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Physics 2 > Module 3 : Ultrasonic Field > Flashcards

Module 3 : Ultrasonic Field Flashcards

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1
Q

beam uniformity - near fields

A
  • non uniform beams as the result of the interference between the wavelets
  • many frequencies emitted from low Q probes also make near field less uniform
  • all interference
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2
Q

beam uniformity - far field

A
  • far fields have uniform beams

- mostly wave form diverging

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3
Q

beam uniformity - intensity

A
  • can represent beam uniformity to intensity changes
  • near field
    + less uniform intensity
  • far field
    + intensity levels out and drops off do to attenuation
    + more uniform intensity
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4
Q

beam shape - grating lobes

A
  • off axis beams in array probes

- result of length and width vibration of crystal resulting in cross talk

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5
Q

beam shape - side lobes

A
  • off axis beams in mechanical probes
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6
Q

beam shape - main beam

A
  • contains most of the energy and the grating lobes are weaker
  • result of radial mode vibration in the single disc probe
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7
Q

2 zones in beam

A
  • fresnel

- Fraunhofer

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8
Q

fresnel zone

A
  • near field

- constant beam width

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9
Q

Fraunhofer zone

A
  • far field

- diverging beam width

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10
Q

beam shape - natural focus

A
  • natural narrowing occurs at one near zone length NZL
  • point is called the transition zone
  • beam is 1/2 crystal diameter at this point
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11
Q

usable beam shape

A
  • length of beam is equal to 2 near zone lengths

- narrow enough to provide reasonable lateral resolution

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12
Q

near zone length equation

A
  • NZL = D^2 / 4 wavelength

+ d = diameter

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13
Q

Near zone length equation for soft tissue

A
  • NZL = D^2 x f / 6
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14
Q

near zone length relation ships in soft tissue

A

increase frequency = increase near zone length

increase diameter = increase near zone length

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15
Q

near zone length relation ships

A

increase frequency = decrease wavelength = decrease SPL = increase NZL
= increased axial resolution

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16
Q

far field divergence equation

A

sin 0 = 1.22 x wavelength / D

+ d= diameter

17
Q

Fairfield relationships

A
  • increasing frequency or diameter = decrease angle of divergence = better lateral resolution
18
Q

beam shape - array probe

A
  • diameter controlled by aperture

- as depth of focus increases the aperture increases to maintain and relatively consistent beam width at the focus

19
Q

focal zone and near zone length relationship

A
  • as we push the near zone length deeper aperture/diamter gets larger
  • lateral resolution takes a hit at the probe
20
Q

focusing

A
  • prime resin for focusing is to improve our lateral resolution
  • want to decrease beam width
  • improve sensitivity
21
Q

two main types of focusing

A
  • mechanical

- electronic

22
Q

mechanical focusing

A
  • internal and external

- focus is fixed has 3 focal lengths

23
Q

internal mechanical focusing

A
  • curve applied to crystal itself
  • curved crystal will help focus the sound
  • concave curve
  • crystal thickness 1/2 wavelength
24
Q

external mechanical focusing

A
  • accomplished with acoustic lens or mirror

- fixed

25
Q

mechanical focal lengths

A
  • short
  • medium
  • long
26
Q

short focal length

A

1-4cm

weak

27
Q

medium focal length

A

4-10cm

28
Q

long focal length

A

7-19cm

strong

29
Q

electronic focusing

A
  • in array probes
  • variable and operator controlled
  • TRANSMIT FOCUSING
30
Q

transmit focusing

A
  • if all elements in an array are excited at the same time then act as single flat disc
  • we can apply a delay to crystal to steer the probe but delays can also focus
  • when delays are added they can converge at a focal point
  • increasing delays will increase focus
31
Q

large delays

A
  • sharper but less NZ
32
Q

disadvantage to transmit focus

A
  • divergence in far field increases
  • near zone length decreased
  • these are over come by dynamic aperture and frequency
33
Q

dual focusing

A
  • refers to the use of both mechanical and electronic focusing in probe
  • beam is 3D so we need to focus in z-axis ro elevational plane
34
Q

elevational plane

A
  • z axis

- mechanical lens focuses this

35
Q

multiple focus

A
  • possible to have more than one transmit focus on image
  • multiple focus expands the focal region creating a long focus
  • requires multiple pulses per scan line with each pulse focused at different depth
  • frame rate reduced but resolution optimized
36
Q

receive focus

A
  • time delays applied to the recited echo to allow for constructive interference
  • does not effect frame rate and is not operator controlled
  • done dynamically as echoes come back from deeper depth
  • goal is to bring echoes into phase so don’t cancel
  • DYNAMIC RECIEVE FOCUS
37
Q

slice thickness

A
  • another way to describe elevational plane
  • depend on beam width perpendicular to image plane
  • cystic structure smaller than slice thickness demonstrate false debris from echoes in off axis beam
  • fixed and requires a curved element or lens to help reduce thickness at a fixed depth
38
Q

effective beam shape

A
  • most effective = in NZL and central
  • next effective = in far field but central
  • next effective = in far field but off axis
  • next effective = of central beam shape in far field
  • least effective = deeper than far field and off axis
39
Q

controls of effective beam shape

A
  • determine the sensitivity of the system
  • gain
  • power
  • suppression (reject)

Physics 2 (8 decks)

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