Transcript MedPhys961Ultrasound_05

Medical Physics
Ultrasound
Option 9.6.1
2006
Syllabus - Contextual Outline
Contextual Outline
The use of other advances in technology, developed from our understanding of the
electromagnetic spectrum, and based on sound physical principles, has allowed medical
technologists more sophisticated tools to analyse and interpret bodily process for diagnostic
purposes. Diagnostic imaging expands the knowledge of practitioners and the practice of
medicine. It usually uses non-invasive methods for identifying and monitoring diseases or
injuries via the generation of images representing internal anatomical structures and organs of
the body.
Technologies, such as ultrasound, compute axial tomography, positron emission tomography
and magnetic resonance imaging, can often provide clear diagnostic pictures without surgery. A
magnetic resonance image (MRI) scan of the spine, for example, provides a view of the discs
in the back, as well as the nerves and other soft tissues. The practitioner can look at the MRI
films and determine whether there is a pinched nerve, a degenerative disc or a tumour. The
greatest advantage of these techniques are their ability to allow the practitioner to see inside
the body without the need for surgery.
This module increases students’ understanding of the history of physics and the implications of
physics for society and the environment.
Syllabus 9.6.1
The properties of
ultrasound waves can
be used as diagnostic
tools
Syllabus 9.6.2
The physical
properties of
electromagnetic
radiation can be
used as diagnostic
tools
Syllabus 9.6.3
Radioactivity can be used as a diagnostic tool
Syllabus 9.6.4
The magnetic field
produced by
nuclear particles
can be used as a
diagnostic tool
Ultrasound
X-rays
Medical
Physics
MRI
Endoscopy
Nuclear - PET
Individual
1 minute
Group
2 minutes
Ultrasonography
• Ultrasonography is the
process of obtaining
medical images using
high frequency sound
waves.
• The person who carries
out the procedure is
usually a medical
technologist - a
sonographer.
Syllabus 9.6.1
The properties of
ultrasound waves can
be used as diagnostic
tools
About Ultrasound
The properties* of ultrasound
waves make them useful medical
diagnostic tools
•Pass through soft tissues
•Reflect from tissue boundaries
•Short wavelength => resolution
Sonography uses reflected sound
to “look” inside the body.
Ultrasound Imaging - Basic Principle
High-frequency sound waves are
passed into the body.
The waves are reflected at
boundaries between different
tissues and organs in the body.
Using known information about
the speed of sound in the
tissues, and the measured time
for the echo to be received, the
distance from the transmitter to
organ can be calculated, and
used to create an image.
Ultrasound Imaging - Basic Principle
The principle of ultrasound is
similar to SONAR and RADAR.
Some animals use sound waves to
produce a mental image of their
surroundings to navigate and to
locate prey. e.g. bats, some birds,
dolphins.
Advantages of ultrasound
Ultrasound is non-invasive.
Ultrasound is non-ionising.
It is therefore very safe.
Australian Ultrasound
Reason for using short wavelengths
• An image of an object cannot be produced if
the object is smaller than a few wavelengths
of the wave being used to examine it
because there is little reflection of the wave
• Electron microscopes can produce images of
much smaller objects than a light microscope
because the wavelength of electrons is
much less that that of light
• Some bats use ultrasound to navigate and to
locate their prey - the high frequencies
allowing them to locate small insects in flight
organ
Reason for using short wavelengths
• Objects that are larger than a few
wavelengths produce strong reflection of the
waves
reflected wave
incident wave
What is ultrasound?
• Ultrasound is any sound having a
frequency greater that the upper limit
of human hearing
• The human hearing range covers
frequencies from 20 hertz to 20
kilohertz
• Ultrasound used in medical imaging
typically has frequencies from 2 MHz
to 10 MHz
• Ultrasound travels about 1500 m s–1
in soft tissues
• The sound waves produced have
wavelengths of about 1 mm
Ultrasound machine 1970
• identify the differences between ultrasound and sound in normal hearing range
Frequency and wavelength of ultrasound waves
1. Ultrasound travels at 340 m s-1 in air and
1585 m s-1 in muscle. Calculate the
wavelength in air and in muscle tissue of
ultrasound having a frequency of 2 MHz
Answer
In air
 = v/f = 340 / 2 x 106 = 1.7 x 10-4 m
 = 0.17 mm
In muscle
 = v/f = 1585 / 2 x 106 = 7.9 x 10-4 m
 = 0.79 mm
• identify the differences between ultrasound and sound in normal hearing range
Ultrasound propagation and properties
• Velocity of sound in most soft tissues is about 1500 m/s
• This is faster than the speed of sound in air (~340 ms-1)
• Velocity of sound in bone is >> than in soft tissue
• Velocity of sound = frequency x wavelength
• Ultrasound (medical) has frequencies > 2 MHz
• This is much higher than normal audible sounds (maximum 20 kHz)
• Wavelength of ultrasound is therefore < 1.5 mm
• The shorter the ultrasound wavelength, the better the resolution,
however tissue penetration is poorer for shorter wavelengths
• identify the differences between ultrasound and sound in normal hearing range
Contrasting audible sound waves and ultrasound waves
Compared to sounds detectable by the human ear
ultrasound . . .
has a frequency that is
higher
has a wavelength that is
shorter
A significant difference between sound in air
and ultrasound in human tissue is . . .
the speed at which the waves travel.
In air v ~ 340 m s-1
In human tissue v ~ 1500 m s-1
• identify the differences between ultrasound and sound in normal hearing range
Frequency and wavelength of ultrasound waves
•
Waves can be used to produce an
image of objects with a minimum
diameter about equal to the
wavelength of the wave.
•
Use of high frequencies, and hence
short wavelengths produces an image
with good resolution - that is, images
of small objects can be produced
•
A 10 MHz wave can produce clear
images of objects similar in size to the
wavelength of the wave in tissue. If v =
1585 m s-1 this is . . .
1981
 = v/f = 1585 / 10 x 106
 = 1.585 x 10-4 m
 = 0.16 mm millimetres across
• identify the differences between ultrasound and sound in normal hearing range
3-D ultrasound
Ultrasound Images
Normal iris
Iris with tumour
• gather secondary information to observe at least two ultrasound images of body organs
The Piezoelectric Effect
Ultrasound is produced by a rapidly vibrating crystal transducer*
(*a transducer converts energy from one form to another e.g. a loudspeaker)
The piezoelectric effect
The piezoelectric effect is the conversion of electrical to mechanical energy or
mechanical to electrical energy by certain types of crystals.
• describe the piezoelectric effect and the effect of using an alternating potential difference with a piezoelectric crystal
The Piezoelectric Effect
Ultrasound is produced by a rapidly vibrating crystal transducer*
(*converts electrical energy to sound energy)
When a voltage is applied across opposite faces of certain crystals, the distances between
atoms in crystal lattice changes slightly deforming the crystal.
crystal
+
crystal
–
An alternating voltage causes the crystal to vibrate at the
frequency of the applied voltage, producing sound in the
surrounding medium.
+
–
crystal
• describe the piezoelectric effect and the effect of using an alternating potential difference with a piezoelectric crystal
–
+
The Piezoelectric Effect
Watch alarms, telephones and
other electronic buzzers use the
piezoelectric effect to make sound.
[Demonstration - piezoelectric buzzer]
If the frequency is greater than 20
kHz, ultrasound is produced.
Quart watches use a rapidly
vibrating crystal to keep time
accurately.
• describe the piezoelectric effect and the effect of using an alternating potential difference with a piezoelectric crystal
The Piezoelectric Effect
Piezoelectric materials can
transform pressure changes into
voltages - the reverse of the
principle behind the production of
ultrasound.
This property allows the same
material to be used as a detector of
ultrasound, to convert pressure
changes caused by the reflected
wave into voltages that can be
processed and analysed
electronically.
• describe the piezoelectric effect and the effect of using an alternating potential difference with a piezoelectric crystal
The Piezoelectric Effect - Summary
Ultrasound is produced by a
piezoelectric material
Producing ultrasound
A piezoelectric crystal converts
variations in electrical voltage to
mechanical vibrations - producing
ultrasound
Detecting ultrasound
The same transducer converts the
reflected vibrations of the
ultrasound into electrical signals for
computer processing
• describe the piezoelectric effect and the effect of using an alternating potential difference with a piezoelectric crystal
Tutorial Questions
Describe the production of ultrasound used for medical imaging.
Answer
Ultrasound is produced using a piezoelectric crystal transducer, which
converts high frequency alternating potential differences into mechanical
vibrations of the crystal at a corresponding frequency. These vibrations are
used to create pressure variations that propagate through the surrounding
medium. These pressure variations, if the frequency exceeds 20 kHz, are
called ultrasound.
Tutorial Questions
Describe the piezoelectric effect.
Answer
The piezoelectric effect occurs when a voltage is applied across opposite
faces of certain crystals, causing the the crystal lattice to change size
slightly. The effect is reversible, with pressure variations that deform the
crystal slightly resulting in the production of a voltage across opposite
faces.
Tutorial Question
How is the piezoelectric effect used to detect ultrasound?
Answer
Ultrasound returning to the transducer deform
the piezoelectric crystal in the transducer
slightly, producing an alternating voltage
across opposite faces. This is called the
piezoelectric effect.
The voltage variations correspond to the
varying intensity of the ultrasound returning to
the crystal.
Tutorial Question
Compare the properties of medical ultrasound with sound in the normal
hearing range. (10 lines - 4 marks)
Answer
The sounds are similar because they are both longitudinal waves requiring a medium
through which to propagate. Both types of waves can be reflected from a boundary between
two media having different acoustic impedances.
Ultrasound has frequencies extending up from the upper limit of human hearing, which has a
range from 20 Hz to 20 kHz.
Medical ultrasound frequencies fall in the range 2 MHz to 10 MHz and therefore have
frequencies much greater than those that humans can hear.
Ultrasound has a much shorter wavelength, of the order of a millimetre, than the sounds that
humans can hear.
Both have the same speed in the same medium. Medical ultrasound has a velocity of
approximately 1500 m s-1 in soft human tissues whereas sound in air has velocity of about
340 m s-1.
Acoustic Impedance
• Acoustic impedance is the product of density and acoustic velocity*
Z = 
The logical units for acoustic
impedance are
kg m–3 x m s–1 or kg m–2 s–1
This unit is given the special
name - a rayl
Z = acoustic impedance (rayls)
 = density (kg m–3)
v = acoustic velocity (m s–1) *speed of sound in the medium
• define acoustic impedance … and identify that different materials have different acoustic impedances
Acoustic Impedance
Bone has a density of 2 x 103 kg m–3.
The speed of sound in bone is 4080 m s–1.
Calculate the acoustic impedance of bone.
Z = 
Answer
Z
= v
Z
= 2 x 103 kg m–3 x 4080 m s–1
= 8.16 x 10 6 rayls
• define acoustic impedance … and identify that different materials have different acoustic impedances
Acoustic Impedance
Source: Butler Physics 2
Substance
Aluminium
Iron
Copper
Gold
Glass
Ice
Bone
Water
density (kg m–3)
2.7 x 103
7.8 x 103
8.9 x 103
19.3 x 103
2.4 – 2.8 x 103
0.917 x 103
1.7 – 2.0 x 103
1.00 x 103
Substance
Blood (plasma)
Blood (whole)
Seawater
Mercury
Ethanol
Air
Helium
CO2
density (kg m–3)
1.03 x 103
1.05 x 103
1.025 x 103
13.6 x 103
0.79 x 103
1.29
0.179
1.98
Use the information in these
tables to calculate the acoustic
impedance of water and blood
and compare these to bone.
Z = 
Conclusion . . .
Material
Velocity (m s–1) Material
Velocity (m s–1)
Air (0°C)
Fat
Mercury
Brain
Water (50°C)
Liver
331
1450
1450
1541
1540
1549
1561
1570
1585
1620
4080
6400
Kidney
Blood
Muscle
Lens of eye
Skullbone
Aluminium
Answers
Water
Z = 1.00 x 103 x 1540
= 1.54 x 106 R
Blood
Z = 1.05 x 103 x 1570
= 1.65 x 106 R
Bone
Z = 2.0 x 103 x 4080
= 8.16 x 106 R
Bone has an acoustic impedance approximately 5
times that of blood and water
• define acoustic impedance … and identify that different materials have different acoustic impedances
Calculating Acoustic Impedance
Acoustic Properties of Biological Materials
Material
air
fat
water
average soft tissue
liver
kidney
blood
muscle
skull bone
Density
(kg/m^3)
Velocity of
Sound (m/s)
1.21
952
1000
1058
1065
1038
1025
1076
1912
Compare the acoustic
impedances of bone, soft
tissue, fat, blood and air
Z = 
330
1450
1480
1540
1550
1560
1570
1580
4080
Answers
Bone
Z = 1.9 x 103 x 4080
= 7.8 x 106 R
Blood
Z = 1025 x 1570
= 1.61 x 106 R
Soft tissue
Z 1.06 x 103 x 1540
= 1.63 x 106 R
Air
Z = 1.21 x 330
= 399 R
Fat
Z = 9.52 x 102 x 1450
= 1.38 x 106 R
Blood and soft tissues have approximately the same
acoustic impedance. Fat has the smallest acoustic
impedance of these tissues and bone has the greatest
acoustic impedance. The acoustic impedance of less
than 0.1% that of the human tissues.
• solve problems and analyse information to calculate the acoustic impedance of a range of materials, including bone, muscle, soft
tissue, fat, blood and air and explain the types of tissues that ultrasound can be used to examine
Acoustic impedance of non-biological materials
For interest only!
Z = 
Reference: File Wave Reflection
http://freespace.virgin.net/mark.davidson3/reflection/reflection.html
• solve problems and analyse information to calculate the acoustic impedance of a range of materials, including bone, muscle, soft
tissue, fat, blood and air and explain the types of tissues that ultrasound can be used to examine
Acoustic Impedance and Reflection
fat
muscle
Represented as
Io
Ir
It = I o – Ir
Consider two different tissues - such as fat and muscle.
A boundary or interface exists between the two tissues.
Sound travelling and meeting the interface will be partly reflected
and partly transmitted.
• describe how the principles of acoustic impedance and reflection and refraction are applied to ultrasound
Acoustic Impedance and Reflection
• If two tissues have the same
acoustic impedance, no
reflection of ultrasound takes
place at a boundary between
them
• The greater the difference in
acoustic impedance between
two tissues at a boundary, the
greater the reflection
• Identify the two tissues in this
table, a boundary between
which would produce the
greatest reflection and the least
reflection
Acoustic
Impedance
(rayls)
Material
air
fat
water
soft tissue (av.)
liver
kidney
blood
muscle
skull bone
Answer
Greatest reflection:
Least reflection:
400
1380000
1480000
1630000
1650000
1620000
1610000
1700000
7800000
fat/skull bone
blood/kidney OR
kidney/soft tissue
• describe how the principles of acoustic impedance and reflection and refraction are applied to ultrasound
Acoustic Impedance and Reflection
• The ultrasound machine
measures the time for
the incident wave to
reach the boundary and
return to the detector
• Since the time and speed
are known, the distance
(d) can be calculated
• 2d = v x t
incident
reflected
d
v
transmitted
• describe how the principles of acoustic impedance and reflection and refraction are applied to ultrasound
Acoustic Impedance and Reflection
• Images are clearer if there is a
strong reflection (a large
difference in acoustic
impedance at the reflecting
boundary)
incident
reflected
incident
reflected
transmitted
transmitted
• Ideally the ultrasound should
strike tissue boundaries normal
to the surface so that it reflects
directly back to the transducer
Reflected ray does not
strike detector
• describe how the principles of acoustic impedance and reflection and refraction are applied to ultrasound
Acoustic Impedance and Refraction
• Ultrasound meeting a
tissue boundary at an angle
other than 90° are refracted
on crossing the boundary
• This complicates the
processes of detection and
analysis
• Ultrasound waves reflected
perpendicular to the
boundary are simpler to
analyse
Analysis of refracted
waves is more
complex
• describe how the principles of acoustic impedance and reflection and refraction are applied to ultrasound
Reflection - Quantitative
Definition
The ratio of the reflected
intensity of ultrasound at a
tissue boundary to the
original intensity of the
ultrasound at the boundary
is equal to the ratio of the
square of the acoustic
impedance difference to
the square of the sum of
the acoustic impedances
• define the ratio of reflected to initial intensity as . . .
Write this definition in symbolic
form if the two tissues have acoustic
impedances Z1 and Z2 and the
reflected intensities is Ir and the
incident intensity is Io
Ir [Z 2  Z1 ]

2
Io [Z 2  Z1 ]
2
Acoustic Impedance and Reflection
Z  v
Io
Ir
Z1  1v1
It = Io – Ir
Z2  2v2
Ir Z2  Z1 

2
Io Z2  Z1 
2
• identify that the greater the difference in acoustic impedance between two materials, the greater is the reflected
proportion of the incident pulse
Acoustic Impedance and Reflection
Ir Z2  Z1 

2
Io Z2  Z1 
2
Io
Ir
Z1  1v1
It = I o – Ir
Z2  2v2
Compare the proportion of the ultrasound signal
reflected at a muscle/fat boundary with the
proportion reflected at a muscle/bone boundary.
What is illustrated by these calculations?
Answer
muscle-fat boundary Ir/Io = 0.01
I r Z2  Z1 

I o Z2  Z1 2
2
6
6
I r 1.3810  1.710 

 1.08102
2
I o 1.38106 1.7106 
2
Muscle/bone Ir/Io = 0.41

• identify that the greater the difference in acoustic impedance between two materials, the greater is the reflected
proportion of the incident pulse
Calculating Acoustic Impedance
Acoustic Properties of Biological Materials
Material
air
fat
water
soft tissue (av.)
liver
kidney
blood
muscle
skull bone
Density (kg/m^3)
1.21
952
1000
1058
1065
1038
1025
1076
1912
Velocity of Sound
(m/s)
Acoustic Impedance
(rayls)
330
1450
1480
1540
1550
1560
1570
1580
4080
400
1380000
1480000
1630000
1650000
1620000
1610000
1700000
7800000
Explain the types of tissues that ultrasound can be used to examine.
Discuss in class and make appropriate notes!
• solve problems and analyse information to calculate the acoustic impedance of a range of materials, including bone, muscle, soft
tissue, fat, blood and air and explain the types of tissues that ultrasound can be used to examine
Acoustic Impedance and Reflection
• Air between the ultrasound scanner
head and the body, causes most of
the sound energy to be reflected
from the skin surface due to the poor
impedance match.
• A gel with approximately the same
acoustic impedance as flesh is
placed between the scanner head
and the body.
The gel
• ensures most sound energy enters
the body
• makes it easier to move the
ultrasound head over the body
Ultrasound does
not enter the
body
• identify that the greater the difference in acoustic impedance between two materials, the greater is the reflected
proportion of the incident pulse
Acoustic Impedance and Reflection
• Acoustic energy is reflected at interfaces between tissues with different
acoustic impedances (Z)
• Acoustic impedance = product of density and acoustic velocity (Z=v)
• The unit of acoustic impedance is the rayl
• The proportion of acoustic reflection increases as the difference in
acoustic impedances increases
• For soft tissue/air, soft tissue/bone and bone/air interfaces, almost total
reflection occurs
• identify that the greater the difference in acoustic impedance between two materials, the greater is the reflected
proportion of the incident pulse
Problem Solving
Determine the proportion of
ultrasound reflected at a boundary
between fat and kidney tissue.
Material
air
fat
water
soft tissue (av.)
liver
kidney
blood
muscle
skull bone
Density (kg/m^3)
1.21
952
1000
1058
1065
1038
1025
1076
1912
Velocity of Sound
(m/s)

330
1450
1480
1540
1550
1560
1570
1580
4080
Ir [Z 2  Z1 ]2

Io [Z 2  Z1 ]2
Z = 
Answer
Fat
Z = 952 x 1450
= 1.38 x 106 R
Kidney
Z = 1.038 x 103 x 1560
= 1.619 x 106 R
Proportion of reflected ultrasound
Ir [Z 2  Z1 ]2

Io [Z 2  Z1 ]2

Ir/Io = (1.619 - 1.38)2/(1.619 + 1.38)2
Ir/Io = 6.35 x 10-3
• solve problems and analyse information using [the above equations]
Bone Density Measurement Using Ultrasound
•
Why measure bone density?
•
•
•
•
Low bone density is associated with
osteoporosis - risk of breaks
Two methods are currently used to
measure bone density
• X-rays (Called DXA or DEXA – “Dual
x-ray absorption”) – Measures spine,
hip or total body.
• Ultrasound – measurements are taken
at the heel - safer than DEXA proportion of ultrasound transmitted
through the heel is a measure of bone
density
During an ultrasound exam, two soft
rubber pads come in contact with either
side of the heel. These pads send and
receive high-frequency sound waves
through the heel bone
Ultrasound is not as reliable as DEXA
• identify data sources, gather, process and analyse information to describe how ultrasound is used to measure bone
density
Reflection of Ultrasound and A-scan Use
• The earliest ultrasound scans
used a simple ray - effectively
one-dimensional that entered the
body and was reflected back. The
intensity of the reflected ray was
displayed on an intensity vs time
graph.
• This is called an A-scan.
•
Using the A-Scan mode, the
distance to each boundary between
different tissues could be calculated
from the known speed of sound in
the tissues.
• describe the situations in which A scans, B scans, and phase and sector scans would be used and the reasons for the
use of each
Reflection of Ultrasound and A-scan Use
• A-scans are now obsolete
• A-scans were useful in
measuring the thickness of
tissues such as the cornea of
the eye.
• Improvements in technology
have made A-scans
obsolete.
• describe the situations in which A scans, B scans, and phase and sector scans would be used and the reasons for the
use of each
Reflection of Ultrasound and B-scan Use
The B-scan mode was developed to show directly
on a display the distances of each tissue boundary
from the surface of the body.
B-scan results could be combined to produce a 2-D
section (see right)
• describe the situations in which A scans, B scans, and phase and sector scans would be used and the reasons for the
use of each
Reflection of Ultrasound and A-scan Use
•
•
The B-scan mode enables a true 2-D image to be produced.
It is therefore useful because it enables the size and extent (as well as
thickness) of a particular organ or tumour to be determined.
Question
1. A simple B-scan was produced
using a single ultrasound beam that
was moved across a patient’s
abdomen from the patient’s right to
left just below the navel.
Describe* the B-scan image
produced from the accumulated
data.
Answer
The image is 2-D.
It is a transverse slice from right to left
with the section showing parts of body
structures from the front to the back.
Parts of the body closer to the navel or
closer to the feet than the scan path are
not visible in the image
* “Provide characteristics and features”
• describe the situations in which A scans, B scans, and phase and sector scans would be used and the reasons for the
use of each
Reflection of Ultrasound and A-scan Use
• A convex array scanner
produces a sector shaped beam
• The image produced is a twodimensional slice
• This is a common type of scan
used in obstetrics
Advantage
• This array permits a large
imaging area through a small
window
• describe the situations in which A scans, B scans, and phase and sector scans would be used and the reasons for the
use of each
Reflection of Ultrasound and Sector scan Use
Animation
A convex array transducer
Below - typical sector scan
QuickTime™ and a
Cine pak decomp resso r
are nee ded to s ee this picture.
• describe the situations in which A scans, B scans, and phase and sector scans would be used and the reasons for the
use of each
Ultrasound and Phased Arrays
• A linear array produces parallel wavefronts from a line of
transducers
• The resulting image is a sectional slice parallel to the transducer
array
• ADVANTAGE: This type of scan results in accurate linear
distances being displayed i.e. correct proportions are maintained
• describe the situations in which A scans, B scans, and phase and sector scans would be used and the reasons for the
use of each
Ultrasound and Phased Arrays
• A steerable beam is created by some
modern ultrasound scanners
• Successive transducers produce circular
wavefronts with a slight delay between
each wave
• Interference between the waves results
in a strong linear wavefronts
• The direction of propagation is controlled
by changing the time delay between
transducers
•
The advantage of this is that the beam does not
have to be steered manually by the operator - the
process is automatic
• describe the situations in which A scans, B scans, and phase and sector scans would be used and the reasons for the
use of each
Ultrasound and Phased Arrays
Animation: Phased array used to create a steerable beam
QuickTime™ and a
Cinepak decompressor
are needed to see this picture.
• describe the situations in which A scans, B scans, and phase and sector scans would be used and the reasons for the
use of each
The Doppler Effect
The Doppler effect refers to the property of waves that results in a change in frequency of the
wave when the source and the observer are moving relative to each other.
The Doppler effect can
be heard when moving
vehicles are producing a
constant pitch sound as
they pass by.
QuickTime™ and a
Cinepak decompressor
are needed to see this picture.
• describe the Doppler effect in sound waves and how it is used in ultrasonics to obtain flow characteristics of blood
moving through the heart
The Doppler Effect
The Doppler Effect and Sound Waves
The Doppler effect can be the result of…
• movement of the source relative to the observer
• movement of the observer relative to the source
• movement of both objects at different velocities in a common
reference frame
In medical ultrasound imaging, the relative movement is due to the
movement of a tissue inside the body from which the sound is
reflecting, relative to the ultrasound head.
e.g. Blood flow, heart beat, breathing
• describe the Doppler effect in sound waves and how it is used in ultrasonics to obtain flow characteristics of blood
moving through the heart
The Doppler Effect and Sound Waves
• A first hand investigation to
demonstrate the Doppler effect
• piezoelectric buzzer
• oscilloscope (use computer software
- Audacity*)
* Audacity is downloadable freeware and can be
used as an oscilloscope for investigation of sounds
• describe the Doppler effect in sound waves and how it is used in ultrasonics to obtain flow characteristics of blood
moving through the heart
The Doppler Effect and Sound Waves
•
•
The Doppler effect results in an increase in the frequency of a sound wave when the
source is moving relatively towards the observer, compared to when there is no
relative motion.
It results in a decrease in the frequency of a sound wave when the source is moving
relatively away from the observer, compared to when there is no relative motion.
• describe the Doppler effect in sound waves and how it is used in ultrasonics to obtain flow characteristics of blood
moving through the heart
The Doppler Effect
• The Doppler effect is used in
medical imaging to produce a
Doppler ultrasound image
showing whether or not
movement, such as blood flow
or heartbeat is normal.
• Doppler ultrasound images are
normally colour coded, with
different colours representing
different velocities relative to the
ultrasound head.
• describe the Doppler effect in sound waves and how it is used in ultrasonics to obtain flow characteristics of blood
moving through the heart
The Doppler Effect
A Doppler ultrasound image uses colour coding to show
different rates of movement of the tissues being imaged.
• describe the Doppler effect in sound waves and how it is used in ultrasonics to obtain flow characteristics of blood
moving through the heart
The Doppler Effect
Colour Doppler image showing leakage of blood through a hole
in the septum separating the left and right ventricles
• describe the Doppler effect in sound waves and how it is used in ultrasonics to obtain flow characteristics of blood
moving through the heart
The Doppler Effect
• describe the Doppler effect in sound waves and how it is used in ultrasonics to obtain flow characteristics of blood
moving through the heart
The Doppler Effect
• describe the Doppler effect in sound waves and how it is used in ultrasonics to obtain flow characteristics of blood
moving through the heart
The Doppler Effect
• describe the Doppler effect in sound waves and how it is used in ultrasonics to obtain flow characteristics of blood
moving through the heart
The Doppler Effect
•
Guess what this baby is doing!
QuickTime™ and a
Cinepak decompress or
are needed to s ee this pic ture.
• describe the Doppler effect in sound waves and how it is used in ultrasonics to obtain flow characteristics of blood
moving through the heart
Doppler ultrasound - heart blood flow
QuickTime™ and a
Cinepak decompressor
are needed to see this picture.
Red indicates blood flow
towards the US detector and
blue indicates blood flow
away from the US detector
[Coincidentally the opposite of
red/blue shift in astronomy]
• identify data sources and gather information to observe the flow of blood through the heart from a Doppler ultrasound
video image
Doppler Ultrasound and Cardiac Problems
• Doppler ultrasound can be
•
•
•
•
used to detect
Leakage of blood through
heart walls - holes
Backflow of blood through
faulty valves
Poor blood flow due to fat
deposits in arteries
Irregular flow due to heart
malfunction
QuickTime™ and a
Cinepak decompressor
are needed to see this picture.
• outline some cardiac problems that can be detected through the use of the Doppler effect
Ultrasound Advantages and Disadvantages
Advantages of using ultrasound
•
•
•
•
It is non-invasive – does not require surgical
procedures
Ill patients can be examined without sedation, and
relatively quickly and conveniently
Since sound is non-ionising it does not damage
DNA, cells and tissues
It is relatively cheap (compared with other scanning
technologies)
Disadvantages of using ultrasound
•
•
•
•
The images obtained are highly dependent on the
operator’s skill
Images are not as easy to interpret as x-rays or MRI
It is difficult to produced clear images with obese
patients (due to sound absorption and reflection
from fat)
The presence of air and bone obscure objects
behind them because both reflect ultrasound
strongly at boundaries with other tissues
Ultrasound Advantages and Disadvantages
•
•
•
•
Ultrasound is used for medical
imaging because
Ultrasound is extremely safe and can
be used for obstetrics and it can show
tumours and some soft tissue injuries.
Ultrasound provides a real-time image,
and the sonographer can change the
way the scan is done to show a
desired part of the body most clearly
Ultrasound technology is relatively
cheap and widely available
Ultrasound’s disadvantage is that the
image does not show fine detail visible
in an X-ray or MRI scan
Review and PFAs
• H1. evaluates how major
advances in scientific
understanding and
technology have changed
the direction or nature of
scientific thinking
Review and PFAs
• H2. analyses the ways in
which models, theories and
laws in physics have been
tested and validated
Review and PFAs
• H3. assesses the impact of
particular advances in
physics on the
development of
technologies
Review and PFAs
• H4. assesses the impacts
of applications of physics
on society and the
environment
• The impact on society of
the application of our
knowledge of ultrasound
has been very significant
• Improved diagnosis of pre-
natal medical problems e.g.
spina bifita, cleft palate,
foetus developmental
problems - benefits
individuals and society by
reducing treatment costs
• Safe, non-invasive imaging
technology which is cost
effective
Review and PFAs
• H5. identifies possible
future directions of physics
research
A word from the creator
This PowerPoint presentation was prepared
by
Greg Pitt
of
Hurlstone Agricultural High School
Please feel free to use this material as you see
fit, but if you use substantial parts of this
presentation, leave this slide in the presentation.
Share resources with your fellow teachers and students.

Tutorial Questions
Describe the production of ultrasound used for medical imaging.
Answer
Ultrasound is produced using a piezoelectric crystal transducer, which
converts high frequency alternating potential differences into mechanical
vibrations of the crystal at a corresponding frequency. These vibrations are
used to create pressure variations that propagate through the surrounding
medium. These pressure variations, if the frequency exceeds 20 kHz, are
called ultrasound.

Tutorial Questions
Describe the piezoelectric effect.
Answer
The piezoelectric effect occurs when a voltage is applied across opposite
faces of certain crystals, causing the the crystal lattice to change size
slightly. The effect is reversible, with pressure variations that deform the
crystal slightly resulting in the production of a voltage across opposite
faces.

Tutorial Questions
How is the piezoelectric effect used to detect ultrasound?
Pressure variations produced by the ultrasound deform the piezoelectric
crystal slightly, producing an alternating voltage across opposite faces.
The voltage variations correspond to the varying intensity of the ultrasound
returning to the crystal.

Tutorial Questions
Compare the properties of medical ultrasound with sound in the normal
hearing range.
The sounds are similar because they are both longitudinal waves. Both types of waves can be
reflected from a boundary between two media having different acoustic impedances.
Ultrasound has frequencies extending up from the upper limit of human hearing, which has a
range from 20 Hz to 20 kHz.
Medical ultrasound frequencies fall in the range 2 MHz to 10 MHz and therefore have frequencies
much greater than those that humans can hear.
Ultrasound has a much shorter wavelength, of the order of a millimetre, than the sounds that
humans can hear.
Medical ultrasound has a velocity of approximately 1500 m s-1 in soft human tissues whereas
sound in air has velocity of about 340 m s-1.

Tutorial Questions
Define acoustic impedance.
Acoustic impedance (Z) of a medium is the product of the density of the
medium () and the speed of sound (v) in the medium. Hence…
Z  v

Tutorial Questions
Do all materials have the same acoustic impedance?
Explain your answer.
No.
The acoustic impedance (Z) of a medium is the product of the density of the
medium () and the speed of sound (v) in the medium.
Z  v
Two materials may have the same density and the speed of sound may
different in them. Hence their acoustic impedances will differ.
Tutorial Questions
Do materials in which the speed of sound is the same always have the same acoustic
impedance? Explain your answer.
No
The acoustic impedance (Z) of a medium is the product of the density of the medium () and the
speed of sound (v) in the medium hence two media through which the speed of sound is the
same, but which have different densities, have different Z values.
The speed of sound in fat and mercury is the same, however mercury is much more dense, and
therefore has a higher acoustic impedance*
* There is no need to remember such detail
- This example is for the purpose of
illustration only.
Z  v
Material
Velocity
(ms–1)
Air (0°C)
331
Fat
1450
Mercury
1450
Brain
1541
Water (50°C) 1540
Liver
1549
Material
Kidney
Blood
Muscle
Lens of eye
Skullbone
Aluminium
Velocity
(ms–1)
1561
1570
1585
1620
4080
6400
Tutorial Questions
Explain why is ultrasound not very useful in the diagnosis of adult brain disorders.
Bone has a significantly higher acoustic impedance than soft tissue and therefore any
ultrasound entering the scalp will be reflected strongly from the scull, resulting in very little
energy entering the brain.
The homogeneous nature of tissue in the brain also results in very little reflection from
different areas of the brain, so even if ultrasound did enter the brain, it would be difficult to
produce any image of structures within the brain itself.
Ultrasound is sometimes used to investigate foetal brains because the skull in early
development is softer cartilage, rather than calcified bone, and therefore ultrasound can
penetrate the foetal brain more readily.
Tutorial Questions
Calculate the acoustic impedance of bone and blood
Z  v
Bone
Z = 2 x 103 x 4080 = 8.16 x 106 R
Blood
Material
Velocity
(ms–1)
Air (0°C)
331
Fat
1450
Mercury
1450
Brain
1541
Water (50°C) 1540
Liver
1549
Substance
103
Z = 1.05 x
x 1570
= 1.65 x 106 R
muscle density = 1070 kg m–3
fat density = 1070 kg m–3
Aluminium
Iron
Copper
Gold
Glass
Ice
Bone
Water
Material
Kidney
Blood
Muscle
Lens of eye
Skullbone
Aluminium
density
(kgm–3)
Substance
2.7 x 103
7.8 x 103
8.9 x 103
19.3 x 103
2.4 – 2.8 x 103
0.917 x 103
1.7 – 2.0 x 103
1.00 x 103
Blood (plasma)
Blood (whole)
Seawater
Mercury
Ethanol
Air
Helium
CO2
Velocity
(ms–1)
1561
1570
1585
1620
4080
6400
density
(kgms–3)
1.03 x 103
1.05 x 103
1.025 x 103
13.6 x 103
0.79 x 103
1.29
0.179
1.98
Tutorial Questions
Predict whether any ultrasound energy striking an interface between water and aluminium
would be reflected. Justify your prediction.
Z  v
Water
Z = 1 x 103 x 1540 = 1.54 x 106 R
Aluminium
Material
Velocity
(ms–1)
Air (0°C)
331
Fat
1450
Mercury
1450
Brain
1541
Water (50°C) 1540
Liver
1549
Substance
Z = 2.7 x 103 x 6400
= 1.728 x 107 R
Since the acoustic impedance of aluminium is much
greater than that of water, most of the energy would
be reflected at the boundary.
Aluminium
Iron
Copper
Gold
Glass
Ice
Bone
Water
Material
Velocity
(ms–1)
Kidney
1561
Blood
1570
Muscle
1585
Lens of eye 1620
Skullbone
4080
Aluminium 6400
density
(kgm–3)
Substance
2.7 x 103
7.8 x 103
8.9 x 103
19.3 x 103
2.4 – 2.8 x 103
0.917 x 103
1.7 – 2.0 x 103
1.00 x 103
Blood (plasma)
Blood (whole)
Seawater
Mercury
Ethanol
Air
Helium
CO2
density
(kgms–3)
1.03 x 103
1.05 x 103
1.025 x 103
13.6 x 103
0.79 x 103
1.29
0.179
1.98
Tutorial Questions
Calculate the density of fat and muscle given their acoustic impedances
fat
muscle
1.38 x 106 R
1.7 x 106 R
Z  v
fat
1.38 x 106 =  x 1585
 = 870 kg m–3
muscle
1.7 x 106 =  x 1585
 = 1070 kg m–3
Material
Velocity
(ms–1)
Air (0°C)
331
Fat
1450
Mercury
1450
Brain
1541
Water (50°C) 1540
Liver
1549
Substance
Aluminium
Iron
Copper
Gold
Glass
Ice
Bone
Water
Material
Kidney
Blood
Muscle
Lens of eye
Skullbone
Aluminium
density
(kgm–3)
Substance
2.7 x 103
7.8 x 103
8.9 x 103
19.3 x 103
2.4 – 2.8 x 103
0.917 x 103
1.7 – 2.0 x 103
1.00 x 103
Blood (plasma)
Blood (whole)
Seawater
Mercury
Ethanol
Air
Helium
CO2
Velocity
(ms–1)
1561
1570
1585
1620
4080
6400
density
(kgms–3)
1.03 x 103
1.05 x 103
1.025 x 103
13.6 x 103
0.79 x 103
1.29
0.179
1.98
Tutorial Questions
Question
Explain how medical ultrasound is produced. Identify the effect.
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Do we have
to
memorise
this?
NO!