Doppler physics ppt

1. DOPPLER PHYSICS

2. PRINCIPLE OF DOPPLER ULTRASOUND
• BASIS – ‘ DOPPLER EFFECT’ • DISCOVERY –
Physicist Christian Johann Doppler in 1842 •
‘When a sound source and reflector are moving
towards each other, the sound waves are spaced
closer together and reach the receiver at a
higher frequency than that at which they were
originally emitted ’.

3. DOPPLER SHIFT
• Doppler shift is the difference between the
transmitted and received frequencies
• Transmitted and received frequencies are in
the MHz range
• Doppler shift frequencies often in audible
range

4. DOPPLER SHIFT
• WHEN RECEIVED FREQUENCY= TRANSMITTED
FREQUENCY, NO DOPPLER SHIFT

5. • Relationship between Doppler shift (or just
Doppler) frequency, FD and reflector velocity, v is
FD = 2 f0 v cos theta
c
fo is the ultrasound frequency, or the transmitted
beam frequency.
v is the reflector velocity (m/s; cm/s)
q is the Doppler angle
c is the speed of sound
DOPPLER EQUATION

6. DOPPLER ULTRASOUND
MODES/INSTRUMENTATIONS
• Continuous wave Doppler
• Pulsed wave Doppler
• Spectral doppler
• Colour doppler
• Power doppler

7. CONTINUOUS WAVE DOPPLER
• Simplest form
• Uses separate transmit & receive crystals that
continuously transmit and receive ultrasound.
• Able to detect the presence and direction of flow,
but are unable to distinguish signals arising from
vessels at different depths. ( THEREFORE IT LACKS
RANGE RESOLUTION.)
• Portable and inexpensive.
• Uses- CARDIAC SCANNERS( HIGH VELOCITIES IN
AORTA ), bedside to ascertain flow in superficial
vessels.

8. CONTINUOUSWAVE DOPPLER

9. PULSED-WAVE (PW) DOPPLER
• Uses brief pulses of ultrasound energy using
only one crystal. • The echo delay time( Te)
can be converted into distance and the DEPTH
of the echo source can be determined • The
sensitive volume from which flow data are
sampled can be controlled in terms of shape,
depth, and position.

10. PULSED WAVE DOPPLER
• PULSE REPETITION FREQUENCY ( PRF)- No. of
pulses transmitted per second • With increasing
scanning depth, PRF decreases as more time is
needed for the echoes to return • At a minimum,
the PRF must beat at least twice the frequency of
the Doppler signal to construct the signal
successfully. • This sets the upper limit to the
flow velocities to be accurately recorded • Use
higher PRF setting for high flow velocities, low
PRF for slow venous flow.

12. DUPLEX ULTRASOUND SCANNING
• Duplex ultrasound instruments are real-time
B-mode scanners with built-in Doppler
capabilities.
• B-mode Imager( Outline anatomic structures)
• Pulsed-Doppler ( Flow and movement
patterns)

13. SPECTRAL DOPPLER
• Ultrasound is emitted in pulse similar to B-
mode • Gate is used to determine the interval
after emission when returning signals are
received and therefore the depth from which
the sample is taken can be assessed. • The
Doppler shift data are displayed in graphic form
as a time-varying plot of the frequency
spectrum of the returning signal.

15. SPECTRAL ANALYSIS
• shows the Doppler shift spectrum and
direction ,usually displayed as the maximum
velocity rather than the frequency .
• Provides QUANTITATIVE INFORMATION.

16. SPECTRAL ANALYSIS
• The resulting Doppler frequency spectrum
displays the following : Variation with time of
the Doppler frequencies present in the
volume sampled.
• The envelope of the spectrum, represents the
maximum frequencies present at any given
point in time.
• The width of the spectrum at any point,
indicates the range of frequencies present.

17. DOPPLER SPECTRUM ASSESSMENT
• Assess the following 1. Presence of flow 2.
Direction of flow 3. Amplitude 4. Window 5.
Pulsatility

18. DOPPLER SPECTRUM ASSESSMENT
SENSITIVITY IMPROVEMENT
• Increasing the power or gain • Decreasing the
velocity scale • Decreasing the reject or filter •
Slowly increasing the SV size.

19. DOPPLER SPECTRUM ASSESSMENT
• DIRECTION OF FLOW • Pulsed doppler uses
quadrature phase detection to provide
bidirectional doppler information.
• FLOW CAN EITHER BE • Mono-phasic • Bi-
phasic • Tri-phasic • Bidirectional

25. DOPPLER SPECTRUM ASSESSMENT
• AMPLITUDE: The spectrum displays the echo
amplitude by varying the brightness of the
display. The amplitude of the echoes are
determined by • Echo intensity • Power • Gain •
Dynamic range

26. DOPPLER SPECTRUM ASSESSMENT
• WINDOW :
• Received doppler shift consists 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

27. SPECTRUM BROADENING
• Loss of spectral window is called SPECTRAL
BROADENING.
• OCCURS-• As the blood decelerates in
diastole • SV placed close to the vessel wall •
In small vessels ( parabolic velocity profile )

28. SPECTRAL BROADENING
• Tortuous vessels • Low flow states • Excessive
gain/power/dynamic range.
• IT IS HALLMARK OF DISTURBED AND/OR
TURBULENT FLOW.

30. DOPPLER SPECTRUM ASSESSMENT
• PULSATILITY • Measures the difference
between the maximum and minimum
velocities within the cardiac cycle • Indices are
unit less • All increase in value as the flow
pulsatility increases • Can be measured
without knowledge of the doppler angle

32. DOPPLER INDICES
• S/D ratio(SDR): systolic velocity / diastolic velocity
• Pulsatility index (PI) (Gosling index): (PSV - EDV)
/ TAV
• Resistive index (RI) (Pourcelot index): (PSV - EDV)
/ PSV
• PSV: peak systolic velocity
• EDV: end diastolic velocity
• TAV: time-averaged velocity

33. COLOUR DOPPLER
• Based on pulsed doppler technique.
• Doppler shifts- converted to colour and the
moving blood is displayed in colors that
corresponds to its velocity and direction.

34. COLOUR DOPPLER
• Positive Doppler shifts are encoded as red and
negative shifts are encoded as blue. • Velocity of
the flow is represented in shades of color -
faster – brighter.

36. COLOUR DOPPLER
COLOUR BOX
• Operator adjustable area within the US image
• Affects Image resolution & quality ( change in
box size)- Frame rate decreases with increasing
box size • As small & superficial as possible .
• Deep color box – slower PRF results in aliasing.

38. COLOUR DOPPLER
• Advantages : It provides an overall view of the
flow in an organ or structure
• Provides directional information about flow
• Provides velocity information about flow and
shows turbulent type flow
• Limitations :• SEMI QUANTITATIVE • Angle
dependence • Aliasing • Artifacts caused by
the noise • Poor temporal resolution.

39. SPECTRAL DOPPLER
ADVANTAGES:
• depicts quantitative flow at one site
• allows calculations of velocity and indices
• good temporal resolution

42. POWER DOPPLER
• Also known as Energy Doppler / Amplitude
Doppler. • In this mode, colour is assigned to the
power /strength/ energy of the Doppler signal
rather than the Doppler frequency shift. • Flow is
usually displayed with one colour .
USES • To detect very slow flow, flow in small
vessels, or where the transducer angling is
awkward. • Power Doppler can be used in
conjunction with contrast agents of varying
brightness

44. POWER DOPPLER
• Advantages :Angle independent
• No aliasing
• Improved signal to noise ratio
• More sensitive to detect low flow
• Better able to define boundaries
• Limitations : No directional information
• Poor temporal resolution (due to relatively low
PRF ).
• Due to the low PRF, power Doppler is particularly
sensitive to flash artifact

45. OPTIMISATION OF COLOR FLOW
DOPPLER EXAMINATION
1. Transducer Frequency:
• For superficial structures 7-10MHZ is used.
• For deep abdominal structures– 3MHZ –
5MHZ is optimal.
• Choice of transducer frequency is paramount
because the intensity of the scattered sound
varies in proportion to the 4th power of the
Doppler frequency.

46. OPTIMISATION OF COLOR FLOW
DOPPLER EXAMINATION
2. Doppler Angle: • Strongest signals of Doppler
results when the motion is parallel to beam. • A
Doppler angle of 900 does not display flow
because no doppler shift detected.

47. OPTIMISATION OF COLOR FLOW
DOPPLER EXAMINATION
3. Sample Volume. • Ideal sample volume–
2/3rd of the vessel width positioned in the
center of the vessel. • If sample volume is more:
- Spectral broadening • If sample volume is less:
Measured velocity is too low.

48. OPTIMISATION OF COLOR FLOW
DOPPLER EXAMINATION
4. Wall Filters. • Cut off of the low frequency
noises, a cleaner high velocity blood flow signal
is displayed. • If set too high ,the blood flow is
discarded ;if set low, noise will be more.
5. Doppler Gain. • Controls the amplitude of the
colour display in colour or power Doppler mode
& the spectral display in pulse Doppler mode.

49. OPTIMISATION OF COLOR FLOW
DOPPLER EXAMINATION
6. Velocity scale. • Controls the range of
frequencies displayed • Too high scale-the
dynamic range is too large and low velocity
signals are missed simulating an area of
thrombosis • If scale is too low the dynamic
range is too small to display the high velocity
signals accurately resulting in aliasing.

50. DOPPLER ARTIFACTS
• 1.Artifacts caused by technical limitations:
• Aliasing
• Blooming artifact/colour bleed
• Directional ambiguity
• Partial volume artifact
• Absence of flow due to high velocity settings
• 2.Artifacts caused by patient anatomy
• Pseudo flow
• Flash artifact
• Mirror image artifact
• 3.Artifacts caused by machine factors
• Edge artifact-along cortical bone
• Twinkling artifact-beneath calculi

51. DOPPLER ARTIFACTS
• ALIASING : An inaccurate display of colour or
spectral Doppler velocity when the velocity
range exceeds the scale available to display it.

53. How to reduce ALIASING ?
• Drop the baseline
• Increase the available velocity range.
• Decrease the Doppler frequency shift by using
a lower insolating frequency or
• Increasing the Doppler angle.

54. • Power Doppler has no aliasing because it has
no directional or velocity component.
Advantage: - • 1. useful for localizing the highest
velocity region. • 2. It is used in identifying the
abnormal area in TIPSS & in localizing AV
fistulae. Disadvantage:- high velocity may not be
accurately measured.

55. DOPPLER ARTIFACTS
• Blooming Artifact ( Colour Bleed ).
• Caused by abnormally high gain settings.
• Causes the obscuration of thrombus or plaqu
es in the vessel. -
Also seen with ultrasound contrast agents

57. DOPPLER ARTIFACTS
• Flash Artifact • Manifests as a color signal due
to transducer or patient motion • It is seen in
left lobe of liver due to cardiac pulsation and
in hypoechoic areas such as cysts and fluid
collections.

59. DOPPLER ARTIFACTS
• Mirror Image artifact:
• displays objects on both sides of a strong refle
ctor
• The reflectors (diaphragm, pleural surface an
d aortic wall) directs some of the echoes to a
second reflector before it returns them to the
transducer resulting
in multipath reflection. Eg Duplication of sub
clavian artery (pleura reflector).

61. DOPPLER ARTIFACTS
• Twinkling artifact – Appears as discrete focus
of alternating colors behind echogenic object
e.g: Renal Calculi, bladder calcification and
cholesterol crystals in the gall bladder • FB like
iron fillings, aneurysm coil produce this
artifact.

63. GUIDELINES FOR AN OPTIMAL
COLOUR FLOW DOPPLER
EXAMINATION.
• The colour flow box should be kept as small &
superficial as possible • Adjust the gain and
filter settings • Adjust the velocity scale(PRF)
and baseline according to the flow conditions.

64. GUIDELINES FOR AN OPTIMAL
COLOUR FLOW DOPPLER
EXAMINATION
• Obtain an optimal Doppler angle by adjusting
the beam steering and probe position. ( 60 deg
or less) • Adjust the pulsed Doppler sample
volume size(gate) appropriately (2/3rd of the
velocity diameter) • AVOID TRANSDUCER
MOTION.

65. RECENT INNOVATIONS
IN DOPPLER US TECHNIQUES
• • Doppler US is experiencing technical
innovations that also contribute to improved B-
mode imaging. 1 Extended field of view imaging 2
(a) wideband, wide dynamic range systems-
improve the sensitivity of Doppler US; (b) Via
incorporation of custom-designed integrated
circuitry.- Doppler US processing on even
handheld US scanners. • 3 Contrast agents have
become much more than simple “echo
enhancers” for Doppler instrumentation.

66. REFERENCES
• 1. Diagnostic Ultrasound, Carol M Rumack and
others, 4th ed.
• 2.Clinical sonography-a practical guide:Roger C
Sanders, Barbara Hall-Terracciano-5th ed.