Transcript Ultra_Machines

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.
• 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:
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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 combinations of analog and
digital electronics.
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 applicationspecific integrated circuits (ASICs) that
provide transmit/receive switches, digital-toanalog 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.
Pulser
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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.
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In digital beam-former systems, a digital-to analogconverter 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-tonoise 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.
Transmit/Receive Switch
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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).
Pulse Echo Operation
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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