ultrasound physics ch 21

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lollybebe
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ultrasound physics ch 21
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2014-07-26 21:51:27
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ch 21
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  1. What is an artifact?
    an error in imaging
  2. Artifacts include reflections that are described as: (5)
    • 1.) not real
    • 2.) not seen on the image
    • 3.) incorrect shape or size
    • 4.) incorrect position
    • 5.) incorrect brightness
  3. Artifacts results from: (4)
    • 1.) violation of assumptions
    • 2.) equipment malfunction or poor design
    • 3.) the physics of ultrasound
    • 4.) operator poor
  4. What does the term Hyperechoic mean?
    portions of an image that are brighter than surrounding tissues, or tissues that appear brighter than normal.
  5. What does the term Hypoechoic mean?
    Portions of an image that are not as bright as surrounding tissues, or tissues that appear less bright than normal
  6. What does the term Anechoic mean?
    An extreme form of hypoechoic, meaning entirely without echoes (echo-free).
  7. What does the term Isoechoic mean?
    Describes structures with equal echo brightness.
  8. What does the term Homogeneous mean?
    A portion of tissue or an image that has similar echo characteristics throughout.
  9. What does the term Heterogeneous mean?
    A portion of tissue or an image that has differing echo characteristics throughtout.
  10. Artifacts appear when these assumptions are not true. What are the 6 assumptions?
    • 1.) Sound travels in a straight line.
    • 2.) Sound travels directly to a reflector and back.
    • 3.) Sound travels in soft tissue at exactly 1,540 m/s.
    • 4.) Reflections arise only from structures positioned in the beam's main axis.
    • 5.) The imaging plane is very thin.
    • 6.) The strength of a reflection is related to the characteristics of the tissue creating the reflection.
  11. What does reverberation appear like on the display? (2)
    appears on the display as multiple, equally spaced echoes caused by the bouncing of the sound wave between two strong reflectors positioned parallel to the ultrasound beam.

    Resemble a ladder or Venetian blind.
  12. What assumption does reverberation break?
    Assumption #2. Sound travels directly to a reflector and back, is invalid.
  13. Reverberations are created when a...
    are created when a sound wave bounces back and forth between two strong reflectors.

    The first and second reflections are real, but the remaining echoes (which appear deeper) do not correctly correspond to true anatomic structures.
  14. What are the characteristics of Reverberation?
    • 1.) appear in multiples
    • 2.) appear equally spaced
    • 3.) are located parallel to the sound beam's main axis.
    • 4.) are located at ever-increasing depths.
  15. Comet tail is also known...
    ring down artifact
  16. What does Comet tail artifact appear like on the image?
    appears as a solid hyperechoic line directed downward.
  17. Comet tail artifact is described as...
    reverberation with the spaces squezzed out.
  18. What assumption does Comet Tail break?
    Assumption #2. Sound travels directly to a reflector and back, is invalid.
  19. What are the characteristics of Comet Tail artifact? (2)
    • 1.) appears as a single long hyperechoic echo.
    • 2.) is located parallel to the sound beam's main axis.
  20. What causes Comet tail artifact?
    1.) when a sound wave bounces back and forth between two very closely spaced objects.

    2.) when the reflecting surfaces are located in a medium with a very high progagation speed, such as a mechanical heart valve.

    3.) Can also happen when small structures such as gas bubbles causes the sound wave to resonance or vibrate.
  21. What does Shadowing appear like in the image?
    appears as a hypoechoic or anechic region extending down from a highly attenuating structure. Shadows are the same color as the image background.
  22. What assumption does Shadowing break?
    Assumption #6. The intensity of a reflection is related to the tissue creating the reflection.
  23. What are the characteristics of Shadowing? (4)
    • 1.) Hypo- or anechoic (background color).
    • 2.) The result of too much attenuation.
    • 3.) Located beneath the structure with abnormally high attenuation.
    • 4.) Prevents visualization of true anatomy on the scan.
  24. What causes or creates Shadowing?
    When too much attenuation occurs, deep reflecting surfaces do not appear on the image. 

    Shadows appear when the attenuation is higher in the tissue above the shadow than in the surrounding tissue.
  25. What does Enhancement appear like on the image?
    appears as a hyperechoic region beneath tissues with abnormally low attenuation.,
  26. What assumption does Enhancement break?
    assumption 6, the intensity of a reflection is related to the characteristics of the tissue.
  27. What are the characteristics of Enhancement? (3)
    • 1.) Hyperechoic (appears the same as the foreground color).
    • 2.) The result of too little attenuation.
    • 3.) Located beneath a structure with abnormally low attenuation.
  28. What can Enhancement do?
    Can be clinically useful. It may provide valueable diagnostic information that helps to characterize tissue.
  29. What structures can Enhancement be caused by? (2)
    1.) a breast cyst that has a lower attenuation rate than surrounding tissues.

    2.) Gallbladder
  30. Focal enhancement is a special form of what?
    enhancement
  31. Focal enhancement can also be called...
    focal banding
  32. What does Focal Enhancement appear like on the image?
    a side- to- side region (horizontal region) that appears brighter than tissues at other depths.
  33. Focal banding has the same appearance as...
    an incorrect TGC setting.
  34. Where is banding most prominent at?
    at the focus
  35. What is focal enhancement caused by?
    When a beam is strongly focused, the intensity in the focal zone is increased. Reflections from the focal zone may be abnormally strong. The structures at the focus appear brighter than those at other depths.
  36. What assumption does Focal Enhancement break?
    assumption #6, the intensity of a reflection is related to the characteristics of the tissue.
  37. What are the characteristics of Focal Enhancement? (2)
    • 1.) A hyperechoic side-to-side region (appears the same as the foreground color).
    • 2.) Results from increased intensity at the focus.
  38. What is Mirror Image artifact created by?
    created when sound reflects off a strong reflector (mirror), and is redirected toward a second structure. This redirection causes a replica, or second copy, of the structure to incorrectly appear on the image.
  39. Where does the real structure appear in a mirror image artifact?

    Where does the false or artifact appear in a mirror image artifact?
    real- before the mirror

    artifact- appears deeper than the real image
  40. What two types of ultrasound does mirror image artifacts appear?
    1.) grayscale- B-mode

    2.) color doppler
  41. What assumption does Mirror Image artifact break?
    assumption 1- sound travels in a straight line.

    assumption 2- sound travels directly to a reflector and back to the transducer.
  42. What are the characteristics of Mirror Image? (4)
    • 1.) A second copy of a true reflector.
    • 2.) The artifact appears deeper than the true reflector.
    • 3.) A bright reflector, the mirror, lies on a straight line between the artifact and the transducer.
    • 4.) True reflector and artifact are equal distances from the mirror.
  43. What is Crosstalk?
    Crosstalk artifact is a mirror image artifact that appears on a spectral Doppler display
  44. What is Speed Error created by?
    created when a sound wave propagates through a medium at a speed other than that of soft tissue. The correct number of reflectors are displayed, but they appear at incorrect depths.

    appear as a step-off, as if structures are split or cut.
  45. Speed Error can also be called?
    Range error artifact
  46. When the medium's speed is SLOWER than that in soft tissue what happens? (6)
    • 1.) sound travels slower than the ultrasound system expects.
    • 2.) Pulses return from their trip in the body-- slow, slow, slow.
    • 3.) Go-return time is too long.
    • 4.) System assumes reflectors are far from the transducer.
    • 5.) Reflectors placed too deep on image.
    • 6.) Distances are overestimated (reported number is too large).
  47. When the medium's speed is FASTER than that in soft tissue what happens? (6)
    • 1.) Sound travels faster than the ultrasound system expects.
    • 2.) Pulses return from their trip in the body--fast, fast, fast.
    • 3.) Go-return time is too short.
    • 4.) System assumes reflectors are close to the transducer.
    • 5.) Reflectors are located too shallow on image.
    • 6.) Distances are underestimated (reported number is too small).
  48. What assumption does speed error artifact break?
    assumption 3, sound travels at a speed of exactly 1,540 meters per second.
  49. EXAMPLE OF SPEED ERROR
    Two pins in a test object are 100 mm apart. On the display of an ultrasound system, they appear 90 mm apart. How will these two pins appear on the display if they were 50 mm apart in the test object?
    The two pins will appear 45 mm apart: the ultrasound system underestimates true distances by 10%, so 100 mm is displayed as 90 mm. Thus, 50 mm will be underreported by 10% and the pins will appear 45 mm apart on the display.
  50. What are the characteristics of speed error artifact? (3)
    • 1.) correct number of reflectors
    • 2.) improper depth
    • 3.) appears as a step-off
  51. Lobe artifacts are created from what?
    when sound energy is transmitted in a direction other than along the beam's main axis.
  52. Lobe artifacts degrade what?
    degrade lateral resolution
  53. What are the characteristics of Lobe artifact? (2)
    • 1.) A second copy of the true reflector.
    • 2.) The artifact and the true reflector are located side by side at the same depth.
  54. What assumption does artifact Lobes break?
    assumption 2, reflections arise from structures located along the beam's main axis.
  55. What type of lobes do single crystal mechanical transducer create?
    side lobes
  56. What type of lobes do array transducers create?
    grating lobes
  57. What are two methods to eliminate lobes?
    1.) subdicing

    2.) Apodization
  58. What is subdicing?
    Grating lobe artifact can be reduced by dividing each PZT element into small pieces.
  59. What is apodization?
    grating lobes can be further reduced by exciting the subdiced elements with different voltage. Elements closer to the center of the sound beam are excited with higher voltages, while the outermost elements, which are farther away from the beam's center, are excited with lower voltages. This process of differential excitation is called apodization.
  60. How is Refraction created?
    • created when a sound pulse changes direction during transmission.
    • Refraction occurs when a sound wave strikes a boundary obliquely and the media on either side of the boundary have different propagation speeds.
  61. What does refraction degrade?
    refraction degrades lateral resolution.
  62. What assumption does refraction break?
    assumption 1, sound travels in a straight line.
  63. What are the characteristics of Refraction?
    • 1.) A second copy of the reflector.
    • 2.) The copy is side by side or at the same depth as the true reflector.

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