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

duplex ultrasound scanning uses:

• B-mode

• pulse-doppler

measurement of doppler frequency is dependent on:

• direction of blood flow

• direction the sound wave propagates

doppler spectrum assessment - you must assess the following:

• presence of flow - sensitivity

• direction of flow

• amplitude

• window

• pulsatility

doppler spectrum assessment - sensitivity improvement

• increase power or gain

• decrease velocity scale

• decrease the reject or filter

• slowly increase the SV (sample volume ) size/range gate]

  • range gate must be 1/3 the diameter of the lumen

doppler spectrum assessment - direction of flow

• pulsed doppler uses phase detection to provide bidirectional doppler info

• flow can be: monophasic, biphasic, triphasic, bidirectional

monophasic flow

flow on just one side of the baseline (does not cross the baseline)

biphasic flow

flow starts on one side of the baseline and then crosses to the other side (crosses baseline once)

triphasic flow

flow starts on one side of the baseline, then crosses to the other side, then returns back to the original side of the baseline (crosses baseline twice)

bidirectional flow

flow which occurs simultaneously on both sides of the baseline

doppler spectrum assessment - amplitude

the spectrum displays echo amplitude by varying the brightness of the display

the amplitude of the echoes are determined by:

• echo intensity

• power

• gain

• dynamic range

doppler spectrum assessment - window

narrow range of frequencies will result in a narrow display line and the clear underneath the spectrum is the window

a thicker the line on the spectral display will represent a wide or narrow range of receiving frequencies?

wide range of receiving frequencies

a narrow range of frequencies will show what kind of sonic window?

a large sonic window

spectral analysis

a tool that breaks the complex signal into its basic building blocks

what are the 2 things spectral analysis does?

• portrays the concentration of RBCs based on shading

• identifies individual velocities making up the reflected doppler signal

spectral analysis of plug flow shows

a narrow range of velocities

spectral analysis of parabolic flow/laminar flow shows

a wide range of velocities

spectral analysis of turbulent and disturbed flow shows

a very wide range of velocities

what are the 2 methods of analyzing flow/spectral analysis?

• fast fourier transform (FFT)

• autocorrelation (color doppler)

fast fourier transform (FFT)

• a digital mathematical technique used to process pulsed and continuous-wave doppler signals

• analyzes returning signals in order to produce spectral displays

advantages of FFT

• exceedingly accurate

• displays all individual velocity components that make up the complex reflected signal

if a vessel has a high resistance, there will be

forward flow in systole with flow reversal in early diastole and no flow in late diastole

if a vessel has a low resistance, there will be

unidirectional flow (forward flow) in systole and diastole

resistance depends on

the organ that the vessel is supplying

laminar flow on spectral display

• narrow

• well-defined spectral trace

• clear spectral window

turbulent flow on spectral display

• spectral window is completely lost

• spectral broadening

• reversed systolic component

• erratic flow

disturbed flow on spectral display

• spectral broadening

• filling in of spectral window

spectral broadening

vertical thickening of the spectral trace, often indicative of pathology

narrow spectral tracings are generally seen in:

large diameter vessels

broad spectral tracings are seen in:

small vessels

high pulsatility/resistance waveform

• triphasic

• sharp systolic peak

• reversed diastolic flow

low pulsatility/resistance waveform

• broad systolic peak

• forward flow in diastole

mixed pulsatility/resistance waveform

• sharp systolic peak

• forward flow in diastole

diagnostic indices

mathematical formulas developed to describe the pulsatile nature of the arterial doppler waveform and/or the amount of downstream resistance

what are the 2 indices?

• resistivity index (RI)

• pulsatility index (PI)

resistivity index (RI)

a quantitative doppler derived measurement of the resistance present within a vascular segment

pulsatility index (PI)

a quantitative doppler derived assessment of the pulsatile nature of the doppler waveform in a vessel segment