Principles of Doppler ultrasound - Wake Forest Baptist · PDF filePrinciples of Doppler ultrasound
Display of Motion & Doppler Ultrasound
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Transcript of Display of Motion & Doppler Ultrasound
Display of Motion &Doppler Ultrasound
Resident Class
Hemodynamics
PlugLaminarDisturbedTurbulent
Blood Flow Characterization
Plug Flow
Type of normal flowConstant fluid speed across
tubeOccurs near entrance of flow
into tube
Laminar Flowalso called parabolic flowparabolic flowfluid layers slide over one anotheroccurs further from entrance to
tubecentral portion of fluid moves at
maximum speedflow near vessel wall hardly moves
at allfriction with wall
FlowDisturbed FlowDisturbed Flow
Normal parallel stream lines disturbedprimarily forward particles still flow
Turbulent FlowTurbulent Flowrandom & chaoticindividual particles flow in all directionsnet flow is forwardOften occurs beyond obstruction
such as plaque on vessel wall
Flow, Pressure & ResistancePressure
pressure difference between ends of tube drives fluid flow
Resistancemore resistance = lower flow rateresistance affected by
fluid’s viscosity vessel length vessel diameter
flow for a given pressure determined by resistance
Flow Variations
pressure & flow in arteries fluctuate with pulse
pressure & flow in veins much more constantpulse variations dampened by arterial
system
Flow Rate MeasurementsVolume flow rate
Volume of liquid passing a point per unit time
Example100 ml / second
Flow Rate MeasurementsLinear flow rate
Distance liquid moves past a point per unit time
Example10 cm / second
Flow Rate Measurements
Volume Flow Rate = Linear flow rate X Cross Sectional Area
Flow 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 Ratesconstant volume flow rate in all
parts of closed system
Sure! Any change in flow rate would
mean you’re gaining or losing
fluid.
Stenosisnarrowing in a vesselfluid must speed up in stenosis
to maintain constant flow volumeno net gain or loss of flow
turbulent flow common downstream of stenosis
StenosisIf narrowing is short in length
Little increase in overall resistance to flow Little effect on volume flow rate
If narrowing is long Resistance to flow increased Volume flow rate decreased
Doppler Shiftdifference between received & transmitted
frequencycaused by relative motion between sound
source & receiverFrequency shift indicative of reflector
speed
IN
OUT
Doppler Exampleschange in pitch of as object
approaches & leaves observertrainAmbulance siren
moving blood cellsmotion can be presented as sound or as an image
Doppler Angle
angle between sound travel & flow
0 degreesflow in direction of sound
travel
90 degreesflow perpendicular to sound
travel
Flow ComponentsFlow vector can be separated into two vectors
Flow parallel to sound
Flow perpendicular to sound
Doppler SensingOnly flow parallel to sound
sensed by scanner!!!
Flow parallel to
sound
Flow perpendicular to sound
Doppler SensingSensed flow always < actual flow
Sensed flow
Actual flow
Doppler Sensingcos() = SF / AF
Sensed flow(SF)
Actual flow(AF)
Doppler Equation
wherefD =Doppler Shift in MHzfe = 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
Relationships
positive shift when reflector moving toward transducerechoed frequency > operating frequency
negative shift when reflector moving away from transducerechoed frequency < operating frequency
2 X fo X v X cosf D = fe - fo = ------------------------- c
Relationships
Doppler angle affects measured Doppler shift
2 X fo X v X cosf D = fe - fo = ------------------------- c
cos
Simplified (?) Equation
Solve for reflector velocityInsert speed of sound for soft tissueStick 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:
Doppler 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
Continuous Wave Doppler
Audio presentation onlyNo imageUseful as fetal dose monitor
Continuous Wave Doppler
2 transducers usedone continuously transmits
voltage frequency = transducer’s operating frequency typically 2-10 MHz
one continuously receivesReception Area
flow detected within overlap of transmit & receive sound beams
Continuous Wave Doppler:Receiver Function
receives reflected sound wavesSubtract signals
detects frequency shifttypical shift ~ 1/1000 th of source frequency
usually in audible sound range
Amplify subtracted signalPlay directly on speaker
- =
Pulse Wave vs. Continuous Wave Doppler
Continuous Wave Pulse Wave
No Image Image
Sound on continuously
Both imaging & Doppler sound pulses generated
Doppler Pulsesshort pulses required for imaging
minimizes spatial pulse lengthoptimizes axial resolution
longer pulses required for Doppler analysisreduces bandwidthprovide purer transmitted frequency
important for accurate measurement of frequency differences needed to calculate speed
Color-Flow Display Features
Imaged electronically scanned twiceimaging scan processes echo intensityDoppler scan calculates Doppler shifts
Reduced frame ratesonly 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
Duplex Doppler Gatesoperator indicates active Doppler region on
displayregions are called gatesgates
only sound in gate analyzed for frequency shiftcan be isolated based on delay time after pulse
Gate
Spectral Display
shows range of frequencies receivedamplitude of each
frequency indicated by gray shade
can be displayed real timefast Fourier Transform
(FFT) technique Elapsed Time
Frequency
frequencyrange
Spectral Broadeningdisplay indicates
range of frequencies
corresponds to range of speeds of blood cells
range indicative of type of flow laminar, disturbed, turbulent
Time
Frequency
frequencyrange
Pulse Wave DopplerAllows range selectivityrange selectivitymonitor Doppler shift (frequency difference)
at only selected depth(s)ability to separate flow from >1 vessel or
localize flow within vessel
Spectral vs. Color-Flowspectral Display shows frequency range directly Color Doppler’s color represents complete
spectrum at each pixel
Elapsed Time
Frequency
frequencyrange
Power DopplerAKA
Energy DopplerAmplitude DopplerDoppler 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