Chapter 10 Sound Beams

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Chapter 10Sound Beams
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Sound Beams

• Shape
• Regions
• Crystal characteristic vs. depth of focus
• Crystal characteristics which determine the
  degree of beam spread (divergence)

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Shape of a Sound Beam

• At the starting point the beam diameter is equal
  to the transducers diameter
• The beam narrows until it reaches its smallest
  diameter
• The Focus
• The beam then progressively expands or diverges

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Anatomy of a Sound Beam

• Focus
• Near zone
• Focal length
• Far zone
• Focal zone

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Focus

• The location where the beam diameter is the
  narrowest
• For a disc-shaped crystal the beam diameter at
  the focus ½ the width of the beam as it leaves
  the transducer

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Near Zone

• Aka, near field or Fresnel zone
• The region from the transducer to the focus
• The ultrasound beam gradually narrows within the
  near zone
• The focus is located at the end of the near zone

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Focal Length

• The distance from the transducer to the focus
• Aka, focal depth or near zone length (NZL)

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Far Zone

• The region starting at the focus and extending
  deeper
• Aka, far field or Fraunhofer zone
• The sound beam diverges within the far zone
• At the beginning of the far zone, the focus, the
  beam is ½ as wide as at the transducer
• At two near zone lengths from the transducer the
  beam width is the same as the active element
• At depths greater than two near zone lengths, the
  beam is wider than the diameter of the active
  element

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Focal Zone

• The region around the focus where the beam is
  relatively narrow
• Half of the focal zone is located within the near
  field and half is located within the far field
• Reflections arising within the focal zone create
  images that are more accurate than those from
  other depths

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Focal Depth Determination

• Fixed focus transducer focal depth is
  determined by two characteristics of
  the active element
• Transducer diameter
• Frequency of the sound

(X) mm
(X) MHz
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Crystal Diameter vs. Focal Depth

• Focal depth is directly related to diameter2
• Larger diameter active element ? deeper focal
  depth
• Smaller diameter active element ? shallower focal
  depth

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Transducer Frequency vs. Focal Depth

• Directly related
• Higher frequency transducer ? deeper focal depth
• Lower frequency transducer ? shallower focal depth

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Focal Depth (NZL) - Calculation
Multiply each side by 10-3
Round 6.16 ? 6.0
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NZL - Calculation
Calculate the NZL given an element diameter of 1
cm frequency of 5 MHz
Unmodified Equation
Modified Equation
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The Problem

• Given the physics, i.e., high frequency ? deep
  focal depth
• High frequency transducers are utilized to image
  superficial structures
• Therefore, high frequency transducers are
  manufactured with very small diameter crystals to
  shorten the focal depth

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Sound Beam Divergence

• Facts
• Sound beam diameter is smallest at the end of the
  near zone
• The sound beam diverges at depths greater than
  the focus
• Two transducer characteristics determine the
  spread of the beam in the far field
• Transducer diameter
• Frequency of the sound

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Transducer Diameter vs. Sound
Beam Divergence

• Crystal diameter beam divergence in the far
  field
• Inversely related
• Smaller crystals
• Greater beam divergence in the far field
• Larger crystals
• Less beam divergence in the far field
• Larger diameter crystals improve lateral
  resolution in the far field due to less beam
  divergence

Larger diameter crystals create less beam
divergence in the far field
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Frequency vs. Sound Beam Divergence

• Frequency beam divergence in the far field
• Inversely related
• Higher frequency
• Less beam divergence in the far field
• Higher frequencies have better lateral resolution
  in the far field due to less beam divergence
• Lower frequency
• Greater beam divergence in the far field

Higher frequency transducer creates less beam
divergence in the far field
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Whats Important
Factors affecting beam divergence in the far field
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Huygens Principle

• Large active elements may be thought of
  as millions of tiny, distinct sound sources
• Each of these sound sources creates a Huygens
  wavelet with a V-shape
• V-shaped waves are known as
• Spherical waves
• Diffraction patterns
• Huygens wavelets

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Spherical Waves

• Sound waves produced by very small sources
  diverge in the shape of a V
• The V-shaped wave is created when the source is
  about the size of the sounds wavelength

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Huygens Principle Beam Shape

• Huygens wavelets interfere constructively
  destructively
• Explains beam shape based upon in-phase
  out-of-phase wavelets interfering with each other
• Constructive interference leads to hourglass
  shaped main sound beam
• Destructive interference occurs in areas where
  the sound beam is absent

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Lateral Resolution

• The ability to identify two structures as two
  distinct structures, which are close together
  side-by-side, or perpendicular to the sound
  beams main axis
• In other words, what is the minimum distance that
  two structures (reflectors) , positioned
  side-by-side, can be apart and still produce two
  distinct echoes on the image

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Lateral Resolution

• Units of distance
• mm, cm
• Smaller numbers are preferable, indicating more
  accurate images
• Determined by width of the sound beam
• Lateral resolution (mm) Beam diameter (mm)
• Narrower beams have better lateral resolution
• Beam diameter varies with depth, therefore,
  lateral resolution changes with depth
• Synonyms/Mnemonic LATA
• Lateral
• Angular
• Transverse
• Azimuthal

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Lateral Resolution vs. Focus

• Lateral resolution is best at the focus (end of
  near zone)
• Lateral resolution is good within the focal
  zone

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Lateral Resolution vs. Beam Diameter

• Lateral resolution is best at the narrowest part
  of the beam
• When two structures are closer together than the
  width of the beams diameter, only one reflection
  will be seen on the image (suboptimal lateral
  resolution at red arrows)

Seen on image
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Lateral vs. Axial Resolution

• In clinical imaging
• Axial resolution is superior to lateral
  resolution due to pulses being shorter than
  they are wide
• Numerical value is less for axial resolution than
  it is for lateral resolution
• Higher frequencies improve both axial lateral
  resolution
• Improved axial resolution because of shorter
  pulses
• Improved lateral resolution because of less beam
  divergence in the far field

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Focusing

• Improves lateral resolution by concentrating
  sound energy into a narrower beam
• Three methods
• External focusing with a lens
• Utilized with single element transducers
• Internal focusing with a curved active element
• Utilized with single element transducers
• Phased array focusing with the electronics of
  the ultrasound system
• Utilized with array, multiple element, transducers

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Focusing

• Fixed, internal external, focusing techniques
  are also known as conventional or mechanical
  focusing
• With fixed focusing techniques, focusing cannot
  be changed
• Electronic focusing is adjustable

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External Focusing

• Fixed focusing technique
• Lens is placed in front of the piezoelectric
  material
• Increased prominence in the arc of the lens
  increases the degree of focusing, narrowing the
  beam in the focal zone

Active Element
Lens
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Internal Focusing

• Most common form of fixed focusing
• Curved piezoelectric crystal concentrates sound
  energy into a narrower beam
• As curvature becomes more pronounced, the degree
  of focusing increases

Curved PZT crystal
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Phased (Electronic) Focusing

• Phased array means adjustable, or multiple
  focusing
• More versatile than fixed focusing techniques
• Electronics of the US system focus the sound beam
• Sonographer can alter the focusing
  characteristics of the beam
• Utilized only with multi-element transducers

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Effects of Focusing

• Four distinct alterations occur to the sound beam
• Beam diameter narrows in the near field and in
  the focal zone
• Focus is moved closer to the transducer, i.e.,
  near field length (focal depth) is reduced
• Beam diameter beyond the focal zone widens
• Improved lateral resolution in near focal zones
• Degraded lateral resolution beyond the focal zone
• Size of the focal zone is reduced
• Since the near field length is reduced, the focal
  zone will also be reduced

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Summary of Beam Features