Ultrasound Medical Imaging

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Ultrasound Medical Imaging

• Imaging Science Fundamentals

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Sound waves

• Sound waves are mechanical waves. Not in
  electromagnetic spectrum!
• Just like light waves, sound waves transmit
  energy, and can be described in terms of
  wavelength, period, speed, and amplitude
• Also can be described in terms of pressure,
  density, particle displacement

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Sound waves (2)

• Frequency, period, and amplitude are determined
  by the source
• Wave speed and changes in density, pressure, and
  particle displacement are determined by the
  medium
• Wavelength depends on both source and medium

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Some relationships
The higher the frequency, the shorter the period.
Units 1/s The higher the frequency, the
shorter the wavelength. Units m
Note Propagation speed depends on the density
and stiffness of the medium
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• Human hearing range 20 Hz to 20KHz.

• Sound requires a medium in which to travel. In
  the following diagram assume particles are
  separated by springs.

When a particle is pushed, the disturbance is
transmitted to the others by springs.
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• Driving force may be sinusoidal ? particles
  oscillate back and forth

pressure
time
Period
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Sound and Human Hearing
Fast-rate changes (oscillations) in air pressure
arriving at ear are detected by eardrum and
transmitted to the brain, where they are
interpreted as sounds.
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Ultrasound

• Ultrasound is any sound emitted at a frequency gt
  20 KHz.
• Medical ultrasound uses freqencies in the MHz
  range
• Sound speed in tissue c ? 1540 m/s
• similar to speed of sound in water
• need to know c, because it is used to generate
  image
• In lungs, because of gas, propagation speed is
  lower. In bone, because its a solid, propagation
  speed is higher.
• Ultrasound doesnt penetrate lungs bone very
  well

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• What kinds of waves are used in imaging?
• Pulsed ultrasound. A few cycles of ultrasound
• Produced by applying electric pulses to
  transducer

Pulse duration
Pulse repetition period
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• Imaging with ultrasound
• Sound wave is altered by tissue through which it
  passes.
• At boundaries between structures (e.g., different
  tissue types) it is partially reflected
• To determine distance, we need to know
  propagation speed pulse round-trip time (from
  signal emission to echo detection) then
  determines distance
• c 2d/t gt d (tc)/2

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• As round-trip increases, reflectors distance
  increases as well
• For c 1540 m/s 1.54 mm/ms

130 ms
39 ms
13 ms
cm
0 1 2 3 4 5 6 7 8
9 10
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• Imaging with ultrasound
• Ultrasound pulse simultaneously encounters
  several scatterers ? several echoes are generated
  simultaneously

Interference
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• Imaging with ultrasound
• Interference produces a dotted pattern
  (speckles)
• does not directly represent echoes, but rather
  how they recombine at the detector