Chapter 9 Flashcards
_ or _is the location
where the beam is the narrowest
Focus or focal point
For a disc-shaped crystal, the width of the
beam at the focus is _
½ the width of the
beam as it leaves the transducer
Near zone AKA
Fresnel zone
The region from the transducer to the focus
Near zone/ fresnel zone
The beam _ within the near zone
gradually narrows
For a continuous wave, disc-shaped
crystal, the diameter of the sound beam
as it leaves the transducer is
the same as
the diameter of the active element.
At the end of the near zone, the beam _
narrows to only ½ the width of the active
element
The focus is located
at the end of the near zone
The distance from the transducer to the
focus
Focal length
Focal length AKA
Focal depth or near zone length
The region that starts at the focus and
extends deeper
Far zone
Far zone AKA
Fraunhofer zone or far field
Within the far zone, the beam
diverges
spreads out
At the beginning of the far zone, the
beam is
only ½ as wide as it is at the
transducer.
When the beam is two near zone lengths
from the transducer, the beam is
the same size as the active element.
At depths more than 2 near zone lengths,
the beam is
wider than the active
element.
The region around the focus where the
beam is relatively narrow
Focal zone
Reflections arising from the focal zone
create images that are
more accurate
than those from other depths.
The _ is the distance from the
transducer to the narrowest part of the
beam (the focus)
focal depth
With a fixed focus transducer, two factors
combine to determine the focal depth:
- Transducer diameter
2. Frequency of sound
Relationship between transducer diameter and focal depth
Directly
Relationship between frequency and focal depth
Directly
Shallow focus:
_ diameter PZT
_ frequency
Smaller
Lower
Deep focus:
_ diameter PZT
_ frequency
Larger
Higher
Higher frequency sound creates a _ focus
Deeper
Higher frequency sound creates a deeper
focus. Transducer manufacturers are aware of
this and overcome it by
creating very
small diameter, high frequency crystals
Focal depth=
diameter^2 x frequency/6
The gradual spread of the ultrasound
beam in the far field.
Beam divergence
Two factors combine to determine beam
divergence:
- Transducer diameter
2. Frequency of sound
Relationship between crystal diameter and beam divergence
Inversely
Relationship between frequency and beam divergence
Inverse
Less divergence:
_ diameter
_ frequency
Larger
Higher
More diveregence:
_ diameter
_ frequency
Smaller
Lower
Sound waves produced by very small sources (tiny pieces of PZT) diverge in
the shape of a V
The v-shaped wave is created when the sound source is
about the size of the sound’s wavelength
Spherical wave AKA
Diffraction patterns
Huygens’ wavelets
US transducers with large PZT crystals create
sound beams shaped like
an hourglass
Small sound sources create beams that are
V shaped
Huygens’ Principle state
that a large active
element may be thought of as millions of tiny,
distinct sound sources. Each of these tiny
particles is a Huygen’s source and creates a
Huygen’s wavelet with a V-shape.
The hourglass shape produced by a large
crystal is the result o
f interference of the
many Huygens’ sound waves emitted
from these numerous sound sources.
Huygens principle: Some of these wavelets are in phase and
interfere _, creating _
Constructively
an hourglass shaped sound beam.
Huygens principle:
Destructive interference occurs where
the wavelets are out-of-phase
and the sound beam is cancelled.
