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MEDT8002 Blood flow measurement by Doppler technique Hans Torp Institutt for sirkulasjon og medisinsk bildediagnostikk Hans Torp NTNU, Norway

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Pulsed 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

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Ultralyd Doppler Ultralyd probe

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Lyden forandrer frekvens ved bevegelse o o 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 Torp NTNU, Norway

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Fartskontroll på E6 ved Heggstadmoen Motorsykkelens turtall 0.5*(f1+f2) = 140.3 Hz Dopplerskift: +/- 9.55 Hz ~ +/- 6.8 % Motorsykkelens fart: 340 * 6.8/100 *3.6 = 83.3 km/h Fartsgrense = 80 km/h Frekvensspekter før passering Frekvensspekter etter passering

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Ultralyd Doppler Ultralyd probe Dopplerskift fd = fo v/c Dopplerskift fd = fo v/c + fo v/c = 2 fo v/c

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Signal processing for CW Doppler fo frequency fo+fd 0frequencyfd 0 Hans Torp NTNU, Norway Matlab: cwdoppler.m

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Blood velocity calculated from measured Doppler-shift f d = 2 f o v cos( ) / c v = c/ 2f o / cos ( ) f d fd : Dopplershift fo : Transmitted frequency v : blood velocity : beam angle c : speed of sound (1540 m/s ) Hans Torp NTNU, Norway

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time Velocity CW/PW Doppler blood flow meter PEDOF developed in Trondheim 1976 Blood velocity Mitral inflow Normal relaxation Delayed relaxation Hans Torp NTNU, Norway

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Angelsen & Kristoffersen PW&CW Dopper PEDOF - 1976 Holen Velocity -> Pressure gradient Liv Hatle: Clinical practice Cardiac Doppler in Trondheim

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Continous Wave Doppler Signal from all scatterers within the ultrasound beam Pulsed Wave Doppler Signal from a limited sample volume Hans Torp NTNU, Norway Matlab: pwdoppler.m

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a) b) 10-08/12pt Hans Torp NTNU, Norway Signal from a large number of red blood cells add up to a Gaussian random process

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G ( ) e 10-11/12pt Hans Torp NTNU, Norway Power spectrum of the Doppler signal represents the distribution of velocities

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Doppler spektrum Hans Torp NTNU, Norway

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Doppler spectrum analysis: Different velocities separated in frequency

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Nyquist limit in Pulsed wave Doppler Nyquist-limit

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Doppler spectrum Subclavian Artery Velocity waveform restored by stacking

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Tidsvindu-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.

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Resolution in Doppler spectrum analysis Δ T Δ v Δv *ΔT = λ / 2 Minimum dot area in the spectrum display: Δv *ΔT Ultrasound wavelength: λ

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Doppler spectrum Velocity resolution Blood flow velocity fo=2 MHz ~ λ = 0.76mm ΔT = 10 msec Δv = 3.6 cm/sec ~ 0.7 % of peak syst. velocity Myocardial velocity fo=3.5 MHz ~ λ = 0.4 mm ΔT = 20 msec Δv = 2 cm/sec ~ 25 % of peak syst. velocity Δv *ΔT = λ / 2 5 m/sec 10cm/sec

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Aliasing in pulsed wave Doppler Velocity waveform restored by baseline shift or stacking

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Tracking Spectrum algorithm Pulse no 1 2.. … N Hans Torp NTNU, Norway Pulse no 1 2.. … N Conventional spectrumTracking spectrum

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Blood velocity spectrum Subclavian Artery Conventional spectrum Tracking spectrum -100 -50 50 100

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PW Doppler Tracking spectrum 0.3m/s 2 m/s 5 m/s. Hans Torp NTNU, Norway CW Doppler

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Thermal noise SNR considerations PW Doppler Hans Torp NTNU, Norway Doppler shift frequency [kHz] Ultrasound pulse frequency [MHz] Doppler shift frequency [kHz] Power Pulse bandwidth 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 by thermal index or ISPTA

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Summary spectral Doppler Complex demodulation give direction information of blood flowComplex demodulation give direction information of blood flow Smooth window function removes sidelobes from clutter- signalSmooth window function removes sidelobes from clutter- signal PW Doppler suffers from aliasing in many cardiac applicationsPW Doppler suffers from aliasing in many cardiac applications SNR increases by the square of pulse length in PW DopplerSNR increases by the square of pulse length in PW Doppler

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