Doppler physics and instrumentation
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Doppler Physics and Instrumentation. Topics. Doppler effect Doppler equation Doppler Modes Doppler Artifacts. Scattering of Ultrasound by RBC’s. Red blood cells Diameter: 7µm Raleigh scatterers Smaller than ultrasound wavelength (0.1-0.7 mm). Scattering of Ultrasound by RBC’s.

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Doppler physics and instrumentation

Doppler Physicsand Instrumentation


Topics

Topics

  • Doppler effect

  • Doppler equation

  • Doppler Modes

  • Doppler Artifacts


Scattering of ultrasound by rbc s

Scattering of Ultrasound by RBC’s

  • Red blood cells

    • Diameter: 7µm

  • Raleigh scatterers

    • Smaller than ultrasound wavelength (0.1-0.7 mm)


Scattering of ultrasound by rbc s1

Scattering of Ultrasound by RBC’s

  • The Intensity of the Raleigh Scatterers is

  • determined by:

  • Transducer frequency

  • Scatterer density

  • Scatterer size

  • Scatterer acoustic impedance


The doppler effect

The Doppler Effect

Doppler shift is a change in frequency caused by the

relative movement of the sound source or the reflector.

Transducer is the sound wave source.

Red Blood Cells are the reflector

RBC’s can be:

  • stationary

  • moving towards the transducer

  • moving away from the transducer


Doppler shift

Doppler Shift

fo

fr

=

fd

fr

fo

fo

-

=

=

0

fr

Stationery Reflectors


Doppler shift1

Doppler Shift

fr

fo

>

fd

fr

fo

=

-

+ve shift

=

fo

fr

Reflectors moving towards

Sound source


Doppler shift2

Doppler Shift

fr

fo

<

fd

fr

fo

=

=

=

- ve shift

fo

fr

Reflectors moving away

from sound source


Doppler equation

Doppler Equation

2f0vcos

fd =

c

  • fd = Doppler shifted frequency

  • fo = transducer frequency

  • v = blood velocity

  •  = beam flow angle

  • c = speed of sound in tissue (1540 m/s)


Doppler physics and instrumentation

Cos 

1.00

Cos 

0

90


Angle to flow

Angle to Flow


Beam flow angle

Beam Flow Angle

  • Ø = 0°

    • Parallel to flow - Optimum shift

  • Ø = 1° to 89°

    • Shift is reduced

  • Ø = 90°

    • Perpendicular to flow - No shift


Beam flow angle1

Beam Flow Angle

  • Best Doppler angle is parallel to flow and closest to 0

  • Angle is effected by

    • organ position

    • patient position

    • transducer position


Reflector velocity

Reflector Velocity

The Doppler shift can be used to calculate the velocity

of a column of moving RBCs if the following is known:

  • fo = transducer frequency

  •  = beam flow angle

  • c = speed of sound in tissue (1540 m/s)


Reflector velocity1

Reflector Velocity

fdc

v =

2f0cos


Doppler modes

Doppler Modes

  • Pulsed Wave Doppler

  • Continuous Wave Doppler

  • Color Doppler

  • Power Doppler


Pulsed doppler

Pulsed Doppler

A

C

A = PRP

B = Pulse Duration

C= Reception Time

B


Pulsed doppler1

Pulsed Doppler

  • The # of cycles per pulse is determined by:

  • Strength of the excitation voltage.

  • Electro-mechanical efficiency.

  • The damping characteristics.


Pulsed doppler2

Pulsed Doppler

Pulsed Duration = period x cycles per pulse

PRP = PD + Reception Time

PRF = # of pulses per second

PRP = 1/PRF

Duty Factor = PD

PRP


Range equation

Range Equation

Distance = velocity x time


Range equation1

Range Equation

Velocity = speed of sound in soft tissue.

Distance = reflector distance.

Time = time it takes the sound wave to reach reflector.


Range equation2

Range Equation

Time that can be measured is the go – return time.

The actual time = go-return time

2


Range equation3

Range Equation

Reflector distance = velocity x go-return time

2


Pulsed doppler3

Pulsed Doppler


Aliasing

Aliasing

  • The inability of a PD transducer to detect large Doppler shift is known as aliasing.


Aliasing1

Aliasing

  • The sampling rate = PRF

  • Maximum PRF is determine by the go- return time.

  • Deeper vessels requires longer go-return time

  • and thus a lower PRF.


Aliasing2

Aliasing

  • Low sampling rate results large signal

  • changes occurring between samples

  • Acquired sample lacks information

  • regarding these fast changes.


Aliasing3

Aliasing

Low Sampling Rate

Received information

sampled too infrequently

Measurement Errors

(aliasing)


Pulsed doppler4

Pulsed Doppler

Receiver Gated

Receiver circuits only open for a short interval during every pulse cycle.


Pulsed doppler5

Pulsed Doppler

Receiver Gated

Receiver circuits only open for a short interval during every pulse cycle.


Spectral analysis

Spectral Analysis

  • Returning signal from volume of RBCs contains

  • range of frequencies.

  • These range of frequencies are called the

  • FREQUENCY SPECTRUM.

  • Analysis of this spectrum will separate this

  • complex signals into its component parts.


Spectral analysis1

Spectral Analysis

  • The component frequencies are converted

  • into velocity information.

  • This allow for quantitative analysis of the

  • range of RBC’s velocities.


Fast fourier transform

Fast Fourier Transform

  • Digital method of spectrum analysis.

  • Mathematical technique. Complex wave is

  • broken down into a series of simpler sine wave.

  • Analog Doppler signal is digitized in a ADC


Fast fourier transform1

Fast Fourier Transform

  • 5- 10 microseconds samples of signal are

  • processed using the FFT algorithm.

  • Digital component frequencies are converted

  • back into a analog signal by a DAC.

  • Signal is displayed.


Spectral display

Spectral Display

Doppler Signals

Analysis

FFT

Quadrature Phase

Detector

Positive

Shift

Negative

Shift

Channel A

Channel B

Audio Channel A

Audio Channel B

Spectral Display

Below the Baseline

Spectral Display

Above the Baseline


Spectral display1

Spectral Display

  • X- axis – time information

  • Y- axis – frequency/velocity information.

  • Z- axis – amplitude information.


Spectral display2

Spectral Display

Frequency,

y-axis

Amplitude

Z- axis

Sonic Window

Time, x axis


Spectral display3

Spectral Display

  • Velocity Measurements

  • Peak systolic velocity

  • End-diastolic velocity

  • Mean velocity – calculated by taken the area

  • under the curve.


Spectral display4

Spectral Display

Systolic Peak

Velocity

Velocity

Mean Velocity

End Diastolic Velocity

Time


Doppler spectrum assessment

Doppler Spectrum Assessment

  • Assess the following:

  • Presence of flow

  • Direction of flow

  • Amplitude

  • Window

  • Pulsatility


Doppler spectrum assessment1

Doppler Spectrum Assessment

Check for Flow

Flow

Detected

No Flow

Detected

Check

Sensitivity

Check

SV Placement

Check Beam-

flow angle

Sensitive

Decreased

Sensitivity

Improve

Sensitivity


Doppler spectrum assessment2

Doppler Spectrum Assessment

  • Sensitivity can be improved by:

  • Increasing power or gain.

  • Decreasing the velocity scale.

  • Decreasing the reject or filter.

  • Slowly increasing the SV size.


Doppler spectrum assessment3

Doppler Spectrum Assessment

  • Direction of Flow

  • Pulsed Doppler use quadrature phase

  • detection to provide bidirectional Doppler

  • information.


Doppler spectrum assessment4

Doppler Spectrum Assessment

  • Flow can either be:

  • Mono-phasic

  • Bi-phasic

  • Tri-phasic

  • Bidirectional


Spectral display5

Spectral Display

Mono-phasic Flow

Flow on just on side

of the Baseline.

Frequency

Time


Spectral display6

Spectral Display

Bi-phasic Flow

Flow start on one

side of the Baseline

and then crosses to

the other.

Frequency

Time


Spectral display7

Spectral Display

Tri-phasic Flow

Flow start on one side

of the baseline side,

then crosses to the

other, then returns to

the original side.

Frequency

Time


Spectral display8

Spectral Display

Bidirectional Flow

Flow which occurs

simultaneously on

both sides of the

baseline.

Frequency

Time


Doppler spectrum assessment5

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


Spectral display9

Spectral Display

Low amplitude

Frequency

Time


Spectral display10

Spectral Display

Mid amplitude

Frequency

Time


Spectral display11

Spectral Display

High amplitude

Frequency

Time


Doppler spectrum assessment6

Doppler Spectrum Assessment

  • Window

  • Received Doppler shift consist of a range of

  • frequencies.

  • Narrow range of frequencies will result in a

  • narrow display line.

  • The clear area underneath the spectrum is

  • called the window.


Spectral display12

Spectral Display

Velocity

A narrow range of frequencies

results in large clear window.

Sonic Window

Time


Spectral display13

Spectral Display

Velocity

A broad range of frequencies

results in diminished window.

Sonic Window

Time


Spectrum broadening

Spectrum Broadening

Loss of the Spectral window is called

Spectral Broadening.


Spectrum broadening1

Spectrum Broadening

  • Occurs usually:

  • As the blood decelerates in diastole

  • If sample volume is placed to close to the vessel wall

  • In small vessels (parabolic velocity profile)


Spectrum broadening2

Spectrum Broadening

  • Tortuous vessels.

  • Low flow states..

  • Excessive gain/power/dynamic range


Spectrum broadening3

Spectrum Broadening

It is hallmark of disturbed and/or

turbulent flow.


Spectrum broadening4

Spectrum Broadening

  • Pulsatility

  • Measures the difference between the maximum

  • and minimum velocities within the cardiac cycle.

  • Indices are unit less.

  • All increase in value as flow pulsatility increases.

  • Can be measured without knowledge of the Doppler

  • angle.


Spectral display14

Spectral Display

Maximum

Velocity

Resistive Index

RI = Max – Min

Max

Frequency

Mean Velocity

Minimum Velocity

Time


Spectral display15

Spectral Display

Maximum

Velocity

Pulsatility Index

PI = Max – Min

Mean

Frequency

Mean Velocity

Minimum Velocity

Time


Spectral display16

Spectral Display

Maximum

Velocity

A/B Ratio

A/B ratio= Max

Min

Frequency

Mean Velocity

Minimum Velocity

Time


Spectral analysis parameters

Spectral Analysis Parameters

Sample Volume

Center SV in center of vessel to

avoid spectral broadening.


Spectral analysis parameters1

Spectral Analysis Parameters

Beam Flow Angle

  • Vascular – BFA should be between 30

  • and 60 degrees.

  • Echocardiography – BFA should be close

  • to 0 degree.


Spectral analysis parameters2

Spectral Analysis Parameters

Sample Volume Size

  • Start with large sample volume to improve

  • sensitivity.

  • Decrease SV size to reduce spectral broadening

  • once flow established.


Spectral analysis parameters3

Spectral Analysis Parameters

Power

  • Increase power to improve

  • sensitivity.

  • Once flow is established decrease

  • power to improve S/N ratio.

  • Excess power will cause spectral

  • broadening and mirroring.


Spectral analysis parameters4

Spectral Analysis Parameters

Gain

  • Increase gain to improve

  • sensitivity.

  • Once flow is established decrease

  • gain to improve S/N ratio.

  • Excess gain will cause spectral

  • broadening and mirroring.


Spectral analysis parameters5

Spectral Analysis Parameters

Dynamic Range

  • Compress the dynamic range to improve

  • S/N ratio.

  • Over compression will mask true spectral

  • broadening.


Spectral analysis parameters6

Spectral Analysis Parameters

Baseline

  • Center baseline initially when searching for

  • flow.

  • Once direction is established, shift to optimize

  • display.


Spectral analysis parameters7

Spectral Analysis Parameters

Velocity Scale

PRF

  • Set low to maximize sensitivity.

  • Once flow is detected, increase to

  • reduce aliasing.


Spectral analysis parameters8

Spectral Analysis Parameters

Filters

  • Remove high amplitude, low frequency

  • signals from the display.

  • Low filters improve sensitivity.

  • High filters improve S/N ratio.

  • Filters can be set at various levels from

  • 50 to 800 Hz.


Spectral analysis parameters9

Spectral Analysis Parameters

Sweep Speed

  • Changes the length of time displayed on

  • X-axis of the spectral display.

  • Set sweep speed should be set to allow

  • display of several cardiac cycles at a time.


Spectral artifacts

Spectral Artifacts

Mirroring

  • Duplication of the Doppler waveform on the other side of the baseline.

  • Duplicated waveform is identical in terms of frequency and temporal components.

    .

  • Duplicate image has diminished amplitude.


Spectral artifact

Spectral Artifact

Mirroring

Frequency

Time


Spectral artifacts1

Spectral Artifacts

Causes of mirroring

  • Doppler angle close to 90 degrees.

  • Excessive gain.


Spectral artifacts2

Spectral Artifacts

Doppler angle close to 90 degrees

Doppler beam intersect the flow so that the flow

is simultaneously sampled both as it approaches

and as it leaves the sample volume.


Spectral artifacts3

Spectral Artifacts

Excessive gain

  • The quadrature phase detector has two channels.

  • Each channel is sensitive to the amplitude of the

    incoming signal.

  • High Doppler gain will saturate the quadrature detector causing cross talk between the two channels.


Spectral artifacts4

Spectral Artifacts

Aliasing

  • Pulsed Doppler has a limit to the maximum Doppler shift frequency that can be sampled unambiguously.

  • This limit is called the Nyquist Limit.

  • The Nyquist Limit = PRF/2


Spectral artifacts5

Spectral Artifacts

Aliasing

Aliasing only occurs when the Doppler shifts exceed the Nyquist limit. To correct aliasing you can either:

1. Raise the Nyquist Limit

2. Lower the Doppler Shift.


Spectral artifacts6

Spectral Artifacts

Correcting

Aliasing

Lower the

Doppler Shift

Increase

Nyquist Limit

Increase the

Velocity Scale

High PRF

Mode

Lower Operating

Frequency

Increase Doppler

Flow Angle


Spectral artifacts7

Spectral Artifacts

2f0vcos

fd =

c

f0

fd

< Nyquist Limit


Spectral artifacts8

Spectral Artifacts

1.00

Cos 

0

90


Spectral artifacts9

Spectral Artifacts

Increase in  Decrease in Cos 


Spectral artifacts10

Spectral Artifacts

2f0vcos

fd =

c

fd

< Nyquist Limit

 Cos 


Cw doppler

CW DOPPLER

  • Contains two crystals or two sets of crystals.

  • One for transmitting

  • One for receiving


Cw doppler1

CW DOPPLER

  • The transmit crystal is continuously driven by a voltage generator.

  • Oscillating frequency of the voltage generator regulates the frequency of the emitted wave.


Cw doppler2

CW DOPPLER

  • In the sound beam the transmitted and reflected sound waves overlap over a relatively narrow range.

  • The region of overlap is called the flow sensitive zone.


Cw doppler3

CW DOPPLER

  • The flow sensitive zone varies with different transducers.

  • Low frequency transducers have deep flow sensitive zones

  • High frequency transducers have superficial flow sensitive zones.


Doppler physics and instrumentation

CW DOPPLER

Transmitting

Crystal

Flow sensitive Zone

Receiving

Crystal


Cw doppler4

CW DOPPLER

  • Doppler shift can be located at any depth in the flow sensitive zone of beam.

  • The Doppler receiver is unable to determine the exact location of the Doppler shift.

  • Thus CW lacks range resolution.

  • Because it is continuously sample returning echoes it have no limitations

    on measuring high flow velocities.


Color flow imaging

Color Flow Imaging

  • Pulsed Doppler Technique

  • Uses 200 – 500 sample gates along each scan line.

  • Multiple pulses are sent down each scan line.

  • The number of pulses that are sent down each scan line is called the Packet Size or Ensemble Length.


Doppler physics and instrumentation

Color Flow Imaging

Multiple Pulses

Multiple Gates


Color flow imaging1

Color Flow Imaging

  • Doppler data evaluated using autocorrelation.

  • Autocorrelation is a technique that compare the echo from each pulse with the echo from the previous pulse.

  • Autocorrelation requires a minimum of 3 pulses per scan line.


Color flow imaging2

Color Flow Imaging

  • This technique can only produce an estimate of the mean frequency shift and is mean velocity.

  • Increasing the line per frame provides an image with more resolution at the expense of the frame rate.


Color flow imaging3

Color Flow Imaging

Color Resolution

Frame rate

Number of lines in

Gray scale imaging


Color flow imaging4

Color Flow Imaging

  • Echo versus color threshold is a processing control that allows for determining whether a pixel will display color information or gray scale information.

  • Echoes having greater amplitudes than the threshold value will be displayed in gray scale.

  • Echoes having amplitude less than the threshold value will displayed color.


Color flow imaging5

Color Flow Imaging

  • To produce the color flow image, the mean Doppler shift is encoded according to a preset color map.

  • This color information is superimposed on the gray scale anatomic scan in real time.


Color flow maps

Color Flow Maps

  • Velocity color map

  • Variance color map


Color flow imaging6

Color Flow Imaging

Velocity Color Map

  • One color use to encode shift in one channel.

  • Another color to encode shift in the other channel.

  • As the Doppler shifts increase, they are assigned lighter shades of these colors.


Velocity color bar

Velocity Color Bar

Increasing flow velocity toward the transducer

Zero flow

Increasing flow velocity away from the transducer


Color flow imaging7

Color Flow Imaging

Variance Color Map

  • One color use to encode shift in one channel.

  • Another color to encode shift in the other channel.


Color flow imaging8

Color Flow Imaging

  • As the Doppler shifts increase, they are assigned lighter shades of these colors.

  • A third color, usually green is used to display variance.


Variance color bar

Variance Color Bar

Variance Color


Aliasing4

Aliasing

  • Color Doppler can alias.

  • As the Doppler shift increases the color will be encoded with a light shade of one color into the

    light shade of the other color.


Aliasing5

Aliasing

Increasing flow velocity toward the transducer

Wrap Around

Zero flow

Increasing flow velocity away from the transducer


Aliasing6

Aliasing

With the correct setup for normal flow velocities at the aliasing threshold, then a focal region of aliased signal will indicate the location of high velocity.


Aliasing7

Aliasing

Laminar

Color indicates flow presence, direction, and mean velocity.


Color flow imaging9

Color Flow Imaging

  • Estimate of mean velocity only

  • Shows direction of flow by color assignment

  • Shows changes in mean velocity by changing shades or different color


Color flow imaging10

Color Flow Imaging

  • Flow is angle dependence.

  • Potential for aliasing artifacts.

  • Show variance.


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