physics.txt

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Anonymous
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28635
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physics.txt
Updated:
2010-08-01 12:03:15
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physics mid term
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chapt 12,13.16,17
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  1. Transducer Arrays
    comprises a single slap of PZT cut into a collection of separate pices called elements. Each active element is connected by a wire to its own electronic circuitry
  2. Mechanical Transducer
    • single,circular: disc shaped
    • fan or sector-shape
    • fixed or mechanical focusing
    • damage - entire image lost
  3. Types of array transducers
    • Linear
    • annular
    • convex
  4. Linear phased arrays
    • small footprint
    • 100-300 elements, side by side, rectangular and narrow
    • electronic steering (phasing)
    • electronic beam focus
    • damage - inconsistent focusing
  5. Phased array
    always means adjustable or multi-focus
  6. Beam former
    the electonics within the ultrasound system that create a pattern
  7. Annular phased arrays
    • disc shaped element
    • mechanical steering
    • multiple focal zones (not adjustable)
    • inner circle - shollow scan
    • sector shape image
    • damage - horizontal or side to side band
  8. Linear sequential arrays
    • large footprint
    • 120 - 250 piezoelectric elements:
    • some bur not all crystals are fired simultaneously
    • transmit and receive focusing (electronic)
    • rectangular image
    • damage - vertical line
  9. Convex (curved) arrays
    • 120-250 rectangular elements
    • some but not all crystals are fired simultaneously
    • electronic beam focus
    • blunt sector shape
    • damage - vertical line
    • dynamic receiving focus
  10. Vector arrays
    • 120-250 rectangular elements
    • small footprint
    • electronic beam steering
    • electronic focusing
    • trapezoidal image
  11. Resolution
    accuracy in imaging
  12. Slice thickness
    elevational resolution
  13. Image resolution
    • three dimensional space:
    • side to side
    • shallow to deep
    • above and below
  14. Side lobes
    off axis sound beams from a single element transducer
  15. Grating lobes
    off axis sound beams created by array transducers
  16. Apodization
    stronger electrical signals are used to excite the innner crystals, annd progressiely weaker electrical spikes excite the outer crystals. this diminishes side lobes
  17. Variable aperture (dynamic aperture)
    changing the number of crystals along the face of the probe used to transmit pulses and receive reflctions

    (number of elements used to receive reflected echoes.
  18. Temporal resolution
    • accuracy in time
    • the ability to precisely position moving structures from instant to instant

    determined by frame rate

    higher frame rate improves temporal resolution
  19. Frame rate
    the ability of the system to create numerous frames each second

    measured in Hz

    determined by speed of sound and imaging depth
  20. Tframe
    the time it takes to make a single image

    inversely related to frame rate
  21. sonographer setting that control frame rate
    imaging depth

    number of pulses in each picture
  22. Imaging depth
    shallow imaging increases frame rate and improves temporal resolution
  23. Number of pulses per image
    pulses per frame and frame rate are inversely related

    • multi vs. single focus
    • sector size
    • lines per angle of sector
  24. Multi focus
    negative effect on temporal resolution

    improves accuracy of the individual image

    superior lateral resolution
  25. Sector size
    increase in sector size decreases temporal resolution
  26. Line density
    space between sound beams

    high line density decreases temporal resolution

    improved spatial resolution
  27. dynamic range
    a method of reporting the extent to which a signal can vary and still be accurately measured

    measured in dB

    relative measurement or ratio
  28. dynamic range of components
    • transducer 120
    • receiver 100 - 120
    • scan converter 40 - 50
    • display 20 - 30
    • archive 10 - 30
  29. Harmonic imaging
    the creation of an image from sound reflctions at twice the frequency of the transmitted sound

    arise from no -linear behavior
  30. Fundamental frequency
    sound created by the transducer and transmitted into the body
  31. Fundamental image
    the image created by rocessing reflections that have the same frequency as the transmitted sound
  32. Harmonic image
    image created by processing reflections that are twice the fundamental frequency
  33. Linear behavior
    proportional or symmetrical

    systems respond in an even manner
  34. Non-linear behavior
    irregular or disproportionate

    system behaves unevenly
  35. Contrast agents
    microbubbles that are entrapped in a shell that are ingested or injected into the circulation

    have a different acoustic fingerprint than blood or tissue and therefore create strong reflections that actually "light up" blood chambers and vessels
  36. Contrast agent requirements
    • 1. safe
    • 2. metabolically inert
    • 3. long lasting
    • 4. strong reflector of ultrasound
    • 5. small enough to pass through capillaries
  37. Contrast harmonics
    when an ultrasound pulse interacts with microbubbles, a small amount of energy in converted from the fundamental frequency to the harmonic frequency

    contrast harmonics are created during reflection
  38. Resonance
    uneven behavior caused when the microbubble within the sound beam grows or shrinks in relation to the pressure
  39. Mechanical index (MI)
    the amount of contrast harmonics produced is estimated by MI

    MI=peak negative pressure/sqaure roote of frequency

    high frequency -less pressure- lower MI

    high mechanical index - bubbles expand and break apart
  40. Contrast agent characteristic
    nature of the outer shell

    gas that fills the microbubbles
  41. Tissue harmonics
    as a sound wave travels in the body, a miniscule amount of energy is converted from the fundamental frequency to the harmonic frequency

    faster through compressions - slower through rarefactions

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