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KREMKAU, Chapter 3, Transducers

Terms in this set (66)

Diffraction
The spreading out of sound (or light) into a pie shape when produced by a small source
Huygen's Principle
Explains why diffraction does not occur by stating:

1) each point on the surface of the transducer is a separate and individual point of sound.
2) sound wavelets interfere and overlap.
3) overlapping and interference of wavelets cause the u/s beam to assume its hourglass shape
How do you describe the focal point of a beam?
The point in which the beam is at it's narrowest
What is the shape of a sound beam?
Hourglass
How is the beam width related to the transducer width?
At the face of the transducer the beam is the same width.
Convergence
The narrowing pattern of the beam
The distance from the face of the transducer to the focal point
NZL (near zone length)
OR Focal Depth
OR Fresnel's Zone
Where does the highest variation of intensity in the beam occur?
In the near zone
Divergence
The expansion of the sound beam that occurs after the focal point
The point beyond the focus to the end of the beam
Far zone
OR Fraunhofer zone
Focal zone
3 dB of total power
What is the relationship between the focal diameter and intensity?
Inversely related
Focal length
The length of space between the transducer's face and focus
What are the two factors that affect the NZL?
1) The transducer's diameter
2) Frequency
1/2 of the transducer's aperture
diameter of the focal point
At double NZL (At the far zone of the beam) the ______________ is equal to _____________.
diameter of the beam, diameter of the transducer
What is the relationship between wavelength and NZL?
Inversely related
The longer the NZL the ________ the resolution
Better
Far zone
The area of space beyond the focus where the beam starts to enlarge or widen
How are NZL and divergence related?
Inversely related
What are the two factors that affect the far zone?
1) frequency
2) crystal diameter
How does the crystal diameter affect the far zone?
Larger crystals produce beams that diverge less
Describe the x, y, and z axis
x-axis = LARRD
y-axis = width of the beam
z-axis = beam thickness
What happens to the beam when frequency is increased?
The beam becomes narrower and diverges less
Spatial resolution
the ability to distinguish the structures clearly from a spatial perspective
The aperture and NZL are related how?
Directly related (higher the aperture, longer the NZL)
How are aperture and NZL related?
Directly
How are aperture and divergence related?
Inversely
How are aperture and frequency related?
Directly
How are frequency and NZL related?
Directly
How are frequency and divergence related?
Inversely
How is the speed of sound through PZT and NZL related?
Directly
How is the speed of sound through PZT and divergence related?
Inversely
Axial resolution
The ability to distinguish 2 structures as separate and distinct from each other when located along the beam's path
LARRD
Longitudinal Axial Radial Range Depth
LATA
Lateral Angular Transverse Azimuthal
The LAARD is associated with which axis?
X-axis
The LATA is associated with which axis?
The y-axis
In detail resolution what is associated with the z-axis?
Elevation resolution (slice thickness OR partial volume)
What units are used for axial resolution?
Distance
What 2 factors affect LARRD
SPL & frequency
What is the equation for axial resolution?
SPL/2
What is the equation for SPL?
# of cycles x wavelength
How can you improve axial resolution?
Use a higher frequency, shorten the spl, redue the pd, reduce the # of cycles
Lateral resolution
The ability to distinguish 2 structures as separate and distinct from one another when located perpendicular to the beams path
The lateral resolution is equal to ______
the beam diameter
Slice thickness resolution is also known as
Elevation resolution, out of plane focusing
Slice thickness resolution
the resolution that is 90 degrees to a 2D scan
where is slice thickness resolution the best?
in the near zone
Name the resolutions from best to worst.
Axial, Lateral, Elevational
What is the piezoelectric effect?
The conversion of acoustic pressure energy into electrical energy or vice versa
Direct piezoelectric effect
From acoustic pressure to electrical energy
Indirect piezoelectric effect
From electrical energy to acoustic energy
Natural piezoelectric materials
Amber, tourmaline, quartz, Rochelle salt
How are piezoelectric materials made piezoelectric?
By placing them in a strong electrical field and heating to a high temperature
What is the Curie point?
The point at which the ceramic material will depolarize
Impedance of skin is ________ than impedance of PZT
20 times less
What does the matching layer do?
Decreases the differences in impedences
What is the ideal thickness of the matching layer?
1/4 the wavelength
Q Factor
a number that indicates to what extent the transducer can operate at the center or main frequency
US transducer have a ________ Q factor
low
What is the equation for the Q (quality) factor
operating frequency divided by the bandwidth or about the # cycles in pulse
What two factors determine the operating frequency?
1) Propagation speed (C) in the piezoelectric material
2) Thickness of piezoelectric crystal
What is the typical thickness of the piezoelectric material?
.2mm - 1mm
What is the best way to change the beam width?
Focus the beam
What is the range of LATA?
1mm-10mm
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