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Scattering from Individual Red Blood Cells

Scattering from Individual Red Blood Cells. Scattering from Ensembles of Red Blood Cells. Scattering from Ensembles of Red Blood Cells. The Doppler Effect. Moving source. Moving receiver. ‘Pulse-echo’: . Continuous wave Doppler System: getting directionality I.

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Scattering from Individual Red Blood Cells

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  1. Scattering from Individual Red Blood Cells

  2. Scattering from Ensembles of Red Blood Cells

  3. Scattering from Ensembles of Red Blood Cells

  4. The Doppler Effect Moving source Moving receiver ‘Pulse-echo’:

  5. Continuous wave Doppler System:getting directionality I

  6. Continuous wave Doppler System:getting directionality II

  7. Pulsed wave Doppler: Approach Question: what is really being measured here?

  8. Pulsed wave Doppler: Basic Equations

  9. Pulsed wave Doppler:Signals Reference oscillator

  10. Measurement limitations: aliasing Well sampled At Nyquist (PRF/2) Aliased

  11. Measurement limitations: depth vs velocity limits

  12. Measurement limitations: velocity resolution limits Velocity resolution  Frequency resolution Fres = 1/(total time) = PRF/(N) where N=number of pulses Using the Doppler equation… Vres = c*PRF/(2*Ftrans*N) Increasing N therefore improves resolution, but total time must be less than that of expected changes to flow (e.g. due to cardiac cycle)

  13. Measurement limitations: pulse length

  14. Measurement limitations: tranverse motion Fundamental trade-off between localization and velocity resolution

  15. A closer look at spectra and their relation to hemodynamics

  16. Measurement limitations: tranverse motion

  17. Measurement of Haemodynamic indices

  18. Colour flow: how it works

  19. Colour flow: The autocorrelation function

  20. Colour flow: how it works Demodulated Audio (time domain) Audio (freq. domain) For pulsed wave Doppler we had: And calculated full spectra (i.e. velocity distributions in a time window) with: For colour flow, we use the autocorrelator to calculate the mean velocity (Jensen ’96): N=# pulses Is=Iaudio

  21. Demodulated Audio (time domain) Audio (freq. domain) Power Doppler: how it works For power Doppler, we use the audio frequency data to calculate the mean power: N=# pulses Is=Iaudio The power is proportional to the volume of moving blood

  22. Colour flow: ‘clutter’ filtering Prior to power and velocity estimation, a high pass filter (e.g. FIR, IIR, regression) is applied to remove the tissue signals - the ‘clutter’ filter can have a fundamental impact on flow detection

  23. Flow imaging processing overview Velocity Est. Thresholding/ Flow decision Velocity image Raw Data ‘ensemble’ Clutter filter Power Est. Power image Variance Est. Variance image Power est. Bscan - Ensembles generally 4-16 pulses along a beam direction - process repeated across image plane

  24. Colour flow: limitations • - noise and small number of pulses results in vessel detection limits • - ‘flash’ artefacts due to transient tissue/transducer motion • filtering results in increased loss of slower flow • As with pulsed-wave Doppler: • -aliasing • -depth vs velocity limits (more stringent due to 2D image acquisition)

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