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SPI > Ch 9 Sound Beams, Ch 10 Axial & Lateral Resolution > Flashcards

Ch 9 Sound Beams, Ch 10 Axial & Lateral Resolution Flashcards

1
Q

What are the five regions of a sound beam?

A
  1. focus
  2. near zone
  3. focal length or near zone length
  4. far zone
  5. focal zone
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2
Q

What is the focus/focal point?

A

Location where beam is narrowest, beam is half the width that it was when it left transducer

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3
Q

What is the near zone AKA?

A

near field or Fresnel zone

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4
Q

What is the near zone?

A

Region from transducer to focus, beam gradually converges within near zone

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5
Q

What is focal length AKA?

A

focal depth, near zone length

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6
Q

What is focal length?

A

Distance from transducer to focus

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7
Q

What is the far zone AKA?

A

far field or Fraunhofer zone

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8
Q

What is the far zone?

A

Region that starts at the focus and extends deeper, beam diverges in far zone

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9
Q

When does the beam return to the same width as the active element?

A

Two near zone lengths from transducer

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10
Q

What is focal zone?

A

Region around the focus where the beam is relatively narrow, half is in near field, half in far field

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11
Q

Which region has reflections that produce the most accurate images?

A

Focal zone

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12
Q

What are adjustable focus systems called?

A

phased array

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13
Q

What characteristics of a fixed focus transducer determine the focal depth?

A
  1. transducer diameter
  2. frequency of sound
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14
Q

How does transducer diameter affect the focal depth?

A

Transducer diameter and focal depth are directly related
Larger diameter = deeper focus

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15
Q

How does frequency affect focal depth?

A

Frequency and focal depth are directly related
Higher frequency = deeper focus

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16
Q

How are clinically useful transducers created if higher frequency creates a deeper focus?

A

Manufacturers make very small diameter, high frequency crystals

17
Q

What characteristics of a transducer determine the spread of the beam in the far field?

A
  1. transducer diameter
  2. frequency of sound
18
Q

How does transducer diameter affect beam divergence in the far field?

A

Crystal diameter and beam divergence are inversely related
Smaller diam crystals = more divergence in far zone

19
Q

Larger diameter crystals _______ lateral resolution in the far field

A

Improve

20
Q

How does frequency alter beam divergence in the far field?

A

Frequency and beam divergence are inversely related
Lower frequency = more divergence in far zone

21
Q

Higher frequency sound ______ lateral resolution in the far field.

A

improves

22
Q

Less divergence =
More divergence =

A

Less: larger diameter, higher frequency
More: smaller diameter, lower frequency

23
Q

What are v-shaped waves known as?

A

spherical waves, diffraction patterns, or Huygens’ wavelets

24
Q

What is Huygens’ Principle?

A

A large active element is made of millions of tiny sound sources, each one is a Huygens’ source & creates a Huygens’ wavelet. The hourglass sound beam created by a large crystal is a result of interference of the Huygens’ wavelets

25
Q

What is axial resolution?

A

Ability of a system to display two structures that are very close together when they are parallel to sound beam’s main axis

26
Q

What determines axial resolution?

A

Spatial pulse length, shorter pulses = improved axial res
Also pulse duration

27
Q

What is axial resolution AKA?

A

Longitudinal, range, radial, or depth resolution
LARRD

28
Q

Can axial resolution be adjusted?

A

No, SPL is fixed

29
Q

What are typical values for axial resolution?

A

0.1-1.0 mm, lower values=shorter pulses & improved image accuracy

30
Q

What is the relationship between the numerical value of axial resolution and image quality?

A

Lower numerical values indicate a shorter pulse which create more accurate images
lower=better quality

31
Q

What is the equation for axial resolution in soft tissue?

A

ax res (mm) = (0.77 x # cycles in pulse)/frequency

32
Q

What allows some transducers to have better axial resolution than others?

A
  1. less ringing (less cycles/pulse)
  2. higher frequency (shorter wavelength)
33
Q

Better axial resolution is achieved with what 5 things?

A
  • shorter SPL
  • shorter pulse duration
  • higher frequencies
  • fewer cycles/pulse
  • lower numerical values
34
Q

What is the ability to accurately distinguish two structures lying close together called?

A

resolution

35
Q

If there are more cycles in a pulse, the numerical value of range resolution is

A

greater

36
Q

If a new pulsed transducer has many more cycles in its pulse, the image accuracy

A

degrades

37
Q

_____ frequency transducers generally have the best range resolution.

A

High

38
Q

Which of the following transducers has the poorest axial resolution?
A. 1.7 MHz, 4 cycles/pulse
B. 2.6 MHz, 3 cycles/pulse
C. 1.7 MHz, 5 cycles/pulse
D. 2.6 MHz, 2 cycles/pulse

A

C

SPI (9 decks)

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