Chapters 9 Transducers

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Chapters 9Transducers
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Transducers

• Convert one form of energy to another
• Perform two functions
• Transmission
• Electrical energy is converted into sound
  (acoustic energy)
• Reception
• Acoustic energy is converted into electrical
  energy

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Piezoelectric Effect

• Describes the property of certain materials to
  create a voltage when they are mechanically
  deformed
• Reverse piezoelectric effect
• Materials also change shape when a voltage is
  applied to them
• Piezoelectric (ferroelectric) materials
• Natural occurring
• Quartz, Tourmaline
• Man-made
• Lead zirconate titanate (PZT)

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Ultrasound TransducerBasic Construction
Modern ultrasound transducers share many of the
characteristics of a simple, single crystal
transducer
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Ultrasound Transducer

• Case
• A cylindrical tube
• Metal or plastic
• Protects the transducers internal components
• Insulates the patient from electrical shock
• Electrical shield
• Thin metallic barrier which lines the inside of
  the case
• Prevents extraneous electrical signals in the
  environment from entering the transducer,
  preventing electrical noise from contaminating
  the image

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Ultrasound Transducer

• Acoustic insulator
• Thin barrier of cork or rubber
• Isolates internal components from the case
• Prevents vibrations in the case from inducing an
  electrical voltage in the element of the
  transducer
• Active element
• Generally PZT
• Sound beam characteristics are related to the
  dimensions of the active element
• Piezoelectric material is ½ wavelength thick
• Wire
• Electrically connects the active element and the
  ultrasound system

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Ultrasound Transducer

• Matching layer
• Placed at the face of the transducer, in front of
  the active element
• Increases the efficiency of sound energy transfer
  between the active element and the body
• Acoustic impedance of PZT is 20 times greater
  than the acoustic impedance of the skin
• Matching layer has an acoustic impedance between
  the active element and the skin
• Some modern transducers utilize more than one
  matching layer
• Protects the active element
• ¼ wavelength thick

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• Matching Layer Why
• Differences in acoustic impedance result in
  reflection at boundaries
• Without a matching layer, the vast majority of
  sound energy would reflect back into the PZT and
  not enter the body
• Matching Layer Gel
• Gel (couplant) has an impedance between the
  matching layer impedance and the impedance
  of the media
• Matching layer and gel increase the efficiency of
  sound energy transfer between the PZT and the
  skin
• In decreasing order of impedance
• PZT gt Matching layer gt Gel gt Skin

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Matching Layer

• Why ¼ wavelength thick?
• ¼ wavelength thickness puts the sound wave 90
  out of phase
• Some reflection will occur at the far boundary of
  the matching layer, i.e. reflection back into the
  matching layer, which again will change by 90
• This makes the reflected wave 180 out of phase
• Destructive interference occurs, cancelling out
  the reflected wave
• Consequences of not having ¼ wavelength matching
  layer
• Reflection going straight into the active element
  and that reflection being placed on image

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Ultrasound Transducer

• Backing material
• Bonded to the back of the active element
• Plays essential role in pulse creation
• Reduces the ringing of the pulse (damping)
• Restricts the extent of active element
  deformation
• Damping shortens the duration and length of the
  pulse wave
• Enhances axial resolution
• Commonly made of epoxy resin which is impregnated
  with tungsten filaments

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Backing Material Why?

• Without backing (damping) material
• PZT crystal would ring for a long time
• Pulse would be longer in length and duration
• Long pulses degrade axial resolution
• Characteristics of damping material
• High degree of sound absorption
• Acoustic impedance similar to PZT
• Facilitates the movement of the sound energy into
  the damping material, away from the patient
• Consequences of using backing material are
• Decreased sensitivity
• Wide bandwidth
• Low quality factor

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Decreased Sensitivity

• Transducers are less able to convert low-level
  sound reflections into meaningful electrical
  signals during reception
• Backing material
• Reduces active element vibration during
  transmission
• Reduces vibration during reception

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Wide Bandwidth

• The range, difference, between the highest
  lowest frequencies in a pulse
• The longer a tone rings, the purer the tone,
  ringing at its resonant frequency
• Ringing of the PZT is restricted by damping
  material, resulting in
• Shorter pulses
• Sound with many different frequencies, i.e.,
  larger bandwidth
• Example
• 3 MHz transducer with 4 MHz bandwidth
• Range of frequencies 1 to 5 MHz

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Bandwidth
Imaging vs. Continuous-wave Probes

• Continuous-wave Doppler therapeutic US
• Narrow bandwidth
• Imaging
• Wide bandwidth

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Low Quality Factor (Q-factor)

• A unitless number related to bandwidth
• Q-factor main frequency / bandwidth
• Wide bandwidth low Q-factor
• Imaging transducers
• Narrow bandwidth high Q-factor
• CW Doppler
• Therapeutic ultrasound
• Example
• 3 MHz with 4 MHz bandwidth
• Q-factor ¾ 0.75

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Imaging vs. Non-imaging Transducers
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PZT Materials

• Piezoelectric properties of PZT are not naturally
  occurring
• Created by exposing PZT to a strong electrical
  field during heating
• Process is called polarization
• Temperature to which PZT is heated is called the
• Curie point or temperature
• Curie temperature for PZT is 572 F or 300 C
• PZT can be depolarized as well
• DO NOT heat sterilize ultrasound transducers

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Ultrasound Transducers

• Sterilization vs. Disinfection
• Sterilization
• The destruction of all microorganisms by exposure
  to extreme heat (autoclave), chemical agents or
  radiation
• Disinfection
• Application of a chemical agent to reduce or
  eliminate infectious organisms
• US transducers rarely penetrate mucous membranes,
  therefore are less likely to transmit infection
• Disinfect ultrasound transducers with CidexTM
  (activated gluteraldehyde) or other cold
  germicides

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Transducer Frequencies

• Continuous-wave transducer frequency is
  determined by the frequency of the electrical
  sound exciting the active element

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Transducer Frequencies

• Pulsed-wave transducer frequency is determined by
  two characteristics of the active element
• Speed of sound in the PZT
• Thickness of the PZT

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Speed of sound in PZT vs. Frequency

• Directly related
• Faster speed of sound
• Higher frequency
• Slower speed of sound
• Lower frequency

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PZT Crystal Thickness vs. Frequency

• Inversely related
• Pulsed-wave transducer
• PZT crystal thickness is equal to ½ the
  wavelength of sound in the PZT
• Thinner PZT crystal
• Higher frequency
• Thicker PZT crystal
• Lower frequency