cause variation in measured velocities.
- This range increases with increasing Doppler angle.
