Ch. 9 Physics: Sound Beams Flashcards
As sound travels, the width of the beam______
changes
How does the beam change?
Begins the same diameter as the transducer (aperture)
Narrows progressively
Then expands again
What are the zones of the beam?
Near zone
focus
focal zone
far zone
What is another name for the near zone?
Fresnel zone
The region from the transducer to the focus.
near zone or fresnel zone
The sound beam is at its most narrow in this section of the beam?
Focus or focal point
Structures imaged at this point are the highest quality and most accurate.
Focus or focal point
How big is the focus of the original beam’s diameter?
one-half
What section of the beam has the highest spatial peak intensity?
Focus
Region from the focal point and beyond, also called the Fraunhofer zone.
Far zone
True or false
At depths more than two near zones lengths, the beam is still the same size as the active element
False
At depths more than two near zones lengths, the beam is wider than the active element
What happens to the beam after the focal point?
The beam diverges, or spreads out, after the focal point
Which region is the most important region in diagnostic imaging, where the most accurate images are created?
Focal zone
What is the exact center of the focal zone called?
Focus
Also known as focal depth or near zone length.
Focal length
The distance from the transducer to the focus, also is the length of the near zone.
Focal length
The diameter of the crystal at the transducer face is 12 mm. The near zone length measures 8 cm. The focal zone is 4 cm long.
What is the diameter of the sound beam as it exits the transducer?
What would the beam diameter measure at 8 cm?
What would the beam diameter measure at 16 cm?
What distance would mark the beginning of the focal zone?
When would the beams diameter be larger than 12 mm?
- 12mm
-6mm
-12mm
-6 cm
-At a length greater than 16 cm, or after the second near zone length
What are the two factors that effect the depth on fixed focus?
Transducer diameter (aperture)
Frequency of the sound beam
Crystal diameter is _______ related to focal depth.
Directly
Frequency is ________ related to focal depth.
Directly
A smaller crystal diameter has a ______ focal depth.
Shallow
A bigger crystal diameter has a ______ focal depth.
Deeper
Low frequency has a _______ focal depth
shallow
Higher frequency has a ______ focal depth
deeper
What describes the gradual spread of the sound beam in the far field?
Divergence
Is it more or less divergence that has better resolution in deeper tissues?
Less divergence
Is it more or less divergence that has better resolution in deeper tissues?
Less divergence
Divergence of the sound beam is _________ related to:
Transducer diameter (aperture)
Frequency of the sound beam
Inversely
Smaller element diameter =________ divergence in the far field
Greater
Larger element diameter = _________ divergence in the far field (improves lateral resolution deeper)
Less
Lower frequency = _______ divergence in the far field
Greater
Higher frequency =
______ divergence in the far field
Less
Aperture (Beam) Diameter & Frequency of Sound are BOTH _________ RELATED to Divergence
Inversely
Aperture (Beam) Diameter & Frequency of Sound are BOTH _________ RELATED to Depth
Directly
Which of the following probes would create the most divergence?
Small diameter, high frequency
Large diameter, low frequency
Large diameter, high frequency
Small diameter, low frequency
Small diameter, low frequency
Two 12MHz transducers have apertures measuring 6 mm and 9 mm.
Which would have the best lateral resolution in the far field?
The 9 mm diameter – larger = less divergence
Two 5 mm diameter crystals have frequencies of 4Mhz and 6MHz. Which will produce the most divergence?
4 MHz – lower = more divergence
Which of the following probes would create the most divergence?
2 mm, 8 MHz
6 mm, 6 MHz
2 mm, 4 MHz
8 mm, 6 MHz
2 mm, 4 MHz
True or False:
Lateral resolution is inversely related to beam diameter?
True
true or False:
Larger diameter crystals have better lateral resolution than smaller crystals?
True
Which of the following probes would create the shallowest focus?
3 MHz, 4 mm crystal diameter
3 MHz, 6 mm crystal diameter
6 MHz, 4 mm crystal diameter
6 MHz, 6 mm crystal diameter
3 MHz, 4 mm crystal diameter
True or False:
Frequency is directly related to focal depth
True
True or False:
Aperture diameter is directly related to focal depth
True
True or False:
Lower frequency transducers diverge more
True
True or False:
Smaller apertures diverge less
False
Sound waves produced by very small sources (tiny pieces of PZT) and all diverge to form the shape of a V.
spherical waves
diffraction patterns
Huygen’s wavelets
States that a large active element may be thought of as millions of tiny sound sources each producing its own V-shaped waves called wavelets.
Huygen’s principle
Explains that the shape of an imaging transducer’s sound beam is based upon the idea that in-phase and out-phase wavelets are interfering with each other.
Huygen’s principle
