doppler instruments

types of wave doppler

• continuous wave doppler

• spectral doppler (duplex)

• spectral and color doppler (triplex)

• power doppler

pulse wave (PW) and continuous wave (CW) both present

doppler shift information in visual form (spectral display) and in audible form

an analog spectral display shows:

• displayed as a graph

• horizontal axis: time

• vertical axis: velocity

a digital spectral display shows:

• above baseline: positive doppler shift, toward the transducer

• below baseline: negative doppler shift, away from transducer

aliasing

occurs when doppler shift information exceeds the Nyquist limit

aliasing results in

the highest velocity amplitudes being displayed on the spectral analysis as heading in the opposite direction of flow

what are the 5 ways you can adjust spectral aliasing?

• adjust PRF (velocity) scale

• use baseline shift

• use a lower frequency transducer

• use a CW transducer

• select a new ultrasonic view with a shallower sample volume

aliasing - adjust PRF (velocity) scale

• raising the scale, raises the PRF

• advantages:

  • raises the Nyquist limit

• disadvantages:

  • less sensitive to low velocities so venous flow may disappear on image

  • aliasing artifact may persist when extremely high velocities are present

aliasing - baseline shift

• shift the baseline so the entire velocity scale is devoted to one direction

• advantages:

  • displays the high velocity flows in the proper direction

• disadvantages:

  • if using speaker, the audio signal will still arise from the incorrect speaker

  • if velocity is very high, there will still be wraparound

aliasing - use a lower frequency transducer

• advantages:

  • doppler shift is directly related to transducer frequency

  • lower frequency sound produces lower frequency doppler shifts which means less likely to exceed the Nyquist limit

• disadvantages:

  • lower image resolution (axial and lateral resolutions degrade)

aliasing - use a different ultrasonic window

• using a different ultrasonic window with a shallower sample volume depth

• advantages:

  • higher PRF which increases the Nyquist limit

• disadvantages:

  • none

aliasing - CW transducer

• change to continuous wave doppler

• advantages:

  • no aliasing occurs with CW doppler

• disadvantages:

  • range ambiguity

Nyquist limit/Nyquist frequency

the highest doppler frequency or velocity that can be measured without the appearance of aliasing

the sonographer can adjust the Nyquist limit by

manipulating the PRF

if blood is flowing towards the transducer (positive shift), will it be above or below the baseline?

above the baseline

if blood is flowing away from the transducer (negative shift), will it be above or below the baseline?

below the baseline

nondirectional doppler

• determines only the presence of moving blood by detecting doppler shift

• used to assess fetal HR during labor and delivery

crossing detector

• provides an estimate of doppler signal frequency vs time

• output may be to a chart recorder or spectral display

bidirectional doppler

• distinguishes flow toward or away from the transducer signal

• requires stereo headphones or speakers and/or graph

doppler detector

contains demodulator and phase quadrature detector

demodulation

the process of extracting the lower frequency from the higher frequency to determine the doppler shift

phase quadrature detector

determines whether blood was flowing toward or away from the transducer based on the doppler shift

continuous wave doppler

• simplest form

• uses separate transmit and receive crystals that continuously transmit and receive ultrasound

• able to detect presence and direction of blood flow

• range ambiguity

• portable and inexpensive

range ambiguity

unable to distinguish signals arising from vessels at different depths — we don’t know exactly where the signal came from

in CW doppler, the frequency of the emitted ultrasound wave is determined by

the frequency of the stimulating electrical current

advantages of CW doppler

can measure very high velocities with no aliasing

disadvantages of CW doppler

• range ambiguity (no depth localization)

• lack of TGC

what shape is a waveform from CW doppler?

bullet-shaped

characteristics of CW doppler transducers

• contains only 2 crystals

• cannot perform 2D imaging

• no damping/backing material

• very sensitive

advantages of PW doppler

• select exact location of doppler interrogation

• duplex imaging is possible

• measures peak velocity

disadvantages of PW doppler

• difficult to measure high velocities

• aliasing can occur

characteristics of PW doppler transducers

• only 1 crystal is needed

• sample volume (gate) position determines location for sampling

range gate

incorporated into the instrument, which represents the time during which the ultrasound machine “listens” to the returning echoes