Chapter 9 Sound Beams Flashcards
At the starting point, how wide is the sound beam width?
The sound beam’s width is the same as the transducer diameter.
As the sound beam gets further from the transducer, what happens to it?
The sound beam narrows (converges), like a funnel.
After the sound beam narrows, what happens to it’s shape?
The sound beam expands (diverges)
What is the location where the beam diameter is the narrowest called?
Focus or Focal Point
What is the region from the transducer to the focus called?
Near Zone
aka Fresnel Zone, or Near Field
At the far end of he near zone, what is the diameter of the near zone?
1/2 the width of the active element
The distance from the transducer to the focus is called what?
Focal Length
aka Focal depth or Near Zone length
The region starting at the focus and extending deeper is called what?
Far Zone
aka Fraunhofer Zone
What is the width of the sound beam at the focus?
1/2 of transducer diameter
At the beginning of the far zone what is the beams width?
1/2 as wide as it is at the transducer
When the beam is two near zone lengths from the transducer, how wide is it?
The beam is as wide as the active element again
What happens to the sound beam width beyond two near zone lengths from the transducer?
The beam widens beyond the width of the active element, and continues expanding.
The region around the focus where the beam is relatively narrow is called what?
Focal Zone
Why is the focal zone important to sonographers?
Reflections arising from the focal zone create images of superior detail, compared to other depths.
What marks the end of the near zone?
Focus
aka focal point
At what point does the sound beam first begin to widen?
Focus
aka focal point
The focal zone is found where?
Half in the near zone
Half in the far zone
What is the beams width at the end of the near zone?
1/2 the transducer diameter
What is the beams width at 2 near zone lengths?
beam diameter equals transducer diameter
Is focal depth adjustable by sonographers on most modern ultrasound systems? If so, what is this called?
Yes. Adjustable focus systems are called Phased Array
For a fixed-focus transducer, what two characteristics determine the focal depth?
- Transducer Diameter
2. Sound Frequency
What is the relationship between transducer diameter and focal depth?
Directly related
What is the relationship between frequency and focal depth?
Directly related
A larger diameter results in?
a deeper focus
Higher frequency of sound results in?
deeper focus
What is the relationship between diameter and near zone length?
directly related
What is the relationship between frequency and near zone length?
directly related
What is the relationship between wavelength and near zone length?
inversely related
(Smaller|Larger) Crystals produce beams that spread out (more|less) in the far field
Small|more
Larger|less
Since larger diameter crystals create sound beams that diverge less in the far field, what would be improved in the far field?
Lateral resolution
What is the relationship between frequency and beam divergence?
Inversely related
Since high frequency sound beams diverge less in the far field, what would be improved in the far field?
Lateral resolution
Less divergence involves what two characteristics?
- Larger diameter (PZT)
2. Higher frequency
More divergence involves what two characteristics?
- Smaller Diameter (PZT)
2. Lower Frequency
Shallow focus involves what two factors?
- Smaller Diameter (PZT)
2. Lower Frequency
Deep focus involves what two characteristics?
- Larger Diameter (PZT)
2. Higher Frequency
Very small pieces of PZT are called what?
Huygen’s Sources
Very small pieces of PZT create sound waves in the shape of a “V” called what?
Spherical Waves
Diffraction Patterns
Huygens’ Wavelets
This explains the shape of an imaging transducer’s beam based upon in-phase and out-of-phase wavelets interfering with each other.
Huygens’ Principle
