Ch 9 Sound Beams, Ch 10 Axial & Lateral Resolution

Question 1

What are the five regions of a sound beam?

Answer 1

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

Question 2

What is the focus/focal point?

Answer 2

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

Question 3

What is the near zone AKA?

Answer 3

near field or Fresnel zone

Question 4

What is the near zone?

Answer 4

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

Question 5

What is focal length AKA?

Answer 5

focal depth, near zone length

Question 6

What is focal length?

Answer 6

Distance from transducer to focus

Question 7

What is the far zone AKA?

Answer 7

far field or Fraunhofer zone

Question 8

What is the far zone?

Answer 8

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

Question 9

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

Answer 9

Two near zone lengths from transducer

Question 10

What is focal zone?

Answer 10

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

Question 11

Which region has reflections that produce the most accurate images?

Answer 11

Focal zone

Question 12

What are adjustable focus systems called?

Answer 12

phased array

Question 13

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

Answer 13

1. transducer diameter
2. frequency of sound

Question 14

How does transducer diameter affect the focal depth?

Answer 14

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

Question 15

How does frequency affect focal depth?

Answer 15

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

Question 16

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

Answer 16

Manufacturers make very small diameter, high frequency crystals

Question 17

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

Answer 17

1. transducer diameter
2. frequency of sound

Question 18

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

Answer 18

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

Question 19

Larger diameter crystals _______ lateral resolution in the far field

Answer 19

Improve

Question 20

How does frequency alter beam divergence in the far field?

Answer 20

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

Question 21

Higher frequency sound ______ lateral resolution in the far field.

Answer 21

improves

Question 22

Less divergence =
More divergence =

Answer 22

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

Question 23

What are v-shaped waves known as?

Answer 23

spherical waves, diffraction patterns, or Huygens’ wavelets

Question 24

What is Huygens’ Principle?

Answer 24

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

Question 25

What is axial resolution?

Answer 25

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

Question 26

What determines axial resolution?

Answer 26

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

Question 27

What is axial resolution AKA?

Answer 27

Longitudinal, range, radial, or depth resolution LARRD

Question 28

Can axial resolution be adjusted?

Answer 28

No, SPL is fixed

Question 29

What are typical values for axial resolution?

Answer 29

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

Question 30

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

Answer 30

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

Question 31

What is the equation for axial resolution in soft tissue?

Answer 31

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

Question 32

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

Answer 32

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

Question 33

Better axial resolution is achieved with what 5 things?

Answer 33

- shorter SPL - shorter pulse duration - higher frequencies - fewer cycles/pulse - lower numerical values

Question 34

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

Answer 34

resolution

Question 35

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

Answer 35

greater

Question 36

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

Answer 36

degrades

Question 37

_____ frequency transducers generally have the best range resolution.

Answer 37

High

Question 38

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

Answer 38

C