Transcript Slide 1

The Physics of Sound


Sound travels through the air (and different media) in
waves, called Sound Waves
These waves cause the air to oscillate (vibrate) back and
forth
Light
1. Does NOT require a medium
2. Transverse wave
3. Electromagnetic field
Sound
1. REQUIRES a medium
2. Longitudinal wave
3. Mechanical vibration
Longitudinal vs. Transverse Waves


Longitudinal waves – molecular displacement is
along direction in which waves travel
Compression – regions of high molecular density
(molecules in high pressure areas compress)


Rarefraction – regions of low molecular density
(molecules in low pressure areas expand)
Transverse waves – molecular displacement in
direction perpendicular to wave (guitar string)

Longitudinal waves – travel in solids & liquids
Soft tissue – more like liquids
 US primarily travels as longitudinal wave


Transverse waves – cannot pass through fluids;
found in the body only when ultrasound strikes
bone
Not Just Any Wave
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The waves (ripples) created by throwing a rock
into the pond are Transverse Waves
Sound waves are NOT transverse waves
Sound waves ARE Longitudinal Waves
Transverse Waves


Pretend like you are at a Canucks game.
Everyone do the wave starting from the left to
the right!
What direction is the wave traveling?

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The wave travels to the right
What direction is the displacement caused by the
wave?

Displacement is vertical; Perpendicular to the travel
direction.
Transverse Waves
 NOT SOUND WAVES !!!
Longitudinal Waves
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Do the wave again, but this time, instead of
moving your arms up and down, move them
side to side.
What direction is the wave traveling?

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The wave travels to the right
What direction is the displacement caused by the
wave?

Displacement is horizontal; Parallel to the travel
direction
Sound Production

How does one make sound?


Vocal cords, speakers, headphones etc.
What do these all have in common?

They all vibrate the air!
Sound Waves
compression
rarefaction
amplitude
sin wave
time
Sound Reproduction

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
Speakers take an electronic signal, and
reproduce sound.
By far most common type of speaker is the
Dynamic Speaker.
Other types of speakers include piezoelectric
speakers, plasma arc speakers and electrostatic
speakers.
The “Sonic” Spectrum
Infrasound:
< 20 Hz
Sound:
20 Hz – 20,000 Hz (20kHz)
Ultrasound:
> 20 kHz (~1013 Hz maximum)
Human Hearing
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Human range: 20-20,000 Hz
Human ear is most sensitive in the 1,000 4,000 Hz range.
Less sensitive in lower frequencies.
Hearing Range
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Hearing range in human is 20 Hz to 20 kHz
Infrasound is the sound the frequency of which
is lower than 20 Hz
Ultrasound is the sound the frequency of which
is higher than 20 kHz
Nature Of Ultrasound
The sound is no longer audible
︰﹥20 kHz.
 The medical ultrasound is in the 1~20 MHz
region.

Ultrasound
 Ultrasound
in Nature
 Medical applications
Diagnostic
Treatment
 Industrial Applications
Why Use Ultrasound?

Ultrasound is very safe. There is no firm
evidence that it does any harm to the body (or
the baby in the case of pregnancy scans).

X-rays are potentially dangerous, particularly to
young children and pregnant women (they
damage the unborn baby).
What is Ultrasound?

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Located in the Acoustical Spectrum
May be used for diagnostic imaging, therapeutic
tissue healing, or tissue destruction
Thermal & Non-thermal effects
We use it for therapeutic effects
Can deliver medicine to subcutaneous tissues
(phonophoresis)
Ultrasound

Sinusoidal waveform
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Therapeutic ultrasound waves range from 750,000 to
3,000,000 Hz (0.75 to 3 MHz)
Displays properties of
wavelength,
 frequency,
 Amplitude

What is Ultrasound?
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Ultrasound is simply sound
that has a very high frequency.

Humans are not able to hear
ultrasound, though some
animals can hear them.

Sounds with frequencies above
20 000 hertz are called
ultrasounds.
EQUIPMENT
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
TRANSDUCERS ARE USED TO
GENERATE THE ULTRASONIC ENERGY
THE MAJOR COMPONENT OF AN
ULTRASOUND TRANSUCER IS THE
PEIZOELECTRIC ELEMENT
PIEZOELECTRIC MATERIAL

ARE CAPABLE OF CONVERTING ONE
FORM OF ENERGY INTO ANOTHER

ABLE TO GIVE OFF AND RECIEVER SOUND
ENERGY
TRANSDUCERS

DIFFER IN SIZE, SHAPE &
FREQUENCIES

SIZE AND SHAPE TO IMPROVE
CONTACT WITH DIFFERENT BODY
STRUCTURES
Transducer
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A device that converts one form of energy to another
Piezoelectric crystal: a crystal that produces (+) and (-)
electrical charges when it contracts or expands

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Reverse (indirect) piezoelectric effect: occurs when an
alternating current is passed through a crystal resulting in
contraction & expansion of the crystal
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Crystal of quartz, barium titanate, lead zirconate, or titanate housed
within transducer
US is produced through the reverse piezoelectric effect
Vibration of crystal results in high-frequency sound waves
Fresnal zone (near field) – area of the ultrasound beam on the
transducer used for therapeutic purposes
FREQUENCIES


LOW FREQUENCY OF 1 MHz TO HIGHER
FREQUENCIES OF 12 TO 20 MHz.
THE SMALLER THE OBJECT TO BE
IMAGED, THE HIGHER THE
TRANDUCER FREQUENCY
Transducers


It is not only used as a transmitter, but
also a receiver.
For different applications, we need to
choose the transducer resonant
frequency, diameter and focal length.

Piezoelectric Effect—
Certain substances change their
dimensions when an electrical charges
on their surfaces when deformed.
Such substances are called piezoelectric.
Ex. Quartz.
Metal case
Transducer
Matching
layer
Coaxial
connector
Backing
Probe
Acoustic
insulator

Backing—
For imaging purpose, we are interested
in generating short pulse of US and
this is achieved by the transducer.
Make sure that its acoustic impedance
is the same as that of the transducer.

Matching Layer—
In order to improve the transmission
from the transducer into the tissue
efficiently, matching layer may be used.
Ideal thickness is a quarter of a
wavelength thick.
d=λ/4
Metal case —
The backing is enclosed within a metal
case in order to provide a means of
handling the transducer and to provide
electrical shielding to prevent electrical
interference.
 Acoustic Insulator —
EX. rubber or cork.
To avoid transmission of US into the
metal case.

Generation of Ultrasound

Pizoelectric effect - generated by pizoelectric crystals


occurs when an electric current is passed through the crystal
crystal expands & contracts at frequencies that produce ultrasound
pizoelectric crystal in transducer
head
ultrasound
transducer
Wavelength
(l)
Generation of Ultrasound
 Properties
of ultrasound
 higher
the sound frequency, less the propagation wave
diverges
 ultrasound
 like
beams are well collimated (straight line)
electromagnetic energy, ultrasound energy is…
 transmitted
through a medium………or
 totally reflected back toward the point of generation……..or
 refracted (bent)………or
 absorbed or attenuated (loose energy)
Generation of Ultrasound

in tissues, ultrasound is transmitted, absorbed,
reflected, or refracted
absorption of ultrasound energy generates heat
 at higher F’s, more tissue friction must be overcome
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the more friction that must be overcome, the more heat is generated
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the more friction that must be overcome, less energy left for
propagation
higher frequencies of ultrasound penetrate less deep before being
absorbed
3 MHz frequency used to treat tissues at depths of 1 cm to 2 cm
1 MHz frequency used to treat tissues > 2 cm from the surface
Transverse wave propagation
Longitudinal wave representation
Longitudinal wave propagation
Wave Properties
l
Wavelength: The distance  between identical points on
the wave.
l
Amplitude: The maximum displacement A of a point on the
wave.
l
A wave varies in time and space.
y ( x, t )  A cos[( 2 /  ) x  t )]
Wavelength

y
x
A
Sound Wave Properties
l
Displacement: The maximum relative displacement s of a
point on the wave. Displacement is longitudinal.
l
Maximum displacement has minimum velocity
s ( x, t )  smax cos[( 2 /  ) x  t )]
ds / dt  smax sin[( 2 /  ) x  t )]
Molecules “pile up” where the relative velocity is maximum (i.e., ds/dt = smax)
Wavelength

s
DPmax=rvsmax
x
smax
Speed of Sound
V=λf
=λ/T
one wavelength per period
Frequency

Frequency: number of times an event occurs in 1
second; expressed in Hertz or pulses per second
Hertz: cycles per second
 Megahertz: 1,000,000 cycles per second

In the U.S., we mainly use ultrasound frequencies of 1, 2
and 3 MHz
 1 = low frequency; 3 = high frequency
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 frequency =  depth of penetration
 frequency = sound waves are absorbed in more
superficial tissues (3 MHz)
Frequency
Pitch/Hz
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Perceived as “Pitch”.
Equal to the number of
complete cycles that occur
in one second.
 one cycle = one
compression and one
rarefaction.
Measured in Hertz (Hz).
Frequency
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A cycles is one complete variation in the
acoustic variables
Frequency is the number of cycles that occurs in
one second
The frequency is measured in unit of Herz (Hz,
kHz and MHz)
Half of the cycles is rarefaction and the other
half is compression.
Period
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Period is the time that it takes for one cycle to
occur
In ultrasound the unit for period is microsecond
( s )
Period is the reciprocal of the frequency
Velocity

The speed of sound wave is directly related to the
density ( velocity =  density)

Denser & more rigid materials have a higher velocity of
transmission

At 1 MHz, sound travels through soft tissue is 1540
m/sec and 4000 m/sec through compact bone
C  f
1
C
kr
r  density(kg / m )
3
k  compressib ility
Medium
Velocity（m/s）
Air
330
Water
1480
Blood
1570
Fat
1460
Muscle
1580
Bone
3500
Soft tissue（mean）
1540
Measurement of Dd & Dt
Amplitude

The Amplitude measures the displacement of
the air molecules.
The Sine Wave (Pure Tone)
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Horizontal Axis: time in seconds
Vertical Axis: molecular movement
Compression: upward movement
Rarefaction: downward movement
Amplitude: height of wave; intensity
Sound Wave Basics
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Two main components of a sound wave that
affects what we hear are amplitude and
frequency.
Amplitude determines how loud it is.
Frequency determines the “pitch”.
Wave Properties
II
1. Reflection
2. Refraction
3. Interference
4. Diffraction
Influences on the Transmission of
Energy

Reflection – occurs when the wave can’t pass
through the next density

Refraction – is the bending of waves as a result of
a change in the speed of a wave as it enters a
medium with a different density

Absorption – occurs by the tissue collecting the
wave’s energy
Reflection
Flat surfaces
Parabolic surfaces
Ellipsoidal surfaces
Ends of musical instruments
Reflection from flat surfaces
Optical board
Echo / Echolocation
Acoustical Radar
Direction of Wave
is
Perpendicular
to Wave Front
Source moving
The Doppler Effect
Demonstration
Graphs with description
Doppler Effect
observer stationary
Doppler Effect
source stationary

If the source of sound is moving
Toward the observer   seems
smaller
 Away from observer   seems larger

f observer
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If the observer is moving
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 v 
 f source
 
 v  vs 
Toward the source   seems smaller
Away from source   seems larger
 v  vo 
f observer  
 f source
 v 
If both are moving
 v  vo 
 fsource
fobserver  
 v  vs 
Doppler Example Audio
Doppler Example Visual
Examples: police car, train, etc. (Recall:
v is
Frequency and Amplitude
Reflection of Ultrasound & Sonography

Ultrasound is reflected at the interface of different tissues

reflection amount & time until reflection returns to transducer can be charted
w/ computer

image construction: sonogram (depth, density, & position of tissue structures)
Amount of Ultrasonic Reflection (Acoustic Impedance)
Interface
water-soft tissue
soft tissue - fat
soft tissue - bone
soft tissue - air
Energy Reflected
.2%
1%
15-40%
99.9%
highly reflective surfaces include:
1) muscle tendon junctions
2) intermuscular interfaces
3) soft tissue-bone
Attenuation
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Decrease in a wave’s intensity resulting from absorption,
reflection, & refraction
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 as the frequency of US is  because of molecular friction the waves
must overcome in order to pass through tissues
US penetrates through tissue high in water content & is absorbed
in dense tissues high in protein
 Absorption =  Frequency (3 MHz) , and
 Penetration =  Absorption (1 MHz) , so
 Penetration =  Frequency +  Absorption (1 MHz)
Tissues  water content = low absorption rate (fat)
Tissues  protein content = high absorption rate (peripheral
nerve, bone)

Muscle is in between both
Attenuation: Acoustic Impedance

Determines amount of US energy reflected at tissue interfaces

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If acoustic impedance of the 2 materials forming the interface is the same, all
sound will be transmitted
The larger the difference, the more energy is reflected & the less energy that
can enter the 2nd medium
US passing through air = almost all reflected (99%)
US through fat = 1% reflected
Both reflected/refracted @ m. interface
Soft-tissue: bone interfaced = much reflected
As US energy is reflected @ tissue interfaces with different
impedances, intensity is increased creating a Standing Wave (hot spot)
Attenuation of Ultrasound


The higher the tissue H2O content, the less the attenuation
The higher the tissue protein content, the more the attenuation
attenuation
of 1 MHz beam
Blood
 Fat
 Muscle
 Skin
 Tendon
 Cartilage
 Bone

3% / cm
13% / cm
24% / cm
39% / cm
59% / cm
68% / cm
96% / cm
Exponential Attenuation
1.0
Quantity
of
Ultrasound
(fraction of
beam being
further
propagated)
The quantity of the ultrasound
beam decreases as the depth of the
medium (tissue) increases.
.5
.25
.125
1st
Half
Value
2nd
Half
Value
Tissue depth
3rd Half
Value
4th Half
Value
Attenuation of Ultrasound
Half
value thickness (centimeters)
tissue
depth at which 1/2 of the sound beam
of a given frequency is attenuated
1 MHz
2 MHz
3 MHz
Fat
15.28
5.14
2.64
Muscle
2.78
1.25
.76
Bone
.04
.01
.004
Physiological Effects of Ultrasound

Non-thermal effects
 cavitations
 alternating expansion & compression of small gas bubbles
 may cause u cell membrane & vascular wall permeability
 (u nutrient and oxygen delivery)
 unstable cavitation may cause tissue damage
 unstable cavitation – large, violent changes in bubble volume
 microstreaming
 bubble rotation r fluid movement along cell membrane boundaries (u nutrient
and oxygen delivery)
 changes in cell permeability & ion flux r d healing time

Possible therapeutic benefits of non-thermal effects
 difficult to make distinction from thermal benefits
 u capillary density & u cell permeability
 u fibroblastic activity and associated collagen production
 u cortisol production around nerve bundles r d inflammation
Non-thermal Effects of Ultrasound
Cavitation
gas buble expansion
gas buble compression
Microstreaming
bubble rotation &
associated fluid
movement along
cell membranes
Ultrasound Adverse Effects & Contraindications

Adverse effects associated with ultrasound
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potassium leakage from red blood cells
u platelet aggregation r d microscopic blood flow
damage to tissue endothelium
Contraindications to ultrasound
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thrombophlebitis or other blood clot conditions
fractures ? (studies exist suggesting ultrasound may help)
epiphyseal injuries in children
vascular diseases (embolus formation - plaque rupture)
spinal column injuries (treat low back pain with caution)
cancer (danger of metastases)
do not apply directly over heart (pacemaker concerns)
do not apply to reproductive organs (pregnancy)

Effective Radiating Area (ERA): area of the sound
head that produces ultrasonic waves; expressed in square
centimeters (cm2)

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

Represents the portion of the head’s surface area that
produces US waves
Measured 5 mm from face of sound head; represents all areas
producing more than 5% of max. power output
Always lesser area than actual size of sound head
Large diameter heads – column beam
Small diameter heads – more divergent beam
Low frequency (1 MHz) – diverge more than 3 MHz
Treatment Duration: time for total treatment
Intensity Output & Power

Power: measured in watts (W);
amount of energy being produced by the transducer
 Intensity: strength of sound waves @ a given location
within the tissues being treated
 Spatial Average Intensity (SAI): amount of US
energy passing through the US head’s ERA;




expressed in watts per square centimeter (W/cm2) (power/ERA)
Changing head size affects power density (larger head results in
lower density)
Limited to 3.0 W/cm2 of maximum output
Sound Level
 I0
is called the reference intensity
It is taken to be the threshold of hearing
 I0 = 1.00 x 10-12 W/ m2
 I is the intensity of the sound whose level is to
be determined


b is in decibels (dB)
Threshold of pain: I = 1.00 W/m2; b = 120
dB
 Threshold of hearing: I0 = 1.00 x 10-12 W/
m2 ; b = 0 dB

Intensity of sounds

Some examples (1 pascal  10-5 atm) :
Sound Intensity Pressure
Intensity
amplitude (Pa)
(W/m2)
level (dB)
Hearing threshold
3  10-5
10-12
0
Classroom
0.01
10-7
50
City street
0.3
10-4
80
Car without muffler 3
10-2
100
Indoor concert 30
1
120
Jet engine at 30 m.
100
10
130
Lecture 22, Exercise 4
Plane Waves
A: You are driving along the highway at 65
mph, and behind you a police car, also
traveling at 65 mph, has its siren turned on.
B: You and the police car have both pulled
over to the side of the road, but the siren is
f
f’
still turned on.
v
In which case does the frequency of the
siren seem higher to you?
vo
vs
(A) Case A
Audio Fundamentals

Acoustics is the study of sound
Generation,
transmission, and reception of sound
waves
Sound wave - energy causes disturbance in a
medium

Example is striking a drum
Head
of drum vibrates => disturbs air molecules
close to head
Regions of molecules with pressure above and
below equilibrium
The Physics Of
Sound
Why do we hear what we hear?
(Turn on your speakers)
Sound is made when something vibrates.



The vibration disturbs
the air around it.
This makes changes in
air pressure.
These changes in air
pressure move through
the air as sound waves.
The sound waves
cause pressure
changes against
our ear drum
sending nerve
impulses to our
brain.
Intensity
Loudness/dB




Perceived as “Loudness”.
Intensity is expressed as the sound pressure level (SPL),
which is a function of distance the vibrating object is
displaced (amplitude), which depends on energy
applied.
Measured in decibels (dB). One dB is 1/10th of a bel.
Decibels are logarithmic units. The reference used is
.0002 dynes/cm2, roughly the smallest pressure that will
move the TM.
Intensity (cont.)
Why logarithms?


To compress the very large range of pressure our
ears can hear in to a small range of numbers for
convenience.
0-140 dB represents a sound pressure range of
1:1,000,000,000 units (a ratio of 10 million to 1!)
Intensity
(cont.)


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
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0 dB is typically the softest volume that
can be heard, but sound energy is also
present below 0 dB.
Human intensity range is 0-140.
140 dB is the threshold of pain.
170-180 dB causes tissue damage.
180 dB+ can cause death!
INVERSE SQUARE LAW

Doubling the distance from a sound source
decreases intensity by 6 dB.
Doubling the Noise Source…

A combination of two different noise
sources of equal loudness will increase the
intensity by 3dB

For example, if noise source “A” is 93 dBA and
noise source “B” is 93 dBA, the combined result
of “A” and “B” is 96 dBA.
Duration
Time




Perceived as “Time”.
Can last from thousandths of a second, to
several hours or all day!
Occupational noise exposure varies over time.
Can be constant or intermittent with
continuous (steady-state) or impulse noise.
Sending/Receiving

Receiver
A
microphone placed in sound field moves
according to pressures exerted on it
Transducer transforms energy to a different
form (e.g., electrical energy)

Sending
A
speaker transforms electrical energy to
sound waves
Signal Fundamentals


Pressure changes can be periodic or aperiodic
Periodic vibrations
cycle
- time for compression/rarefaction
cycles/second - frequency measured in hertz (Hz)
period - time for cycle to occur (1/frequency)

Frequency ranges
pression is 10-6 Hz
cosmic rays are 1022 Hz
human perception [0, 20kHz]
barametric
Wave Lengths

Wave length is distance sound travels in one
cycle
20
Hz is 56 feet
20 kHz is 0.7 inch


Bandwidth is frequency range
Transducers cannot linearly produce human
perceived bandwidth
Frequency
range is limited to [20 Hz, 20 kHz]
Frequency response is not flat
Hazardous Noise Levels
Defined As

Continuous or steady state noise > 84 dBA


Impulse/Impact noise > 140 dB peak SPL



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Generator, Aircraft Noise, etc.
Explosions or weapons fire
Two or more objects hitting together
Intensity and duration are the two main factors that
determine if a particular sound is hazardous
If it is loud enough for long enough, most people will
suffer hearing loss. Often takes many years!
Sensitivity of the Human Ear (Review)


Frequency Range: 20 - 20,000 Hz
Intensity Range: 0 - 140 dB SPL


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Primary speech frequencies: 500 - 4000 Hz

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

Referred to as the dynamic range
Sounds >140dB lose tonal quality
Frequencies above and below add quality to speech, but little
intelligibility
Consonant Sounds – Primarily high freq’s, convey 80% of
meaning of speech
Vowel Sounds – Primarily low freq’s, convey 80% of energy
of sounds
Threshold = The lowest intensity that the human ear
can hear
Introduction
 Sonography comes from the LATIN
sonus ( Sound)
 Graphein come from the GREEK (to
write)
 Ultrasoundsonography,
Ultrasonography , means imaging
with ultrasound.
 Diagnostic
sonography is medical
Ultrasound provides a windows
into the body
Pulse
Echoes
Scan lines
One pulse of ultrasound
generates a single scan
line(series of echoes) as it
travel through tissue.
Sonography



Ultrasonography means imaging with ultrasound
Diagnostic sonography is medical two
dimensional cross-sectional and three
dimensional anatomic and flow imaging using
ultrasound
An interactive process nivolving the sonologist,
patient, instrument, transducer, sonographer.
The Basis Of
Sonography


A ultrasound wave consists of a
mechanical disturbance of a medium
（gas, liquid or solid） which passes at
a fixed speed.
The type of ultrasound wave is
longitudinal.


The disturbance propagates through a
medium at a speed which depends on
the compressibility and density.
The speed in the soft tissue is
approximately 1540 m/s.
Attenuation
Attenuation is the reduction in US intensity during
passage through medium.
 The mechanisms are︰
1.Absorption
2.Scatter
3.Refraction
4.Beam divergence
5.Reflection

The dependence of attenuation on
frequency has an effect on pulse
spectrum shape and therefore on the
shape of the ultrasound pulse itself.
μ=k*f

Interface
I(incident)
I(transmit)
I(reflect)
Z1
Z2
Specular Reflection

R=

T=1－R=
I r / I i  (Z 2  Z1 ) /( Z 2  Z1 )
Z : Tissue Impedance
Z＝ρc
2
2
I t / I i  4Z1Z 2 /(Z1  Z 2 )
2

A large difference in acoustic impedence
leads to a high degree of reflection.
For example, tissue-bone or tissue-air
Display




A-Mode
B-Mode
M-Mode
Doppler

The most popular material for medical
US transducers is know as PZT﹙lead
zirconate titanate, PbZrTiO3 ﹚.
d=λ/2
Signal process & control
TGC
GAIN
Doppler

Doppler shift－
fd

2v cos 
 f0 
c
fd= Doppler shifted frequency
f0= transducer frequency
v = blood velocity
θ= angle of insonification
c = sound velocity in tissue
Advantage of US



No radiation
Cheap
Convenience
Comparison of imaging by X-radiation
to ultrasonic imaging
Advantage 1. Excellent resolution
2. Distinguishes bone
boundaries well
Disadvantage
1. Hazard(esp. to dividing cells
2. Won’t differentiate soft tissues well
1. Noninvasive, safe at low
powers 2. Differentiates soft tissues
1. Resolution not as good as xrays. 2. Won’t penetrate air or
bone areas
A-SCAN
The principle of the ultrasonic A-scanner
A-scan display
B-SCAN
The principle of the ultrasonic B-scanner
Uses of Ultrasound in Medicine

Ultrasound is used for
examining soft tissue inside
the body.

Parts of the body that may
be examined include muscles
and unborn babies.

Blood flow can also be
monitored using ultrasound.
© 2000 ATL Ultrasound
Ultrasound images courtesy of ATL
The Power of Ultrasound

Modern ultrasound
equipment can produce

3D images

Colour enhancement to
show blood flow

Digital files for
examination on
computers
© 2000 ATL Ultrasound
Ultrasound images courtesy of ATL
How Does It Work?

Medical ultrasound systems use very high frequencies several megahertz (mega means million or 106).

A sound is a wave it has all the usual wave properties
(reflection, refraction, diffraction). Ultrasound imaging
makes use of the fact that sound can be reflected.

The idea is just like that used in radar and sonar.
More about how it works…
l
A thin layer of jelly is placed between
the probe and the skin to make sure all
the sound enters the body.

The probe contains a transmitter and
a receiver.
l
A pulse of ultrasound is sent out by the
transmitter.
l
The pulse is reflected from a surface
and returns to the receiver.
l
The ultrasound machine measures
how long it takes for the pulse to return
Ultrasound
probe
skin
Body tissue
(muscle etc)
How the image is created…

Millions of sound waves are
transmitted every second.

As the waves reflected at
different times, the computer
in the ultrasound machine
calculates how far the wave
travelled before being
reflected (using d=vt).

Using this information the
computer builds up an image
of the inside of the patient.
© 2000 ATL Ultrasound
Ultrasound images courtesy of ATL