doppler

• doppler shift aka

  doppler frequency
• f(t) = transducer frequency

  V = velocity of blood flow

  cos 0 = cosine of theta (angle of insonation)

  c = propagation speed of sound in media

  A) 2f(t)

  B) V

  C) cos 0

  D) c

  
• what is doppler effect?

  the apparent change in frequency and wavelength of a sound wave caused by the motion of the sound source relative to the observer
• what is doppler shift used for?

  to measure blood velocity = speed + direction
• if received freq is < transmitted freq, what kind of Doppler shift?

  negative
• if received freq is > transmitted freq, what kind of Doppler shift?

  positive
• what's demodulation?

  isolates the frequency shift from the received signal, allowing the Doppler system to determine the direction of blood flow
• speed vs velocity

  speed = magnitude

  velocity = magnitude + direction

  
• according to the Doppler shift equation, which variable changes?

  velocity
• CW Doppler requires 2 crystals
• CW Doppler requires 2 crystals where:

  • 1 continuously transmits the sound wave
  • 1 continuously receives the sound wave
• advantages of CW Doppler

  able to measure high velocities
• disadvantages of CW Doppler

  • lacks TGC
  • no anatomical image - only accurate waveforms
  • range ambiguity
• PW Doppler requires at least 1 crystal
• PW Doppler requires at least 1 crystal where it:• alternates between sending + receiving signals
• advantages of PW Doppler

  range resolution
• disadvantages of PW Doppler

  aliasing
• what's the most common error associated with Doppler ultrasound?

  aliasing
• what's Nyquist limit?

  the maximum velocity that is displayed without aliasing
• formula for Nyquist limit

  Nyquist limit = PRF / 2
• 2 ways to avoid aliasing

  increase Nyquist limit

  reduce Doppler shift
• CW transducers are used to detect:

  • low amplitude waveforms
  • very large Doppler shifts
• CW transducers do not use backing material causing:• narrow bandwidth• high sensitivity
• 2 different color maps can be used:• Velocity• Variance
• wall filter aka

  high pass filter
• Ghosting & Clutter• occurs when anatomy moves (heart contractions, blood vessel wallmovement) creating low frequency Doppler shifts that are picked up adisplayed
• on spectral waveforms, ghosting & clutter is called:

  clutter
• on color Doppler, ghosting & clutter is called:

  ghosting artifact
• power Doppler pros

  • increased sensitivity to low flow (venous flow or small vessels)
  • no aliasing
  • unaffected by Doppler angle
• power Doppler cons

  • no velocity measurement
  • susceptible to flash artifact
  • lower frame rates
• power Doppler is aka

  energy mode or color angio
• crosstalk artifact usually occurs due to

  • PW gain set too high
  • angle of insonation is near 90°