IMAGE DATA ACQUISITION

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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.

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• A method for indicating output power in terms of
  a thermal index (TI) and mechanical index (MI) is
  usually provided.

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Transmit/Receive Switch

• The transmit/receive switch, synchronized with
  the pulser, isolates the high voltage used for
  pulsing (150 V) from the sensitive amplification
  stages during receive mode, with induced voltages
  ranging from 1 V to 2 mV from the returning
  echoes.
• After the ring-down time, when vibration of the
  piezoelectric material has stopped, the
  transducer electronics are switched to sensing
  small voltages caused by the returning echoes,
  over a period up to about 1000 msec (1 msec).

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Pulse Echo Operation

• In the pulse echo mode of transducer operation,
  the ultrasound beam is intermittently
  transmitted, with a majority of the time occupied
  by listening for echoes.
• The ultrasound pulse is created with a short
  voltage waveform provided by the pulser of the
  ultrasound system.

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• This event is sometimes called the main bang. The
  generated pulse is typically two to three cycles
  long, dependent on the damping charac