Lecture presented in Archamps (Salève Building) by :
2012 :15th SESSION of ESMP
Jean-Martial MARI (INSERM Lyon)
European School of Medical Physics - Archamps1
Ultrasound sessionUltrasound Basic Principles
Jean Martial Mari
Christian Cachard
TransducerHervé Liebgott Ultrasonic Doppler Mode
Piero Tortoli
Quality assurance of US equipment
Jean Martial Mari
40 years of medical ultrasoundNico de Jong
European School of Medical Physics - Archamps2
Ultrasound sessionSpecific applications
Contrast AgentNico de Jong
Post Image Processing
Hervé Liebgott
IntraVascular UltraSoundNico de Jong
Ultrasound ElastographyJean Martial Mari
Therapeutic Ultrasound Jean Martial Mari
European School of Medical Physics - Archamps3
Ultrasound session
TherapeuticJean Martial Mari
Diagnosis
Low intensity
High intensityEuropean School of Medical Physics - Archamps
European School of Medical Physics - Archamps4
Basic Principles of
Ultrasound
Basic Principles of
Ultrasound
Christian CACHARDChristian.cachard@creatis;univ-lyon1.fr
CREATIS
Université Lyon 1
European School for Medical Physics 2012
Teresa RobinsonConsultant Clinical Scientist
Head of the Vascular Studies Unit, United Bristol Healthcare NHS Trust
Jean Martial [email protected]
INSERM Lyon
European School of Medical Physics - Archamps5
Basic principles of ultrasound
1: Overview and history
2: Sound waves
3: Ultrasound generation
4: Ultrasound in tissue
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1
2
3
4
5
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Ultrasound scanning
Scanner
Probe
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European School of Medical Physics - Archamps9
The place of ultrasound in medical imaging
– 4 IRM
– 8 scanners
– 10 angiographic
– 10 PET
– One hundred ultrasound scanners
Public Hospitals in Lyon (2000)
Speci
fic ro
oms
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The place of ultrasound in medical imaging• real time imaging
IN L
IVE
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The place of ultrasound in medical imaging
• Ultrasound has the last ten years been the fastest growing
imaging modality for non-invasive medical diagnosis.
• Of all the various kinds of diagnostic produced in the world,
one of four is an ultrasound scan.
• Reasons for this are the ability to image soft tissue and blood
flow
– the real time imaging capabilities,
– the harmlessness for the patient and the physician (no radiation)
– the low cost of the equipment.
– no special building requirements as for X-ray, Nuclear, and Magnetic
Resonance imaging.
• Limitations are that ultrasound imaging cannot be done
through bone or air (limitations on chest imaging).
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Diagnostic Ultrasound
Echoes
return
Processed into
picture
Pictures
analysed
Sound waves
directed into
patient
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Mechanical wave
Sound is a mechanical wave
�Created by a vibrating object
�Propagated through a medium
Vacuum chamber
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The acoustic pressure
Sound is a pressure wave
The acoustic pressure is the change of pressure around the static
(ambient) pressure
Acoustic pressure
amplitude
t
p (Pa)
105 Ambient pressure
• Ultrasound energy is exactly like sound
energy, it is a variation in the pressure
within a medium.
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• Ultrasound scanner work as sonar
• Probe ⇔⇔⇔⇔ Microphone + Loudspeaker
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Ultrasound
Frequency of Sound
20 Hz
20 000 000 Hz
2 000 000 Hz
20 000 HzAudible Sound
Diagnostic Medical
Ultrasound
Infrasound20 Hz
20 MHz
2 MHz
20 kHz
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target
2
tcd
⋅=
d = ?
c= 1500 m/s
Timetf = 100 µs
Depth
or time
d = ?
d = 7. 5 cm
Transducer or probe
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target
2
tcd
⋅=
d
c= 1500 m/s
Time
0.75 cm < depth < 15 cm
10 µs < tf: time of flight < 200 µs
tf
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Reflection and transmission
One transmitted pulse gives rise to a train of received echoes. Time
⇓⇓⇓⇓Depth
We can calculate where the echoes have come from by timing how long they take to get back.
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64 to 512
Transducer elements
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Depth /
timeWidth
Amplitude
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Depth /
time
0.75 cm < depth < 15 cm
10 µs < time of flight < 200 µs
TPRF PRF: Pulse Repetition Frequency
TPRF >> maximum time of flight
PRF = 1/ TPRF
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Echoes from ONE pulse
The echo
amplitudes are
converted to shades of grey
A-Mode
(amplitude)
B-Mode
(brightness)
Amplitude
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Transducer
One
Frame
One
Frame
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Visible scan
lines (48)
Visible scan
lines (48)
1970’
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Thyroid mass
B (Brightness) Mode Images
Ovarian Cyst
Popliteal artery
Gall bladder & stone
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Popliteal artery Gall bladder & stone
Linear probe Sectorial probe
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• B-mode
� M-mode
(or TM mode,
Time Motion)
time
t
1
t
2
t
3
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Imaging modes
mode A
mode TMmode B
Signal enveloppe
Time (s)
Scan plane
Distance (cm)
Dep
th (cm
)
Dep
th (cm
)
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Doppler
Spectral Doppler
““““Ultrasonic Ultrasonic Ultrasonic Ultrasonic Doppler ModesDoppler ModesDoppler ModesDoppler Modes””””
Piero TortoliPiero TortoliPiero TortoliPiero Tortoli
““““Ultrasonic Ultrasonic Ultrasonic Ultrasonic Doppler ModesDoppler ModesDoppler ModesDoppler Modes””””
Piero TortoliPiero TortoliPiero TortoliPiero Tortoli
Colour Doppler
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History of Ultrasound
• 1794 Lazzaro Spallanzani discovered high frequency ‘ultra’sound by demonstrating ability of bats to navigate by echo reflection
• 1876 Francis Galton invented a whistle that generated sound above the limit of human hearing
• 1880 Pierre Currie discovered the piezo-electric effect in certain crystals.
It was then possible for the generation
and reception of ultrasound
It was then possible for the generation
and reception of ultrasound
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““““Medical Medical Medical Medical Ultrasound Ultrasound Ultrasound Ultrasound
Development over Development over Development over Development over 40 years40 years40 years40 years””””
Nico de JongNico de JongNico de JongNico de Jong
““““Medical Medical Medical Medical Ultrasound Ultrasound Ultrasound Ultrasound
Development over Development over Development over Development over 40 years40 years40 years40 years””””
Nico de JongNico de JongNico de JongNico de Jong
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Intravascular Imaging
““““Intravascular Intravascular Intravascular Intravascular ImagingImagingImagingImaging””””Nico de JongNico de JongNico de JongNico de Jong
““““Intravascular Intravascular Intravascular Intravascular ImagingImagingImagingImaging””””Nico de JongNico de JongNico de JongNico de Jong
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Basic Principles of Ultrasound
1: Overview & History
2: Sound Waves
3: Ultrasound generation
4: Ultrasound in Tissue
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Waves
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Wave Motion
Up and down
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Wave Motion
Stadium wave
Transverse wave
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Transverse Wave
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Longitudinal Wave
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Elastic deformable
medium – gas, liquid
or solid.
Elastic deformable
medium – gas, liquid
or solid.
Molecules do not travel from one
end of the medium to the other.
No flow of particles
Molecules do not travel from one
end of the medium to the other.
No flow of particles
Pressure amplitude
Depth
rare
fact
ion
com
pre
ssio
n
wave velocity c
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At each spatial position , the material points are oscillating around their
equilibrium position (particle velocity v)
Molecules do not travel from one end of the medium to the other.
At each spatial position , the material points are oscillating around their
equilibrium position (particle velocity v)
Molecules do not travel from one end of the medium to the other.
Depth
wave velocity c
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The Nature of a Sound Wave
• Sound is a mechanical wave
– Created by a vibrating object
– Propagated through a medium
• Sound is a longitudinal wave
– Motion of particles is in a direction parallel to direction
of energy transport
• Sound is a pressure wave
– Consists of repeating pattern of high and low pressure
regions
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The Frequency of a wave
T = 1 / fT = 1 / f
Peak excess pressure
=
amplitude (A) of wave
Peak excess pressure
=
amplitude (A) of wave
( )ftAA π2sin0=
Wave equation
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Wavelength, λλλλ = c T = c / fWavelength, λλλλ = c T = c / f
Wavelength, λ
Time
Period,
T=1/f Depends on sourceDepends on source
Depends on speed of sound, c
(depends on material)
Depends on speed of sound, c
(depends on material)
Wavelength and Frequency
Distance
Pressure rare
fact
ion
com
pre
ssio
n
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Air c=330m/s
Water c=1480m/s
If source= 3 MHz λλλλ=1480/ 3 .106= 493 µm
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Ultrasound Pulse
• The majority of ultrasound is emitted as pulses
Pressure
Time
Length of pulse is about 3 to 5 period
F = 3 MHz
1 µs < Tp < 1.66 µs
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t . c
2d =
Range Equation
• It is possible to predict the
distance (d) of a reflecting
surface from the transducer
if the time (t) between
transmission & reception of
the pulse is measured and the
velocity (c) of the ultrasound
along the path is known
d
c
t
Pulse Echo
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Intensity
The intensity associated with the wave is
defined as the power flowing through a
unit area (measured in W/m2 or mW/cm2 )Power
1m
1m
Ii (t,r)p (t,r)2
=cρ
• Time averaged intensity (I) for a sinusoidal wave (where
Po is the peak-pressure amplitude)
c
PI
ρ2
2
0=
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pression
intensité
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Intensity
time
Intensity
Isptp (Temporal
Peak)
Isppa (Pulse
Average)
Ispta (Temporal
Average)
PRFsppa
spta
TI
I ττττ=
TPRFτ
≈ 1/200
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Isptp
Ispta
Isppa
True time axis
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Basic Principles of Ultrasound
1: Overview & History
2: Sound Waves
3: Ultrasound Generation
4: Ultrasound in Tissue
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Sources of Sound Waves
Sound production
requires a
vibrating object
Vocal chords
Audio speaker system Collision!
Piezoelectric Element
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Ultrasound Generatiom
Pierre & Jacques Currie
Piezoelectric Piezoelectric
effect discovered effect discovered
in 1880in 1880 Quartz
+
-Expansion
+
-Contraction
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Ultrasound Detection
• Apply force to
piezoelectric material
• Result is electrical
charge proportional to
force
• The frequency of the
force applied will effect
the frequency with
which a voltage is
generated
Force
Force
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Piezoelectric materials
• Quartz is a naturally occurring piezoelectric
material.
• Lead Zirconate Titanate (PZT) is the
synthetic ceramic material traditionally used
for transducers.
– Can be customised according to the specialist
properties required.
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Transducer
• …any device that transforms one kind of
energy into another
– E.g. electrical to mechanical
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• The information obtained from ultrasound
scanning depends in large part on the beam
characteristics, which in turn are governed
by transducer design.
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““““Ultrasound Ultrasound Ultrasound Ultrasound transducerstransducerstransducerstransducers””””Franco BertoraFranco BertoraFranco BertoraFranco Bertora
““““Ultrasound Ultrasound Ultrasound Ultrasound transducerstransducerstransducerstransducers””””Franco BertoraFranco BertoraFranco BertoraFranco Bertora
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Beam Shape
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Beam Shape
w
λ
W>>λ
w
λ
W
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Constructive
and destructive
interference, in
accordance with
Huygens’
principle
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Plane Disc Source
Near Field Far Field
Non-uniform
beam
Non-uniform
beamUniform
beam
Uniform
beam
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Ultrasound beam from a plane disc
source
Continuous beam, single frequencyContinuous beam, single frequency
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Intensity Variation
Intensity
Distance from probe
Near field Far field
Intensity varies
in space and
time
Intensity varies
in space and
time
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Intensity
Distance from probe,z
( )[ ]zzrI
I z −+=2/1222
0
sinλ
π
)12(4
)12(4 222
max+
+−=
m
mrZ
λ
λ
λ
λ
n
nrZ
2
222
min
−=
λ
2
max'r
Z =
Solve a 3D
geometrical
problem
Near
Field
length
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Component elements
PZT
Lens
Matching
LayerBacking
Layer
Electrodes
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Lens
• A narrow ultrasound beam is desirable to
allow closely spaced targets to be resolved.
• Improvement to the beam can be made by
focussing
• An acoustic lens attached to the face of a
flat surface produces a curved wavefront by
refractions at its outer surface.
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Multi-Element Linear Array
Elements can be excited
individually or in groups
Elements can be excited
individually or in groups
Scan lines
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Different shapes & sizes
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Electronic Beam Steering
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• Spatial Resolution
• Temporal Resolution
• Contrast Resolution
Imaging Resolution
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Spatial Resolution
Spatial (in space)
• axial (along the beam)
• lateral (across the beam)
– azimuth (in the scan
plane)
– elevation or slice
thickness (perpendicular
to the scan plane)
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Axial Resolution
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Axial Resolution
• The minimum
reflector spacing
along the axis of the
ultrasound beam that
results in separate,
distinguishable echoes
on the display.
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Axial Resolution
• Example 3MHz
transducer
• Example 10MHz
transducer
mmHz
sm
f
c5.0
000,000,3
/1540===λ
mmHz
sm
f
c15.0
000,000,10
/1540===λ
Pulse length =number of cycles x λPulse length =number of cycles x λ
ra =3 x 0.5 = 1.5 mm
ra =3 x 0.15 = 0.45 mm
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Lateral Resolution
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Lateral Resolution
• The ability to
distinguish two closely
spaced reflectors that
are positioned
perpendicular to the
axis of the ultrasound
beam.
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Elevational Resolution
(slice thickness)
• Works in a direction
perpendicular to the
image plane.
• Dictates the thickness
of the section of tissue
that contributes to
echoes visualised on
the image.
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Slice Thickness Resolution
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Temporal Resolution
• The time interval between pulses
– limits the temporal resolution
– it is usually set so that there is sufficient time
for the most distant echo to return to the
transducer before the next pulse is launched
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Temporal Resolution
pulse
Interval to
allow echoes
to return
Sequence of pulses from
transducer
Typical PRF = 5kHz
TPRF = 0.2 ms
Typical PRF = 5kHz
TPRF = 0.2 ms
Time
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Contrast Resolution
The ability to display regions of differing echo size
Low
High
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Basic Principles of Ultrasound
1: Overview & History
2: Sound Waves
3: Ultrasound Generation
4: Ultrasound in Tissue
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Speed of Sound
Speed of Sound
Speed of Sound
Speed of Sound
Impedance
Impedance
ImpedanceImpedance
ReflectionReflectionReflectionReflection
RefractionRefractionRefractionRefraction
Attenuation
Attenuation
Attenuation
Attenuation
NonNonNonNon----linear effectslinear effe
ctslinear effe
ctslinear effe
cts
SafetySafetySafetySafety
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Matter
• It is helpful, for ultrasound purposes, to
imagine that matter is composed of
– tiny particles
– joined together by springs
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Matter
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Properties
• Stiffness of substance K (bulk modulus)
– strength of the spring
• Density of substance ρ
– mass & separation of particles
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Movements
• Each particle is connected to each of its
neighboring particles by ‘springs’
• A single movement of ONE particle will
move all the others
• Repetitive movements will move all the
others repetitively - but after a time delay
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0 u
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0 udu
vdu
=dt
particle velocity
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0 u
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x
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Speed of Sound
• Low density and high stiffness
high speed of sound
• High density and low stiffness
low speed of sound
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Mathematically…
ρ
kc =
Bulk modulus
Density
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Speed of Sound
• Air 330m/s
• Water 1480m/s
• Fat 1460m/s
• Blood 1560m/s
• Muscle 1600m/s
• Bone 4060m/s
Average soft
tissue value =
1540m/s
This can lead to small errors in the estimated distance travelled because of the
variation in the speed of sound in different tissues.
This can lead to small errors in the estimated distance travelled because of the
variation in the speed of sound in different tissues.
Programme the
ultrasound
machine with...
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Speed of Sound
Speed of Sound
Speed of Sound
Speed of Sound
Impedance
Impedance
ImpedanceImpedance
ReflectionReflectionReflectionReflection
RefractionRefractionRefractionRefraction
Attenuation
Attenuation
Attenuation
Attenuation
NonNonNonNon----linear effectslinear effe
ctslinear effe
ctslinear effe
cts
SafetySafetySafetySafety
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Reflection at Boundaries
Incident
wave
Transmitted wave
Reflected
wave
• At the boundary
between tissues
ultrasound is partially
reflected
• The relative proportions
of the energy reflected
and transmitted depend
on the acoustic
impedance between the
two materials
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Acoustic Impedance
Small masses
Weak springs
Low impedance
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Large masses
Strong springs
High impedance
Acoustic Impedance
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Mathematically
Bulk modulusDensity x
kz ρ=
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The Proof…
• Acoustic Impedance
analogous to electrical
resistance.
• So using Ohm’s lawP = local pressure
v = local particle velocity v
Pz =
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Similar Values
Acoustic Impedance
• Air 0.0004 x 106 rayls
• Lung 0.18 x 106
• Fat 1.34 x 106
• Water 1.48 x 106
• Blood 1.65 x 106
• Muscle 1.71 x 106
• Skull Bone 7.80 x 106
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Speed of Sound
Speed of Sound
Speed of Sound
Speed of Sound
Impedance
Impedance
ImpedanceImpedance
ReflectionReflectionReflectionReflection
RefractionRefractionRefractionRefraction
Attenuation
Attenuation
Attenuation
Attenuation
NonNonNonNon----linear effectslinear effe
ctslinear effe
ctslinear effe
cts
SafetySafetySafetySafety
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Reflection
Z1
Z2
Pi , vi
Pt , vt
Pr , vrReplace v with P/z
112 z
P
z
P
z
P rit −=
rit
rit
vvv
PPP
−=
+=
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112 z
P
z
P
z
P rit −=
ririt PPPz
zP
z
zP +=−=
1
2
1
2
riri PzPzPzPz 1122 +=−
rrii PzPzPzPz 2112 +=−
( ) ( )2112 zzPzzP ri +=−
12
12
zz
zz
P
P
i
r
+
−= Reflection
Coefficient
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Blood Fat
Z1=1.65 Z2= 1.34
10.034.165.1
34.165.1=
+
−==
i
r
P
PR
10% reflected 90% transmitted
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64.071.18.7
71.18.7=
+
−==
i
r
P
PR
Muscle Bone
Z1=1.71 Z2= 7.8
64% reflected 36% transmitted
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Calcification
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999.034.10004.0
34.10004.0−=
+
−==
i
r
P
PR
Fat Air
Z1=1.34 Z2= 0.0004
99.9% reflected 0.01% transmitted
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Bowel Gas
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Strong
Reflection
Weak
Reflection
Very weak
Reflection
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Reflection
• Specular reflection
– from large flat boundaries
• Diffuse reflection
– from small structures
• Rayleigh Scattering
– from very small structures
StrongStrong
WeakWeak
Very weakVery weak
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Non-perpendicular
Incidence
Specular Reflection
θθθθi = θθθθr
Perpendicular
Incidence
Reflected
beam
travels off
at an angle
Strong orientation dependenceStrong orientation dependence
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Diffuse Reflection
Reflected waves
travel in various
directions away
from the interface
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Diffuse Reflection
Reflected waves
travel in various
directions away
from the interface
Some orientation dependenceSome orientation dependence
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Rayleigh Scattering
Particles size
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Rayleigh Scattering
Particles size
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Echo Amplitude - Beware!
The amplitude of the echoes (image grey level)
does not have a simple relationship with the tissue (unlike X-ray CT [Hounsfield numbers]).
• Echo size depends on
– relative acoustic impedances across boundary
– shape and orientation of boundary
– size of structure compared with λ
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Speed of Sound
Speed of Sound
Speed of Sound
Speed of Sound
Impedance
Impedance
ImpedanceImpedance
ReflectionReflectionReflectionReflection
RefractionRefractionRefractionRefraction
Attenuation
Attenuation
Attenuation
Attenuation
NonNonNonNon----linear effectslinear effe
ctslinear effe
ctslinear effe
cts
SafetySafetySafetySafety
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Refraction
θt
θi
• At boundaries between
tissues with different
velocities i.e. c1≠c2• The beam direction is
changed (if it is not
incident normally to the
boundary) i.e. θi≠ θ t• Beam distortion leads to
misregistration
• A problem with fat and
muscle
C1
C2
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Snell’s Law
θt
θi
C1
C2
2
1
sin
sin
c
c
t
i =θ
θ
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Refraction - Snell’s Law
C1=1600m/s
θt
60o
Fat
C2=1460m/s
E.g. Muscle to fat with an angle
of incidence of 60o
o
t
t
52
1460
1600
sin
60sin
=
=
θ
θMuscle
i.e 8 degrees
difference
i.e 8 degrees
difference
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Image Degradation
Real path
Real target
Path as assumed by
machine
Displayed image
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Speed of Sound
Speed of Sound
Speed of Sound
Speed of Sound
Impedance
Impedance
ImpedanceImpedance
ReflectionReflectionReflectionReflection
RefractionRefractionRefractionRefraction
Attenuation
Attenuation
Attenuation
Attenuation
NonNonNonNon----linear effectslinear effe
ctslinear effe
ctslinear effe
cts
SafetySafetySafetySafety
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Attenuation
The energy of the ultrasound beam is reduced with distance
Energy is lost from the beam by:
Absorption (conversion into heat)
Scattering (reflection out of beam confines,
refraction, divergence)
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Attenuation in an image
Bright
deep to
the cyst
Dark
deep to
the defect
in the
phantom!
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Attenuation Coefficient
The intensity, Iz, of an ultrasound beam is related
to distance from the source, z, thus:
Where I0 is the intensity at z = 0, the transducer face.
Iz
z
z
z eIIα−= 0
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Attenuation Coefficient
Attenuation is approximately exponential,
the slope of the logarithmic graph is constant.
Attenuation coefficient is quoted in dB/cm
In addition, for soft tissue,
attenuation is proportional to frequency.
The attenuation coefficient for soft tissue is
0.5 - 0.7 dB/cm/MHz
The attenuation coefficient for soft tissue is
0.5 - 0.7 dB/cm/MHz
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Attenuation - compensation
Average echo
Amplification
factor
Echo train
after compensation
Time
(distance)
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depth
gain
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Speed of Sound
Speed of Sound
Speed of Sound
Speed of Sound
Impedance
Impedance
ImpedanceImpedance
ReflectionReflectionReflectionReflection
RefractionRefractionRefractionRefraction
Attenuation
Attenuation
Attenuation
Attenuation
NonNonNonNon----linear effectslinear effe
ctslinear effe
ctslinear effe
cts
SafetySafetySafetySafety
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Non-Linear Propagation
Under conditions of relatively high pressure amplitude
the speed of sound is NOT CONSTANT
but varies over the propagation path (z)
)(2
1)( 0 zvA
Bczc ÷
++=
c
pv
ρ =
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Non-Linear Propagation
)(2
1)( 0 zvA
Bczc ÷
++=
Material B/A
water (30°C) 5.2
blood 6.3
liver 7.6
spleen 7.8
fat 11.1
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High pressure High pressure
Low pressure Low pressure
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After a sufficient distance, the faster
moving high-pressure parts of the wave
catch up to the slower low-pressure parts.
The result is a
sawtooth wave
The distorted wave has many harmonic frequencies
Higher
speed
Higher
speed
Lower
speed
Lower
speed
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Higher
speed
Higher
speed
Lower
speed
Lower
speed
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Propagation non linéaire
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Harmonics
f0
Fundamental
2f0
2nd
3rd
3f0
Frequency
Amplitude
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Harmonic Imaging
A relatively recent innovation in
diagnostic ultrasound imaging is
Tissue Harmonic Imaging
Discovered by accident it uses the
effects of non-linear propagation.
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• Native harmonic frequencies are used to
improve images
• How?
– By tuning the receiver to the harmonic frequency (2Fo)
rather than the transmitted frequency Fo
• Benefits
– Reduces clutter (noise), increases resolution at depth,
improves sensitivity
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Speed of Sound
Speed of Sound
Speed of Sound
Speed of Sound
Impedance
Impedance
ImpedanceImpedance
ReflectionReflectionReflectionReflection
RefractionRefractionRefractionRefraction
Attenuation
Attenuation
Attenuation
Attenuation
NonNonNonNon----linear effectslinear effe
ctslinear effe
ctslinear effe
cts
SafetySafetySafetySafety
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Safety
• Thermal
– tissue heating, cell death for T > 42oC
• Mechanical
– cavitation bubbles for pressures > threshold
• unfortunately threshold is frequency dependent
Possible damage from ultrasound:
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Important Intensities
• ISPTA (spatial peak temporal average)
– describes tissue heating potential
• ISPTP (spatial peak temporal peak)
– describes cellular damage potential Isptp (Temporal
Peak)
Isppa (Pulse
Average)
Ispta (Temporal
Average)
TPRFτ
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Example: Acoustical Outputs
Mode Pr(Mpa) ISPTA(mW/cm2) ISPPA (W/cm
2) Power(mW)
B 1.68 18.7 174 18
M 1.68 73 174 3.9
PD 2.48 1140 288 30.7
CF 2.59 234 325 80.5
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Mechanical and Thermal Indices
• Thermal index
– relates to temperature
– potential for heating effects (metabolic rate)
• Mechanical Index
– relates to pressure
– potential for bubble effects (cavitation)
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Thermal Index
• TI is the ratio between:
– the power exposing the tissue, W
– the power required to cause a 1oC temperature
rise, Wdeg
TI =Wdeg
W
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Mechanical Index
• MI describes the likelihood of the negative
pressure causing bubble activity
MIP-d
f
=
P-d is the ‘derated’ pressure at the site in the body
f is the frequency of the pulse
megapascals
(megahertz) 1/2
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MI and TI in practice
important not so important
contrast agents
lung (cardiac)
bowel gas (abdominal)
absence of gas bodies
(most soft tissue studies)
TI 1st trimester
fetal skull and spine
ophthalmic
fever
poor perfusion
good perfusion
(liver, spleen)
cardiac
vascular
MI
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Guidelines
• MI>3 Possibility of minor damage to
neonatal lung or intestine
• MI>0.7 Theoretical risk of cavitation.
• TI>0.7 Restrict exposure time of a fetus
• TI>1.0 Eye scanning not recommended
• TI>3.0 Fetal scanning not recommeded
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Speed of Sound
Speed of Sound
Speed of Sound
Speed of Sound
Impedance
Impedance
ImpedanceImpedance
ReflectionReflectionReflectionReflection
RefractionRefractionRefractionRefraction
Attenuation
Attenuation
Attenuation
Attenuation
NonNonNonNon----linear effectslinear effe
ctslinear effe
ctslinear effe
cts
SafetySafetySafetySafety
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Scanner settings
•Fixed settings : MI, TCG, gain
•Adjustement: focus, sector
size
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References
• Ultrasound Imaging, Bjorn A. J. Angelsen,
ISBN 82-995811-0-9, Emantec AS,
Trondheim, Norway,
www.ultrasoundbook.com
• Ultrasound in Medecine, Institute of
Physics, Publishing Bristol and Philadelphia
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It’s all over….
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