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