MEDT8002

Blood flow measurement by Doppler technique

Hans Torp

Institutt for sirkulasjon og medisinsk bildediagnostikk

Hans TorpNTNU, Norway

Pulsed and continuous wave DopplerPulsed and continuous wave Doppler

• Dopplershift from moving scatterersDopplershift from moving scatterers

• Stocastic signal modelStocastic signal model

• Clutter filteringClutter filtering

• Spectrum analysisSpectrum analysis

• Two dimensional signal modelTwo dimensional signal model

Ultralyd DopplerUltralyd Doppler

Ultralyd probe

Lyden forandrer frekvens ved bevegelseLyden forandrer frekvens ved bevegelse

oo

Bilens hastighet: 70 km/time ~ 6% av lyd-hastighet

Ét halvtone-trinn i 12-toneskalaen: 2 = 5.94 %

Endring i frekvens (turtall): 6% + 6% = 12 %

1/12

Hans TorpNTNU, Norway

Fartskontroll på E6 ved HeggstadmoenFartskontroll på E6 ved Heggstadmoen

0 50 100 150 200 250 3000

50

100

150

200

250

300

frekvens [Hz]Motorsykkelens turtall 0.5*(f1+f2) = 140.3 HzDopplerskift: +/- 9.55 Hz ~ +/- 6.8 %Motorsykkelens fart: 340 * 6.8/100 *3.6 = 83.3 km/h

Fartsgrense = 80 km/h

Frekvensspekter før passeringFrekvensspekter etter passering

Ultralyd DopplerUltralyd Doppler

Ultralyd probe

Dopplerskift fd = fo v/cDopplerskift fd = fo v/c + fo v/c = 2 fo v/c

Signal processing Signal processing for CW Dopplerfor CW Doppler

fofrequency

fo+fd0 frequencyfd0

Hans TorpNTNU, Norway

Matlab: cwdoppler.m

Blood velocity calculated from Blood velocity calculated from measured Doppler-shiftmeasured Doppler-shift

fd = 2 fo v cos() / c

v = c/2fo/cos() fd

fd : Dopplershiftfo : Transmitted frequencyv : blood velocity

: beam angle c : speed of sound (1540 m/s )Hans Torp

NTNU, Norway

time

Velocity

CW/PW Doppler blood flow meterCW/PW Doppler blood flow meter

PEDOF developed in Trondheim 1976

Blood velocity Mitral inflow

Normal relaxation

Delayed relaxation

Hans TorpNTNU, Norway

Angelsen & Kristoffersen PW&CW Dopper PEDOF - 1976

HolenVelocity -> Pressure gradientLiv Hatle: Clinical practice

Cardiac Doppler in TrondheimCardiac Doppler in Trondheim

Continous Wave DopplerContinous Wave Doppler

ø

Single transducerPW

Double transducerCW

ø

transmit

recieve

Velocity profile, v

Artery

Range cell

Observation region in overlap of beams

Signal from all scattererswithin the ultrasound beam

Pulsed Wave DopplerPulsed Wave Doppler

Signal from a limited sample volume

ø

Single transducerPW

Double transducerCW

ø

transmit

recieve

Velocity profile, v

Artery

Range cell

Observation region in overlap of beams

Hans TorpNTNU, Norway

Matlab: pwdoppler.m

a)

b)

10-08/12pt

Hans TorpNTNU, Norway

Signal from a large number of red blood cellsSignal from a large number of red blood cellsadd up to a Gaussian random processadd up to a Gaussian random process

G ()e

10-11/12pt

Hans TorpNTNU, Norway

Power spectrum of the Doppler signal Power spectrum of the Doppler signal represents the distribution of velocitiesrepresents the distribution of velocities

Doppler spektrumDoppler spektrum

frequ

ency

time

spec

tral

am

plit

ude

Hans TorpNTNU, Norway

Doppler spectrum analysis:Doppler spectrum analysis:Different velocities separated in frequencyDifferent velocities separated in frequency

Nyquist limit in Pulsed wave DopplerNyquist limit in Pulsed wave Doppler

Nyquist-limit

DopplerDoppler spectrum spectrumSubclavian ArterySubclavian Artery

Velocity waveform restoredby stacking

Tidsvindu-lengde ved spektralanalyseTidsvindu-lengde ved spektralanalyse

Performance of the spectrum sonogram as a function of window length. Upper left: 150 points, upper right: 64 points, lower left: 32 points, and lower right: 16 points. The window type is Hamming and the degree of overlap is 75% in all pictures.

Resolution in Resolution in Doppler spectrum analysisDoppler spectrum analysis

-100

-50

50

100

Δ T

Δ v

Δv *ΔT = λ / 2

Minimum dot area in the spectrum display: Δv *ΔT

Ultrasound wavelength: λ

Doppler spectrumDoppler spectrumVelocity resolutionVelocity resolution

Blood flow velocityfo=2 MHz ~ λ = 0.76mmΔT = 10 msecΔv = 3.6 cm/sec ~ 0.7 % of peak syst. velocity

Myocardial velocityfo=3.5 MHz ~ λ = 0.4 mmΔT = 20 msecΔv = 2 cm/sec ~ 25 % of peak syst. velocity

Δv *ΔT = λ / 2

5 m/sec

10cm/sec

Aliasing in pulsed wave DopplerAliasing in pulsed wave Doppler

Velocity waveform restoredby baseline shift or stacking

Tracking Spectrum algorithm Tracking Spectrum algorithm

Pulse no1 2 .. … N

Hans TorpNTNU, Norway

Pulse no1 2 .. … N

Conventional spectrum Tracking spectrum

Blood velocity spectrumBlood velocity spectrumSubclavian ArterySubclavian Artery

Conventional spectrum Tracking spectrum

-100

-50

50

100

PW DopplerPW DopplerTracking spectrumTracking spectrum

0.3m/s

2 m/s

5 m/s

.

Hans TorpNTNU, Norway

CW DopplerCW Doppler

Thermal noise

SNR considerations PW DopplerSNR considerations PW Doppler

Hans TorpNTNU, Norway

Doppler shift frequency [kHz]

Ult

raso

und

puls

e fr

eque

ncy

[MH

z]

Doppler shift frequency [kHz]

Pow

er

Pulsebandwidth B

Sample vol. size ~ pulse length ~1/ B

Thermal noise level ~ B (matched filter)

Blood signal spectral peak ~1/ B(blood vessel larger than sample vol.)

Spectral SNR ~ 1/ B^2

Double pulse length give + 6 dB SNR

What is the best pulse length?

Energy per pulse limited bythermal index or ISPTA

Summary spectral DopplerSummary spectral Doppler

• Complex demodulation give direction information of blood Complex demodulation give direction information of blood flowflow

• Smooth window function removes sidelobes from clutter-Smooth window function removes sidelobes from clutter-signalsignal

• PW Doppler suffers from aliasing in many cardiac PW Doppler suffers from aliasing in many cardiac applicationsapplications

• SNR increases by the square of pulse length in PW Doppler SNR increases by the square of pulse length in PW Doppler