doppler ultrasound resident categorical course. laminar flow parabolic flowalso called parabolic...
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Doppler UltrasoundDoppler Ultrasound
Resident Categorical Course
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Laminar FlowLaminar Flow
• also called parabolic flowparabolic 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
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Turbulent FlowTurbulent Flow• random & chaotic• individual particles flow in all directions• net flow is forward• Often occurs beyond obstruction
such as plaque on vessel wall
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Flow, Pressure & ResistanceFlow, Pressure & Resistance• Quantity of flow is function of
Pressure Resistance
• Pressure Heart provides pulsating pressure
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Flow and PressureFlow and Pressure
Low Pressure
Low Flow
High Pressure
High Flow
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Resistance to FlowResistance to Flow• more resistance = lower flow
rate
• resistance affected by
fluid’s viscosity vessel length vessel diameter
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Resistance to FlowResistance to Flow
Less ViscosityMore Flow
More ViscosityLess Flow
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Resistance to FlowResistance to Flow
Shorter VesselMore Flow
Longer VesselLess Flow
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Resistance to FlowResistance to Flow
Larger DiameterMore Flow
Smaller DiameterLess Flow
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George DavidAssociate Professor
Flow VariationsFlow 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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George DavidAssociate Professor
Normal VesselNormal 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 MeasurementsFlow Rate Measurements• Volume flow rate
Volume of liquid passing a point per unit time
• Example 100 ml / second
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Flow Rate MeasurementsFlow Rate Measurements• Linear flow rate
Distance liquid moves past a point per unit time
• Example 10 cm / second
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Flow Rate MeasurementsFlow Rate Measurements
Volume Flow Rate = Linear flow rate X Cross Sectional Area
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Flow Rate MeasurementsFlow Rate MeasurementsVolume Flow Rate = Linear flow rate X Cross-sectional Area
Same Volume Flow Rate
High VelocitySmall Cross-section Low Velocity
Large Cross-section
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Volume Flow RatesVolume Flow Rates• constant volume flow rate in
all parts of closed system
Any change in flow rate would mean you’re gaining or
losing fluid.
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StenosisStenosis
• 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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George DavidAssociate Professor
StenosisStenosis
• 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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George DavidAssociate Professor
Doppler ShiftDoppler Shift
• difference between received & transmitted frequency
• caused by relative motion between sound source & receiver
• Frequency shift indicative of reflector speed
IN
OUT
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George DavidAssociate Professor
Doppler AngleDoppler 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 AngleDoppler Angle
Angle between direction of sound and direction of fluid flow
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Doppler SensingDoppler Sensing• Flow vector can
be separated into two vectors
• Only flow parallel to sound sensed by scanner!!!
• Sensed flow always < actual flow
Flow parallel to
sound
Flow perpendicular to sound
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Doppler SensingDoppler Sensing
cos() = SF / AF
Sensed flow(SF)
Actual flow(AF)
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George DavidAssociate Professor
Doppler EquationDoppler Equation
fD =Doppler Shift in MHz
fe = echo of reflected frequency (MHz)
fo = operating frequency (MHz)v = reflector speed (m/s) = angle between flow & sound propagationc = speed of sound in soft tissue (m/s)
2 X fo X v X cosf D = fe - fo = ------------------------- c
v
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RelationshipsRelationships
• Positive Doppler shift reflector moving toward transducer echoed frequency > operating frequency
• Negative Doppler shift reflector moving away from transducer echoed frequency < operating frequency
2 X fo X v X cosf D = fe - fo = ------------------------- c
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RelationshipsRelationships
• Doppler angle affects measured Doppler shift
• Larger angle Smaller cosine Small Doppler shift
2 X fo X v X cosf D = fe - fo = ------------------------- c
cos
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Simplified (?) EquationSimplified (?) Equation
• Solve for reflector velocity
• Insert speed of sound for soft tissue
• Stick in some units
2 X fo X v X cosf D = fe - fo = ------------------------- c
77 X fD (kHz)v (cm/s) = -------------------------- fo (MHz) X cosSimplified:
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George DavidAssociate Professor
Doppler RelationshipsDoppler Relationships
• higher reflector speed results in greater Doppler shift
• higher operating frequency results in greater Doppler shift
• larger Doppler angle results in lower Doppler shift
77 X fD (kHz)v (cm/s) = -------------------------- fo (MHz) X cos
Constant
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George DavidAssociate Professor
Continuous Wave DopplerContinuous Wave Doppler
• Audio presentation
• 2 transducers used one continuously transmits one continuously receives
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• 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
Continuous Wave Doppler:Receiver Function
Continuous Wave Doppler:Receiver Function
- =
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Pulse Wave vs. Continuous Wave Doppler
Pulse Wave vs. Continuous Wave Doppler
Continuous Wave Pulse Wave
No Image Image
Sound on continuously
Both imaging & Doppler sound pulses generated
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Doppler PulsesDoppler 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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George DavidAssociate Professor
Color-Flow Display FeaturesColor-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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• operator defines active Doppler region (gate)
• only sound in gate analyzed
Duplex Doppler GatesDuplex Doppler Gates
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• 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
Spectral DisplaySpectral Display
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Absolute Speed MeasurementAbsolute Speed Measurement• Absolute speed measurements
must include Doppler angleDoppler angle angle between flow & sound propagation Indicated by operator Accuracy affects flow speed accuracy
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George DavidAssociate Professor
Relative Speed MeasurementRelative 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 DopplerColor 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-FlowSpectral vs. Color-Flow
• spectral Display shows detailed frequency data for single location
• Color Doppler’s color represents complete spectrum at each location in window
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Power DopplerPower 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