2. OUTLINE
• Doppler Principles
• Pulsed and Continuous Doppler
• Aliasing and Nyquist Criteria
• Spectral Analysis
• Colour flow imaging
• Power Doppler
• Doppler Artifacts
3. Waves from a moving source
Source moving this way
4. Doppler Effect
Change in the perceived frequency of sound
emitted by a moving source.
The basis of Doppler ultrasonography is the fact
that reflected/scattered ultrasonic waves from a
moving interface will undergo a frequency shift.
5.
6. • In diagnostic ultrasound, the Doppler effect is used to
measure blood flow velocity.
• When the emitted ultrasound beam strikes moving
blood cells, the latter reflect the pulse with a specific
Doppler shift frequency that depends on the velocity and
direction of blood flow
7. Positive shift
Received freq >
transmitted freq
Flow towards the
transducer
Negative shift
Transmitted freq >
received freq
Flow away from
transducer
IF RECEIVED FREQUENCY=TRANSMITTED FRQUENCY,NO DOPPLER SHIFT
8. Doppler equation
= Doppler shift frequency (kHz)
= Ultrasound transmission frequency (MHz)
= Blood cell velocity (cm/sec)
= Cos of angle between US & flow direction
= Speed of sound in soft tissue (1540 m/sec)
9. Doppler shift depends on the cosine
of the angle between the sound
beam and the direction of motion
11. The size of the Doppler signal is dependent on:
• Blood velocity: as velocity increases, so does the Doppler
frequency
• Ultrasound frequency: ultrasound
frequencies give increased Doppler
frequency.
• Angle of Insonation
12. Continuous Doppler
• Uses two crystals, one to send
and one to receive.
• Uses continuous transmission and
reception of ultrasound. Doppler signals
are obtained from all vessels in the path of
the ultrasound beam (until the ultrasound
beam becomes sufficiently attenuated due
to depth)
• Unable to determine the specific location
of velocities within the beam and cannot
be used to produce color flow images.
• Used in adult cardiac scanners to
investigate the high velocities in the aorta
13. 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.
14.
15. Pulsed Doppler
• The transducer both sends and receives the signal.
The returned signal is gated so that only information about the
desired depth is computed
•
• Pulses — just like real time scanning
• Need to "gate" analysis of received pulse, so we know where the
moving objects are.
• This allows measurement of the depth (or range) of the flow site.
Additionally, the size of the sample volume (or range gate) can be
changed. Pulsed wave ultrasound is used to provide data for
Doppler sonograms and color flow images
16. Continuous doppler Pulsed doppler
Separate crystal for transmitting &
receiving Single
Can measure high velocities
Range ambiguity
Single crystal transmits & receives
Range resolution
Can't measure very high velocities
19. 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.
20. Color Flow Imaging
• . This technique can only produce an estimate of the mean frequency
shift and mean velocity.
• Increasing the line per frame provides an image with more resolution at
the expense of the frame rate.
22. 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,
31. .
Box should be as small & superficial as possible
while still providing necessary information.
A deep color box Will result in a slower PRF,
which may produce aliasing of depicted color
flow.
Colour Box
Color box is an operator-adjustable area within US image in which all color Doppler
information is displayed. Because frame rate decreases as box size increases, image
resolution & quality are affected by box size and width
33. Aliasing
• Aliasing is production of artificial low
frequency signals when the sampling rate is
less than twice the doppler signal
frequency. When the Doppler shifts exceed
a value Nyquist frequency, velocities are
perceived as going in opposite direction
34. Nyquist Sampling Limit
• The Maximum Doppler frequency that can be sampled is
1/2 the PRF
• Example, if PRF = 8 kHz
— Max Doppler frequency is 4 kHz
• Example, if PRF = 4 kHz
— Max Doppler frequency is 2 kHz
35. Adjustments to be made to avoid aliasing
• Increasing the PRF
• Moving color or spectral baseline up or down.
• Decreasing Doppler shift frequency (changing angle of
insonation).
• Using a lower-frequency transducer.
37. Doppler Spectrum Assessment
• Decreasing the velocity
scale.
• Decreasing the reject or
filter. •
• Slowly increasing the SV size
Sensitivity can be improved by:
• Increasing power or gain.
44. 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
45. 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.
49. 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)
52. 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.
53. ■ High Pulsatility/Resistance
Waveform
.Tnptasicwaveform
• Low Puls atility/Res i sta nee
Waveform
• Mixed Pulsatility/Resistance
Waveform
sharp systolic peak +
reversed diastolic flow
(e.g.) extremity artery in
resting stage.
Broad systolic peak +
forward flow in diastole
(e.g.)
ICA, renal, vertebral, celiac.
" 1 : ! 1 1 forward
flow in diastole.
(e.g.) ECA & SMA (during fasting)
Spectrum Analysis
57. Doppler indices are :
PI
RI
SYSTOLIC / DIASTOLIC RATIO
Acceleration time(AT) and acceleration index(Al)
SPECTRAL BROADENING
• These indices can thus serve as a semiquantitative
parameter for the evaluation of stenoses
58. Pulsatility Index
It is defined as the maximum height of
the waveform, S, minus the
minimum diastolic, D (which may be
negative), divided by the mean height,
M,
Stenoses or occlusions in arteries will
alter the Doppler waveform and the
pulsatility index.
59. Pourcelot's Resistance index (RI)
• The resistance indices, in particular the Pourcelot index,
reflect wall elasticity as well as the peripheral resistance
of the organ supplied
• In vessels with greater peripheral resistance, the
Pourcelot index is higher and end-diastolic velocity
decreases.
• It is defined as follows
where E is end diastolic velocity. The value of RI can be
calculated by the scanner and displayed on the screen.
61. Spectral Broadening
• There have been several definitions of spectral broadening
(SB) described over the years in an attempt to quantify the
spread of frequencies present within a spectrum, One
such definition is as follows:
Increased SB indicates the presence of arterial disease
62. • SPECTRAL DOPPLER • COLOUR DOPPLER
Depiction of Doppler shift information in
waveform
Utilize the Doppler shift
information to show blood flow
in color
63. • SPECTRAL DOPPLER
Advantages :
Depicts quantitative
flow at one site
Allows calculations of
velocity and indices
Good temporal
resolution
• COLOUR DOPPLER
Advantages :
Overall view of
flow
Directional
information
about flow
Averaged
velocity
information
about flow
64. Power Doppler
Power or intensity of Doppler signal is
measured rather than Doppler shift.
Limitations
No direction / velocity information
Slow frame rate
65. Power Doppler
A color-coded map of Doppler shifts superimposed onto a B-
mode ultrasound image
Color flow imaging have to produce several thousand color
points of flow information for each frame superimposed on
the B-mode image.
Color flow imaging uses fewer, shorter pulses along each
color scan line of the image to give a mean frequency shift
and a variance at each small area of measurement. This
frequency shift is displayed as a color pixel.
66. Power Doppler
The transducer elements are switched rapidly between B-mode
and color flow imaging to give an impression of a combined
simultaneous image.
The pulses used for color flow imaging are typically three to four
times longer than those for the B-mode image, with a corresponding
loss of axial resolution.
Assignment of color to frequency shifts is usually based on
direction (for example, red for Doppler shifts towards the
ultrasound beam and blue for shifts away from it) and
magnitude (different color hues or lighter saturation for higher
frequency shifts).
72. Wall Filter
• Filters eliminate typically low
frequency high-intensity noise
that may arise from vessel wall
motion
73. Spectral Filter
• Color duplex US image obtained a high wall setting shows loss
of the low-velocity-flow component of the spectral waveform immediately above the
baseline, Higher-velocity flow is well depicted, and accurate flow quantification can
still occur. In the evaluation of the liver vasculature, this is likely to become relevant
only when flow velocity is very low and falls within the range of velocities that are
filtered out
74. Spectral Filter
Color duplex US image demonstrates
how the spectral waveform
progressively fills in toward the
baseline
77. Angle Correction
Angle correction refers to adjustment of
Doppler angle & is used to calibrate
velocity scale for the angle between US
beam and blood flow
being measured
78. •The angle of insonation should also be between 450- 600.
• Flow may appear to be reversed when the beam-flow angle
changes about 90 0 .
•complete loss of flow may be evident when the beam-flow angle
is 90 0 .
86. Mirror image artifact
• any vessel adjacent to a highly
reflective surface, such as the
lung, subdiaphragmatic region of
the liver and the supraclavicular
region
88. Twinkling artifact
• Rapidly fluctuating
mixture of
Doppler signals
(red and blue
pixels) that imitate
turbulent
89. Colour in non vascular structures (Colour
flash artifact)
• Manifests as a colour
signal due to
transducer or patient
motion
• Hypoechoic areas
such as a cyst or a duct are
susceptible to colour flash
artifact
