(Obstetrical
I will discuss the following aspects. Please scroll down and
start reading.
•Introduction to sound and ultrasound
•ultrasound probe
•Fundamentals of Ultrasound
•Introduction to wave
•Frequency, Wavelength, Resolution, and Depth
•Beam focusing
•Sending and receiving ultrasound
•Interaction of ultrasound with body tissues
•Imaging by ultrasound
•Doppler Ultrasound
•3D Ultrasound
Sound (1)- a disturbance in pressure that propagates
through a compressible medium.
Sound (2) the auditory sensation produced by transient
or oscillatory pressures
acting on the ear.
The first definition is what we mean by sound in
these notes. The second definition is what is
meant by sound in everyday speech.
The human ear can hear between frequencies
of about 20 Hz to 20,000 Hz.
Bats!
batsLike
are gifted
withsound,
a system of
normal
locating
things
with
sound.
First
they
ultrasound echoes off
emit sound.
objects
Bats navigate using ultrasound
Bats: Navigating with ultrasound
• If a bat hears an echo 0.01 second after it makes a chirp, how
far away is the object?
• Clue 1: the speed of sound in air is 330 ms
• Clue 2: The speed of sound equals the distance travelled
divided by the time taken
•
•
We can also use
Answer:
distance = speed x time
ultrasound
to look inside
Put the numbers in:the body…
distance = 330 x 0.01 = 3.3 m
• But this is the distance from the bat to the object and back
again, so the distance to the object is 1.65 m.
Generation of Ultrasound Waves
There is a special material called a “piezo
electric crystal”. This material has a very
special property. When a voltage is applied to
an piezo electric crystal it expands. When the
voltage is removed, it contracts back into its
original thickness.
Receiving Ultrasound
1-When a piezo electric crystal is
compressed, it generates a voltage
The ultrasound machine then very
2-The
crystal
then
generates
a
quickly switches to a listening
voltage
that
corresponds
to
the
mode by monitoring the voltage
intensity
ofthe
thepiezo
ultrasound
wave that
across
electric crystal.
hits it.
obstacle
The above examples show only one crystal for
clarity. In reality, ultrasound probes are
composed of a large number of individual piezo
ultrasound
electric crystals. The information gathered from
probe
the crystals are processed by a computer to
display the images on a screen.
piezo electric
crystals
Fundamentals of Ultrasound
This section covers some basic notation
and terminology used in acoustics, and
some of the
fundamental physical principles.
What is a wave?
• A wave is any disturbance that transmits
energy through matter or empty space.
• Waves can be Transverse, Longitudinal
• Some waves combine both transverse and
longitudinal motions
TRANSVERSE WAVES
• Transverse Waves are waves in which the
particles vibrate perpendicularly to the
direction the wave is traveling.
• Transverse waves are made up of crests
and troughs.
• Water waves, waves on a rope, and
electromagnetic waves are examples of
transverse waves.
• http://www.acs.psu.edu/drussell/Demos/w
aves/wavemotion.html
Transverse Waves
direction the wave
longitudinal WAVES
• Compression Waves are waves in which the
particles vibrate back and forth along the path
that the waves moves.
• longitudinal waves are also known as
Compression waves.
• Compression/longitudinal waves are made up of
compressions and rarefactions.
• Waves on a spring are longitudinal waves.
• http://www.acs.psu.edu/drussell/Demos/waves/wavemotion.html
Longitudinal Waves
direction the wave
Ultrasound waves are longitudinal,
compressional waves, that can be periodic
or pulsed,
propagate at roughly 1500 m/s in water or
biological tissue, can leave the medium
unchanged
(diagnostic ultrasound), but at higher
intensities can also change it (therapeutic
ultrasound).
Is soft tissue solid or liquid?
Should we treat biological tissue as liquid or
solid, at least, as far as ultrasound is
concerned?
Some tissues are obviously solid - bones, for
instance - but what about soft tissues such as
skin or muscle?
1- they are very malleable and consist largely of
water .
2- One of the differences between a solid and a
liquid is that a solid has rigidity and can support
a shear force.
What is shear force?
• Shear Force : A good example of shear force is
seen with a simple scissors. The two handles
put force in different directions on the pin that
holds the two parts together. The force
applied to the pin is called shear force.
ImagineWhy
gluing
of your
hand
do wethe
treatpalm
soft tissue
as a fluid
whento
it isa table and then
tryingactually
to push
an elastic
yoursolid? The pragmatic
reason is that this approximation has proven to be
hand along
the table
You can
move your hand a bit
reasonably
accuratetop.
and useful
over half
century
of ultrasound
as thea skin
deforms
butstudies.
it willAnother
soon motivation
is that wave propagation in fluids is
reach a much
pointsimpler
where
you can't
pushmathematically
it any further (without
to visualise
and model
In
ultrasound
imaging,
soft
tissue
is
than
wave
propagation
in
solids.
The
tearing the skin). Your skin will
reasonusually
why we get away
with it, though,
is a
because
modelled
as
fluid.
be supporting
a
shear
force
it
seems
to
behave like a
treating tissue as a fluid is equivalent to
solid. ignoring shear waves, and there are good reasons
why shear waves can usually be neglected
• Remove
your hand
from the table and your skin will
in ultrasound
imaging
return to the same shape it was originally - it is an
elastic solid. This suggests we should treat soft tissue as
an elastic solid.
Wavelength, frequency and wave speed
• Wavelength, (lambda)
• A wavelength is the distance between any
point on a wave to an identical point on the
next
wave.on a 0.1-1 mm scale for medical
typically
ultrasound.
• A wave with a shorter wavelength carries
more energy than a wave with a longer
wavelength does.
Frequency
• Frequency is the number of waves
Medical in a given amount of time.
produced
ultrasound typically uses frequencies
from 1-15 MHz.
Wave speed, c.
• Wave Speed is the speed at which a
wave travels.
• Sounds waves travel faster in a medium if
the temperature is increased.
• Sound travels at about 340 m/s in air and
1500 m/s in water.
•vf
• (Speed = frequency x wavelength)
Ultrasound Intensity
One property of propagating waves is that they transfer energy from one
point to another without the transfer of matter. In acoustics, this flow of
energy is called the acoustic intensity.
Instantaneous acoustic intensity, I(x,t), in a time-varying
acoustic field, is a vector
defined as
I(x,t) = pa(x,t).ua(x,t)
[J/s/m2 = W/m2]
where pa is the acoustic pressure and ua the acoustic particle
velocity. (Is this a plausible definition of intensity? Recall that,
in general terms, pressure = force per unit area, velocity=
distance per unit time and `work done' = force X distance.
Acoustic intensity is a measure of power per unit area = work
done per unit time per unit area = pressure X velocity.)
Safety limits
Maximum ultrasound intensities recommended by the US Food and Drug
Administration (FDA) for various diagnostic applications.
Use
(Intensity)max (mW/cm2)
Cardiac
430
Peripheral vessels
720
Opthalmic
17
Abdominal
94
Fetal
94
Reflection, Refraction and Scattering
Acoustic impedance:
Materials in which the density, 0 , and sound speed
c0 are constant. If this were the case in soft tissue
then ultrasound imaging would not work. There need
to changes in the sound speed or the density in order
for the ultrasound waves to be reflected. More
precisely, the characteristic
acoustic impedance of the material, 0 c0, must vary
between different tissue types.
characteristic acoustic impedance, Z = 0 c0
Reflection and transmission coefficients
Boundary conditions When a wave reaches a
boundary, part will be reflected and part
transmitted. These three parts (incident wave,
reflected wave, transmitted wave) must obey
two boundary conditions:
(1) Continuity of pressure The acoustic pressure
must be the same on both sides of the
boundary. There must be no net force.
(2) Continuity of normal particle velocity The particle
velocities normal to the boundaries
must be equal. The fluid must stay in contact.
Normal incidence pressure reflection
and transmission coefficients
When an acoustic pressure wave with amplitude
pi is normally incident on an interface (a
change in characteristic acoustic impedance),
a wave with amplitude pr will be reflected and
another wave, with amplitude pt will be
transmitted. These wave amplitudes define
the pressure reflection and transmission
coefficients, R and T:
Oblique incidence pressure reflection
and transmission coefficients
Incident
wave
θi
θr
Reflected wave
Z1
Z2
Transmitted wave
θt
Refraction
• When a wave moves from one medium to
another, the wave’s speed and wavelength
changes. As a result, the wave bends and
travels in a new direction.
Refraction (snell’s law)
• When a wave moves from one medium to
another, the wave’s speed and
wavelength changes. As a result, the wave
bends and travels in a new direction.
• c : sound velocity
c1
c1
c2
c1 > c2
c2
c1 < c2
This phenomenon causes artifacts in medical echo image.
Scattering and diffraction
- Scattering refers to the reflection of sound from
surfaces or heterogeneities in a
medium. It is quite a general term and includes reflection
and diffraction.
- Diffraction is usually used to refer to the `leakage' of
sound into `shadow zones'.
Diffraction is the reason you can hear someone talking in
the next room even though
you can't see them; the sound waves `bend' around the
corners more than light waves do
as they have a much longer wavelength. (Dif fraction is
quite different from refraction
and the two should not be confused.)
More about transducer
• With only air behind the crystal, ultrasound transmitted back into the
cylinder from the crystal is reflected from the cylinder’s opposite end.
• The reflected ultrasound reinforces the ultrasound propagated in the
forward direction from the transducer.
• This reverberation of ultrasound in the transducer itself contributes
energy to the ultrasound beam (i.e., it increases the efficiency).
• It also extends the time over which the ultrasound pulse is produced.
• Extension of the pulse duration (decreases bandwidth, increases Q) is no
problem in some clinical uses of ultrasound such as continuous wave
applications.
• However, most ultrasound imaging applications utilize short pulses of
ultrasound, and suppression of ultrasound reverberation is desirable.
• Backing of transducer with an absorbing material (tungsten powder
embedded in epoxy resin) reduces reflections from back, causes damping
at resonance frequency
– Reduces the efficiency of the transducer
– Increases Bandwidth (lowers Q)
Fresnel (or near) zone & Fraunhofer (or
far) zone
• Plane wave
– Line sound source, infinite length
– No diffusion attenuation
Sound source
Fresnel (or near) zone & Fraunhofer
(or far) zone
• Spherical wave
– Point sound source
– Diffuse sound field
Point source
Fresnel (or near) zone & Fraunhofer
(or far) zone
• Practical condition –ultrasonic element– Finite element size (about 0.3mm)
– Not plane wave, not spherical wave
D2
D
: wavelength
= 0.437mm
D:diameter
= 0.3mm
4
Plane wave
Near field
(Fresnel zone)
Spherical wave
Far field
(Fraunhofer zone)
Fresnel zone
= 0.052mm
NFL for 2 MHz
(=0.77 mm)
NFL for 4 MHz
(=0.385 mm)
Diameter NFL
Diameter NFL
1 cm
3.2 cm
1 cm
6.4 cm
2 cm
13 cm
2 cm
26 cm
4 cm
52 cm
4 cm
104 cm
If the diameter doubles, NFL increases by 4.
If the frequency doubles, NFL doubles.
Divergence in far field
• (The ‘sin’ is a function of the angle)
• Larger diameter diverges less
• Higher frequency (smaller wavelength) diverges
less
What is the divergence angle for a 2 cm diameter, 3 MHz
transducer?
1.2
sin  
d
c
1540m / s
1,540,000mm / s
 

0.51mm
f 3,000,000 / s
3,000,000 / s
1.2  0.51mm
sin  
 0.036
20mm
1
o
  sin 0.036  1.75
Focusing,
Methods
Transducers
can be designed
to produce either a
focused or non-focused beam, as shown in the
following figure. A focused beam is desirable for
most imaging applications because it produces pulses
with a small diameter which in turn gives better
visibility of detail in the image. The best detail will be
• Focusing reduces the beam width in the focal
obtained
for structures within the focal zone. The
zone
distance
between the transducer and the focal zone
• Methods
is the focal depth.
– Lens
– Curved element
– Electronic
focusing technique
• Acoustic Lens
Acoustic lens
sound velocity : c1
c1 < c2
Ultrasonic
element
Focal point
wavefront
Human body
Sound velocity : c2
Weak point : a fixed focus
The Principle of Electronic Focusing
with an Array Transducer
• Focusing is achieved by not applying the electrical
pulses to all of the transducer elements simultaneously.
The pulse to each element is passed through an
electronic delay. Now let's observe the sequence in
which the transducer elements are pulsed in the figure
above. The outermost element (annular) or elements
(linear) will be pulsed first. This produces ultrasound
that begins to move away from the transducer. The
other elements are then pulsed in sequence, working
toward the center of the array. The centermost element
will receive the last pulse. The pulses from the
individual elements combine in a constructive manner
to create a curved composite pulse, which will converge
on a focal point at some specific distance (depth) from
the transducer.
focusing technique
• Electronic focus (transmission)
Array of ultrasonic Element
Delay circuit
Focal point
Desired focal length
by control of delay circuit
focusing technique
• Electronic focus (receiving)
delay
Array of ultrasonic Element
+
Point scatterer
High S/N
The same principle as radar
scanning techniques - grouping Element
array
Control of
beam direction
linear
convex
Switched array method
linear
annular
Phased array
method
mechanical
scan
linear
Offset sector
sector
Probe form
linear
convex
sector
Region of
image
thyroid, breast
Abdominal
region
heart
scanning techniques…
• Performed with transducer arrays
– multiple elements inside transducer
assembly arranged in either
• a line (linear array)
• concentric circles (annular array)
Linear Array Scanning
• Two techniques for activating groups of linear
transducers
– Switched Arrays
• activate all elements in group at same time
– Phased Arrays
• Activate group elements at slightly different
times
• impose timing delays between activations of
elements in group
Linear Switched Arrays
• Elements energized as
groups
– group acts like one large
transducer
• Groups moved up & down
through elements
– same effect as manually
translating
– very fast scanning possible
(several times per second)
• results in real time image
Linear Switched Arrays
Linear Phased Array
• Groups of elements energized
1
– same as with switched arrays
• voltage pulse applied to all
elements of a group
BUT
• elements not all pulsed at
same time
2
Linear Phased Array
• timing variations allow beam
to be
– shaped
– steered
– focused
Above arrows indicate
timing variations.
By activating bottom
element first & top last,
beam directed upward
Beam steered upward
Linear Phased Array
Above arrows indicate
timing variations.
By activating top
element first & bottom
last, beam directed
downward
Beam steered downward
By changing timing variations between pulses, beam
can be scanned from top to bottom
Linear Phased Array
Focus
Above arrows indicate
timing variations.
By activating top &
bottom elements earlier
than center ones, beam
is focused
Beam is focused
Linear Phased Array
Focus
Focal point can be moved toward or away
from transducer by altering timing
variations between outer elements & center
scanning techniques - grouping Element
array
Control of
beam direction
linear
convex
Switched array method
linear
annular
Phased array
method
mechanical
scan
linear
Offset sector
sector
Probe form
linear
convex
sector
Region of
image
thyroid, breast
Abdominal
region
heart
scanning techniques…
Control
of beam direction : phased array
Scanning
: sector
Heart image
scanning techniques…


Control of beam direction : switched array
Scanning : linear
Thyroid image
scanning techniques…
Control
of beam direction : switched array
Scanning
: offset sector
Liver image
Doppler ultrasound
Doppler ultrasound is based upon the Doppler
Effect. When the object reflecting the ultrasound
waves is moving, it changes the frequency of the
echoes, creating a higher frequency if it is moving
toward the probe and a lower frequency if it is
moving away from the probe. How much the
frequency is changed depends upon how fast the
object
is moving. Doppler ultrasound measures
A Doppler
flow
themeasures
change in frequency of the echoes to calculate
meter
how fast
the speed
of redan object is moving. Doppler ultrasound
has
been
used mostly to measure the rate of
blood
cells
.
blood flow through the heart and major arteries.
In a similar way, there are many different ways a ultrasound probe can “look “•at
things. These ways are called “modes
Or we might scan
the whole area,
upscanning
and down,
look horizontally
when
theleft
seaand
right, in many dimensions when absorbing scenery such as
the one below in Sri Lanka.
A mode (Amplitude mode)
When we look at things with our
eyes,
there
are
various
ways
in
B mode (Brightness mode) including
which
we
“look
“
real time, 2 dimensional, B mode
times, we might choose to look only straight ahead like when we
MAt mode
(Motion
mode)
read a notice on a wall.
A Mode Scanning
•The A mode is the
simplest form of
ultrasound imaging and is
not frequently used.
•One use of the A scan is
to measure length. For an
example,
ophthalmologists can use
it to measure the diameter
of the eye ball.
Imagine that the red circle below
is the eye ball and you want to
measure the diameter of it.
•An
scanning
can be used.
Theultrasound
time differencemachine
between the
first bump in
and“A
thescan”
secondmode
bump represents
how long
the
As probe
the
ultrasound
waveisreaches
wave took
theon
first
to one
travel
wall end
of
between
theof
eye,
the
some
two
ofwalls.
the ultrasound
Longer theislength,
reflected
longer
backis
The
placed
the
eye
ball.
the
into
time
thedifference.
probe. TheThe
returned
speed wave
of ultrasound
is recorded
in the
on the
eye line
is known
as a bump.
to be 1500
The stronger
meters per
is
•Anthe
ultrasound
sent
the
probe
and (given
at
the
same
second
returned
(yes,wave,
thatwave
ishigher
fast).isSo
the
if height
youfrom
know
of the
the
bump.
time
difference
The height
of the
by
bump
the interval
is called
between the
bumps),
can
how
wavestands
traveled
which
iscalculate
what
the
“A”far
of the
“A scan”
forbetween
instance,
a two
lineAmplitude
from you
the
left
of the
screen
starts
to be
drawn.the two
walls of the eye, giving you the eyeball length.
This line moves horizontally
measuring time.
using the eye
ball as an example, the probe is placed on one end. Like
BAgain
Mode
Scanning
The
inBthe
scan
A scan,
in thewhen
form the
discussed
wave meets
doesn’t
the
amount
first wall,
to much
a part….ofjust
theawave
few dots
is
The first B scan line is kept on the screen. Then at a slightly different level,
ofreflected
differentback
brightness
into thealong
probe.
a line.
However,
However,
this if
time,
a B scan
instead
is done
of a bump,
at different
the
the B scan is repeated.
strength
levels
ofof
the
the
object,
returning
you will
wave
get
isB
arecorded
two dimensional
by a bright
image
dot.
The
on the
brightness
screen asof
•In its
form,
the
mode
is
very
similar
In simplest
this way, a two
dimensional
(2 scan
D) image
of the object
is formed
on the
the
shown
dot represents
below. Firstthe
a Bstrength
scan is done
of the
atreturning
the top ofwave.
the structure
The brighter
chosen,
thee.g.
dot,
screen.
to the
scanismode.
Justwave.
like
the
A “B”
scan,
wave
of
the A
stronger
the returning
letter
of “Ba
scan”
represents
the The
eye.
ultrasound is sent out inBrightness.z
a pencil like narrow path.
And again like the A scan, the horizontal line
represents the time since the wave was released.
M Mode Scanning
• M stands for motion. This approach is used
for the analysis of moving organs. It is based
on A-mode data from a single ultrasound
beam that are represented as function of
time.
3D Ultrasound Imaging
• 3D ultrasound is a data set that contains a
large number of 2D planes (B-mode
images).
• This is analogous to assuming that a page
of a book is one 2D plane, and the book
itself is the entire data set.
http://www.3d-4d-ultrasounds.com/images/gallery/before-after.jpg
• Once the Volume is acquired using a
dedicated 3D probe you can “Walk”
through the volume in a manner similar to
leafing through the pages of a book,
meaning you can walk through the various
2D planes that make up the entire volume.
• This is also known as translation and the
planes are reconstructed using a computer.
http://www.doctorscareclinic.com/html/ultrasound.html
3D from Conventional 2D Ultrasound
Each US image represents one slice of the body
and by taking therefore multiple cross sectional
scans and putting them “side-by-side” you can
2D Images
render a 3D image or you could view Volume
any one of
theData
2D slices. Construction
Position
Engine
k
(x,y,z)
i
US Probe
Tracking
Device
Kane, Physics in Modern Medicine, CRC Press
j
Workstation
Volume
Rendering
Engine
3D imaging allows you to get a better look at
the organ being examined and is best used for:
• Early detection of cancerous and benign tumors
• examining the prostate gland for early detection
of tumors
• looking for masses in the colon and rectum
• detecting breast lesions for possible biopsies
• Visualizing a fetus to assess its development,
especially for observing abnormal development
of the face and limbs
• Visualizing blood flow in various organs or a
fetus
Summary of how ultrasound imaging works
1. The ultrasound machine transmits high-frequency (1 to 5
megahertz) sound pulses into your body using a probe.
2. The sound waves travel into your body and hit a
boundary between tissues (e.g. between fluid and soft
tissue, soft tissue and bone).
3. Some of the sound waves get reflected back to the
probe, while some travel on further until they reach
another boundary and get reflected.
4. The reflected waves are picked up by the probe and
relayed to the machine.
5. The machine calculates the distance from the probe to the
tissue or organ (boundaries) using the speed of sound in
tissue (5,005 ft/s or1,540 m/s) and the time of the each
echo's return (usually on the order of millionths of a
second).
6. The machine displays the distances and intensities of the
echoes on the screen, forming a two dimensional image
like the one shown below.
Ultrasound image of a growing fetus (approximately 12 weeks
old) inside a mother's uterus. This is a side view of the baby,
showing (right to left) the head, neck, torso and legs.