Doppler Ultrasound

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Doppler Ultrasound

• Resident Categorical Course

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Laminar Flow

• also called parabolic flow
• fluid layers slide over one another
• central portion of fluid moves at maximum speed
• flow near vessel wall hardly moves at all
• friction with wall

3
Turbulent Flow

• random chaotic
• individual particles flow in all directions
• net flow is forward
• Often occurs beyond obstructionsuch as plaque on
  vessel wall

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Flow, Pressure Resistance

• Quantity of flow is function of
• Pressure
• Resistance
• Pressure
• Heart provides pulsating pressure

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Flow and Pressure
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Resistance to Flow

• more resistance lower flow rate
• resistance affected by
• fluids viscosity
• vessel length
• vessel diameter

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Resistance to Flow
Less Viscosity More Flow
More Viscosity Less Flow
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Resistance to Flow
Longer Vessel Less Flow
Shorter Vessel More Flow
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Resistance to Flow
Larger Diameter More Flow
Smaller Diameter Less Flow
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Flow Variations

• Large fluctuation in pressure flow in arteries
  with pulse
• Less fluctuation in pressure flow in veins
• pulse variations dampened by arterial system

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Normal Vessel

• Distensible
• Expands contracts with
• pressure changes
• Changes over cardiac cycle
• Vessel expands during systole
• Vessel contracts during diastole

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Flow Rate Measurements

• Volume flow rate
• Volume of liquid passing a point per unit time
• Example
• 100 ml / second

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Flow Rate Measurements

• Linear flow rate
• Distance liquid moves past a point per unit time
• Example
• 10 cm / second

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Flow Rate Measurements

• Volume Flow Rate Linear flow rate X Cross
  Sectional Area

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Flow Rate Measurements

• Volume Flow Rate Linear flow rate X
  Cross-sectional Area

High Velocity Small Cross-section
Low Velocity Large Cross-section
Same Volume Flow Rate
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Volume Flow Rates

• constant volume flow rate in all parts of closed
  system

Any change in flow rate would mean youre gaining
or losing fluid.
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Stenosis

• narrowing in a vessel
• fluid must speed up in stenosis to maintain
  constant flow volume
• no net gain or loss of flow
• turbulent flow common downstream of stenosis

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Stenosis

• If narrowing is short in length
• Little increase in flow resistance
• Little effect on volume flow rate
• If narrowing is long
• Resistance to flow increased
• Volume flow rate decreased

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Doppler Shift

• difference between received transmitted
  frequency
• caused by relative motion between sound source
  receiver
• Frequency shift indicative of reflector speed

OUT
IN
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Doppler Angle

• angle between sound travel flow
• 0 degrees
• flow in direction of sound travel
• 90 degrees
• flow perpendicular to sound travel

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Doppler Angle

• Angle between direction of sound and direction of
  fluid flow

q
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Doppler Sensing

• Flow vector can be separated into two vectors
• Only flow parallel to sound sensed by scanner!!!
• Sensed flow always lt actual flow

Flow parallel to sound
Flow perpendicular to sound
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Doppler Sensing

• cos(q) SF / AF

Actual flow (AF)
q
Sensed flow (SF)
q
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Doppler Equation
2 X fo X v X cosq f D fe
- fo -------------------------
c
q
v

• fD Doppler Shift in MHz
• fe echo of reflected frequency (MHz)
• fo operating frequency (MHz)
• v reflector speed (m/s)
• q angle between flow sound propagation
• c speed of sound in soft tissue (m/s)

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Relationships
2 X fo X v X cosq f D fe
- fo -------------------------
c

• Positive Doppler shift
• reflector moving toward transducer
• echoed frequency gt operating frequency
• Negative Doppler shift
• reflector moving away from transducer
• echoed frequency lt operating frequency

q
q
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Relationships
2 X fo X v X cosq f D fe
- fo -------------------------
c
q

• Doppler angle affects measured Doppler shift
• Larger angle
• Smaller cosine
• Small Doppler shift

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Simplified (?) Equation
2 X fo X v X cosq f D fe
- fo -------------------------
c
77 X fD (kHz) v (cm/s)
-------------------------- fo
(MHz) X cosq
Simplified

• Solve for reflector velocity
• Insert speed of sound for soft tissue
• Stick in some units

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Doppler Relationships
Constant
77 X fD (kHz) v (cm/s)
-------------------------- fo
(MHz) X cos?
?

• higher reflector speed results in greater Doppler
  shift
• higher operating frequency results in greater
  Doppler shift
• larger Doppler angle results in lower Doppler
  shift

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Continuous Wave Doppler

• Audio presentation
• 2 transducers used
• one continuously transmits
• one continuously receives

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Continuous Wave DopplerReceiver Function

• receives reflected sound waves
• Subtract signals
• detects frequency shift
• typical shift 1/1000 th of source frequency
• usually in audible sound range
• Amplify subtracted signal
• Play directly on speaker

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Pulse Wave vs. Continuous Wave Doppler
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Doppler Pulses

• Different Imaging Doppler pulses
• short pulses required for imaging
• Accurate echo timing
• minimizes spatial pulse length
• optimizes axial resolution
• longer pulses required for Doppler analysis
• reduces bandwidth
• provide purer transmitted frequency
• important for accurate measurement of frequency
  differences needed to calculate speed

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Color-Flow Display Features

• Imaged electronically scanned twice
• imaging scan processes echo intensity
• Doppler scan calculates Doppler shifts
• Reduced frame rates
• only 1 pulse required for imaging
• additional pulses required when multiple focuses
  used
• several pulses may be required along a scan line
  to determine Doppler shift

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Duplex Doppler Gates

• operator defines active Doppler region (gate)
• only sound in gate analyzed

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Spectral Display

• Displays real-time range of frequencies received
• amplitude of each frequency indicated by
  brightness
• display indicates range of frequencies received
• corresponds to range of speeds of blood cells
• indicative of type of flow
• laminar, turbulent

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Absolute Speed Measurement

• Absolute speed measurements must include Doppler
  angle
• angle between flow sound propagation
• Indicated by operator
• Accuracy affects flow speed accuracy

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Relative Speed Measurement

• relative measurements can be useful
• Doppler angle not required
• indications of spectral broadening do not require
  absolute measurements
• ratio of peak-systolic to end-diastolic relative
  flows independent of angle

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Color Doppler

• User defines window superimposed on gray scale
  image
• For each location in window scanner determines
• flow direction
• mean value
• Variance
• window size affects frame rate
• larger window slower scanning
• more Doppler pulses required

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Spectral vs. Color-Flow

• spectral Display shows detailed frequency data
  for single location
• Color Dopplers color represents complete
  spectrum at each location in window

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Power Doppler

• AKA
• Energy Doppler
• Amplitude Doppler
• Doppler angiography
• Magnitude of color flow output displayed rather
  than Doppler frequency signal
• flow direction or different velocities not
  displayed

"Color Power Angio" of the Circle of Willis