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MEDT8002 Blood flow measurement by Doppler technique Hans Torp Institutt for sirkulasjon og medisinsk bildediagnostikk Hans Torp NTNU, Norway Pulsed and continuous wave Doppler • Dopplershift from moving scatterers • Stocastic signal model • Clutter filtering • Spectrum analysis • Two dimensional signal model Ultralyd Doppler Ultralyd probe Lyden forandrer frekvens ved bevegelse o o Bilens hastighet: 70 km/time ~ 6% av lyd-hastighet 1/12 Ét halvtone-trinn i 12-toneskalaen: 2 = 5.94 % Endring i frekvens (turtall): 6% + 6% = 12 % Hans Torp NTNU, Norway Fartskontroll på E6 ved Heggstadmoen 300 250 200 150 100 50 0 0 50 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 100 150 frekvens [Hz] 200 250 300 Frekvensspekter før passering Frekvensspekter etter passering Ultralyd Doppler Ultralyd probe Dopplerskift fd == fo v/c + fo v/c = 2 fo v/c Signal processing for CW Doppler fo 0 fo+fd frequency 0 fd frequency Matlab: cwdoppler.m Hans Torp NTNU, Norway Blood velocity calculated from measured Doppler-shift fd = 2 fo v cos() / c v = c/2fo/cos() fd Hans Torp NTNU, Norway fd : fo : v: Dopplershift Transmitted frequency blood velocity : beam angle c: speed of sound (1540 m/s ) CW/PW Doppler blood flow meter Blood velocity Mitral inflow Velocity time Normal relaxation Hans Torp NTNU, Norway PEDOF developed in Trondheim 1976 Delayed relaxation Cardiac Doppler in Trondheim Angelsen & Kristoffersen Liv Hatle: Clinical practice PW&CW Dopper PEDOF - 1976 Holen Velocity -> Pressure gradient Continous Wave Doppler Double transducer CW recieve Pulsed Wave Doppler Single transducer PW transmit Range cell ø Artery ø Velocity profile, v Observation region in overlap of beams Matlab: pwdoppler. Hans Torp NTNU, Norway Signal from all scatterers within the ultrasound beam Signal from a limited sample volume 10-08/12pt Signal from a large number of red blood cells add up to a Gaussian random process a) Hans Torp NTNU, Norway b) 10-11/12pt G () e Power spectrum of the Doppler signal represents the distribution of velocities Hans Torp NTNU, Norway Doppler spektrum Hans Torp NTNU, Norway Doppler spectrum analysis: Different velocities separated in frequency Nyquist limit in Pulsed wave Doppler Nyquist-limit Doppler spectrum Subclavian Artery Velocity waveform restored by stacking 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. Resolution in Doppler spectrum analysis Δv *ΔT = λ / 2 ΔT Minimum dot area in the spectrum display: Δv *ΔT 100 Ultrasound wavelength: λ 50 Δv Doppler spectrum Velocity resolution Δv *ΔT = λ / 2 5 m/sec Blood flow velocity fo=2 MHz ~ λ = 0.76mm ΔT = 10 msec Δv = 3.6 cm/sec ~ 0.7 % of peak syst. velocity 10cm/sec Myocardial velocity fo=3.5 MHz ~ λ = 0.4 mm ΔT = 20 msec Δv = 2 cm/sec ~ 25 % of peak syst. velocity Aliasing in pulsed wave Doppler Velocity waveform restored by baseline shift or stacking Tracking Spectrum algorithm Pulse no 1 2 .. … Conventional spectrum Hans Torp NTNU, Norway N Pulse no 1 2 .. … Tracking spectrum N Blood velocity spectrum Subclavian Artery 100 50 -50 -100 Conventional spectrum Tracking spectrum PW Doppler Tracking spectrum CW Doppler 0.3m/s 2 m/s . 5 m/s Hans Torp NTNU, Norway What is the best pulse length? Energy per pulse limited by thermal index or ISPTA Pulse bandwidth B Sample vol. size ~ pulse length ~1/ B Thermal noise level ~ B (matched filter) Ultrasound pulse frequency [MHz] SNR considerations PW Doppler Blood signal spectral peak ~1/ B Spectral SNR ~ 1/ B^2 Doppler shift frequency [kHz] Power (blood vessel larger than sample vol.) Thermal noise Double pulse length give + 6 dB SNR Hans Torp NTNU, Norway Doppler shift frequency [kHz] Summary spectral Doppler • Complex demodulation give direction information of blood flow • Smooth window function removes sidelobes from cluttersignal • PW Doppler suffers from aliasing in many cardiac applications • SNR increases by the square of pulse length in PW Doppler