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Transcript

  • 1. BASIC ULTRASONIC PRINCIPLES
  • 2. Sound?
    Sound is a mechanical, longitudinal wave that travels in a straight line
    Sound requires a medium through which to travel
  • 3. Acoustic spectrum
  • 4. Amplitude
    Basic Ultrasound Physics
    oscillations/sec = frequency - expressed in Hertz (Hz)
  • 5. What is Ultrasound?
    Ultrasound is a mechanical, longitudinal wave with a frequency exceeding the upper limit of human hearing, which is 20,000 Hz or 20 kHz.
  • 6. ULTRASOUND – How is it produced?
    Produced by passing an
    electrical current through
    a piezoelectrical crystal
  • 7. Transducer Construction
  • 8. Human Hair
    Single
    Crystal
    Microscopic view of scanhead
  • 9. Piezoelectric material
    • AC applied to a piezoelectric crystal causes it to expand and contract – generating ultrasound, and vice versa
    • 10. Naturally occurring - quartz
    • 11. Synthetic - Lead zirconate titanate (PZT)
  • Ultrasound Production
    • Transducer contains piezoelectric elements/crystals which produce the ultrasound pulses (transmit 1% of the time)
    • 12. These elements convert electrical energy into a mechanical ultrasound wave
  • The Returning Echo
    • Reflected echoes return to the scanhead where the piezoelectric elements convert the ultrasound wave back into an electrical signal
    • 13. The electrical signal is then processed by the ultrasound system
  • Piezoelectric Crystals
    • The thicknessof the crystal determines the frequency of the scanhead
    Low Frequency
    3 MHz
    High Frequency
    10 MHz
  • 14. Frequency vs. Resolution
    • The frequency also affects the QUALITY of the ultrasound image
    • 15. The HIGHER the frequency, the BETTER the resolution
    • 16. The LOWER the frequency, the LESS the resolution
  • Frequency vs. Resolution
    • A 12 MHz transducer has very good resolution, but cannot penetrate very deep into the body
    • 17. A 3 MHz transducer can penetrate deep into the body, but the resolution is not as good as the 12 MHz
  • BASIC ULTRASONIC PRINCIPLES
    Sound generated above the human hearing range (typically 20 kHz) is called ultrasound.
    However, the frequency range normally employed in ultrasonic nondestructive testingand thickness gaging is 100 kHz to 50 MHz.
    This means it can be reflected off very small surfaces such as defects inside
  • 18. Ultrasound Transducer
    • Acts as both speaker & microphone
    • 19. Emits very short sound pulse
    • 20. Listens a very long time for returning echoes
    • 21. Can only do one at a time
    Microphone
    receives echoes
    Speaker
    transmits sound pulses
  • 22. BASIC CMUT STRUCTURE
    :
    Basic CMUT structure.
  • 23. Operating Principles of CMUTs
    When a DC voltage is applied between the metallized membrane and the substrate of a CMUT, the membrane is attracted toward the bulk by the electrostatic force, and induced stress within the membrane balances the attraction. Driving the membrane with an AC voltage superposed on the bias generates ultrasound. If the biased membrane is subjected to ultrasound, a current is produced due to the capacitance change under constant bias voltage. The amplitude of the current is a function of frequency, bias voltage, and device capacitance. The efficiency of CMUTs is determined by the electromechanical transformer ratio, which can be expressed as the product of the device capacitance and the electric field strength across the gap. Planar fabrication enables building a thin membrane above a sub-micron sealed cavity, which is crucial to obtain high electric fields for improved transducer performance.
     
    Operating Principles of CMUTs
  • 24. When a DC voltage is applied between the metallized membrane and the substrate of a CMUT, the membrane is attracted toward the bulk by the electrostatic force, and induced stress within the membrane balances the attraction. Driving the membrane with an AC voltage superposed on the bias generates ultrasound. If the biased membrane is subjected to ultrasound, a current is produced due to the capacitance change under constant bias voltage.