SPI 2 Flashcards
Beam width
As sound travels the width of the beam changes.
Start- same size as transducer diameter (aperture)
Gets progressively narrower until it reaches smallest diameter
Then diverges
Focus (focal point)
Location where beam reaches its minimum diameter.
Focal depth (focal length, near zone length)
The DISTANCE from the transducer face to the focus.
Near zone (Fresnel zone)
The REGION or zone in between the transducer and the focus
Sound beams converge here
Far zone (Fraunhofer zone)
The REGION or zone deeper than the focus, beyond the near field.
Sound beam diverges here.
Focal zone
Region surrounding the focus where the beam is “sort of narrow”
Picture is relatively good here.
Beam Diameter at transducer
Same as transducer diameter
Beam Diameter at end of near zone
1/2 of transducer diameter
Beam Diameter at 2 near zone lengths
Same as transducer diameter
Beam Diameter deeper than 2 near zone lengths
Greater than transducer diameter
Focal depth is determined by
Transducer diameter (aperture) Frequency
Shallow focus (focal depth)
Small diameter, low frequency
Deep focus (focal depth)
Large diameter, high frequency
Have lower intensity at focus than shallow
Sound beam divergence
Describes the spread of the sound beam in the deep far zone
Sound beam divergence is determined by
Transducer diameter
Frequency of the ultrasound
Less divergence
Narrower beam in far field, large diameter crystal, high frequency, improved lateral resolution in field
More divergence
Wider beam in far field, smaller diameter crystal, low frequency, degraded lateral resolution in far field.
Diffraction
V-shaped wave (Huygen’s wavelet)
Produced by tiny source with a size near the wavelength of the sound.
Waves diverge in this shape as they propogate.
Huygen’s principle
Hourglass shape of sound beam.
Result of the constructive and destructive interference of many sound wavelets emitted from numerous sound sources.
Resolution
The ability to image accurately
Axial resolution
Ability to distinguish two structures that are close to each other front to back, parallel to, or along the beam’s main axis
LARRD
Longitudinal Axial Range Radial Depth
Axial resolution is measured in what units?
Units of distance (mm, cm)
Shorter pulses provide what?
Better axial resolution