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Physics 2 > C3: Ultrasonic Field > Flashcards

C3: Ultrasonic Field Flashcards

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1
Q

what does the term uniformity refer to

A

uniformity of the sound waves, pressure and intensity

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2
Q

describe the uniformity of the near field

A

it has non uniform sound waves due to the interference of wavelets which have not yet formed a wavefront….

it has have higher intensity but non-uniform intensity and non uniform pressure

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3
Q

describe the uniformity of the far field

A

it has uniform sound waves as the wave fronts have been form by now and are mainly diverging…

it has uniform and weaker intensity and has uniform pressure

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4
Q

how do the many frequencies emited from a low Q probe effect the uniformity of the near field

A

makes it less uniform

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5
Q

2 alternative names for the near field and far field.

A

near: fresnel (also focal length)
far: fraunhofer

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6
Q

compare the beam width in the near and far field

A

near: nearly constant beam width
far: divergent beam width

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7
Q

when does the far field end?

A

once all the sound has attenuated

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8
Q

in old flat disc mechanical probes, where was the natural narrowing of the beam located

A

one near field length/near zone length (NZL)/focal length

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9
Q

whats another term for the focal point

A

transition zone

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10
Q

in old flat disc mechanical probes, what is the diameter of the beam at the transition zone

A

1/2 the crystal diameter

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11
Q

in old flat disc mechanical probes, when will the diameter of the beam in the far field be equal to the crystal diameter

A

2 NZL

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12
Q

in old flat disc mechanical probes, what is the usable beam length? why?

A

2 NZL

this distance is narrow enough to provide reasonable lateral resolution. beyond it, the resolution is too poor

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13
Q

what is the general formula for NZL

A

NZL = D^2 / 4 x lambda

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14
Q

what is the formula for NZL in soft tissue

A

NZL = D^2 (f) / 6

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15
Q

if you extend the NZL are you penetrating further?

A

no, you are just moving the focal zone deeper or shallower

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16
Q

in the NZL formula, which variable has the biggest impact

A

the diameter of the crystal b/c its ^2

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17
Q

if you increase frequency or diameter, what happens to the NZL

A

it increases

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18
Q

formula to calculate angle of divergence of the far field

A

sin of the angle = 1.22 (lambda) / D

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19
Q

if you increase the frequency or the diameter of the crystal, how does it effect the lateral resolution (this is for a single disc mechanical probe)

A

it will improve it, but only in the mid and far field because the beam will be skinnier and diverge less…

…however, it will make it slightly wore at the face of the probe (main bang)

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20
Q

how does focusing effect the angle of divergence

A

the more you focus, the more the beam diverges

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21
Q

the term for diameter in an array probe

A

aperture

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22
Q

in an array probe, as we increase the depth of the focus how do we maintain a relatively constant beam width at the focal point

A

you increase the aperture size

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23
Q

will an array probe with a small or large aperture have better lateral resolution

A

small, b/c the focal point can be the narrowest

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24
Q

how does beam width effect sensitivity

A

skinnier beam = better sensitivity

25
Q

what are the 2 main types of focusing

A
  1. mechanical

2. electronic

26
Q

2 types of mechanical focusing

A
  1. internal : focusing is achieved by applying a curve to the crystal itself
  2. external : focusing is achieved by applying a curved lens or mirror
27
Q

what was the only way you could you change the focal point w/ an old single disc mechanical probe

A

change the probe

28
Q

what are the 3 categories of focal lengths for mechanical probes

A
  1. short (strong)
  2. medium (med)
  3. long (weak)
29
Q

why is a short focal length the strongest

A

it will provide a more intense focus b/c the beam and aperture size will be the skinniest

30
Q

list the distances for short, med and long focal lengths for mechanical probes

A

S: 1-4 cm
M: 4-10 cm
L: 7-19 cm

31
Q

what type of probes use electronic focusing

A

array transducers

32
Q

another term for electronic focusing

A

transmit focusing

33
Q

how is transmit focusing achieved

A

by phasing which involves delaying the pulses going to the crystal (pulse delays in the machine)

34
Q

what happens if all elements in an array probe are excited at the same time

A

they act as a flat single disc mechanical probe and we couldn’t adjust the focus

35
Q

what is it called when we apply delays to the crystals

A

phasing

36
Q

how does a time delay/phasing allow us to focus the probe

A

we can excite certain groups of elements at one time, but not others, and excite them in such a way that the wavefronts converge towards a focal region

37
Q

how is the amount of delay applied to the elements related to the strength of the focusing

A

the are directly related

38
Q

what type of focal point would be produced with an increased time delay

A

a narrower, sharper focal point that is closer to the face of the probe

39
Q

what type of focal point would be produced with an decreased time delay

A

a wider, less sharp focal point that is further away from the face of the probe

40
Q

what are the disadvantages of focusing

A

there is greater divergence in the far field
and
the NZL will be greatly decreased

41
Q

whats the relationship between the NZL and focusing

A

more focusing/narrower focal point results in a shorter NZL

42
Q

how can the disadvantages of using transmit focusing be overcome

A

dynamic aperture and frequency

43
Q

what is dual focusing

A

using both mechanical and electrical focusing in a probe

44
Q

how do we focus in the Z axis

A

mechanical focusing only

45
Q

what is multiple focusing

A

having more than one transmit focus on the image, doing this expands the focal regions of the probe creating a long focus

46
Q

how do we have to change the way we pulse the US beam to achieve multiple foci

A

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

47
Q

advantages of multiple foci

A

improves resolution

48
Q

what is receive focus, how does it work

A

focusing the returning echos by applying time delays…. this is done in order to allow for constructive interference and avoid destructive interference

49
Q

does receive focus effect frame rate

A

no

50
Q

is receive focus operator controlled

A

no

51
Q

is receive focus a dynamic process

A

yes, because the aperture size depends on how many crystals the sound waves stimulate at one time (which depends on depth of the returning echos)

52
Q

other terms for receive focus

A

dynamic aperture, echo delays or dynamic receive focus

53
Q

do echos that are returning from reflectors that are close to the probe need more of less time delays

A

more time delays are needed because the echos have less time to diverge before they stimulate the crystal, so they will stimulate fewer elements at one time

54
Q

do echos that are returning from reflectors that are far away from the probe need more of less time delays

A

less, because the echos have more time to diverge before they stimulate the crystals, so will stimulate more crystals at one time

55
Q

what are all the terms that refer to slice thickness?

A

section thickness, volume averaging, partial volume, elevational plane, z axis

56
Q

what does elevational resolution depend on

A

the width of the beam perpendicular to the image plane

57
Q

what is a slice thickness artifact

A

this occurs when a cystic structure is smaller than the width of the beam.

it leads to false echos/debris being placed in the cyst due to off axis echos being picked up…. the cystic structure is poorly resolved

58
Q

another term for effective beam shape

A

usable beam width

59
Q

list the 4 controls that determine the sensitivity of the system and can change the effective beam shape

A

gain
power
suppression (rejection)
TGCs

Physics 2 (7 decks)

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