MEDT 8002 Blood flow measurement by Doppler technique

MEDT 8002 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 E 6 ved Heggstadmoen Motorsykkelens turtall 0. 5*(f 1+f 2) = 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

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

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

Blood velocity calculated from measured Doppler-shift fd = 2 fo v cos( ) / c v = c/2 fo/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 Pulsed Wave Doppler Matlab: pwdoppler. m Hans Torp NTNU, Norway Signal from all scatterers within the ultrasound beam Signal from a limited sample volume

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

10 -11/12 pt 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 Ultrasound wavelength: λ Δv

Doppler spectrum Velocity resolution Δv *ΔT = λ / 2 5 m/sec Blood flow velocity fo=2 MHz ~ λ = 0. 76 mm ΔT = 10 msec Δv = 3. 6 cm/sec ~ 0. 7 % of peak syst. velocity 10 cm/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 -100 Conventional spectrum Tracking spectrum

PW Doppler Tracking spectrum CW Doppler 0. 3 m/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 [k. Hz] Power (blood vessel larger than sample vol. ) Thermal noise Double pulse length give + 6 d. B SNR Hans Torp NTNU, Norway Doppler shift frequency [k. Hz]

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