Ch 9 Sound Beams, Ch 10 Axial & Lateral Resolution Flashcards
What are the five regions of a sound beam?
- focus
- near zone
- focal length or near zone length
- far zone
- focal zone
What is the focus/focal point?
Location where beam is narrowest, beam is half the width that it was when it left transducer
What is the near zone AKA?
near field or Fresnel zone
What is the near zone?
Region from transducer to focus, beam gradually converges within near zone
What is focal length AKA?
focal depth, near zone length
What is focal length?
Distance from transducer to focus
What is the far zone AKA?
far field or Fraunhofer zone
What is the far zone?
Region that starts at the focus and extends deeper, beam diverges in far zone
When does the beam return to the same width as the active element?
Two near zone lengths from transducer
What is focal zone?
Region around the focus where the beam is relatively narrow, half is in near field, half in far field
Which region has reflections that produce the most accurate images?
Focal zone
What are adjustable focus systems called?
phased array
What characteristics of a fixed focus transducer determine the focal depth?
- transducer diameter
- frequency of sound
How does transducer diameter affect the focal depth?
Transducer diameter and focal depth are directly related
Larger diameter = deeper focus
How does frequency affect focal depth?
Frequency and focal depth are directly related
Higher frequency = deeper focus
How are clinically useful transducers created if higher frequency creates a deeper focus?
Manufacturers make very small diameter, high frequency crystals
What characteristics of a transducer determine the spread of the beam in the far field?
- transducer diameter
- frequency of sound
How does transducer diameter affect beam divergence in the far field?
Crystal diameter and beam divergence are inversely related
Smaller diam crystals = more divergence in far zone
Larger diameter crystals _______ lateral resolution in the far field
Improve
How does frequency alter beam divergence in the far field?
Frequency and beam divergence are inversely related
Lower frequency = more divergence in far zone
Higher frequency sound ______ lateral resolution in the far field.
improves
Less divergence =
More divergence =
Less: larger diameter, higher frequency
More: smaller diameter, lower frequency
What are v-shaped waves known as?
spherical waves, diffraction patterns, or Huygens’ wavelets
What is Huygens’ Principle?
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
What is axial resolution?
Ability of a system to display two structures that are very close together when they are parallel to sound beam’s main axis
What determines axial resolution?
Spatial pulse length, shorter pulses = improved axial res
Also pulse duration
What is axial resolution AKA?
Longitudinal, range, radial, or depth resolution
LARRD
Can axial resolution be adjusted?
No, SPL is fixed
What are typical values for axial resolution?
0.1-1.0 mm, lower values=shorter pulses & improved image accuracy
What is the relationship between the numerical value of axial resolution and image quality?
Lower numerical values indicate a shorter pulse which create more accurate images
lower=better quality
What is the equation for axial resolution in soft tissue?
ax res (mm) = (0.77 x # cycles in pulse)/frequency
What allows some transducers to have better axial resolution than others?
- less ringing (less cycles/pulse)
- higher frequency (shorter wavelength)
Better axial resolution is achieved with what 5 things?
- shorter SPL
- shorter pulse duration
- higher frequencies
- fewer cycles/pulse
- lower numerical values
What is the ability to accurately distinguish two structures lying close together called?
resolution
If there are more cycles in a pulse, the numerical value of range resolution is
greater
If a new pulsed transducer has many more cycles in its pulse, the image accuracy
degrades
_____ frequency transducers generally have the best range resolution.
High
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
C
