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Phys 254 > Module Three: The Ultrasonic Field > Flashcards

Module Three: The Ultrasonic Field Flashcards

1
Q

What is divergence?

A

The spreading out of a sound wave from a small source where the smaller the source the more divergence

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2
Q

Huygen’s principle taught us that to limit divergence, we can do what?

A

We can fire a group of elements that together create a wave front

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3
Q

Near fields have non- uniform Beams as the result of what?

A

The interference between the wavelets and far fields have uniform beans

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4
Q

The many frequencies from a low q probe also makes the probe less what?

A

Less uniform

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5
Q

Another way to represent the non-uniformity of the near field is to release it from what?

A

The intensity differences

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6
Q

What is a grating lobe?

A

Off axis beams

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7
Q

Between the main beam and the grating lobes which contains more power?

A

Grating lobes is weaker

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8
Q

In mechanical probes the grating lobes are also called what?

A

Side lobes

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9
Q

The grating lobes are the result of what?

A

The length and width vibration of the crystal resulting in crosstalk

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10
Q

Side lobes are the result of what?

A

Radial mode vibration in the single disc probes

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11
Q

What are the alternate names of the near and far fields?

A
  1. Fresnel (near field)
  2. Fraunhofer (Far field)
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12
Q

The near field is characterized how?

A

A nearly constant beam

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13
Q

The far field is characterized how?

A

By a divergent beam width

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14
Q

At one near field length (NZL), even with old flat disc probes, what happens?

A

There is a natural narrowing of the beam that occurs

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15
Q

What is the transition zone?

A

The natural narrowing of a beam that occurs at one near field length

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16
Q

What is the crystal diameter at one near field length?

A

1/2

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17
Q

What is the usable beam shape?

A

The natural beam shape

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18
Q

What is the formula for near field length?

A

NZL = (D^2)/ (4*lambda)

D= diameter of the crystal

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19
Q

If we were to rewrite the formula for NZL for soft tissue what would it be?

A

NZL = ((D^2)(F))/6

recall that lamda = C/F and C is tissue 1.54. 41.54 is 6*

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20
Q

Looking at the formula for NZL what can we say about the relationship of Diameter and Frequency?

A

It is linear if D or F increase then so does NZL

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21
Q

What is the Fraunhofer zone or far field?

A

The divergent part of the beam

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22
Q

How do we calculate divergence?

A

Sin(x) = (1.22*lambda)/D

D is the diameter of the crystal

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23
Q

If we increase the frequency or the diameter of the crystal what will we do to the angle of divergence?

A

We will decrease the angle of the divergence

Recall just like the NZL formula we should exchange lambda with frequency and Speed of sound to get an alternative formula

sinx = ((1.22*1.54)/((D)(F))

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24
Q

Here we see the effect of diameter on what?

A

The NZL and angle of divergence

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25
Q

Here we see the effect of frequency on what?

A

NZL and the angle of divergence

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26
Q

What is aperture?

A

The part of the probe that controls diameter

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27
Q

As the depth of the focus increases what happens to the aperture?

A

The Apeture increases to maintain a relatively consistent beam width at the focus

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28
Q

We can only focus in which field?

A

Near

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29
Q

What does this picture show us?

A

The manipulation of delayed firing to adjust the focus of the beam. Area of focus is the small black dot

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30
Q

The prime reason for focusing is to do what?

A

Improve our lateral resolution

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31
Q

How would we improve our lateral resolution?

A

decrease the beam width

32
Q

Decreasing beam width would do what?

A

Improve sensitivity

33
Q

Focusing can be divided into how many different parts?

A

Mechanical (both internal and external) and electronical

34
Q

What is internal focusing?

A

Internal focusing that occurs when a curve is applied to the crystal itself

35
Q

What is this picture an example of?

A

Internal focusing

35
Q

What is this picture an example of?

A

Internal focusing

36
Q

External focusing is accomplished with what?

A

An acoustic lens or mirror

37
Q

Explain the two diagrams

A

The left uses internal focusing and the right uses external focusing

38
Q

With older single crystal probes the focus is fixed so that the only way to change it was to do what?

A

change probes

39
Q

Mechanical probes could be categorized into which focal lengths?

A
  1. Short
  2. Medium
  3. Long
40
Q

What is the typical range of a short probe?

41
Q

What is the range of a medium probe?

42
Q

What is the range of long probe?

43
Q

What is transmit focusing?

A

Electronic focusing for the array transducer which is more variable and operator controlled

44
Q

If all the elements in an array probe are excited at the same time then they act how?

A

As a flat single disc

45
Q

We can apply delay to the crystals in a probe to do what?

A

Steer and focus the probe

46
Q

When time delays are used to excite a group of elements then what happens?

A

Wave fronts converge towards the focal region

47
Q

When we use time delays to cause convergence of wave fronts towards a focal region what is it similar to?

A

Using a lens or curved element in a single element transducer

48
Q

Increase the delays of the probe will do what?

A

Increase our focus

49
Q

What is this an example of?

A

Increasing the delays of the probe to increase the focus

50
Q

What are some disadvantages of using focus?

A

The divergence in the far field will increase greatly and the NZL will decrease

51
Q

What does using focus increase?

A

Lateral resolution in the near field and focal point

52
Q

How do we overcome the disadvantages of using focus?2

A
  1. Dynamic aperture
  2. Frequency
53
Q

What is dual focusing?

A

The use of both mechanical and electronic focusing in the probe

54
Q

What is the elevational plane?

A

The third dimension of our probe beam, or the Z axis if you may

55
Q

Is it possible to have more than one focus on an image?

56
Q

Multiple focusing does what?

A

expands the focal regions of the transducer creating a long focus

57
Q

What does multiple focus require?

A

multiple pulses per scan line with each pulse focused at a different depth

58
Q

What happens as a result of using multiple focuses?

A

Frame rate is reduced by resolution is optimized

59
Q

What is receive focus?

A

When array transducers have the ability to focus the received echo signals

60
Q

To focus received echo signals what must happen?

A

Time delays are applied to the received echoes to allow for constructive interference.

61
Q

How does receive focus effect frame rate?

A

It does not effect frame rate and is not operator controlled

62
Q

How is receive focus done?

A

It is done dynamically as echoes come back from deeper depths

63
Q

What is the goal of receive focus?

A

To bring the echoes into phase so they don’t cancel each other out

64
Q

As reflected echoes arrive at various elements they do what?

A

Are amplified and combined in the beam former

65
Q

What is dynamic receive focus?

A

The dynamic process of receiving echoes without canceling each other out

66
Q

What is this an example of?

A

Receive focus

67
Q

What is slice thickness?

A

Another way to describe the elevational plane?

68
Q

Another name for slice thickness is what?

A

Elevation resolution

69
Q

Slice thickness depends on what?

A

The beam width perpendicular to the image plane

70
Q

Axial resolution is dependent on what?

71
Q

What is lateral resolution dependent on what?

A

Beam width

72
Q

Cystic structures that are smaller then the slice thickness may demonstrate what?

A

False debris since echoes are picked up from the off center part of the beam as well as the center

73
Q

Slice thickness is fixed and requires what to help reduce the thickness at a fixed depth?

A

A curved element or lens to help reduce the thickness at a fixed depth

74
Q

Label the beam points

A

A: Target is close on central axis (appears brightest)
B: On axis but in the far field so less intense
C: Target off axis so weaker than B
D: Probably not detected
E: Target may scape detection because it is off axis

75
Q

What controls determine the sensitivity of the system and can change the effective beam shape? 3

A
  1. Gain
  2. Power
  3. Suppression

Phys 254 (19 decks)

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