Ultrasound Medical Imaging
Imaging Science Fundamentals
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
Some relationships:
f =
1
T
The higher the frequency, the shorter the
period. Units: 1/s
λ=
c
f
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.
pressure
• Driving force may be sinusoidal ⇒
particles oscillate back and forth
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.
Ultrasound
• Ultrasound is any sound emitted at a
frequency > 20 KHz.
–Medical ultrasound uses frequencies 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 it’s a solid, propagation
speed is higher.
–Ultrasound doesn’t 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
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 => d = (t*c)/2
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• As round-trip increases, reflector’s
distance increases as well
• For c = 1540 m/s = 1.54 mm/µs:
130 µs
39 µs
13 µs
0
1
2
3
4
5
6
7
8
9
10
cm
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Imaging with ultrasound:
• Ultrasound pulse simultaneously
encounters several scatterers ⇒ several
echoes are generated simultaneously
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Interference
Imaging with ultrasound:
• Interference produces a “dotted”
pattern (“speckles”)
–does not directly represent echoes,
but rather how they recombine at the
detector
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