Transducers

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• Transducers

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• Ultrasound is produced and detected with a
  transducer, composed of one or more ceramic
  elements with electromechanical (piezoelectric)
  properties.
• The ceramic element converts electrical energy
  into mechanical energy to produce ultrasound and
  mechanical energy into electrical energy for
  ultrasound detection.

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• Over the past several decades, the transducer
  assembly has evolved considerably in design,
  function, and capability, from a single-element
  resonance crystal to a broadband transducer array
  of hundreds of individual elements.
• A simple single-element, plane-piston source
  transducer has major components including the
• piezoelectric material,
• marching layer,
• backing block,
• acoustic absorber,
• insulating cover,
• sensor electrodes, and
• transducer housing.

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Piezoelectric Materials

• A piezoelectric material (often a crystal or
  ceramic) is the functional component of the
  transducer.
• It converts electrical energy into mechanical
  (sound) energy by physical deformation of the
  crystal structure.

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• ConverseIy, mechanical pressure applied to its
  surface creates electrical energy.
• Piezoelectric materials are characterized by a
  well-defined molecular arrangement of electrical
  dipoles (Fig. 16-9).

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• An electrical dipole is a molecular entity
  containing positive and negative electric charges
  that has no net charge.
• When mechanically compressed by an externally
  applied pressure, the alignment of the dipoles is
  disturbed from the equilibrium position to cause
  an imbalance of the charge distribution.

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• A potential difference (voltage) is created
  across the element with one surface maintaining a
  net positive charge and one surface a net
  negative charge.
• Surface electrodes measure the voltage, which is
  proportional to the incident mechanical pressure
  amplitude.

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• Conversely, application of an external voltage
  through conductors attached to the surface
  electrodes induces the mechanical expansion and
  contraction of the transducer element.

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• There are natural and synthetic piezoelectric
  materials.
• An example of a natural piezoelectric material is
  quartz crystal, commonly used in watches and
  other time pieces to provide a mechanical
  vibration source at 32.768 kHz for interval
  timing.
• This is one of several oscillation frequencies of
  quartz, determined by the crystal cut and
  machining properties.

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• Ultrasound transducers for medical imaging
  applications employ a synthetic piezoelectric
  ceramic, most often lead-zirconate-titanate
  (PZT).
• The piezoelectric attributes are attained after a
  process of
• Molecular synthesis,
• Heating,
• Orientation of internal dipole structures with an
  applied external voltage,
• Cooling to permanently maintain the dipole
  orientation, and
• Cutting into a specific shape.

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• For PZT in its natural state, no piezoelectric
  properties are exhibited however, heating the
  material past its Curie temperature (i.e., 3280
  C to 3650 C) and applying an external voltage
  causes the dipoles to align in the ceramic.
• The external voltage is maintained until the
  material has cooled to below its Curie
  temperature.
• Once the material has cooled, the dipoles retain
  their alignment.

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• At equilibrium, there is no net charge on ceramic
  surfaces.
• When compressed, an imbalance of charge produces
  a voltage between the surfaces.
• Similarly, when a voltage is applied between
  electrodes attached to both surfaces, mechanical
  deformation occurs.

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• The piezoelectric element is composed of aligned
  molecular dipoles.

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• Under the influence of mechanical pressure from
  an adjacent medium (e.g., an ultrasound echo),
  the element thickness
• Contracts (at the peak pressure amplitude),
• Achieves equilibrium (with no pressure) or
• Expands (at the peak rarefactional pressure),
• This causes realignment of the electrical dipoles
  to produce positive and negative surface charge.

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• Surface electrodes (not shown) measure the
  voltage as a function of time.

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• An external voltage source applied to the element
  surfaces causes compression or expansion from
  equilibrium by realignment of the dipoles in
  response to the electrical attraction or
  repulsion force.

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Resonance Transducers

• Resonance transducers for pulse echo ultrasound
  imaging are manufactured to operate in a
  resonance mode, whereby a voItage (commonly 150
  V) of very short duration (a voltage spike of ?1
  msec) is applied, causing the piezoelectric
  material to initially contract, and subsequently
  vibrate at a natural resonance frequency.
• This frequency is selected by the thickness
  cut, due to the preferential emission of
  ultrasound waves whose wavelength is twice the
  thickness of the piezoelectric material.

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• The operating frequency is determined from the
  speed of sound in, and the thickness of, the
  piezoelectric material.
• For example, a 5-MHz transducer will have a
  wavelength in PZT (speed of sound in PZT is ?
  4,000 m/sec) of

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• A short duration voltage spike causes the
  resonance piezoelectric element to vibrate at its
  natural frequency, fo, which is determined by the
  thickness of the transducer equal to 1/A.

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• To achieve the 5-MHz resonance frequency, a
  transducer element thickness of ½ X 0.8 mm 0.4
  mm is required.
• Higher frequencies are achieved with thinner
  elements, and lower frequencies with thicker
  elements.
• Resonance transducers transmit and receive
  preferentially at a single center frequency.

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Damping Block

• The damping block, layered on the back of the
  piezoelectric element, absorbs the backward
  directed ultrasound energy and attenuates stray
  ultrasound signals from the housing.
• This component also dampens he transducer
  vibration in create an ultrasound pulse width a
  short spatial pulse length, which is necessary to
  preserve detail along he beam axis (axial
  resolution).

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• Dampening of the vibration (also known as
  ring-down) lessens the purity of the resonance
  frequency and introduces a broadband frequency
  spectrum.
• With ring-down, an increase in he bandwidth
  (range of frequencies) of he ultrasound pulse
  occurs by introducing higher and lower
  frequencies above and below the center
  (resonance) frequency.

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• The Q factor describes the bandwidth of the
  sound emanating from a transducer as
• where fo is the center frequency and the
  bandwidth is the width of the frequency
  distribution.

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• A high Q transducer has a narrow bandwidth
  (i.e., very little damping) and a corresponding
  long spatial pulse length.
• A low Q transducer has a wide bandwidth and
  short spatial pulse length.

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• Imaging applications require a broad bandwidth
  transducer in order to achieve high spatial
  resolution along the direction of beam travel.
• Blood velocity measurements by Doppler
  instrumentation require a relatively narrow-band
  transducer response in order to preserve velocity
  information encoded by changes in the echo
  frequency relative to the incident frequency.

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• Continuous-wave ultrasound transducers have a
  very high Q characteristic.
• While the Q factor is derived from the term
  quality factor, a transducer with a low Q does
  not imply poor quality in the signal.

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Matching Layer

• The matching layer provides the interface between
  the transducer element and the tissue and
  minimizes the acoustic impedance differences
  between the transducer and the patient.
• It consists of layers of materials with acoustic
  impedances that are intermediate to those of soft
  tissue and the transducer material.
• The thickness of each layer is equal to
  one-fourth the wavelength, determined from the
  center operating frequency of the transducer and
  speed of sound in the matching layer.

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• For example, the wavelength of sound in a
  matching layer with a speed of sound of 2,000
  m/sec for a 5-MHz ultrasound beam is 0.4 mm.
• The optimal matching layer thickness is equal to
  ¼l ¼ x 0.4 mm 0. 1 mm.
• In addition to the matching layers, acoustic
  coupling gel (with acoustic impedance similar to
  soft tissue) is used between the transducer and
  the skin of the patient to eliminate air pockets
  that could attenuate and reflect the ultrasound
  beam.

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Nonresonance (Broad-Bandwidth) Multifrequency
Transducers

• Modern transducer design coupled with digital
  signal processing enables multifrequency or
  multihertz transducer operation, whereby rhe
  center frequency can be adjusted in he transmit
  mode.
• Unlike the resonance transducer design, the
  piezoelectric element is intricately machined
  into a large number of small rods, and then
  filled with an epoxy resin to create a smooth
  surface.

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• The acoustic properties are closer to issue than
  a pure PZT material, and thus provide a greater
  transmission efficiency of the ultrasound beam
  without resorting to multiple matching layers.
• Multifrequency transducers have bandwidths that
  exceed 80 of the center frequency.

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• Excitation of the multifrequency transducer is
  accomplished with a short square wave burst of
  150 V with one to three cycles, unlike the
  voltage spike used for resonance transducers.
• This allows the center frequency to be selected
  within the limits of the transducer bandwidth.

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• Likewise, the broad bandwidth response permits
  the reception of echoes within a wide range of
  frequencies.
• For instance, ultrasound pulses can be produced
  at a low frequency, and the echoes received at
  higher frequency.

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• Harmonic imaging is a recently introduced
  technique that uses this ability
• lower frequency ultrasound is transmitted into
  the patient, and the higher frequency harmonics
  (e.g., two times the transmitted center
  frequency) created from the interaction with
  contrast agents and tissues, are received as
  echoes.

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• Native tissue harmonic imaging has certain
  advantages including greater depth of
  penetration, noise and clutter removal, and
  improved lateral spatial resolution.

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Transducer Arrays

• The majority of ultrasound systems employ
  transducers with many individual rectangular
  piezoelectric elements arranged in linear or
  curvilinear arrays.
• Typically, 128 to 512 individual rectangular
  elements compose the transducer assembly.
• Each element has a width typically less than half
  the wavelength and a length of several
  millimeters.

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• Two modes of activation are used to produce a
  beam.
• These are the linear (sequential) and phased
  activation/receive modes.

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Linear Arrays

• Linear array transducers typically contain 256 to
  512 elements physically these are the largest
  transducer assemblies.

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• In operation, the simultaneous firing of a small
  group of ? 20 adjacent elements produces the
  ultrasound beam.
• The simultaneous activation produces a synthetic
  aperture (effetive transducer width) defined by
  the number of active elements.

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• Echoes are detected in the receive mode by
  acquiring signals from most of the transducer
  elements.
• Subsequent A-line acquisition occurs by firing
  another group of transducer elements displaced by
  one or two elements.

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• A rectangular field of view is produced with this
  transducer arrangement.
• For a curvilinear array, a trapezoidal field of
  view is produced.

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Phased Arrays

• A phased-array transducer is usually composed of
  64 to 128 individual elements in a smaller
  package than a linear array transducer.
• All transducer elements are activated nearly (but
  not exactly) simultaneously to produce a single
  ultrasound beam.

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• By using time delays in the electrical activarion
  of the discrete elements across the face of the
  transducer, the ultrasound beam can be steered
  and focused electronically without moving the
  transducer.
• During ultrasound signal reception, all of the
  transducer elements detect the returning echoes
  from the beam path, and sophisticated algorithms
  synthesize the image from the detected data.

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BEAM PROPERTIES

• The ultrasound beam propagates as a longitudinal
  wave from the transducer surface into the
  propagation medium, and exhibits two distinct
  beam patterns
• a slightly converging beam out to a distance
  specified by the geometry and frequency of the
  transducer (the near field), and
• a diverging beam beyond that point (the far
  field).

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• For an unfocused, single-element transducer, the
  length of the near field is determined by the
  transducer diameter and the frequency of the
  transmitted sound.

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• For multiple transducer element arrays, an
  effective transducer diameter is determined by
  the excitation of a group of transducer
  elements.
• Because of the interactions of each of the
  individual beams and the ability to focus and
  steer the overall beam, the formulas for a
  single-element, unfocused transducer are not
  directly applicable.

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The Near Field

• The near field, also known as the Fresnel zone,
  is adjacent to the transducer face and has a
  converging beam profile.
• Beam convergence in the near field occurs because
  of multiple constructive and destructive
  interference patterns of the ultrasound waves
  from the transducer surface.

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• Huygens principle describes a large transducer
  surface as an infinite number of point sources of
  sound energy where each point is characterized as
  a radial emitter.
• By analogy, a pebble dropped in a quiet pond
  creates a radial wave pattern.

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• As individual wave patterns interact, the peaks
  and troughs from adjacent sources constructively
  and destructively interfere, causing the beam
  profile to be tightly collimated in the near
  field.

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• The ultrasound beam path is thus largely confined
  to the dimensions of the active portion of the
  transducer surface, with the beam diameter
  converging to approximately half the transducer
  diameter at the end of the near field.

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• The near field length is dependent on the
  transducer frequency and diameter
• where d is the transducer diameter, r is the
  transducer radius, and l is the wavelength of
  ultrasound in the propagation medium.

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• In soft tissue, l 1.54mm/f(MHz), and the near
  field length can be expressed as a function of
  frequency

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• A higher transducer frequency (shorter
  wavelength) will result in a longer near field,
  as will a larger diameter element.

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• For a 10-mm-diameter transducer, the near field
  extends 5.7 cm at 3.5 MHz and 16.2 cm at 10 MHz
  in soft tissue.
• For a 15-mm-diameter transducer, the
  corresponding near field lengths are 12.8 and
  36.4 cm, respectively.

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• Lateral resolution (the ability of the system to
  resolve objects in a direction perpendicular to
  the beam direction) is dependent on the beam
  diameter and is best at the end of the near field
  for a single-element transducer.
• Lateral resolution is worst in areas close to and
  far from the transducer surface.

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• Pressure amplitude characteristics in the near
  field are very complex, caused by the
  constructive and destructive interference wave
  patterns of the ultrasound beam.
• Peak ultrasound pressure occurs at the end of the
  near field, corresponding to the minimum beam
  diameter for a single-element transducer.

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• Pressures vary rapidly from peak compression to
  peak rarefaction several times during transit
  through the near field.
• Only when the far field is reached do the
  ultrasound pressure variations decrease
  continuously.

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• The far field is also known as the Fraunhofer
  zone, and is where the beam diverges.
• For a large-area single-element transducer, the
  angle of ultrasound beam divergence, 0, for the
  far field is given by
• where d is the effective diameter of the
  transducer and l is the wavelength both must
  have the same units of distance.

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• Less beam divergence occurs with high-frequency,
  large-diameter transducers.
• Unlike the near field, where beam intensity
  varies from maximum to minimum to maximum in a
  converging beam, ultrasound intensity in the far
  field decreases monotonically with distance.

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Focused Transducers

• Single-element transducers are focused by using a
  curved piezoelectric element or a curved acoustic
  lens to reduce the beam profile.
• The focal distance, the length from the
  transducer to the narrowest beam width, is
  shorter than the focal length of a non-focused
  transducer and is fixed.

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• The focal zone is defined as the region over
  which the width of the beam is less than two
  times the width at the focal distance
• Thus, the transducer frequency and dimensions
  should be chosen to match the depth requirements
  of the clinical situation.

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Transducer Array Beam Formation and Focusing

• In a transducer array, the narrow piezoelectric
  element width (typically less than one
  wavelength) produces a diverging beam at a
  distance very close to the transducer face.
• Formation and convergence of the ultrasound beam
  occurs with the operation of several or all of
  the transducer elements at the same time.

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• Transducer elements in a linear array that are
  fired simultaneously produce an effective
  transducer width equal to the sum of the widths
  of the individual elements.
• Individual beams interact via constructive and
  destructive interference to produce a collimated
  beam that has properties similar to the
  properties of a single transducer of the same
  size.

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• With a phased-array transducer, the beam is
  formed by interaction of the individual wave
  fronts from each transducer, each with a slight
  difference in excitation time.
• Minor phase differences of adjacent beams form
  constructive and destructive wave summations that
  steer or focus the beam profile.

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Transmit Focus

• For a single transducer or group of
  simultaneously fired elements in a linear array,
• The focal distance is a function of the
  transducer diameter (or the width of the group of
  simultaneously fired elements),
• The center operating frequency, and
• The presence of any acoustic lenses attached to
  the element surface.

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• Phased array transducers and many linear array
  transducers allow a selectable focal distance by
  applying specific timing delays between
  transducer elements that cause the beam to
  converge at a specified distance.

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• A shallow focal zone (close to the transducer
  surface) is produced by firing outer transducers
  in the array before the inner transducers in a
  symmetrical pattern.

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• Greater focal distances are achieved by reducing
  the delay time differences among the transducer
  elements, resulting in more distal beam
  convergence.
• Multiple transmit focal zones are created by
  repeatedly acquiring data over the same volume,
  but with different phase timing of the transducer
  array elements.

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Receive Focus

• In a phased array transducer, the echoes received
  by all of the individual transducer elements are
  summed together to create the ultrasound signal
  from a given depth.
• Echoes received at the edge of the element array
  travel a slightly longer distance than those
  received at the center of the array, particularly
  at shallow depths.

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• Signals from individual transducer elements
  therefore must be rephased to avoid a loss of
  resolution when the individual signals are
  synthesized into an image.
• Dynamic receive focusing is a method to rephase
  the signals by dynamically introducing electronic
  delays as a function of depth (time).

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• At shallow depths, rephasing delays between
  adjacent transducer elements are greatest.

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• With greater depth, there is less phase shift, so
  the phase delay circuitry for the receiver varies
  as a function of echo listening time.

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• In addition to phased array transducers, many
  linear array transducers permit dynamic receive
  focusing among the active element group.

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Dynamic Aperture

• The lateral spatial resolution of the linear
  array beam varies with depth, dependent on the
  total width of the simultaneously fired elements
  (aperture).
• A process termed dynamic aperture increases the
  number of active receiving elements in the array
  with reflector depth so that the lateral
  resolution does not degrade with depth of
  propagation.

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• Side Lobes and Grating Lobes

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• Side lobes are unwanted emissions of ultrasound
  energy directed away from the main pulse, caused
  by the radial expansion and contraction of the
  transducer element during thickness contraction
  and expansion.
• In the receive mode of transducer operation,
  echoes generated from the side lobes are
  unavoidably remapped along the main beam, which
  can introduce artifacts in the image.

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• In continuous mode operation, the narrow
  frequency bandwidth of the transducer (high Q)
  causes the side lobe energy to be a significant
  fraction of the total beam.
• In pulsed mode operation. the low Q broadband
  ultrasound beam produces a spectrum of acoustic
  wavelengths chat reduces the emission of side
  lobe energy.

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• For multielement arrays, side lobe emission
  occurs in a forward direction along the main
  beam.

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• By keeping the individual transducer element
  widths small (less than half the wavelength) the
  side lobe emissions are reduced.
• Another method to minimize side lobes with array
  transducers is to reduce the amplitude of the
  peripheral transducer element excitations
  relative to the central element excitations.

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• Grating lobes result when ultrasound energy is
  emitted far off-axis by multielement arrays, and
  are a consequence of the noncontinuous transducer
  surface of the discrete elements.
• The grating lobe effect is equivalent to placing
  a grating in front of a continuous transducer
  element, producing coherent waves directed at a
  large angle away from the main beam.

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• This misdirected energy of relatively low
  amplitude results in the appearance of highly
  reflective, off-axis objects in the main beam.

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• Spatial Resolution

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• In ultrasound, the major factor that limits the
  spatial resolution and visibility of detail is
  the volume of the acoustic pulse.

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• The axial, lateral, and elevational (slice
  thickness) dimensions determine the minimal
  volume element.

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• Each dimension has an effect on the resolvability
  of objects in the image.

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Axial Resolution

• Axial resolution (also known as linear, range,
  longitudinal, or depth resolution) refers to the
  ability to discern two closely spaced objects in
  the direction of the beam.
• Achieving good axial resolution requires that the
  returning echoes be distinct without overlap.

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• The minimal required separation distance between
  two reflectors is one-half of the spatial pulse
  length (SPL) to avoid the overlap of returning
  echoes, as the distance traveled between two
  reflectors is twice the separation distance.

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• Objects spaced closer than ½ SPL will not be
  resolved.

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• The SPL is the number of cycles emitted per pulse
  by the transducer multiplied by the wavelength.
• Shorter pulses, producing better axial
  resolution, can be achieved with greater damping
  of the transducer element (to reduce the pulse
  duration and number of cycles) or with higher
  frequency (to reduce wavelength).

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• For imaging applications, the ultrasound pulse
  typically consists of three cycles.
• At 5 MHz (wavelength of 0.31 mm), the SPL is
  about 3 x 0.31 0.93 mm, which provides an axial
  resolution of /2(0.93 mm) 0.47 mm.

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• At a given frequency, shorter pulse lengths
  require heavy damping and low Q, broad-bandwidth
  operation.
• For a constant damping factor, higher frequencies
  (shorter wavelengths) give better axial
  resolution, but the imaging depth is reduced.
• Axial resolution remains constant with depth.

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Lateral Resolution

• Lateral resolution, also known as azimuthal
  resolution, refers to the ability to discern as
  separate two closely spaced objects perpendicular
  to the beam direction.

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• For both single element transducers and
  multielement array transducers, the beam diameter
  determines the lateral resolution.

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• Since the beam diameter varies with the distance
  from the transducer in the near and far field,
  the lateral resolution is depth dependent.
• The best lateral resolution occurs at the near
  fieldfar field face.

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• At this depth, the effective beam diameter is
  approximately equal to half the transducer
  diameter.
• In the far field, the beam diverges and
  substantially reduces the lateral resolution.

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• The typical lateral resolution for an unfocused
  transducer is approximately 2 to 5 mm.
• A focused transducer uses an acoustic lens (a
  curved acoustic material analogous to an optical
  lens) to decrease the beam diameter at a
  specified distance from the transducer.

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• With an acoustic lens, lateral resolution at the
  near field-far field interface is traded for
  better lateral resolution at a shorter depth, but
  the far field beam divergence is substantially
  increased.
• The lateral resolution of linear and curvilinear
  array transducers can be varied.

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• The number of elements simultaneously activated
  in a group defines an effective transducer
  width that has similar behavior to a single
  transducer element of the same width.
• Transmit and receive focusing can produce focal
  at varying depths along each line.

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• For the phased array transducer, focusing to a
  specific depth is achieved by both beam steering
  and transmit/receive focusing to reduce the
  effective beam width and improve lateral
  resolution, especially in the near field.

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• Multiple transmit/receive focal zones can be
  implemented to maintain Iateral resolution as a
  function of depth.

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• Each focal zone requires separate pulse echo
  sequences to acquire data.

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• One way to accomplish this is to acquire data
  along one beam line multiple times (depending on
  the number of transmit focal zones), and accept
  only the echoes within each focal zone, building
  up a single line of in-focus zones.
• Increasing the number of focal zones improves
  overall lateral resolution, but the amount of
  time required to produce an image increases and
  reduces the frame rate and/or number of scan
  lines per image.

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Elevational Resolution

• The elevational or slice-thickness dimension of
  the ultrasound beam is perpendicular to the image
  plane.
• Slice thickness plays a significant part in image
  resolution, particularly with respect to volume
  averaging of acoustic details in the regions dose
  to the transducer and in the far field beyond the
  focal zone.

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• Elevational resolution is dependent on the
  transducer element height in much the same way
  that the lateral resolution is dependent on the
  transducer element width.

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• Slice thickness is typically the worst measure of
  resolution for array transducers.
• Use of a fixed focaI length lens across the
  entire surface of the array provides improved
  elevational resolution at the focal distance.

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• Unfortunately, this compromises resolution due to
  partial volume averaging before and after the
  elevational focal zone (elevational resolution
  quality control phantom image shows the effects
  of variable resolution with depth.

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• Multiple linear array transducers with five to
  seven rows, known as 1.5-dimensional (1.5-D)
  transducer arrays, have the ability to steer and
  focus the beam in the elevational dimension.

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• Elevational focusing is implemented with phased
  excitation of the outer to inner arrays to
  minimize the slice thickness dimension at a given
  depth (Fig. 16-25).

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• By using subsequent excitations with different
  focusing distances, multiple transmit focusing
  can produce smaller slice thickness over a range
  of tissue depths.
• A disadvantage of elevational focusing is a frame
  rate reduction penalty required for multiple
  excitations to build one image.

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• The increased width of the transducer array also
  limits positioning flexibility.
• Extension to full 2D transducer arrays with
  enhancements in computational power will allow 3D
  imaging with uniform resolution throughout the
  image volume.

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IMAGE DATA ACQUISITION

• Understanding ultrasonic image formation requires
  knowledge of ultrasound production, propagation,
  and interactions.
• Images are created using a pulse echo method of
  ultrasound production and detection.

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• Each pulse transmits directionally into the
  patient, and then experiences partial reflections
  from tissue interfaces that create echoes, which
  return to the transducer.

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• Image formation using the pulse echo approach
  requires a number of hardware components
• the beam former,
• pulser,
• receiver,
• amplifier,
• scan converter/image memory, and
• display system.

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• Ultrasound equipment is rapidly evolving toward
  digital electronics and processing, and current
  state-of-the-art systems use various
  combin-ations of analog and digital electronics.

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Beam Formers

• The beam former is responsible for generating the
  electronic delays for individual transducer
  elements in an array to achieve transmit and
  receive focusing and, in phased arrays, beam
  steering.
• Most modern, high-end ultrasound equipment
  incorporates a digital beam former and digital
  electronics for both transmit and receive
  functions.

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• A digital beam former controls application-specifi
  c integrated circuits (ASICs) that provide
  transmit/receive switches, digital-to-analog and
  analog-to-digital converters, and
  preamplification and time gain compensation
  circuitry for each of the transducer elements in
  the array.

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• Major advantages of digital acquisition and
  processing include the flexibility to introduce
  new ultrasound capabilities by programmable
  software algorithms and to enhance control of the
  acoustic beam.

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Pulser

• The pulser (also known as the transmitter)
  provides the electrical voltage for exciting the
  piezoelectric transducer elcnwnts, and controls
  the output transmit power by adjustment of the
  applied voltage.
• In digital beam-former systems, a digital-to
  analog-converter determines the amplitude of the
  voltage. An increase in transmit amplitude
  creates higher intensity sound and improves echo
  detection from weaker reflectors.

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• A direct consequence is higher signal-to-noise
  ratio in the images, but also higher power
  deposition to the patient. User controls of the
  output power are labeled output, power, dB,
  or transmit by the manufacturer. In some
  systems, a low power setting for obstetric
  imaging is available to reduce power deposition
  to the fetus. A method for indicating output
  power in terms of a thermal index (TI) and
  mechanical index (MI) is usually provided (see
  section 16.1 1).