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Doppler Physics and Instrumentation

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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  1. Doppler Physicsand Instrumentation

  2. Topics • Doppler effect • Doppler equation • Doppler Modes • Doppler Artifacts

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

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

  5. 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

  6. Doppler Shift fo fr = fd fr fo fo - = = 0 fr Stationery Reflectors

  7. Doppler Shift fr fo > fd fr fo = - +ve shift = fo fr Reflectors moving towards Sound source

  8. Doppler Shift fr fo < fd fr fo = = = - ve shift fo fr Reflectors moving away from sound source

  9. Doppler Equation 2f0vcos fd = c • fd = Doppler shifted frequency • fo = transducer frequency • v = blood velocity •  = beam flow angle • c = speed of sound in tissue (1540 m/s)

  10. Cos  1.00 Cos  0 90 

  11. Angle to Flow

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

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

  14. 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)

  15. Reflector Velocity fdc v = 2f0cos

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

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

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

  19. 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

  20. Range Equation Distance = velocity x time

  21. Range Equation Velocity = speed of sound in soft tissue. Distance = reflector distance. Time = time it takes the sound wave to reach reflector.

  22. Range Equation Time that can be measured is the go – return time. The actual time = go-return time 2

  23. Range Equation Reflector distance = velocity x go-return time 2

  24. Pulsed Doppler

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

  26. 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.

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

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

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

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

  31. 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.

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

  33. 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

  34. 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.

  35. 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

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

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

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

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

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

  41. Doppler Spectrum Assessment Check for Flow Flow Detected No Flow Detected Check Sensitivity Check SV Placement Check Beam- flow angle Sensitive Decreased Sensitivity Improve Sensitivity

  42. 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.

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

  44. Doppler Spectrum Assessment • Flow can either be: • Mono-phasic • Bi-phasic • Tri-phasic • Bidirectional

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

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

  47. 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

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

  49. 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

  50. Spectral Display Low amplitude Frequency Time

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