Transducers

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Transducers
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Transducer

• - device that converts one form of energy into
  another form of energy
• Dx. US transducers
• Converts electrical energy into acoustic energy
  (sound) during transmission
• Coverts acoustic energy to electrical energy
  during reception
• Conversion is accomplished through the
  piezoelectric effect

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Piezoelectric Effect

• piezo is Greek for to press elektron is Greek
  for amber
• - property of certain crystals to emit
  electrical energy when pressure is applied
• In US, the crystal expands contracts with a
  returning sound wave causing an electrical
  voltage to be emitted
• Returning sound wave are converted into
  electrical signals

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Reverse Piezoelectric Effect

• - the property of certain crystals to expand or
  contract when positive or negative electrical
  current is applied
• In US, voltage applied to opposite sides of the
  crystal cause it to expand polarity is reversed
  (AC current) causing the crystal to contract
• Constant change from expansion to contraction,
  contraction to expansion, results in mechanical
  waves (sound) being produced
• Thus, the electrical signal is converted into a
  sound wave

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Curie Point

• - temperature the crystals are heated to while
  in the presence of a strong electrical field
  (Curie temperature ranges from approximately
  300C - 400C).
• - If a crystal gets heated above its Curie
  point, it loses its piezoelectric properties.
• We never autoclave a transducer the
  autoclaving renders the transducer useless

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Transducer Element Characteristics

• The crystal (piezoelectric element) emits the
  sound beam receives echoes.
• Natural piezoelectric material such as quartz,
  tourmaline, Rochelle salt
• Man-made piezoelectric ceramic material lead
  zirconate titanate (PZT), barium titanate, lead
  metaniobate, polyvinylidene difluoride (PVF2).
• PVF2 crystals are being developed to have an
  acoustic impedance closer to that of soft tissue.
  These ceramics are not naturally piezoelectric
  the heating process in a strong electrical field
  causes that effect.

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Synthetic Crystals

• Man-made crystals
• less expensive
• more durable
• more efficient in converting mechanical energy to
  electrical energy
• often combined with non-piezoelectric polymer to
  create a material called piezo-composites
• These composites have lower impedance, improved
  bandwidth, sensitivity resolution.

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Lead Zirconate Titanate (PZT)

• - is the most common piezoelectric material
  found in diagnostic imaging transducers

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Operating Frequency

• - the resonant or natural frequency of the
  crystal
• Operating frequency - depends on 2 factors
• Crystal thickness (inversely related to
  frequency)
• Crystal propagation speed (directly related to
  the frequency)

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Crystal Thickness

• thicker crystal lower frequency
• thinner crystal higher frequency
• crystal thickness ½ ? for the frequency
• Typical diagnostic pulsed ultrasound elements
  are .2 1 mm thick

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Propagation Speed of the Crystal

• higher propagation speed higher frequency
• slower propagation speed lower frequency
• Typical propagation speeds of 4-6 mm/?s
• Frequency (MHz) crystals propagation speed
  (mm/?s)
• 2 x thickness (mm)
• Note The US system determines the PRF the PW
  US crystal determines FREQUENCY of sound
• In CW US, the frequency of sound is determined
  by electrical voltage applied to the element

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Probe Construction
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Probe Construction

• - referred to as the probe, the scanhead, or
  transducer assembly. Most commonly referred to as
  the transducer is comprised of the following
• Active Element
• Damping Material (backing material)
• Matching Layer (facing material )
• Wiring
• Insulating Case

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Probe Construction
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Active Element

• - piezoelectric crystal or composite
• single-element transducer - disk shaped
• linear array transducer - rectangular prism
• annular array - doughnut-shaped rings

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Damping Material (Backing Material)

• - composed of epoxy resin impregnated with
  tungsten bonded to the back of the elements to
  reduce the of cycles in the pulse
• ?? PD SPL ? ? axial resolution
• Z backing material Z of the crystal
• Note Dynamic damping - electronic means to
  suppress the ringing by applying a voltage of
  opposite polarity to the crystal after the
  excitation pulse

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Damping Material

• - limits the crystal from ringing absorbs any
  energy emitted in a backwards direction
• Rear surface of the backing material is slanted
  to prevent reflection of sound energy into the
  crystal
• Limiting the amount of ringing of the crystal, ?
  the transducers bandwidth

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Bandwidth

• - range of frequencies above below the main
  (resonant) frequency
• difference between the highest lowest frequency
  found in a pulse
• measured in MHz

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Bandwidth

• Narrow bandwidth - purer transducer frequency
• Damping material ? the bandwidth because it ? PD
  SPL which in turn ? resolution
• Shorter pulses wider bandwidth lower Q factor
  
• Imaging transducers have wide bandwidth

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• Multihertz transducers have a broad bandwidth
  subdivided into 2 or more frequency ranges for
  transmission and reception
• To change to a different frequency, the
  operator just pushes a button

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Quality Factor (Q Factor or Mechanical
Coefficient)

• High Quality Factor Crystal rings for a long
  time (CW transducers), bandwidth is narrow poor
  axial resolution
• Low Quality Factor Crystal rings for a very
  short time (PW transducers), bandwidth is broad
  good axial resolution
• We use low Q-factor with a value of 2 to 3

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• Q-factor operating frequency ? bandwidth
• Q-factor Resonating Frequency (MHz)
• Bandwidth (MHz)

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Matching Layer (facing material)

• Thin layer of aluminum powder in epoxy resin in
  front (facing) of the crystal
• ? the impedance difference between the crystal
  the skin

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• Piezoelectric elements have Z values gt Zsoft
  tissue
• Z PZT 20X Zsoft tissue
• - creates a large reflection of the sound with
  very little transmission into the body

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Matching Layers

• 2 layers - each with a slightly different Z
• Causes the Z mismatch to ? permitting better
  transmission between crystal skin
• matching layer thickness ¼ ? of crystals
  resonating frequency

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Wiring

• Carries electrical pulse to the crystal
• Transmits voltage from the receiving crystal back
  to the US unit
• Each crystal requires electrical contact

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Insulating Case

• Plastic or metal casing around transducer
• Protects
• Sonographer Pt. from electrical shocks
• Keeps outside interference/electrical noise from
  entering
• Protects the transducers components

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Sound Beam Formation
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Sound Beam Formation

• We do not want the sound beam coming from the
  transducer to be non-directional (diffraction)
  like a light bulb.
• Diffraction causes the sound beam to spread out
  as the waves move further from the transducer

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Huygens Principle

• We want the sound beam to be directional like a
  flashlight. So, the design of the transducer
  permits the sound beam to follow Huygens
  Principle which states that all points on a wave
  are considered a point source for the production
  of spherical secondary wavelets.
• These wavelets combine to produce a new wave
  front that determines the direction of the sound
  beam.

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• The resulting effect of the destructive and
  constructive interference of the sound wavelets
  is a sound beam that is hourglass-shaped with
  most of the energy transmitted along the main
  central beam.
• Huygens Principle explains why the sound beam
  shape does not immediately demonstrate
  diffraction.
• divergence ? with ? diameter crystals

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Sound Beam Shape

• Sound beam produced by the transducer is
  hourglass-shaped
• At its starting point, the sound beam
  transducers diameter
• As the sound travels, the width of the beam
  changes
• Becomes narrower until it reaches its smallest
  diameter then it begins to diverge

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Sound Beam Points of Interest

• Focus (focal point)
• Focal length (near zone length, near field
  length, focal length or focal depth)
• Focal zone (focal area or focal region)
• Near zone (Fresnel zone or near field)
• Far Zone (Fraunhofer Zone or far field)

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Focus (focal point)

• Narrowest area of beam diameter
• ½ the crystals diameter
• Region with highest beam intensity

focus
Sound beam
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Focal length (near zone length, focal length,
near field length, or focal depth)
crystal

• - the distance from the crystal to the beams
  focus.
• The focal length zone is related to wavelength
  and crystal radius or diameter.
• As frequency or crystal diameter (aperture) ?,
  focal length ?.
• At 2X the near zone length, beam width
  crystal diameter

focal length
focus
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Focal zone (focal area or focal region)

• - the region surrounding the focus that has a
  narrow beam
• This area has the maximum sensitivity,
  intensity, and best lateral resolution of the
  beam

Focal zone
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Near zone (Fresnel zone or near field)

• - the region between the transducer focus
• This is where additional focusing can be added
• Longer near zones more additional focusing

Near zone
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Far Zone (Fraunhofer Zone or far field)

• - region beyond the near field where beam starts
  to diverge the intensity is more uniform
• ? ? (or crystal diameter) ? widening of the Far
  Zone

Far Zone
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Note

• Near far field shapes are influenced by
  transducer frequency crystal diameter
• ? frequency or crystal diameter (aperture)
  ? length of the near field
• ?the amount of divergence in the far field

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Focusing
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Focusing

• creates a narrower beam over a specified region,
  resulting in improved image resolution
• Focusing is only performed in the near field
• ? frequency (or crystal diameter) produces a
  narrower beam ?focal length
• focusing ? the focal zone by bringing the focus
  closer to the crystal
• Results in ? resolution distal to the focal zone

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4 Methods of Focusing

1. External focusing
2. Internal focusing
3. Electronic focusing
4. Acoustic mirrors

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External Focusing
LENS

• Acoustic lens placed in front of the crystal to
  focus the sound beam at a pre-determined focal
  zone
• Curvature of the lens determines the focal zone

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Internal Focusing
CRYSTALS

• Piezoelectric elements are shaped concavely to
  produce a focused beam
• Curvature of the crystal determines the focal
  zone

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Electronic Focusing

• Uses the interference phenomena by delaying
  (phasing) the electrical pulses to each crystal
  to cause the wave fronts to converge at variable
  focal points
• The rate of delay in electronic pulses determines
  the focal zone

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Electronic Focusing
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Acoustic mirrors

• Used to focus the beam by the ultrasound beam is
  directed back toward a curved acoustic mirror
  that reflects the sound beam outward
• Curvature of the mirror determines the focal zone

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Resolution
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Resolution

• Capability of making individual parts of closely
  adjacent things distinct
• 3 aspects of resolution in imaging
• Temporal
• Contrast
• Detail

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Temporal

• Ability to distinguish closely spaced events in
  time
• Relates to the US imaging equipments frame rate.

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Contrast

• ability of the equipments gray scale display to
  distinguish between echoes of slightly different
  intensities

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Detail

• - ability to distinguish 2 adjacent objects as
  separate objects rather than 1 merged object.
• Measured in millimeters (mm)
• A function of the transducer
• The ? the resolution , the better the image
  quality

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Detail resolution is subdivided into 3 categories

• Longitudinal (LARD longitudinal, axial,
  range, depth)
• Lateral (LATA - lateral, angular, transverse,
  azimuthal)
• Elevational (Slice Thickness)

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Longitudinal (LARD - longitudinal, axial,
range, depth)

• Ability to distinguish 2 structures that are
  laying one on top of the other parallel to the
  path of sound travel
• Commonly called axial resolution

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Axial Resolution

• Determined by SPL
• Shorter pulses improve resolution
• Axial (LARD) resolution (mm) SPL (mm)
  2
• Axial (LARD) resolution (mm) of cycles x ?
• 2

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2 Ways to Improve Axial Resolution

• Use a transducer with damping material (less
  cycles)
• Use a higher frequency transducer (shorter ?)
• Note axial resolution is typically lt 1.0 mm
  remains constant along the sound path
• Explain why that would be logical

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Axial (LARD)
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Detail resolution is subdivided into 3 categories

• Longitudinal (LARD longitudinal, axial,
  range, depth)
• Lateral (LATA - lateral, angular, transverse,
  azimuthal)
• Elevational (Slice Thickness)

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Lateral Resolution LATA lateral, angular,
transverse, azimuthal

• Resolution perpendicular to beam path
• Minimum distance that 2 structures lying next to
  each other can be separated still produce 2
  distinct echoes
• Lower (mm) better the resolution

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Lateral resolution

• Nearly equal to (but slightly gt) beam diameter
• Beam diameter varies along path (with depth) so,
  lateral resolution varies depending on its
  location along the beam
• Is always best at the focus (beam is the
  narrowest)

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Lateral Resolution

• ? with focusing (? beam diameter)
• ? with a higher frequency transducer (longer near
  field less divergent far field)

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Lateral Resolution
A B C D E
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Detail resolution is subdivided into 3 categories

• Longitudinal (LARD longitudinal, axial,
  range, depth)
• Lateral (LATA - lateral, angular, transverse,
  azimuthal)
• Elevational (Slice Thickness)

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Elevational (Slice Thickness)

• Thickness of the scanned tissue perpendicular to
  the scan plane
• AKA - section thickness, Z-axis, elevational
  axis, or out-of-plane focusing
• Accomplished by the attaching a curved lens that
  has a fixed focal depth. The curve of the lens is
  from front to back of the transducer - different
  from a curved lens used for LATA resolution

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Elevational Resolution

• Slice thickness is usually the size of the
  scanhead close to the array, narrows down to a
  few mm. at the lens focal distance, then
  broadens at beyond the focal distance
• Worst measure of resolution for array transducers
  
• except for annular array transducers
• annular arrays have a cone-shaped beam that
  focuses in 3 dimensions

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Elevational Resolution

• ? slice thickness ? spatial resolution (ability
  to detect display adjacent entities)
• Cause of slice thickness artifact
• ? ability to detect small low-contrast lesions

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Resolutions Compared
Side view
Front view
AXIAL RESOLUTION
Lateral resolution
Slice Thickness