PHY 10

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PHY 10
2012-04-18 20:40:24

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  1. anatomy of sound beam
    • focus
    • near zone
    • focal legnth
    • far zone
    • focal zone
  2. focus
    • narrowest part of beam
    • focal point
    • beam diameter is 1/2 start diameter
  3. near zone
    • near field
    • fresnel sone
    • slowly narrows
    • before focus- 1/2 diameter
  4. focal legnth
    • near zone legnth
    • distance from the transducer to focus
  5. far zone
    • far field
    • fraunhofer zone
    • from the focus on
    • beam diverges
  6. focal zone
    • around the focus where beam is narrow
    • half in near field and half in far field
  7. adjustable focus transducers
    phased array
  8. what characteristics of fixed focus transducers determine focus depth?
    • 1. transducer diameter
    • 2. frequency of sound
  9. how does transducer diameter effect focal depth
    • increase diameter increase depth of focus
    • related directly
  10. how does frequency effect focal depth?
    • higher freq higher depth of focus
    • directly related
  11. how can high freq transducers have shallow focus
    small diameter high freq crystals
  12. what characteristics of a transducer determine spread of the beam in farfield?
    • transducer diameter
    • frequency of sound
  13. how does transducer diameter affect beam divergence in farfield?
    • smaller crystals have greated beam divergence
    • inversely related
  14. what diameter crystal has better lateral resoultion in the far field
    larger diameter- less divergence
  15. how does frequency alter beam divergence in the far field?
    • lower freq have more divergence
    • inversely related
  16. what freq probe has better lateral resoultion?
    high freq
  17. v-shaped waves
    • spherical waves
    • diffraction patterns
    • huygens wavelets
  18. Huygens principle
    • large active element may be thought of as a million tiny distinct sound sources
    • Each with a v shape
    • inphase and out of phase - shape of beam
  19. accuracy in imaging
  20. lateral resoultion
    abilty to distictively identify two structures side by side perpendicular to beam axis
  21. lateral resoultion is determined by
    • width of sound beam
    • narrower beams have better lat res
  22. other names for lateral res
    • lateral
    • angular
    • transverse
    • azimuthal
    • LATA
  23. Lateral resoultion formula
    lateral resoultion = beam diameter
  24. axial and lateral resoultion improved by
    • increase the frequency
    • shorter pules
    • less divergence
  25. three methods of focusing
    • external- lens- fixed
    • internal - curved element- fixed
    • phased array- electronic- only one adjustable
  26. what happens when a beam is focused? 4
    • beam diameter in near field and focal zone reduced
    • focal depth is shallower
    • beam diameter in far zone increases
    • focal zone is smaller