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Basic physics of Ultrasound

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Basic physics of Ultrasound

  1. 1. Nouman Ahmed MIT FMH CM&D
  2. 2.  Ultrasound or ultrasonography is a medical imaging technique that uses high frequency sound waves and their echoes.  Known as a ‘pulse echo technique’  The technique is similar to the echolocation used by bats, whales and dolphins, as well as SONAR used by submarines etc.
  3. 3. 1. The ultrasound machine transmits high-frequency (1 to 12 megahertz) sound pulses into the body using a probe. 2. The sound waves travel into the body and hit a boundary between tissues (e.g. between fluid and soft tissue, soft tissue and bone). 3. Some of the sound waves reflect back to the probe, while some travel on further until they reach another boundary and then reflect back to the probe . 4. The reflected waves are detected by the probe and relayed to the machine.
  4. 4. 5. The machine calculates the distance from the probe to the tissue or organ (boundaries) using the speed of sound in tissue (1540 m/s) and the time of the each echo's return (usually on the order of millionths of a second). 6. The machine displays the distances and intensities of the echoes on the screen, forming a two dimensional image.
  5. 5.  All the energy comes from the transducer  All we “see” are reflections and scatter.
  6. 6. •Sound waves consist of mechanical vibrations containing condensations (compressions) & rarefactions (decompressions)that are transmitted through a medium. •Sound is mechanical. •Sound is not electromagnetic. •Matter must be present for sound to travel
  7. 7. •Infrasound (subsonic) below 20Hz •Audible sound 20-20,000Hz •Ultrasound above 20,000Hz •Non-diagnostic medical applications <1MHz •Medical diagnostic ultrasound >1MHz
  8. 8.  Produces electrical Voltage bursts that are sent to the transducer.  Sends upto 10,000 voltage bursts per second  RVPM(rapid voltage pulse system
  9. 9.  Piezo-electric crystal Converts electric signals to mechanical & vice versa  Transmits pulses of sound into tissue and listens for echos  Most of the time is spent listening for echoes
  10. 10.  Receives echo signal and amplifies it to read through machine
  11. 11.  Puts image data together.  Each retuning data is read in the form of pixel or grain mode.  Signal received is stored in the memory allocation known as “pigeon hole”  Receiver signal is evaluated for two reasons.  Time(location)  Strength(brightnes)
  12. 12.  All bits of data is accumulated until a complete image is formed.  When its done, it is then sent to display on monitor.  Strong echoes are seen as bright spots  Mid level echoes as gray  Weak or no echoes as black spots on the image
  13. 13.  Used to obtain a permanent record of scan  Different recoding devices can be used  Film or digital cameras  Videos  Optical discs  Digital data drives  PACS
  14. 14. Transducer Power on Power off Transducer receiving echoes10-6 sec 10-3 sec
  15. 15.  A mechanical process that is caused by vibration phenomena.  Adjacent formal particles hit later particles and propagate or transfer their energy to other  unit=frequency (Hz)  number of waves repetition in one second.  Infrasounds (below audible range i.e. <20kHz)  Audible sounds(with in human range 20-20kHz)  Ultrasound (above 20,000Hz)
  16. 16.  Waves on the surface of water  Contraction waves in myometrium  Crowd waves  Radio Waves As a sonographer you should know:  Longitudinal waves  Transverse waves
  17. 17.  Displacement of Particles is 90 degree perpendicular to the direction of wave travel.  Electromagnetic radiation travel as tranverse waves.
  18. 18.  Particle Displacement is parallel to the propagation of wave.  Sound is a longitudinal wave.
  19. 19.  Mechanical Wave  Longitudinal Wave  Pressure Wave  Form of Energy  Converge and Diverge  Reflect  Non-Ionizing  Not an electromagnetic wave.
  20. 20.  Waves carry energy from one point to another.  Acoustic Variables show the measurable characteristics of a medium of propagation.  Acoustic Variables are:  Pressure  Density  Particle Motion  Temperature
  21. 21.  Pressure:  Amount of force in a given area.  Unit:  Pascal (Pa)  Pounds per square inch(lbs/in sq)  Newton per square meter(N/m sq)  Density:  Mass per unit volume.  Unit:  Kilogram per cubic meter (kg/m3)  Gram per cubic centimeter (g/cm3)
  22. 22.  Temperature:  Concentration of heat energy.  Unit  Fahrenheit  Celsius  Kelvin  Particle Motion:  Distance moved by particles in a medium
  23. 23. Amplitude Compression Rarefaction
  24. 24.  Compression:  High Density or peak where particles lie in abundance point  Relaxation(rarefaction):  Areas where pressure density is low  Amplitude:  Strength of a wave  In sound, it is called loudness.
  25. 25.  Number of events occurring in a specific duration of time.  Number of Acoustic Variable Cycles that occur in one second.  Number of Vibrations per second.  Units:  Hertz  Cycles per second
  26. 26.  General Abdomen 2 – 5  Gallbladder 3 – 5  Ob/Gyne 3 – 3.5  Adult Heart 2 – 3  Pedriatic Heart 3 – 5  Neck, Breast, Scrotum 5 – 15  Eye 7 – 15  Endoluminal Ultrasound 15 – 30 Frequency is directly proportional to Resolution and Inversely Proportional to Penetration
  27. 27.  Distance over which one complete wave cycle occurs.  Distance between any two consecutive points.
  28. 28.  Wavelength is directly proportional to speed of sound and inversely proportional to the frequency.  By using the speed of in sift tissue (1540m/s or 1.54 mm/µs) we can determine the wavelength of different tissues at different frequencies. Wavelength = Speed of sound Frequency

Notas del editor

  • Piezo electric effect is a property of some crytalline materials eg quartz or PZT (lead zirconate titanate)
    When compressed or expanded these materials show a voltage between the compressed or expanded surfaces (due to the molecular alignment in the crystal lattice)
    Changing polarity is a means of electromechanical conversion