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Doppler Physics and InstrumentationPowerPoint Presentation

Doppler Physics and Instrumentation

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### Doppler Physicsand 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

- The Intensity of the Raleigh Scatterers is
- determined by:
- Transducer frequency
- Scatterer density
- Scatterer size
- Scatterer acoustic impedance

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 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)

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 Angle

- Best Doppler angle is parallel to flow and closest to 0
- Angle is effected by
- organ position
- patient position
- transducer position

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)

Doppler Modes

- Pulsed Wave Doppler
- Continuous Wave Doppler
- Color Doppler
- Power Doppler

Pulsed Doppler

- The # of cycles per pulse is determined by:
- Strength of the excitation voltage.
- Electro-mechanical efficiency.
- The damping characteristics.

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

Distance = velocity x time

Range Equation

Velocity = speed of sound in soft tissue.

Distance = reflector distance.

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

Range Equation

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

The actual time = go-return time

2

Aliasing

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

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.

Aliasing

- Low sampling rate results large signal
- changes occurring between samples
- Acquired sample lacks information
- regarding these fast changes.

Aliasing

Low Sampling Rate

Received information

sampled too infrequently

Measurement Errors

(aliasing)

Pulsed Doppler

Receiver Gated

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

Pulsed Doppler

Receiver Gated

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

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 Analysis

- The component frequencies are converted
- into velocity information.
- This allow for quantitative analysis of the
- range of RBC’s velocities.

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 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

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 Display

- X- axis – time information
- Y- axis – frequency/velocity information.
- Z- axis – amplitude information.

Spectral Display

- Velocity Measurements
- Peak systolic velocity
- End-diastolic velocity
- Mean velocity – calculated by taken the area
- under the curve.

Doppler Spectrum Assessment

- Assess the following:
- Presence of flow
- Direction of flow
- Amplitude
- Window
- Pulsatility

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 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 Assessment

- Direction of Flow
- Pulsed Doppler use quadrature phase
- detection to provide bidirectional Doppler
- information.

Doppler Spectrum Assessment

- Flow can either be:
- Mono-phasic
- Bi-phasic
- Tri-phasic
- Bidirectional

Spectral Display

Bi-phasic Flow

Flow start on one

side of the Baseline

and then crosses to

the other.

Frequency

Time

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 Display

Bidirectional Flow

Flow which occurs

simultaneously on

both sides of the

baseline.

Frequency

Time

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
- 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 Display

Velocity

A narrow range of frequencies

results in large clear window.

Sonic Window

Time

Spectral Display

Velocity

A broad range of frequencies

results in diminished window.

Sonic Window

Time

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 Broadening

- Tortuous vessels.
- Low flow states..
- Excessive gain/power/dynamic range

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 Display

Maximum

Velocity

Resistive Index

RI = Max – Min

Max

Frequency

Mean Velocity

Minimum Velocity

Time

Spectral Display

Maximum

Velocity

Pulsatility Index

PI = Max – Min

Mean

Frequency

Mean Velocity

Minimum Velocity

Time

Spectral Display

Maximum

Velocity

A/B Ratio

A/B ratio= Max

Min

Frequency

Mean Velocity

Minimum Velocity

Time

Spectral Analysis Parameters

Sample Volume

Center SV in center of vessel to

avoid spectral broadening.

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 Parameters

Sample Volume Size

- Start with large sample volume to improve
- sensitivity.
- Decrease SV size to reduce spectral broadening
- once flow established.

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 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 Parameters

Dynamic Range

- Compress the dynamic range to improve
- S/N ratio.
- Over compression will mask true spectral
- broadening.

Spectral Analysis Parameters

Baseline

- Center baseline initially when searching for
- flow.
- Once direction is established, shift to optimize
- display.

Spectral Analysis Parameters

Velocity Scale

PRF

- Set low to maximize sensitivity.
- Once flow is detected, increase to
- reduce aliasing.

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 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

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 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 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 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 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 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 Artifacts

Increase in Decrease in Cos

CW DOPPLER

- Contains two crystals or two sets of crystals.
- One for transmitting
- One for receiving

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 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 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.

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

- 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.

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 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 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 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

- Velocity color map
- Variance color map

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

Increasing flow velocity toward the transducer

Zero flow

Increasing flow velocity away from the transducer

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 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

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.

Aliasing

Increasing flow velocity toward the transducer

Wrap Around

Zero flow

Increasing flow velocity away from the transducer

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.

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 Imaging

- Flow is angle dependence.
- Potential for aliasing artifacts.
- Show variance.

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