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 (0.10.7 mm). Scattering of Ultrasound by RBC’s.
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Doppler Physicsand Instrumentation
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:
fo
fr
=
fd
fr
fo
fo

=
=
0
fr
Stationery Reflectors
fr
fo
>
fd
fr
fo
=

+ve shift
=
fo
fr
Reflectors moving towards
Sound source
fr
fo
<
fd
fr
fo
=
=
=
 ve shift
fo
fr
Reflectors moving away
from sound source
2f0vcos
fd =
c
1.00
Cos
0
90
The Doppler shift can be used to calculate the velocity
of a column of moving RBCs if the following is known:
fdc
v =
2f0cos
A
C
A = PRP
B = Pulse Duration
C= Reception Time
B
Pulsed Duration = period x cycles per pulse
PRP = PD + Reception Time
PRF = # of pulses per second
PRP = 1/PRF
Duty Factor = PD
PRP
Distance = velocity x time
Velocity = speed of sound in soft tissue.
Distance = reflector distance.
Time = time it takes the sound wave to reach reflector.
Time that can be measured is the go – return time.
The actual time = goreturn time
2
Reflector distance = velocity x goreturn time
2
Low Sampling Rate
Received information
sampled too infrequently
Measurement Errors
(aliasing)
Receiver Gated
Receiver circuits only open for a short interval during every pulse cycle.
Receiver Gated
Receiver circuits only open for a short interval during every pulse cycle.
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
Frequency,
yaxis
Amplitude
Z axis
Sonic Window
Time, x axis
Systolic Peak
Velocity
Velocity
Mean Velocity
End Diastolic Velocity
Time
Check for Flow
Flow
Detected
No Flow
Detected
Check
Sensitivity
Check
SV Placement
Check Beam
flow angle
Sensitive
Decreased
Sensitivity
Improve
Sensitivity
Monophasic Flow
Flow on just on side
of the Baseline.
Frequency
Time
Biphasic Flow
Flow start on one
side of the Baseline
and then crosses to
the other.
Frequency
Time
Triphasic Flow
Flow start on one side
of the baseline side,
then crosses to the
other, then returns to
the original side.
Frequency
Time
Bidirectional Flow
Flow which occurs
simultaneously on
both sides of the
baseline.
Frequency
Time
Low amplitude
Frequency
Time
Mid amplitude
Frequency
Time
High amplitude
Frequency
Time
Velocity
A narrow range of frequencies
results in large clear window.
Sonic Window
Time
Velocity
A broad range of frequencies
results in diminished window.
Sonic Window
Time
Loss of the Spectral window is called
Spectral Broadening.
It is hallmark of disturbed and/or
turbulent flow.
Maximum
Velocity
Resistive Index
RI = Max – Min
Max
Frequency
Mean Velocity
Minimum Velocity
Time
Maximum
Velocity
Pulsatility Index
PI = Max – Min
Mean
Frequency
Mean Velocity
Minimum Velocity
Time
Maximum
Velocity
A/B Ratio
A/B ratio= Max
Min
Frequency
Mean Velocity
Minimum Velocity
Time
Sample Volume
Center SV in center of vessel to
avoid spectral broadening.
Beam Flow Angle
Sample Volume Size
Power
Gain
Dynamic Range
Baseline
Velocity Scale
PRF
Filters
Sweep Speed
Mirroring
.
Mirroring
Frequency
Time
Causes of mirroring
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.
Excessive gain
incoming signal.
Aliasing
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.
Correcting
Aliasing
Lower the
Doppler Shift
Increase
Nyquist Limit
Increase the
Velocity Scale
High PRF
Mode
Lower Operating
Frequency
Increase Doppler
Flow Angle
2f0vcos
fd =
c
f0
fd
< Nyquist Limit
1.00
Cos
0
90
Increase in Decrease in Cos
2f0vcos
fd =
c
fd
< Nyquist Limit
Cos
CW DOPPLER
Transmitting
Crystal
Flow sensitive Zone
Receiving
Crystal
on measuring high flow velocities.
Color Flow Imaging
Multiple Pulses
Multiple Gates
Color Resolution
Frame rate
Number of lines in
Gray scale imaging
Velocity Color Map
Increasing flow velocity toward the transducer
Zero flow
Increasing flow velocity away from the transducer
Variance Color Map
Variance Color
light shade of the other color.
Increasing flow velocity toward the transducer
Wrap Around
Zero flow
Increasing flow velocity away from the transducer
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.
Laminar
Color indicates flow presence, direction, and mean velocity.