Resident Physics Lectures

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					Resident Physics Lectures
      01:
      Ultrasound
       Basics
       Principles


                  George David, M.S.
            Associate Professor of Radiology
                Ultrasound Transducer
• Acts as both speaker & microphone
   Emits very short sound pulse
   Listens a very long time for returning echoes

• Can only do one at a time

            Speaker                        Microphone
        transmits sound pulses               receives echoes
             Piezoelectric Principle
• Voltage generated when certain
  materials are deformed by
  pressure
• Reverse also true!
  Some materials change dimensions when
   voltage applied
     » dimensional change causes pressure change
  when voltage polarity reversed, so is           V
   dimensional change
 US Transducer Operation


• alternating voltage (AC) applied
  to piezoelectric element
• Causes
  alternating dimensional changes
  alternating pressure changes

• pressure propagates as sound
  wave
      Ultrasound Basics

• What does your scanner know about
  the sound echoes it hears?
                             I’m a scanner,
                               Jim, not a
                                magician.




                              Acme
                              Ultra-
                              Sound
                               Co.
  What does your scanner
 know about echoed sound?
           How loud is the echo?




inferred from intensity of electrical pulse from transducer
 What does your scanner
know about echoed sound?
   What was the time delay
   between sound broadcast
        and the echo?
What else does your scanner
know about sound echoes?
      • Direction sound was
        emitted
What else does your scanner
know about echoed sound?
      The sound’s pitch or
           frequency
     What Does Your Scanner
      Assume about Echoes
        (or how the scanner can lie to you)

• Sound travels at 1540 m/s
  everywhere in body
   average speed of sound in soft tissue

• Sound travels in straight
  lines in direction
  transmitted
• Sound attenuated equally
  by everything in body
   (0.5 dB/cm/MHz, soft tissue average)
 Luckily These Are Close Enough to
      Truth To Give Us Images

• Sound travels at 1540 m/s
  everywhere in body
   average speed of sound in soft tissue

• Sound travels in straight
  lines in direction
  transmitted
• Sound attenuated equally
  by everything in body
   (0.5 dB/cm/MHz, soft tissue average)
       Ultrasound Display

• B-scan (“Brightness” Mode)
  Image
  series of gray shade dots

• For each dot, scanner
  must calculate
  position
  Gray shade
Images from Echos
         Dot Placement on Image
• Dot position ideally
  indicates source of
  echo
• scanner has no way of
  knowing exact
  location
  Infers location from echo



                               ?
        Dot Placement on Image

• Scanner aims sound
  when transmitting
• echo assumed to
  originate from
  direction of scanner’s
  sound transmission
• ain’t necessarily so

                           ?
             Positioning Dot
• Dot positioned along assumed line
• Position on assumed line calculated
  based upon
  speed of sound
  time delay between sound transmission & echo




                ?
      Distance of Echo from
           Transducer
• Time delay accurately measured by
  scanner

distance = time delay X speed of sound

          distance
          distance =   time delay X speed of sound

    What is the Speed of Sound?
• scanner assumes speed of sound is
  that of soft tissue
  1.54 mm/msec
  1540 m/sec
  13 usec required for echo object 1 cm from transducer
   (2 cm round trip)

                       13 msec
                                       1 cm
   So the scanner assumes the
          wrong speed?
• Sometimes       •Luckily, the speed of
                  sound is almost the
                  same for most body
                  parts

 soft tissue ==> 1.54 mm / msec
 fat ==> 1.44 mm / msec
 brain ==> 1.51 mm / msec
 liver, kidney ==> 1.56 mm / msec
 muscle ==> 1.57 mm / msec
                                     ?
           Gray Shade of Echo
• Ultrasound is gray shade
  modality
• Gray shade should indicate
  echogeneity of object



               ?           ?
How does scanner know what
gray shade to assign an echo?

  • Based upon intensity (volume,
    loudness) of echo



             ?           ?
           Gray Shade

• Loud echo = bright dot
• Soft echo = dim dot
         Complication

• Deep echoes are softer (lower
  volume) than surface echoes.
      Gray Shade of Echo

• Correction needed to
  compensate for sound
  attenuation with distance
• Otherwise dots close to
  transducer would be brighter
Depth Correction
       Echo’s Gray Shade


• Gray Shade determined by
  Measured echo strength
     » accurate
  Calculated attenuation




                            Who am I?
                            Charles Lane
                Attenuation Correction
• scanner assumes              Tissue   Attenuation Coefficient
                                            (dB / cm / MHz)
  entire body has
  attenuation of soft          • Fat             0.6
  tissue                       • Brain           0.6
  actual attenuation varies
   widely in body              • Liver           0.5
                               • Kidney          0.9
                               • Muscle          1.0
                               • Heart           1.1
       Ultrasound Display

• One sound pulse
  produces
  one image scan line
     » one series of gray shade dots
       in a line

• Multiple pulses
  two dimensional image
   obtained by moving direction
   in which sound transmitted
Moving the Sound Beam
• electronically
  phased or pulsed transducer arrays



                         Focus


             Arrows indicate timing
             variations.
             Activating top & bottom
             elements earlier than
             center ones focuses
             beam.
           Scan Patterns

• Linear
  beam translated
    » moved sideways
  produces rectangular image

• sector
  beam pivoted
  produces pie-shaped image
Th’ Th’ Th’ Th’
That’s All Folks