Doppler Ultrasound Resident Categorical Course. Laminar Flow parabolic flowalso called parabolic...
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Transcript of Doppler Ultrasound Resident Categorical Course. Laminar Flow parabolic flowalso called parabolic...
Doppler UltrasoundDoppler Ultrasound
Resident Categorical Course
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
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
Flow, Pressure & ResistanceFlow, Pressure & Resistance• Quantity of flow is function of
Pressure Resistance
• Pressure Heart provides pulsating pressure
Flow and PressureFlow and Pressure
Low Pressure
Low Flow
High Pressure
High Flow
Resistance to FlowResistance to Flow• more resistance = lower flow
rate
• resistance affected by
fluid’s viscosity vessel length vessel diameter
Resistance to FlowResistance to Flow
Less ViscosityMore Flow
More ViscosityLess Flow
Resistance to FlowResistance to Flow
Shorter VesselMore Flow
Longer VesselLess Flow
Resistance to FlowResistance to Flow
Larger DiameterMore Flow
Smaller DiameterLess Flow
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
George DavidAssociate Professor
Normal VesselNormal Vessel
• Distensible Expands & contracts with
» pressure changes
» Changes over cardiac cycle
• Vessel expands during systole
• Vessel contracts during diastole
Flow Rate MeasurementsFlow Rate Measurements• Volume flow rate
Volume of liquid passing a point per unit time
• Example 100 ml / second
Flow Rate MeasurementsFlow Rate Measurements• Linear flow rate
Distance liquid moves past a point per unit time
• Example 10 cm / second
Flow Rate MeasurementsFlow Rate Measurements
Volume Flow Rate = Linear flow rate X Cross Sectional Area
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
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.
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
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
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
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
Doppler AngleDoppler Angle
Angle between direction of sound and direction of fluid flow
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
Doppler SensingDoppler Sensing
cos() = SF / AF
Sensed flow(SF)
Actual flow(AF)
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
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
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
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:
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
George DavidAssociate Professor
Continuous Wave DopplerContinuous Wave Doppler
• Audio presentation
• 2 transducers used one continuously transmits one continuously receives
• 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
- =
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
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
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
• operator defines active Doppler region (gate)
• only sound in gate analyzed
Duplex Doppler GatesDuplex Doppler Gates
• 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
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
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
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
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
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