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Echo Physics and Doppler

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Echo Physics and Doppler

  1. 1. Echo and Doppler Physics Dr George Abraham, Royal Free Hospital.
  2. 2. Waves • Vibrations that transfer energy from place to place without the transfer of matter. • Sound waves travel through a medium and set up patterns of disturbance – vibrations. • Light is a transverse wave and sound is a longitudinal wave.
  3. 3. • Amplitude is maximum disturbance from the resting state • Frequency is the number of waves produced by the source every second
  4. 4. • Wavelength of a wave is the distance between a point on one wave and the same point on the next wave.
  5. 5. What’s the frequency? • Measured in Hz – 1 wave/second • Normal range of human hearing: 20Hz-20kHz • kHz = 10,000; MHz = 10,000,000 • Sound waves travel fastest in solids, then liquids, then gases. • The speed of sound in the heart is 1560 m/s
  6. 6. Velocity of a wave • 𝑉 = 𝑓 λ • Velocity = frequency X wavelength • Frequency = 1/time period • Higher frequencies will thus give a shorter wavelength.
  7. 7. • They can reflect off a smooth surface in a predictable way. • Rough surfaces scatter waves in all directions. • Diffraction happens when waves pass through a gap. • Refraction happens when waves change speed entering a different density medium. What can waves do?
  8. 8. US and Reflection • Frequencies above 20kHz are Ultrasound. • US Machines use frequencies 1.5-7 MHz. • In ultrasound, a detector placed near the source of the US can measure the time taken for the sound to bounce back and hence the distance from the source. • Pulse Repetition Period or PRP is the time between the onset of one pulse till the onset of the next pulse. This parameter includes the time the pulse is “on” and the listening time when the ultrasound machine is “off”. • Pulse Repetition Frequency is the number of pulses/second and relates to frame rate.
  9. 9. Reflection
  10. 10. Diffraction
  11. 11. Refraction Refraction happens when waves pass from one medium to another at an angle and change speed.
  12. 12. Attenuation • The resolution of a recording is the ability to distinguish 2 objects close together. • Higher frequencies have shorter wavelengths hence higher resolution. • As the ultrasound waves pass through a medium they gradually lose energy as heat. • In the body, the depth of penetration is around 200 wavelengths. • At 1 MHz, the penetration is 30 cm • At 5 Mz, the penetration is 6 cm
  13. 13. Acoustic impedance • Tissue density X Propagation velocity • This in addition to the angle of incidence, determines how much of the US is reflected: the strength of the signal.
  14. 14. High frequency beam with a short wavelength and high pulse repetition frequency. Depends on how narrow the beam is: the ‘focus’
  15. 15. US transducers • PZT crystals are materials that vibrate and then transduce an incoming signal to electricity. • Backing materials reduce ringing after the current is gone • Matching layers reduce impedance
  16. 16. How are images produced?
  17. 17. M Mode • A single crystal rapidly alternates between transmission and receiver modes with pulse repetition frequencies >1000Hz. • Thus gives excellent temporal resolution and can be used to visualise rapid movement.
  18. 18. Doppler Shift • As sound waves move from a source towards an observer the waves shorten in wavelength and increase in frequency. • As sound waves move from a source away from an observer the waves lengthen in wavelength and decrease in frequency. • The change between the frequency transmitted and received is the doppler shift • The amount of doppler shift depends on the velocity and this can be calculated
  19. 19. Doppler spectral analysis • The frequency range of doppler shift is • -10 to + 10 kHz hence within the audible range • The spectral display is a speed/time graph with red blood cell velocities detected along the beam at any given time point are depicted ranging from zero to the peak velocity • The brightness of the signal denotes the amplitude of the signal received corresponding to that velocity
  20. 20. CW Doppler
  21. 21. CW Doppler • In CW Doppler the echo machine uses continuous transmission and receiving of US signals by 2 dedicated crystals • The CW Doppler obtains signals along the entire cursor line • It cannot assess flow at any one specific point • It can measure high velocities without aliasing
  22. 22. Aliasing The phenomenon of aliasing means PW doppler displays will not represent the highest velocities accurately. The sampling frequency of the PW probe has to be more than twice the frequency of the wave.
  23. 23. Aliasing
  24. 24. Important Points for Revision • Relationship of velocity to frequency and wavelength • Know the speed of sound in tissues • Understand principles of image formation by measurement of reflected US waves from the transducer • Understand how moving sources give a doppler shift • Know the limitations of PW doppler and CW doppler • Know the simplified Bernoulli equation
  25. 25. References - David Houghton - s-of-ultrasound-and-echocardiography

Notas del editor

  •  Definitions: frequency, wavelength, propagation velocity
    Frequency is number of oscillations per second and is measured in Hertz. 
    Wavelength is the distance between two successive waves. This is measured in metres. 
    Velocity is the average speed of movement of the wave across a medium. This is 1540 m/s in the heart. 
    The wavelength is the propagation velocity divided by the frequency. 
  •  Concept of compression waves
    There are areas in a medium where the particles are closer together which is compression. There are other areas where the particles are further apart. In all of these areas there will be variation in the wave conducted across the medium e.g velocity will be greater or less. 
  • Wavelength is the distance between two identical points on the waves and is measured in metres

    The Ultrasound waves used for echocardiography are between 1.5-7 MHz. The Velocity of these waves in 330 m/s in air and 1570 m/s in blood.

  • Doppler shifts are an increase in the frequency of sound coming from an object moving toward you. This is evidenced in our daily lives e.g if a train is coming toward you the sound it generates increases in frequency. In the same way if blood is coming toward you its frequency increases. If blood is moving away from you its frequency decreases. This is used as the principle for colour doppler in which the areas in which the frequency is changed are colour labelled. This is usually blue for blood moving away from you and red for blood moving toward you. If this is displayed as a graph then velocity toward you is displayed above and velocity toward is displayed below the baseline.