Doppler Physics and Instrumentation. Topics. Doppler effect Doppler equation Doppler Modes Doppler Artifacts. Scattering of Ultrasound by RBC’s. Red blood cells Diameter: 7µm Raleigh scatterers Smaller than ultrasound wavelength (0.1-0.7 mm). Scattering of Ultrasound by RBC’s.

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Doppler Physics and Instrumentation

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Scattering of Ultrasound by RBC’s • Red blood cells • Diameter: 7µm • Raleigh scatterers • Smaller than ultrasound wavelength (0.1-0.7 mm)

Scattering of Ultrasound by RBC’s • The Intensity of the Raleigh Scatterers is • determined by: • Transducer frequency • Scatterer density • Scatterer size • Scatterer acoustic impedance

The Doppler Effect Doppler shift is a change in frequency caused by the relative movement of the sound source or the reflector. Transducer is the sound wave source. Red Blood Cells are the reflector RBC’s can be: • stationary • moving towards the transducer • moving away from the transducer

Beam Flow Angle • Ø = 0° • Parallel to flow - Optimum shift • Ø = 1° to 89° • Shift is reduced • Ø = 90° • Perpendicular to flow - No shift

Beam Flow Angle • Best Doppler angle is parallel to flow and closest to 0 • Angle is effected by • organ position • patient position • transducer position

Reflector Velocity The Doppler shift can be used to calculate the velocity of a column of moving RBCs if the following is known: • fo = transducer frequency • = beam flow angle • c = speed of sound in tissue (1540 m/s)

Doppler Modes • Pulsed Wave Doppler • Continuous Wave Doppler • Color Doppler • Power Doppler

Pulsed Doppler A C A = PRP B = Pulse Duration C= Reception Time B

Pulsed Doppler • The # of cycles per pulse is determined by: • Strength of the excitation voltage. • Electro-mechanical efficiency. • The damping characteristics.

Pulsed Doppler Pulsed Duration = period x cycles per pulse PRP = PD + Reception Time PRF = # of pulses per second PRP = 1/PRF Duty Factor = PD PRP

Aliasing • The inability of a PD transducer to detect large Doppler shift is known as aliasing.

Aliasing • The sampling rate = PRF • Maximum PRF is determine by the go- return time. • Deeper vessels requires longer go-return time • and thus a lower PRF.

Aliasing • Low sampling rate results large signal • changes occurring between samples • Acquired sample lacks information • regarding these fast changes.

Aliasing Low Sampling Rate Received information sampled too infrequently Measurement Errors (aliasing)

Pulsed Doppler Receiver Gated Receiver circuits only open for a short interval during every pulse cycle.

Pulsed Doppler Receiver Gated Receiver circuits only open for a short interval during every pulse cycle.

Spectral Analysis • Returning signal from volume of RBCs contains • range of frequencies. • These range of frequencies are called the • FREQUENCY SPECTRUM. • Analysis of this spectrum will separate this • complex signals into its component parts.

Spectral Analysis • The component frequencies are converted • into velocity information. • This allow for quantitative analysis of the • range of RBC’s velocities.

Fast Fourier Transform • Digital method of spectrum analysis. • Mathematical technique. Complex wave is • broken down into a series of simpler sine wave. • Analog Doppler signal is digitized in a ADC

Fast Fourier Transform • 5- 10 microseconds samples of signal are • processed using the FFT algorithm. • Digital component frequencies are converted • back into a analog signal by a DAC. • Signal is displayed.

Spectral Display Doppler Signals Analysis FFT Quadrature Phase Detector Positive Shift Negative Shift Channel A Channel B Audio Channel A Audio Channel B Spectral Display Below the Baseline Spectral Display Above the Baseline

Spectral Display • X- axis – time information • Y- axis – frequency/velocity information. • Z- axis – amplitude information.

Spectral Display Frequency, y-axis Amplitude Z- axis Sonic Window Time, x axis

Spectral Display • Velocity Measurements • Peak systolic velocity • End-diastolic velocity • Mean velocity – calculated by taken the area • under the curve.

Spectral Display Systolic Peak Velocity Velocity Mean Velocity End Diastolic Velocity Time

Doppler Spectrum Assessment • Assess the following: • Presence of flow • Direction of flow • Amplitude • Window • Pulsatility

Doppler Spectrum Assessment • Sensitivity can be improved by: • Increasing power or gain. • Decreasing the velocity scale. • Decreasing the reject or filter. • Slowly increasing the SV size.

Doppler Spectrum Assessment • Direction of Flow • Pulsed Doppler use quadrature phase • detection to provide bidirectional Doppler • information.

Spectral Display Mono-phasic Flow Flow on just on side of the Baseline. Frequency Time

Spectral Display Bi-phasic Flow Flow start on one side of the Baseline and then crosses to the other. Frequency Time

Spectral Display Tri-phasic Flow Flow start on one side of the baseline side, then crosses to the other, then returns to the original side. Frequency Time

Spectral Display Bidirectional Flow Flow which occurs simultaneously on both sides of the baseline. Frequency Time

Doppler Spectrum Assessment • Amplitude • The spectrum displays echo amplitude by varying the • brightness of the display. • The amplitude of the echoes are determined by: • Echo intensity • Power • Gain • Dynamic range