Basic Physics of Ultrasound

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Basic Physics of Ultrasound
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WHAT IS ULTRASOUND?

• Ultrasound or ultrasonography is a medical
  imaging technique that uses high frequency sound
  waves and their echoes.
• Known as a pulse echo technique
• The technique is similar to the echolocation used
  by bats, whales and dolphins, as well as SONAR
  used by submarines etc.

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In ultrasound, the following events happen

• The ultrasound machine transmits high-frequency
  (1 to 12 megahertz) sound pulses into the body
  using a probe.
• The sound waves travel into the body and hit a
  boundary between tissues (e.g. between fluid and
  soft tissue, soft tissue and bone).
• 3. Some of the sound waves reflect back to the
  probe, while some travel on further until they
  reach another boundary and then reflect back to
  the probe .
• 4. The reflected waves are detected by the probe
  and relayed to the machine.

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• The machine calculates the distance from the
  probe to the tissue or organ (boundaries) using
  the speed of sound in tissue (1540 m/s) and the
  time of the each echo's return (usually on the
  order of millionths of a second).
• 6. The machine displays the distances and
  intensities of the echoes on the screen, forming
  a two dimensional image.

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So.

• All the energy comes from the transducer
• All we see are reflections and scatter.

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SOUND

• Sound waves consist of mechanical vibrations
  containing condensations (compressions)
  rarefactions (decompressions)that are transmitted
  through a medium.
• Sound is mechanical.
• Sound is not electromagnetic.
• Matter must be present for sound to travel

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Compression wave
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CATEGORIES OF SOUND

• Infrasound (subsonic) below 20Hz
• Audible sound 20-20,000Hz
• Ultrasound above 20,000Hz
• Nondiagnostic medical applications lt1MHz
• Medical diagnostic ultrasound gt1MHz

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THE TRANSDUCER

• Piezo-electric crystal
• Converts electric signals to mechanical vice
  versa
• Transmits pulses of sound into tissue and listens
  for echos
• Most of the time is spent listening for echoes

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Power off Transducer receiving echoes
Transducer Power on
10-6sec
10-3sec
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(No Transcript)
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ULTRASOUND PULSESMAKING THE IMAGE

• Echoes occur when pulses of U/S hit reflectors
• A stream of echoes from each pulse return to
  transducer
• Deeper echoes from deeper tissues arrive later
• Stronger echoes arrive from stronger reflectors
• Each transducer has many elements each making
  pulses (150-200)

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Perfect Reflection
Transducer
Object
Distance (d) is proportional to time (t)
If you know the velocity (c) then the distance
is d1/2 (cxt)
Cfxl, so we can work out frequency and
wavelength too
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ULTRASOUND PULSESMAKING THE IMAGE

• The image is a 2D map of reflections displayed as
  a grey scale
• B mode brightness modulation
• Real time is lots of B mode images run together

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SOUND WAVES

• WAVELENGTH IS VERY SMALL
• OBEY THE LAWS OF OPTICS

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SOUND WAVES

• Gathered into a narrow beam
• Reflected
• Refracted
• Scattered
• Absorbed
• Undergo interference

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Components Of B mode Real time machine
TIME IS OF THE ESSENCE