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