Basics of Ultrasound Physics

BASICS OF ULTRASOUND
PHYSICS
Compiled document
Dr. Jonathan R. Chikomele ( Radiology Resident),
Muhimbili University of Health and Allied Sciences,
March 2023

Contents
SNo Topic Page
1 Sound waves production Slide No - 003
2 Sound waves propagation and reflection Slide No- 042
3 Ultrasound Imaging and Instrumentations Slide No- 076
4 Image storage, display and recording Slide No- 101
5 Sound waves resolution and Attenuation Slide No- 118
6 Ultrasound Artifacts Slide No- 159
7 Ultrasound Modes Slide No- 200
8 Doppler Principles Slide No- 211

Ultrasound: Definitions
• Sound waves are mechanical vibratory
disturbances resulting from a
combination of molecular condensations
(compressions) and rarefactions
(decompressions).
• Physical definition
• Mechanical sound energy(pressure)
traveling through a conducting
medium
• Longitudinal wave producing
alternating high pressure
(compression) lower
pressure(rarefaction)
• Sound waves greater than 20 kHz
• Medical definition
• Frequency ranges used in medical
ultrasound imaging are 2-20 MHz
4

• US scanners provide images of soft
tissues for abdominal, obstetrical,
gynaecological, ophthalmic,
cardiac and vascular evaluations.
• Advantages of:
-lt is non invasive and utilise no
ionization radiation.
-real-time nature of the
examination.
-portable nature of the
equipment compared to CT and MRI.
-excellent resolution of superficial
structures.

Sound wave production.
• Definitions of terms:
• Frequency is the number of
vibrations (cycles)per second of
a wave form of energy.
• Cycle is a complete variation of
an acoustic variable, containing
one condensation and one
rarefaction.
• Density is the amount of mass in
a substance for a given volume
• Stiffness is a measure of how
well a material resists being
deformed when it is squeezed
6

• Sound is the result of mechanical
energy producing alternating
compression and rarefaction of the
conducting medium as it travels in the
form of a wave.
• Frequency is number of cycles per
second.
• Unit of frequency is the hertz (Hz).
• Hertz (Hz) is one cycle per second and
• Megahertz (MHz) is one million Hertz
• Sound waves propagate through
media by creating compressions and
rarefactions, corresponding with high-
and low-density regions of molecules
• Frequency of the sound determines
its category-ultrasound,audible and
infrasound.

Sound wave production: definitions
 The period (P) of a sound wave is the time
of one cycle.
 The wave length (λ) is the distance between
two successive points in a pulse that are in
the same state of compression.
 The wavelength of the sound formula is
given as follows: λ = v/f
 Where, f is the frequency of the sound wave
and v is the velocity of the sound wave.
 The amplitude (A) is an indication of the
relative intensity (loudness) of the sound.
 Amplitude refers to the distance of the
maximum vertical displacement of the wave
from its mean position.
8

Sound wave production
• Infrasound – frequencies below
20 Hz.
• Audible sound – frequencies
between 20 – 20000 Hz.
• Ultrasound – frequencies above
20000 Hz.
• Medical diagnostic applications
operate at frequencies above
one megahertz (1 MHz to
20MHz).

• Sound wave propagate through
matter by causing molecules to
vibrate successively along the
sound path.
• Sound waves carry energy from
one point to another.
• Stiffness and density properties
of a material determines the
velocity (speed of propagation).
• Velocity in a pt. averages at 1540
metres per second.

• An increase in density results in
a slight decrease in the velocity.
• Velocity is higher in stiff
materials.
• Velocity is not affected by
frequency.
11

Sound wave production;
transducer
 US is produced by a transducer,
which is a device that converts
one form of energy into
another.
 It is the most expensive part
of any ultrasound unit
• Transducer’s main component is
a vibrating piezoelectric
element :when they are
electronically stimulated
generate the sound pulses used
for diagnostic sonography

transducer
 A basic transducer is a very complex
piece of electrical equipment.
 It is designed to convert an
electronic pulse into an ultrasound
pulse, then convert the sound pulse
into an electronic signal in order to
be processed to produce an image.
 In it simplest form it consists of a
cable connecting it to the ultrasound
machine, a plastic casing which
encloses the electronics, a damping
material, piezoelectric crystals, a
matching layer and a protective
layer.
 The transducer is also known as a
‘probe’.

Parts of the Transducer
Damping Material or Backing Layer
• This layer acts to shorten the
resonance/ringing of the crystal as it
transmits the pulse hence shortening
pulse duration and decrease duty
factor
• This improves the resolution of the
image by decreasing the length of
the pulse, allowing uss to separate
smaller structures along the depth of
the beam.

Sound waves production
 The number of cycles in a pulse is
determined by the amount of
damping that is present in a
transducer,
 Damping means limiting the
duration or decreasing the
amplitude of vibrations as when
damping a transducer element.
 Damping is due to the presence
of the transducer’s backing
material.
 Increased damping causes
reduction in the number of cycles.
15

Sound wave production:
 Damping causes an increase
in the transducer bandwidth
(BW), which is the range of
different frequencies
contained in an ultrasound
pulse.
D> BW
 Increase in the bandwidth is
accompanied by a decrease in
the quality factor (Q)
 Q values are higher in
transducers that have very
little damping.
16

Piezoelectric Crystals
BT, LZT, LT,LM
• These crystals produce an ultrasound pulse
when struck by an electrical impulse, and
convert the returned ultrasound echoes
into an electric signal which is interpreted
by the ultrasound machine to produce the
image.
• The crystals resonate and emit ultrasound,
then stop ringing in order to ‘listen’ for the
return echo – they cannot transmit and
receive at the same time.
• Piezoelectric elements are ceramic.
• Typical ceramic materials are lead zirconate
titanate (LZT), barium titanate (BT), lead
metaniobate (LM) and lead titanate.(LT)
• Lead zirconate titanate is the most used
material.
• They have piezoelectric characteristics
because they are polarized while at a
temperature above their curie point

Parts of the Transducer
• Matching Layer
• Acts to allow the sound to penetrate
through the skin into the tissue.
• There is a very large impedance
mismatch between the transducer,
the air and the skin, so unless a
matching layer is added to the
transducer the sound would bounce
straight back to the crystals and not
penetrate into the tissue.
• Cover
• Acts as a protective cover over
the matching layer and
piezoelectric crystals to protect
them from gel or other liquids or
contaminants.

Piezoelectric Effect
• Piezoelectric effect : refers to the
conversion of electrical energy
(voltage) into mechanical (sound)
energy or the conversion of
mechanical energy into electrical
energy.
• Ultrasound Transducers
• A transducer converts one type of
energy into another
• Based upon the pulse-echo principle
occurring with ultrasound
piezoelectric crystals, ultrasound
transducers convert:
– Electricity into sound = pulse
– Sound into electricity = echo
19

Pulse echo principle
Pulse (transmitted)
• Generated by the transducer
• Sent to body tissues
• Electricity into ultrasound
20
• Echo (returned)
– Produced by body tissues
– Ultrasound into electricity
– Interpreted and processed
echo
pulse

Pulse of sound is sent to soft tissues
• Sound interaction with soft tissue = bio-
effects
• Pulsing is determined by the transducer
or probe crystal(s) and is not operator
controlled
Echo produced by soft tissues
• Tissue interaction with sound = acoustic
propagation properties
• Echoes are received by the transducer
crystals
• Echoes are interpreted and processed by
the ultrasound machine

Transducer operations
 Transducers may be
operated in 3 different
modes, which are:
1.shock – excited mode,
2.burst excited mode or
3.continuous mode.
22

Sound wave production; transducer
Shock - excited mode:
 Transducer excitation is by a brief
driving voltage impulse.
 A driving voltage is the converted
electrical energy used in uss
transducer.
 Results in ultrasound pulses
within the range of element’s
nominal or resonant frequency.
 Resonant: is the ability of an
object to either operate at or
generate energy of a specific
frequency only.
23

Burst – excited mode:
• Is transducer excitation by a
cycle or two cycles of
alternating driving voltages.
• Resulting in ultrasound pulses
equal to one or two or three
selected frequencies (multi –
hertz), within the range of the
resonant frequency.
24

Continuous mode:
• Transducer excitation is by repeating
cycles of alternating driving
voltages.
• Resulting in a non – pulsed/
continuous wave with cycles that
repeat indefinitely at transducer’s
resonant frequency of a
‘broadband’ transducer.
25

• Continuous mode
• continuous alternating current
• Doppler or therapeutic US
• 2 crystals – one talks and the other
listens
• Pulsed mode
• Diagnostic US
• Crystal same talks and then listens

transducer
 The resonant frequency of a
transducer is related to the
piezoelectric element’s
thickness and sound velocity
within it.
 The thinner the element, the
higher the resonant frequency
of the transducer.
27

transducer
 Transducer receives echoes
returning from the object being
studied.
 Upon receiving the return echo,
converts its acoustic pressure into
electrical signal.
 A transducer may have a single
piezoelectric element or may
have (configured as) an array.
 Array: is transducer assembly
containing several piezoelectric
elements.
28

 In a single piezoelectric element
transducer, transmission and
receiving function cannot be
performed simultaneously.
 In shock – excited and burst
excited modes, the transducer
utilizes same piezoelectric
element during separate transmit
and receive intervals.
 In continuous mode, transducer
requires separate piezoelectric
elements for transmit and receive
functions.
29

Huygens's Principle
• States that every point on a
wave front is a source of
wavelets that spread out in the
forward direction at the same
speed as the wave itself.
• The new wave front is tangent to
all of the wavelets.

Sound wave production; Huygen’s principle
 Numerous points on the surface
of piezoelectric element serve as
sources of small individual sound
waves.
 Acoustic interference occurs when
two or more sound waves are
produced at same time.
 Their amplitudes at any point in
space may be added together to
determine their combine effect.
32

Sound wave production; Huygen’s principle
 If the amplitude of the resultant
wave is large than either of the
origin waves, the waves are said to
interfere constructively.
 If the resultant amplitude is smaller
than either of the origin individual
waves, the waves interfere
destructively.
 Constructive interference: is a
combination of in phase positive and
negative pressures.
 Phase: a point or interval in time.
33

side lobe beams
 Ultrasound transducer crystals
expand and contract to produce
primary ultrasound beams in the
direction of expansion and
contraction.
 Secondary beams occur because
the crystals also expand and
contract radially.
 These radial beams are called
side lobe beams. .
 Side lobe beams are low-intensity
beams that surround the central
beam.

constructive interference:
 Individual waves become
tangent to each other and have
the same phase relationship.
 This results in a sound beam
with the most of the useful
energy present along the
primary path of propagation.
 Constructive interference
outside the primary beam
path, results in side lobes
35

Sound waves production:
 Sound waves in a material
produce particle motion that is
back and forth along the
direction of travel. This motion
is termed longitudinal wave
propagation.
 Artefact: is a modification of
the appearance of a displayed
structure as a result of certain
changes in tissue
characteristics in a patient.
36

Sound waves production: sidelobes:
 Side lobe artifacts occur where
side lobes reflect sound from a
strong reflector that is outside
of the central beam, and where
the echoes are displayed as if
they originated from within the
central beam.
 Side lobe artifacts are echogenic,
linear or curvilinear artifacts.
 Strong reflectors include bowel
gas adjacent to the gallbladder
or urinary bladder
37

Sound waves production; sidelobes:
 Unwanted extraneous energy
components that are not in the
primary direction of the
ultrasound beam.
 Often contributes to image
artefacts.
 Granting lobes are side lobes
produced a multi element
structure of transducer array.
38

Wave parameter equations
1
Period = -------
frequency
velocity
Wavelength = ----------
frequency
Pulse duration = Period X no. of cycles
39

Wave parameter equations
 Spatial pulse length = wavelength X no. of cycles.
Frequency
 Quality Factor = ----------------
Bandwidth
40

Thank you for your attention
Dr. Jonathan R.C

BASICS OF DIAGNOSTIC
ULTRASOUND
Sound waves propagation and reflection
Presenter: Dr. Jonathan R. Chikomele (MMED 1 Radiology)
Supervisor: Dr. Zuhura Nkrumbi (Radiologist)
21st March 2023

Contents
1.Introduction to echo system and instrumentation
2.Basics of sound wave propagations
3. Basic acoustics of sound wave propagation
4.Sound wave pulses
5.Distance measurements (Echo ranging) in USS
6.Concept of acoustic interfaces,impedance and reflection coefficient
7.Accoustic shaddowing

Pulse-echo system and instrumentation
• A pulse-echo system is used for
ultrasound imaging.
• Based upon the pulse echo principle
with ultrasound piezoelectric crystals,
ultrasound transducer coverts:
- Electricity into sound energy=Pulse
- Sound energy into electricity= Echo
• So,P-E system refers to an ultrasound
system that is used for US imaging by
periodically transmitting and receiving
short duration sound pulses.
• It create sound pulses, retrieves
reflections and present audio and
visual information for interpretation
44

• Ultrasound scanners are complex
and sophisticated imaging devices,
but all consist of the following basic
components to perform key
functions:
1.Pulser
2.Beam former
3.Transducer
4.Receiver
5.Memmory(Scan converter)
6.Display

Reflection (transducer)
• A transducer both emits sound waves as
well as detect the ultrasound waves that
are reflected back.
• It converts electrical energy into sound
energy by Piezoelectric effect.
• Piezoelectric effect is the ability of certain
materials to convert electrical energy to
mechanical energy and mechanical
energy to electrical energy
• Ceramic crystals that deform and vibrate
when they are electronically stimulated
generate the sound pulses used for
diagnostic sonography
• The transducer transmits mechanical
sound energy(waves) into the patients.
• Ultrasound waves are produced in pulses

Sound waves Propagation
• In most clinical applications of
ultrasound, brief bursts or pulses of
energy are transmitted into the body
and propagated through tissue.
• Acoustic pressure waves can travel in
a direction perpendicular to the
direction of the particles being
displaced (transverse waves), but in
tissue and fluids, sound propagation is
along the direction of particle
movement (longitudinal waves).
• The speed at which the pressure
wave moves through tissue varies
greatly and is affected by the physical
properties of the tissue.

Sound waves Propagation
• Propagation velocity is largely
determined by the resistance of the
medium to compression, which in
turn is influenced by the density of
the medium and its stiffness or
elasticity.
• Propagation velocity is increased by
increasing stiffness and slightly
reduced by increasing density.
• In the body, propagation velocity may
be regarded as constant for a given
tissue and is not affected by the
frequency or wavelength of the
sound

Sound waves Propagation:
• The speed of sound waves in soft
tissues can range from 1450m/s in
fat to 1580m/s in muscles.
• Therefore the average speed of
1540 + 30m/s is often used
• Range equation assumes a velocity
(speed of 50ml) of 1540 m/sec
(1.54mm/usec).
• Variations in the speed of sound
have an effect on the distance
calibration of pulse-echo system,
resulting in errors
49

Sound waves propagation(Basic acoustics)
• As sound waves pass
between media of different
densities,the following may
occur:
1.Scattering
2.Reflection
3.Refraction/transmission
4.Attenuation

Refraction
• Another event that can occur when
sound passes from a tissue with
one acoustic propagation velocity
to a tissue with a higher or lower
sound velocity is a change in the
direction of the sound wave. This
change in direction of propagation
is called refraction
• Refraction is important because it
is one of the causes of
mislocalization of a structure on an
ultrasound Image.

Attenuation
• As the acoustic energy moves through a
uniform medium, work is performed
and energy is ultimately transferred to
the transmitting medium as heat.
• The attenuation of sound energy as it
passes through tissue is of great clinical
importance because it influences the
depth in tissue, from which useful
information can be obtained
• Absorption is greater in soft tissues
than in fluid, and it is greater in bone
than in soft tissues.
• Sound absorption is a major cause of
acoustic shadowing.
• AIR has very large attenuation
coefficient

Reflection
• Sound waves (Ultrasound waves)
follows the rules of propagation
and reflection similar to those
that govern light waves.
• The ultrasound reflects off the
tissue and return to the
transducer, the amount of
reflection depends on difference
in acoustic impedance between
the media through which sound
travels

Reflection
• Reflected echoes most frequent received are
those that occur at normal incidence in the
interface is perpendicular to the ultrasound
beam.
• The transducer does not detect reflections
that are produced as a result of oblique
incidence.
• Because of oblique incidence, not every
thing below the transducer is seen.
• There are two types of ultrasound wave
reflections: Specular and Non specular
reflection
54

Specular interfaces
• Specular interfaces are smooth
and larger compared to the size
of an ultrasound wave.
• Specular interfaces, may cause
refraction.
• Specular interface examples
include; diaphragm, wall of
vessels, cystic structures and
boundaries of many organs.
55

Non-specular interfaces
• Interfaces that are either smaller than
a wavelength, or not smooth are non-
specular.
• A non-specular interface produces
scattering of the sound.
• Example of non-specular interfaces
include red blood cells, some
microbubble contrast media agents,
liver parenchyma etc.
• Differences in scatter amplitude that
occur from one region to another
cause corresponding brightness
changes on the ultrasound display. .
56

Sound wave pulses in ultrasound
• For sonographic and Doppler
ultrasound, pulse ultrasound is used.
• A Pulse is a collection of two or more
cycles followed by a resting/listening
time
• Each pulse contain cycles of the same
frequency
• Pulse duration (PD) is the time taken
from the start of pulse to the end of
that pulse.
• Measured in ms
57

• Pulse repetition frequency (PRF) or
pulse repetition rate is the rate of
which pulses are transmitted. Its
the number of pulses sent by
ultrasound transducer per second
• Pulse repetition period (PRP) is the
interval between the start of
successive pulses. It’s the time from
the beginning of one pulse to the
beginning of the next.
• PRF is often a function of US DEPTH
control
• High PRF there is short PRP
• Pulse Repetition Period = 1
Pulse Repetition Frequency

• Duty factor: is the ratio of the pulse
duration to pulse repetition period.
• It describes the percentage of the time
that the transducer is emitting sound
waves
DF= (PD in ms/PRP in ms) X100
> Duty factor: Is normally less than 1% in
pulse-echo system
> CW doppler has a duty factor of 100%
59

Distance measurements in ultrasound
• Propagation velocity is a particularly important value in clinical
ultrasound and is critical in determining the distance of a reflecting
interface from the transducer.
• Much of the information used to generate an ultrasound scan is
based on the precise measurement of time and employs the
principles of echo-ranging.
• If an ultrasound pulse is transmitted into the body and the time until
an echo returns is measured, it is simple to calculate the depth of the
interface that generated the echo, provided the propagation velocity
of sound for the tissue is known.
60

Echo ranging
• Range equation in Pulse Echo US is
used to determine the locations /
distances of various interfaces.
> Distance = (go-return time x speed of
sound)/2
>For soft tissues, The distance to an
interface (in mm) equals 0.77
multiplied by the round trip time of
the sound pulse (in microseconds).
• Therefore,the distance between the
transducer and the object causing
the echo can be calculated as
follows;
d=1/2 ct

Application of PRP,PRF and Duty factor

Concept of acoustic interface and Impedance
• The transmitted sound encounters
numerous reflectors which are also
termed acoustic interfaces.
• The acoustic interfaces are
reflecting boundaries formed by
changes in tissue characteristics,
such as from soft tissues to blood
or fat to muscle.
• These changes produce echoes that
return to the transducer.
63

Acoustic Interface and impedance
• Acoustic interface: The reflecting
boundary that is formed between two
materials with different acoustic
impedance characteristics
• These interfaces are responsible for
the reflection of variable amounts of
the incident sound energy.
• The greater the differences in the
acoustic impedance values of the two
materials forming an interface
determine the strength of the
returning echo.
• The amount of reflection or
backscatter is largely determined by
the difference in the acoustic
impedances of the materials forming
the interface
64

Acoustic impedance (Z) of various tissues
• Acoustic impedance
(Z) is determined by
product of the
density (ρ) of the
medium propagating
the sound and the
propagation velocity
(c) of sound in that
medium (Z = ρc).

Acoustic Interface and impedance
• Therefore, Acoustic interface:Is
the reflecting boundary that is
formed between two materials
with different acoustic
impedance characteristic.
• By keeping track of the time
between transmitted pulses and
returning echoes (transit time),
the US system leaves the
locations of the various
interfaces.
66

Reflection coefficient
• Reflection coeffient is a parameter that
describes how much of a wave is
reflected back by an impedance
discontinuity in the transmission medium
• It is a measure of the ratio of the
intensity of the wave reflected off the
boundary between two media to the
intensity of the incident wave
• (a) Weak interfaces have echo reflection
coefficients in the 0.01 (1%) range.
• (b) Medium strength interfaces have
echo reflection coefficients in the 0.5
(50%) range
• (c) Very strong interfaces have echo
reflection coefficients in the 1.0 (100%)
range
67

Reflection coefficient
• Interface materials and echo
strength;
(a) Soft tissue to muscle - weak
(b) Soft tissue to bone - medium
(c)Soft tissue to air - very strong
68

Coupling agents
• A coupling agent is necessary to
ensure good contact between the
transducer and the skin to enable
image formation avoid artefacts
caused by presence of air between
them
• The best coupling agents are water
soluble gels which are commercially
available
• Water and disinfectants are suitable
for very short examinations
• Oil has disadvantage of dissolving
rubber or plastic parts of the
transducer.

The use of Gel in ultrasound
• Without gel,the air tissue interface
at the skin surface results in more
than 99% of the US energy to be
reflected at this level.
• This is primarily due to the very
high accoustic impedence of air.
• The use of Gel between the
transducer and the skin surface
greatly increases the percentage of
energy that is transmitted into and
out of the body there by allowing
imaging occur.

In general, the echo signal amplitude from the insonated tissues
depends on;
• the number of scatterers per unit volume,
• the acoustic impedance differences at the scatterer interfaces,
• the sizes of the scatterers, and
• the ultrasonic frequency.
71

Acoustic shadowing
• Tissues that produce a large
acoustic impedance change, will
result in a strong reflection,
causing a significant reduction in
the amount of sound that is
available for further transmission
through the patient, resulting in
acoustic shadowing.
• Acoustic shadow; absence of
displayed echoes beneath the
interface
72

Acoustic shadowing
• Acoustic shadow is seen in relation to
BONE, gallstone, calcified plague, and
bowel gas.
• Contrast agents can produce acoustic
shadows.
• Matching layers on transducer surface
(front face); reduces internal
reflections at the transducer surface;
increasing transducer efficiency
73

Hyperechoic and Hypoechoic
• Hyperechoic (higher scatter
amplitude) and hypoechoic (lower
scatter amplitude) are terms used for
describing the scatter characteristics
relative to the average background
signal.
• Hyperechoic areas usually have
greater numbers of scatterers, larger
acoustic impedance differences, and
larger scatterers.
74

ULTRASOUND PHSYCICS
JR
ULTRASOUND IMAGING AND INSTRUMENTATION

Introduction
• Ultrasound is the name given to high
frequency sound waves, over 20,000
cycles per second (20 kHz)
• These waves are inaudible to humans
• Can be transmitted in beams and are
used to scan the tissues of the body,
• Ultrasound pulses of the type produced
by the scanners described here are of a
frequency from 2-10 MHz
77

• A pulse-echo system is used for
ultrasound imaging.
• Based upon the pulse echo principle
with ultrasound piezoelectric crystals,
ultrasound transducer coverts:
- Electricity into sound energy=Pulse
- Sound energy into electricity= Echo
• So,P-E system refers to an ultrasound
system that is used for US imaging by
periodically transmitting and receiving
short duration sound pulses.
• It create sound pulses, retrieves
reflections and present audio and
visual information for interpretation
78

The Pulser
• Produces electric pulses that drives the
transducer (T) through the beam former
• It also includes a clock that determines the
pulse repetition frequency (PRF) and
synchronizes the various components of
the instrument
• Excitation voltage from the pulser can be
varied in some systems which affect
amount of acoustic energy leaving the
transducer.
81

Beam Former
• Performs all task necessary
for beam steering, transmit
focusing, dynamic aperture
and any other additional
timing for phase arrays
82

Receivers
• The receiver performs the following
functions so as to provide the initial
processing of the received analog
echo information
- Amplification
- Rejection
- Compensation
- Compression
- Demodulation

Receivers
• Amplification is the
conversion of the small
voltages received from the
transducer to larger ones
suitable for processing and
storage

Receivers
• Rejection eliminates the
smaller amplitude voltages
pulses produced by weaker
echoes (multiple scattering
from within tissue) or
electronic noise
86

. Receivers
• Compensation ( Gain Compensation or
Depth Compensation)
• Equalizes differences in received echo
amplifications because of reflector depth
• The attenuation depend on depth
• Reflectors with equal reflection
coefficients will not results in equal
amplitude echoes arriving at the
transducer if their travel distances are
different.

Receivers
• Compression; Is the process of
decreasing the differences
between the smallest and
largest amplitudes, This is done
by logarithmic amplifiers that
amplify weak inputs more than
strong one

Receivers
• Demodulation; is the
process of converting the
voltage delivered to the
receiver from one form (radio
frequency) to another (video).
• This is done by rectification
and smoothing
89

90
Overall receiver process
• The receiver processes the data streaming from the beam former, steps
includes time gain compensation (TGC), Dynamic range compression,
rectification, demodulation and noise rejection.
• The user has the ability to adjust the TGC and the noise rejection level.

Image storage
• To store echo information (from receiver) in
digital memory the demodulated voltage
amplitudes representing echoes must pass
through an analog-to- digital converter (
Digital scan Converter)
• Digital pre-processing is performed to
assign numbers to echo intensities as a
signal format that can be fed to standard TV
monitor
91

DSC
Analogue to Digital Converter
• The echo amplitude and position
information is normally analog, it does
not represent discrete values.
• A to D converter, convert this analog
information to digital, and it also assigns
discrete shades of gray to the incoming
echo amplitudes- This is called pre
processing
• Selectable pre-processing permits the
operator to vary the texture of the
displayed image, before storing it into
digital memory.
92

DSC
Digital to Analogue Converter
• Digital information stored in digital
memory of DSC has to be
changed to analogue for it to be
displayed on a monitor. This is
done by Digital to Analogue
Converter.
• D to A converter; Determine the
brightness to be displayed for each
gray scale level(post-processing) 93

DSC, D to A converter...
• Selectable post processing permits
the operator to vary the emphasis that
is given to various gray scale ranges.
• It affects both stored and live images
• Other image functions that occur prior
to digital memory are:-
-Write Magnification
-Read magnification
94

DSC, Write-
Magnification
• Operator can change the size of
displayed image.
• This is usually done before storage in
the digital memory.
• In this case there is no reduction in
the number of displayed pixels.
• Controls associated with
magnification include:-
scale, size, field of view, depth, etc
95

DSC, Read -
magnification
• Occurs after the digital memory.
• Permits an operator to enlarge a
selected area of the display by
enlarging each pixel.
• Results in fewer pixels on the
display.
• Image seen are coarse c.f. to those
of write magnification technique.
• Controls associated with read
magnification include
ZOOM,MAG,SIZE
96

DSC’s Digital Memory
• Is an electronic device that stores
discrete signals.
• During conversion process, the
information is temporally stored in the
scan converter’s digital memory
• Typical digital memory is configured with
image-matrix memory size 512×512,
which represents the number of rows and
columns of digital picture elements
(pixels)
97

Cont..
• CINE-LOOP; is a result of employing
multiple digital memories, it is a
real-time recording and play back
of multiple image frames
• Color images used for doppler studies
increase the storage requirements
further because of larger numbers of
bits needed for color resolution
24bits/pixel
98

ULTRASOUND PHSYCICS
Image storage,display and recording
JR

Outline
• Digital scan converter
• Analogue to digital converter
• Digital to analogue converter
• Write magnification
• Image recording

Pulse – echo ultrasound system
Basic components of pulse-echo
systems
• Pulser
• Transducer
• Timer
• Receiver
• Image storage
• Display and record
103

Digital scan converter
• Is the heart of image storage
• It coverts echo-amplitude
information from the review along
with echo position information
from its origin formal into a signal
format that can be seen on
standard TV.
• During the conversion process, the
information is temporality stored in
the scan converters memory.
• To store echo information (from
receiver) in digital memory the
demodulated voltage amplitudes
representing echoes must pass
through an analog-to- digital
converter ( Digital scan
Converter)
104

DSC’s Digital Memory
• Digital memory is an electronic device
that stores discrete signals.
• During conversion process, the
information is temporally stored
in the scan converter’s digital
memory
• Digital memory is configured with an
image-matrix memory size of 512 x
512.
• This represents the No. of rows and
columns of digital picture elements or
pixels
• The greater the No. pixels, the better
the spatial resolution.
105

Digital scan converter (Digital memory)
• Each pixel in a 512 x 512 matrix
represents one of 262,144 discrete
horizontal – vertical echo locations
each displayed as a specific shade
of gray.
• Employing multiple digital
memories permits “cine-loop”
which is real time recording and
play back of multiple image frames.

Digital scan converter (A to D converter)
• To store echo information (from receiver) in digital
memory the demodulated voltage amplitudes
representing echoes must pass through an analog-
to- digital converter ( Digital scan Converter)
• Analog to digital (A to D converter); changes the
echo amplitude and position information from
analog to discrete values; prior to being fed into
scan converters digital memory.
107

Digital scan converter (A to D
converter)
• A to D converters assign discrete
shades of gray to the incoming echo
amplitudes.
• Process called pre-processing
• Digital pre-processing is performed to
assign numbers to echo intensities as
a signal format that can be fed to
standard TV monitor
• Pre-processing enables sonographer
to vary textures of the image.
• Pre-processing occurs before digital
memory, changing selection does not
affect image information once it is
stored.
108

Analog to digital converter
• The maximum number of possible
gray scale levels depends on the
number of bits (binary digits) of
information that can be stored in
digital memory for each horizontal
– vertical location.
• The greater the number of gray
levels, the better the contrast
resolution.
• Typical bit values are 4,5,6,7,8,9
and 10 to provide
16,32,64,128,256,512 or 1024 gray
scale levels respectively.
109

Digital to Analogue
Converter
• TV monitor displays analog information.
• Digital information stored in scan converters
digital memory is fed to a digital analog converter
so that it can be seen on TV monitor.
• D to A converter determines the brightness level
that will be displayed for each gray scale level.
• Digital information stored in digital memory
of DSC has to be changed from digital to
analogue for it to be displayed on a
monitor. This is done by Digital to Analogue
Converter,ans is called Post processing
• D to A converter; Determine the brightness to
be displayed for each gray scale level
110

Post processing
• Post processing permits the
operator to vary emphasis that is
given to various gray scale ranges.
• Post-processing occurs after the
digital memory, it can affect stored
and live images.
• Most US systems have some form
of selectable post processing.

DSC, D to A converter...
• Selectable post processing permits
the operator to vary the emphasis that
is given to various gray scale ranges.
• It affects both stored and live images
• Other image functions that occur prior
to digital memory are:-
-Write Magnification
-Read magnification
112

Write- Magnification
• Operator can change the size of
displayed image.
• Permits the operator to electronically
change the size of the displayed image
prior to storage in the digital memory.
• This is usually done before storage in
the digital memory
• In this case there is no reduction in
the number of displayed pixels.
• Controls associated with write
magnification include: scale, size field
of view, depth and RES.
113

Read -magnification
• Occurs after the digital memory.
• Permits an operator to enlarge a
selected area of the display by
enlarging each pixel.
• Results in fewer pixels on the
display.
• Image seen are coarse c.f. to those
of write magnification technique.
• Controls associated with read
magnification include
ZOOM,MAG,SIZE
114

Image recording
• Various methods are used to RECORD ultrasound
images.
• Hard copy: a method of permanently presenting or
recording image data on paper or film.
• Photographic method – Polaroid films, colour, black
and white.
• Multi-image (multi-format) cameras – can record
multiple images on single sheet of film or
photographic paper.
• Laser imagers: produce multiple images on a single
sheet of film, using non-photographic techniques.
• They capture video frames in digital memories.
• The stored digital information is then used to control
the intensity of laser beam that exposes the film;
using sequential scanning technique

Image recording cont..
• Thermal printers: use heat sensitive paper as a
recording medium.
• The paper passes over a multi-element thermal
head.
• The temperatures of the various elements are
controlled by the TV. Signal information fed from
ultrasound system.
• Use magnetic medium.
• Video tape recorder can store live or frozen
images.
• Magnetic and optical disk recorders are
designed to record frozen images (the
recordings are not hard copy).

ULTRASOUND PHSYCICS
Resolution and Attenuation
JR

RESOLUTION AND ATTENUATION
• In ultrasound,the major factor that
limits the spacial resolution and
visibility of a detail is the volume of
acoustic pulse
•RESOLUTION
Ability to appreciate two closely
placed objects as separate.
It is a wavelength dependent
•It includes
-Spacial resolution
-Temporal resolution
119

Spatial resolution
• Spatial resolution Is the ability to
distinguish two distinct and separate
objects that are close
together/considered in two
dimensions,axial and lateral
• It is divided into:
1.Axial resolution: Which is the ability to
distinguish two separate objects
placed along the axis of the
ultrasound beam
2.Lateral resolution: Which is the ability
to distinguish two separate objects
placed in the direction perpendicular
to the beam axis

Axial resolution
Axial resolution (aka longitudinal, range
or depth resolution)
• Is the minimum required reflector
separation of depth along the
direction of propagation required to
produce separate reflections.
• In ultrasound refers to the ability to
discern two separate objects that are
longitudinally/parallel to each other
along the ultrasound beam.
• Axial resolution is generally around
four times better than lateral
resolution.
121

Axial resolution
• The most important determinant of the
axial resolution is the length of the
transmit pulse to form the beam.
• This is called Spatial Pulse Length(SPL)
• Axial resolution is affected by spatial pulse
length
• The Shorter the SPL the better the
resolution
• Axial resolution is defined by the
equation:
Axial resolution = ½ ⨉ spatial pulse
length.
• The spatial pulse length is determined by
the wavelength of the beam and the
number of cycles (periods) within a pulse.
122

Factor affecting axial resolution
• Transducer damping or backing
material which produce shorter
wavelength results in higher axial
resolution.
• Higher frequencies produce
shorter wave length which results
into a higher axial resolution.
123

Factor affecting axial resolution
• Other operator controlled
determinants of axial resolution:
• Transmit power: the greater the
amplitude of the voltage striking the
crystal, the longer the ringing time
and the longer the SPL, leading to a
slight decrease in axial resolution
• Received gain settings: increases SPL
of the voltage signals generated by
the returning echoes, the higher the
gain the poorer the axial resolution
• Field of view settings: effect the
display of pixels per unit area, a
smaller field of view makes best use of
the available scan converter memory

2. Lateral resolution in ultrasound i.e
Lateral,angular.tranverse ,azimuthal
{LATA}
• Ability to distinguish two separate
objects that are perpendicular to each
other along the ultrasound beam.
• It is primarily determined by the width
of the ultrasound beam.
• If the lateral distance between two
objects is > beamwidth at that depth then
they will be resolved
• The narrowest beamwidth at focus is
achieved by using the highest frequency
for depth and largest possible transducer
aperture:
125

Lateral Resolution
• Lateral resolution (azimuthal
resolution) is the ability to
discern objects lateral to each
other.
• It is the minimum distance that
can be distinguished between
two reflectors located
perpendicular to the direction of
the ultrasound beam. Lateral
resolution is high when the
width of the beam of ultrasound
is narrow.
126

Factors affecting lateral resolution
• Beam width varies with depth.
• With higher frequency the near
field is longer and the beam width
diverges less in the far field.
• The frequency is inversely related
to beam width i.e as frequency
increases beam width decreases.
• Increasing the amount of focal
zone narrow the beam width which
increase the lateral resolution.
• Increasing the depth decreases
lateral resolution.
127

Ultrasound Beam
• The area through which the sound
energy emitted from the ultrasound
transducer travels is known as the
Ultrasound beam.
• It can be subdivided into two regions: a
near field (Fresnel zone) which is
cylindrical in shape and a far field
(Fraunhofer zone) where it diverges
and becomes cone-shaped.
• The actual shape of the beam depends
on a number of factors including the
diameter of the crystal, the frequency
and wavelength, the design of the
transducer and the amount of focusing
applied to the beam.
129

130
• At the transducer, beam width is ~ equal to the width of the
transducer.
• The beam converges to its narrowest width which is half the width of
the transducer at a perpendicular distance from the transducer .
• When entering the Fraunhofer's zone, the beam diverges such that it
becomes the width of the transducer. Here, lateral resolution
decreases.
• Lateral Resolution is high when Fresnel’s zone is long.
• Factors that increase near-zone length include short wavelength, high
frequency transducer and large aperture (wide element width).

Focusing:
• Piezoelectric elements in a transducer
operate at different times and can
narrow the pulsed beam with
improved lateral resolution. This
process of focusing leads to the
creation of a focal region within the
near zone but not the far zone.
• High frequency transducers are
designed with a short focus.
• Low frequency transducers are
designed with a long focus
• Transducer arrays produce sound
beam with adjustable focal zones
131

• Although high frequency
transducers provide improved
resolution, there is great
attenuation.
• Factors contributing to attenuation
include: reflection, divergence,
absorption and scattering.
• Conversion of transmitted energy
to other forms of energy such as
heat (absorption) is the primary
means by which attenuation of
ultrasound occurs in biologic tissue
with scatter comprising the other
significant factor.
132

3. Temporal resolution in ultrasound
• TEMPORAL RESOLUTION is the ability to
image movement and moving structures in
real time.
• Represents the ability of the ultrasound
system to distinguish between
instantaneous events of rapidly moving
structures, depends on:
• Represents the extent to which an
ultrasound system is able to distinguish
changes between successive image
frames over time (i.e. movement).
• The ability to accurately track how an
object has moved over time.
134

. Temporal resolution in ultrasound
• Affected by frame rate.
• Frame : one ultrasound image.
• Frame rate number of frames displayed per second.
• Frame rate increased by shallower depth.
Factors which increase frame rate, and hence improve temporal
resolution include :
• Increased propagation speed of sound waves through the tissue.
• Reduced depth of field (as it shortens pulse travel distance).
• Reduced number of beamlines per field.
135

CONTRAST RESOLUTION
• This is the important ability to
distinguish between different
echo amplitudes of adjacent
structures. This determines the
ability to identify differenences
in adjacent tissues based on
their echogenicity; and subtle
differences in echogenicity are
readily deteriorated by poor
image quality.

• Contrast resolution is improved by
reducing spurious artifacts and also
by reducing speckle in particular in
soft tissues/solid organs that
reduce the ability to see
differences in echogenicity:
- contrast resolution is improved by
a variety of pre-processing
(persistence, spatial compound
imaging, tissue harmonic imaging)
methods, and in particular
optimisation of machine settings
(frequency, depth, gain, TGC, and
dynanmic range)

Resolution
In conclusion, resolution of ultrasound information is affected by
several factors considered above. A thorough understanding of these
factors will enhance both quality and interpretation of data contained
in the images.
138

ATTENUATION
• The amplitude and intensity
of ultrasound waves decrease as
they travel through tissue, a
phenomenon known
as attenuation.
• High frequency transducers
provide improved resolution
however, there is great attenuation
(reduction in sound energy)
139

• Factors contributing to
attenuation include:
• absorption
• scattering
• reflection
• refraction
140

absorption
• Conversion of transmitted energy
to another form of energy such as
heat (absorption) is the primary
means by which attenuation of
ultrasound occurs in biologic tissue.
• Absorption and frequency are
directly related i.e the higher the
frequency the higher the amount
of particle motion,results higher
amount of attenuation and so
energy transferred to heat.
• Attenuation is higher in air and
lower in water.
141

scattering
• If a sound waves hits a surface
with irregularities the sound
wave is redirected in multiple
directions.
• Can be organized i.e the sound
wave hit the surface is smaller
than the size of its wave length
- redirected uniformly.
• Disorganized i.e backscatter
-The sound wave is scattered in
different direction.
• Scattering increases with
increasing frequency.
142

reflection
• A sound waves goes into the
tissue type and a portion of the
sound wave return to the
trasducer as an echo.
• The amount of reflection
depends on the type of tissue
that the sound wave incounter.
• Strong reflection - hyperechoic
• Weak reflection - hypoechoic
• No reflection - anechoic
143

refraction
• Bending of the sound wave as it
travel from one media to
another
144

• Given a fixed propagation distance,
attenuation affects high frequency
ultrasound waves to a greater
degree than lower frequency
waves.
• It describe the reduction in beam
intensity with distance tavelled.
• This dictate the use of lower
frequency transducer for deeper
areas of interest.
145

Attenuation coefficient
• The intrinsic property of a medium to
attenuate sound waves at a
given frequency may be represented
by its attenuation
coefficient (represented by the Greek
letter alpha (α), and
• Measured in decibel per centimeter to
each megahertz i.e dB / [mHz x cm]).
• Attenuation coefficient varies widely
between different tissues and media.
147

Bioeffects of Ultrasound
• Refers to the potential adverse
effects of ultrasounds on human
tissue. These are primarily two;
thermal and mechanical effects.
1. Thermal Effects:
Due to conservation of energy, all
the sound energy attenuated by
tissues must be converted to
other forms of energy of which
the majority is heat
149

• Heat is produced by vibrations
within tissues.
• Heat is negligible when using pulse
echo systems for medical
diagnostic purposes.
2. Mechanical
The mechanical bioeffect of
ultrasound refers to damage caused
by the actual oscillation of the sound
wave on tissue.
150

• The most common is referred to
as cavitation and is caused by
the oscillation of small gas
bubbles within the ultrasound
field. In certain circumstances,
these bubbles may grow in size
or collapse generating very high
energies to adjacent tissue.
• The Mechanical Index (MI) is a
value that attempts to quantify
the likelihood of cavitation by an
ultrasound beam (<1.9 is safe).
151

INTENSITY MEASUREMENT AND BIOEFFECTS:
• Stable cavitation may result in
some stress to cells, but this is
minor.
• Transient cavitation, occurs when
formed bubbles collapse or burst,
there is a potential for cellular
damage here.
• Cavitation is due to gas bubbles
production in tissue especially at
higher US intensity.
• It is confirmed no bio effects at
SPTA intensities below 100mW/cm2
(0.1 watt per square centimeter).
153

INTENSITY MEASUREMENT AND BIOEFFECTS:
• On US’s display, out put power or intensity is indicated as:
- a percentage of the system maximum
- As a decibel (dB) difference
- Estimated SPTA intensity
- Mechanical index
- Thermal index
• The thermal index value is used as a guide in determining frequency of bio
effects.
154

INTENSITY MEASUREMENT AND BIOEFFECTS
(WHERE)
• Spatial peak intensity (SP) the
maximum intensity occurring in an
ultrasound beam averaged over
the pulse repetition [the time from
the beginning of one pulse to the
beginning of the next]
-its measured at the beam centre.
• Spatial average intensity (SA) is the
average intensity across the beam.
• Beam uniformity ratio (BUR) is the
ratio of SP to SA.
• SP has great value than that SA.
155

INTENSITY MEASUREMENT AND BIOEFFECTS
(WHEN)
• Temporal peak intensity is
measured when the pulse is
present.
• Temporal average (TA) intensity is
measured when the pulse is
present and when it is not present.
• For TA method the result is
affected by the duty factor, in case
of pulse-echo measurement TP>TA.
156

INTENSITY MEASUMENT AND BIOEFFECTS
(WHEN)
• TA intensity is measured with a
force balance.
• TP intensity and beam profile
can be measured by a
hydrophone.
• TP is also known as Pulse
average (PA)
157

ULTRASOUND PHSYCICS
ULTRASOUND ARTIFACTS
JR

Introduction to Artifacts
• US image processing assumes that the
detected echoes are originated from
the main ultrasound beam. The
ultrasound beam is composed by a
main ultrasound beam (that narrows
as it approaches the focal zone an
then widens again distal to the focal
zone
• A structure that is strongly reflective
located outside of the main
ultrasound beam may generate
echoes that are detected by the
transducer and may be interpreted as
being originated from within the main
beam

• understanding the physical basis of US image
formation is critical to understanding US
artifacts and thus proper image interpretation.
• Artifacts are frequently encountered at clinical
US, and while some are unwanted, others may
reveal valuable information related to the
structure and composition of the underlying
tissue
• They are essential in making ultrasonography
(US) a clinically useful imaging modality but also
can lead to errors in image interpretation and
can obscure diagnoses.
• The ability of a radiologist to understand the
fundamental physics of ultrasound, recognize
common US artifacts, and provide
recommendations for altering the imaging
technique is essential for proper image
interpretation, troubleshooting, and utilization
of the full potential of this modality

• In ultrasound, perhaps more than in any
other imaging method, the quality of the
information obtained is determined by the
user’s ability to recognize and avoid artifacts
and pitfalls.
• Many imaging artifacts are induced by errors
in scanning technique or improper use of the
instrument and are preventable.
• Artifacts may cause misdiagnosis or may
obscure important findings. Understanding
artifacts is essential for correct interpretation
of ultrasound examinations
• Many of these artifacts can be understood as
deviations from the assumptions made in
generating the image

• First, it is assumed that the ultrasound pulse
travels in a straight line, giving rise to echo
signals within the same narrow beam.
• Second,Echo returns after a single reflection
• Second, it is assumed that the speed of sound in
tissues is constant at 1540 m/sec and that the
time delay between the transmitted pulse and
the received echo is directly related to the depth
at which the echo is generated.
• Echo originates from within main beam utrasound
wave
• Acoustic energy is uniformly attenuated
• Depth of object is related to time for ultrasound
pulse to return to transducer
• However, all of these assumptions are violated to
various degrees, resulting in a range of different
imaging artifacts.

Wave propagation/Multiple echoes artifacts
• US image processing assumes that an echo returns to
the transducer after a single reflection and that the
depth of an object is directly related to the amount of
time that an ultrasound pulse takes to return to the
transducer.
• However, in the presence of two parallel highly
reflective surfaces, the echoes generated from a
primary ultrasound beam may be repeatedly reflected
back and forth before returning to the transducer.
• Therefore, multiple echoes are received and displayed
• Under this category we have
- Reverberation artifacts
- Ring down artifacts
- Mirroring
- Comet tail artifacts

1.Reverberation artifacts
• Reverberation artifacts arise when the beam encounters
two highly reflective interfaces in parallel.
• The echo that returns after a single reflection will be
displayed in its correct location
• Instead of the beam reflecting off a single surface and
producing a strong echo that returns to the transducer, the
ultrasound beam is reflected between the interfaces back
and forth multiple times
• These reflected echoes are interpreted as occurring at
increasing depths since they take longer to be received by
the transducer.
• This appears as multiple bright parallel lines at uniform
intervals that decrease in intensity with increasing depths
• But the sequential echoes will take longer to return, and
the ultrasound processor will recognize them as falsely
deeper in the US field
• In clinical imaging, this is seen as multiple linear reflections
• Reverberation artifact may be minimized by selecting a
different imaging plane to avoid the reflective surfaces or
by changing the angle of insonation

Reverberation artifacts cont..
• In Reverberation artifacts, when the
sound beam returns to the transducer
the sound is not ‘absorbed’ into the
transducer; instead the pressure wave
hits the crystals and causes them to
produce the electrical pulse.
• If there is enough energy remaining in
the wave, it is reflected down into the
tissue again (and again and again) like
an echo.
• A reverberation artifact is the multiple
representation of the same interface, at
different depths in the display.
• The structure displays the exact same
shape, but decreases its intensity as it
displays into the image.

2.Comet Tail Artifact
• Comet-tail artifact is a special subtype of reverberation
artifact caused by highly reflective interfaces that are so
closely spaced that the individual echoes are not
discernible.
• In addition, attenuation of more delayed echoes results
in a progressively decreased amplitude and width with
increasing depths
• Therefore, this artifact is based on the same principles
of reverberation but resembles a comet tail, appearing
as a tapering echogenic triangle or cone distal to a
strongly reflecting structure.
• comet-tail artifacts arise from closely spaced interfaces
that are not resolvable and decrease in width with
depth.
• Comet-tail artifacts are clinically useful, particularly in
identifying cholesterol crystals in adenomyomatosis of
the gallbladder or inspissated colloid in benign colloid
nodules of the thyroid. They may also be seen with
small calcifications and metal objects, such as foreign
bodies and surgical clips

Comet Tail Artifact
• These artifacts are a form of
reverberation .
• In this case, the two reflective
structures are very close and thus
sequential echoes are close together
• They are relatively bright echoes that
taper and decrease with depth, which
gives rise
• to the name ‘comet tail’. They are
typically seen behind surgical clips,
IUCDs, foreign bodies – such as
shotgun pellets – and needles when
performing vascular access.

The ring-down artifact
• The ring-down artifact is often similar in appearance with the
comet tail artifact, and in the past they were thought to be the
same.
• Currently they are known to have separate mechanisms.
• The ring-down artifact results of the resonant vibrations within
fluid trapped between a tetrahedron of air bubbles.
• These vibrations create a continuous sound wave that is
transmitted back and is displayed as a line or series of parallel
bands extending deeper to a gas collection.
• These artifacts are the result of resonance, induced by the
ultrasound beam when it strikes a group of air bubbles; it is
similar to the comet tail artifact in appearance, except that the
ring down artifact does not usually decrease in intensity or
brightness

• Ring-down artifacts appear similar to reverberation
artifacts, and although they may resemble comet tail
artifacts, the mechanism is distinctly different.
• Instead of closely spaced reflective interfaces, ring-down
artifacts arise from resonant vibrations within trapped
tetrahedrons of air bubbles .
• These resonant vibrations produce a continuous, though
decaying, sound wave transmitted back to the receiver,
appearing as a streak or series of parallel bands deep to a
focus of gas

• Whereas artifact from normal air-filled
structures may obscure evaluation of deeper
structures, ring-down artifact can be useful in
identifying abnormal foci of air.
• The presence of ring-down artifact may
indicate significant underlying disease, such as
pneumoperitoneum or portal venous gas.
• Similarly, echogenic foci and ring-down artifact
may also be seen with emphysematous (gas-
forming) infections and abscesses.
• Alternatively, the presence of air can be a
reassuring sign if identified in the appendix in
cases of suspected appendicitis. Ring-down
artifact contributes to the phenomenon of
dirty shadowing, which is discussed later.

Mirror Image Artifacts
• Mirror image artifacts result from the false
assumption that an echo returns to the transducer
after a single reflection.
• As opposed to reverberation artifacts, which occur
between two strong reflectors in parallel, mirror
image artifacts are produced when the beam
encounters a target after being reflected off a single
strong specular reflector
• A portion of the beam is reflected from the target
back along its transmitted course, again off the
specular reflector and back to the transducer.
• The second image of the target is therefore
generated along that path, deeper than the true
location owing to the increased time to echo return.

Mirror Image Artifacts
• The prototypical example is a liver
lesion adjacent to the diaphragm,
whereby the transmitted beam is
reflected off the diaphragm and
encounters a liver lesion that
subsequently reflects it back
toward the diaphragm and then to
the transducer.
• The resultant image contains two
lesions equidistant from but on
opposite sides of the diaphragm. In

Mirror artifact
• This artifact is also generated by the same assumption
that an echo returns to the transducer after a single
reflection.
• In this case, the main US beam encounters a highly
reflective interface(usually the pleural-air interface),
then the reflected echoes encounter the end of a
structure and are reflected back toward the reflective
interface before being reflected to the transducer.
• The image processor displays a duplicated structure
equidistant from but deep to the strongly reflective
interface
• At imaging, this usually happens at the level of the
diaphragm, with the pleural-air interface acting as the
strong reflector

Beam characteristics artifacts
Secondary Lobe Artifacts :
• Secondary lobe artifacts arise from reflections of
unwanted ultrasound energy directed off-axis
• from the main beam and include side lobes and
grating lobes. Most of the sound energy is
transmitted within the center of the primary
beam.
• However, a small amount (approximately 1%) is
emitted at several angles outside the primary
beam as side lobes, due to the radial expansion
and contraction of the individual transducer
elements.
• When these weak side lobes encounter a highly
reflective surface, they can be reflected back to
the transducer. Since US assumes that all
returning echoes arise from the primary beam,
the echoes are falsely displayed within beam
instead of at their true location.

Side Lobe Artifacts
• Because they are so low energy, they are
often not recognizable as discrete
structures when they occur in soft
tissues. However, in anechoic structures,
side lobes are sometimes seen as
hyperechoic objects or diffuse echoes.
• A common example of a side lobe artifact
is the appearance of pseudosludge
within the gallbladder from adjacent gas
in bowel loops reflecting side lobes.
• Side lobe artifacts can be reduced by
repositioning the patient, changing the
transducer angle, or reducing the gain
• In practice, imaging from multiple angles
is helpful in discriminating artifact from a
true finding.

Granting lobe artifacts
• Grating lobe artifact occurs in a similar manner to side lobe
artifacts, in which far off-axis grating lobes result in an error
in positioning the returning echoes. Compared with side
lobes, grating lobes occur at more oblique angles (up to
90°) relative to the primary beam and have a different
origin
• When grating lobe artifacts occur, they are at higher
amplitudes than side lobe artifacts and are closer in
intensity to that of the primary beam.
• These artifacts are dependent on the spacing between
individual transducer elements and will not occur if the
interelement spacing is less than one-half the wavelength .
• Therefore, their effect will depend on the particular
transducer being used and the transducer frequency

Beam-width Artifact
• Beam-width artifact is a manifestation of lateral
resolution, which refers to the ability to discriminate two
closely spaced points at the same depth within the
imaging plane as distinct.
• Lateral resolution is limited by the beam width, which
varies with depth and is narrowest at the focal zone
• Therefore, lateral resolution is optimal at the focal zone
and deteriorates in the near and far fields.
• Beam-width artifact refers to the lateral blurring of a
point target that occurs as echoes from the same target
are insonated at adjacent beam positions.
• Similarly, if two adjacent point targets are separated by a
distance less than the beam width, they will appear as
one.
• The beam width decreases as the wavelength
decreases; thus the beam width is narrower with higher-
frequency transducers. Focusing is a process used to
further improve the beam width so that it is narrowest at
the focal zone.

Velocity error artifacts(Refraction artifacts)
• Refraction is a phenomenon whereby a wave changes direction at an interface
between mediums having different speeds of sound. This change in direction is
predictable and is governed by the Snell law
• The ultrasound display assumes that the beam travels in a straight line, but when
a nonperpendicular incident ultrasound energy encounters an interface between
two materials with different speeds of sound, it changes direction. The degree of
this change depends on both the difference in velocity between the two media
and the angle of the incident ultrasound beam
• Refraction is often most pronounced at highly oblique interfaces, such as the
lateral aspect of a curved surface, and with interfaces between substances such
as fat and muscle with disparate speeds of sound.
• This basic principle underlies
- Misregistration,
- Ghosting, and
- Edge-shadowing artifacts.

Edge Shadowing artifact
• When the ultrasound beam hits
the curving edge of a highly
reflective structure at the ‘critical
angle’ (all sound is refracted, none
is reflected), there is no sound
returns to the transducer from that
line of sight, so we see a narrow
black line from the edge of the
structure down towards the back
of the image.
• This occurs in vessels, breast cysts,
at the edge of the gall bladder, and
sometimes at the edge of muscle
fascia or ligaments.

Edge Shadowing artifact
• Edge shadowing (defocusing) is a refractive artifact
that occurs at the edge of a large curved boundary
with a different speed of sound than that of the
surrounding tissues.
• This is often observed at the lateral edges of a
structure such as a cyst or soft-tissue mass and
appears as hypoechoic parallel lines projecting
distal to the edges of the structure (Fig 11d, 11e).
• As the angle of the boundary increases, the
ultrasound beam both reflects and refracts off the
surface. The intensity of the beam reaching the
tissue immediately distal to the edge of the curve is
therefore decreased and appears as a shadow.
• Clinically, it is important not to confuse edge
shadowing, such as that from a vessel in the renal
sinus, with true shadowing, which would suggest a
stone or other highly reflective focus. Edge
shadowing can be decreased or eliminated by
changing the angle of insonation.

Misregistration
• Misregistration is a basic
application of refraction, where
the actual location of an object
is altered by refraction of the
beam at an interface superficial
to it (Fig 11a).

Ghosting (duplication
• Ghosting (duplication) is a refractive
artifact where a deep structure may
appear in duplicate or triplicate by
refraction of the beam superficial to it.
• Ghosting is most often observed when
imaging pelvic structures through the
rectus abdominis and abdominal wall
fat.
• Because of the differences in the speed
of sound between the tissues and the
oblique angle of the incident beam, the
rectus abdominis acts as a lens, which
results in the interrogation of a deep
structure by two or three separate
beams (Fig 11b, 11c).
• The reflected echoes are refracted
again by the rectus

Speed of Sound Artifacts
• Speed of sound artifacts occur because of the
assumption that the propagation speed of
sound is constant at 1540 m/sec, when in fact
there is variability among different tissues.
• In creating the US image, the depth of an echo
is calculated from the time delay between
transmitting a pulse and receiving an echo,
with an assumption of a constant sound speed
of 1540 m/sec in soft tissue.
• If the speed of sound in the tissue is actually
less, such as in fat (1450 m/sec), it will take
relatively longer for the echo to be received,
and it will be displayed as if it originated from
a more distant target.
• Conversely, if the speed of sound is greater,
the echo will appear closer. This concept
results in several variations of related artifacts.

Speed of Sound Artifacts
• Boundary distortion (speed
displacement artifact) occurs when
a portion of the beam encounters a
region of differential velocity
superficial to a smooth interface,
giving rise to a distorted
appearance of the interface (Fig
12).
• This is commonly encountered
when imaging the liver through a
region of focal fat, where the
portion of the beam traversing the
fat takes longer to return

Attenuation Artifacts
• As an ultrasound wave travels
through tissue, its amplitude
reduces with increasing distance,
which is called attenuation.
• Although absorption is usually the
dominant mechanism to cause
attenuation, acoustic reflection,
scattering, and divergence of the
beam also contribute to attenuation.
• Tissues absorb acoustic energy and
convert it to heat in a process highly
dependent on tissue composition
and structure

Acoustic Shadowing
• Acoustic shadowing refers to the
reduction in echo strength distal to a
highly attenuating or reflective object
• clinically, is often seen with
calcifications, bone, and gas, and
appears as clean, partial, or dirty
shadowing.
• Clean shadowing commonly occurs distal
to larger calculi and bone and appears as
a dark anechoic band. In this case, most
of the energy is absorbed and is not
available for secondary reflections.
• Dirty shadowing is commonly seen distal
to a highly reflecting surface such as gas,
whereby multiple secondary reflections
produce low-level echoes that appear
within the shadow, similar to ring-down
artifact.

Increased Through Transmission
(Posterior Acoustic Enhancement)
• Increased through transmission
(posterior acoustic enhancement) refers
to the increased intensity of echoes
relative to surrounding tissues occurring
distal to a low-attenuating structure.
• This can be thought of as an
overcompensation of the TGC, with distal
tissues appearing echogenic
• Increased through transmission is helpful
in distinguishing cystic from solid
structures, but it must be remembered
that a homogeneous solid lesion with a
lower attenuation than the adjacent
tissues will also exhibit increased
through transmission.

Avoiding Artifacts
• Artifacts are an integral part of
ultrasound interaction with tissue;
they cannot be completely
avoided.
• Artifacts do not appear in images
singularly; one image may contain
many types of artifacts, and
artifacts in various combinations.
• For example, side lobe artifacts
may be superimposed on slice
thickness or beam width artifacts.

ULTRASOUND PHSYCICS
Ultrasound Modes
JR

Grey scale imaging modes
• In Ultrasound imaging, different
modes are used to image/examine
different parts of the body.
• These modes are controlled by an
operator
• There are three modes used to
display echoes returning to the
transducer in grey scale ultrasound
imaging,these are:
- A mode.
- B mode and
- M mode or TM mode.

A mode
• This is amplitude – modulated display
(Amplitude modulation)
• A-mode is the simplest type of
ultrasound.
• A single transducer scans a line
through the body with the echoes
plotted on screen as a function of
depth.
• It consists of x and Y axis,where X
represents the Depth and Y the
amplitude
• The displayed spikes are a relative
indication of intensity or strength of
returning echoes.

A MODE
• A scans are used to measure distances
and gives only one dimensional
information
• It represents time required for the
ultrasound beam to strike a tissue and
returnits signal to the transducer
• Therefore spikes represents the
location of tissues
• It’s current use is limited
• Currently its only used in opthalmic
imaging,echoencephalography and
echocardiology

M /TM mode
• Motion mode is also called Time
Motion mode
• M-mode is designed to
document and analyze tissue
motion such as heart valves.
• It is the display of one
dimensionalimage that is used for
analysing moving body parts
• Spikes are converted to dots
• The single sound beam is
transmitted and reflected echoes
are displayed as dots

M mode cont..
• Its most useful in
echocardiography and in
measuring fetal cardiac imaging
i.e documenting fetal heart rate
and activity.
• Limitations include only one
dimension is represented and
short time can be recorded

B mode
• B mode means brightness mode
• Displays in 2 dimensions (2D)
information about the cross
section
• Brightness depends upon the
amplitude or intensity of the
echo
• Large field of view is obtained

Doppler Imaging Modes
• Doppler imaging modes are used
to visualize velocities of moving
tissue i.e. evaluating blood
velocities
• Blood as it flows through a blood
vessel has certain characteristics
that can be evaluated by using
Doppler technique.

Doppler Imaging Modes
• Doppler modes
1. Continuous wave Doppler
2. Pulsed wave Doppler
• Spectral Doppler
• Color Doppler
• Power Doppler
To be discussed in Doppler
principles

ULTRASOUND PHSYCICS
DOPPLER PRINCIPLES
JR

Contents
• Principle of Doppler
- Doppler effect
- Doppler shift
- Doppler equation
• Flow imaging modes
 Continuous mode
 Pulsed wave mode
- Color Doppler,
- Spectral Doppler
- Power Doppler
- Normal Doppler pattern
- Resistance
- Critical stenosis
• Artifact
212

Principles of DOPPLER ULTRASOUND
• BASIS- DOPPLER EFFECT
• DISCOVERY: Physicist Christian Johann Doppler
in 1842
• First described by Christian Doppler in 1842
• Change in frequency and wavelength of the
wave due to motion between the source and
the receiver
• When a sound source and the reflector are
moving towards each other, the sound waves
are closer together and reach the receiver at
higher frequency than they were originally
emitted

Moving sound
• Concept of the sound source and the
receiver/listener
• One or both of the sound source and
receiver may be stationary or moving
• If the sound source moves towards
the listener,the sound perceived to
have a higher frequency/pitch
(POSITIVE DOPPLER SHIFT) and lower
frequency as it moves away of the
listener (NEGATIVE DOPPLER SHIFT)
• This change in frequency of sound due
to the relative motion of the source or
listener is called Doppler effect
• Doppler effect has vast applications in
ultrasonography

Definition of basic terms:
• Doppler effect: refers to change
in the frequency of sound waves
that occurs because of motion of
either a sound source, a sound
reflector, or a sound receiver.
• Doppler imaging systems are
used to locate a moving reflector
or scatterer of sound.
• In blood flow measurements, the
dominant scatterer of sound is
the red blood cell.
215

Clinical Application and indications of Arterial and
Venous Doppler Ultrasound
• Carotid Doppler Ultrasound
• Lower Extremity Doppler
ultrasound
• Portal Vein Doppler Ultrasound
• Renal Arteries Doppler
Ultrasound
• Aorta Doppler ultrasound
• Testis Doppler Ultrasound
• Ovarian Doppler Ultrasound

Assessment and Indication
Vessel Assessment Indication
Carotid a IMT, plaque & stenosis
Atherosclerosis, cerebrovascular
disease
Aorta Aortic aneurysm & disession Aortic aneurysm & disession
Renal a Assement of stenosis Renal artery stenosis
Peripheral a plaque, stenosis & aneurysm
Pseuoaneurysm, Peripheral artery
obstruction
Peripheral v Venous thrombosis
Deep vein thrombosis
Venous insufficiency
219

Main uses of vascular(Doppler) ultrasound:
• Identifying blood vessels
• Confirming the presence of
blood flow and direction.
• Detecting vessels stenosis and
occlusions
• Assessing the perfusion of
organs and tumors
• Characterizing blood flow
dynamics so as to detect
physiological abnormalities
220

Review vascular anatomy
• Anatomical Classification.
• Site
• Size and shape
• Function
- Flow/Supply and
- Drain

Importance of Vascular Ultrasound
• Diagnostic
- patency( stenosis, occlusion, flow
and direction).
• Therapeutic/Procedures
- Transjugular Intrahepatic
Portosystemic Shunt
• Follow up and Screening.
- assessment post organ transplant,
perfusion of organ( Tumors/Mets)

DOPPLER SHIFT
• Doppler shift: the change in frequency of a sound
wave due to motion of a reflector; mathematically,
it is the difference between the transmitted and
received ultrasound frequencies.
• Doppler shift: is the change in frequency between
us waves emitted by the transducer and us waves
returning to the transducer after reflection from
moving rbcs.
• Transmitted and received frequencies are in MHz
range
• Doppler shift frequencies are often in audible
range.
• By measuring the Doppler shift frequency (Doppler
shift) the direction and velocity of blood flow can
be determined.
• Frequency: is a measure of vibrations per second of
a wave form of energy.

Doppler Shift
• Doppler frequency (shift)
• F(d) = F(r) – F(o)
F(o) F(o)
F(r) F(r)

Doppler shift
• During Doppler operation; the
reflected sound has the same
frequency as the transmitted
sound if blood is stationary.
• Has a lower frequency (negative
shift) if blood is moving away
from the transducer.
• Has a high frequency (positive
shift) if blood is moving towards
the transducer.
225

Doppler shift
• By measuring the Doppler shift
frequency (Doppler shift) the
direction and velocity of blood flow
can be determined.
• Frequency: is a measure of
vibrations per second of a wave
form of energy.
• I.e. Doppler shift: is the change in
frequency of a sound wave due to
motion of a reflector;
mathematically, it is the difference
between the transmitted and
received ultrasound frequencies.
226

Remarks:
• An increase in frequency is
termed positive doppler shift;
flow direction is towards the
signal source (transducer).
• A decrease in frequency is
termed negative doppler shift ie
flow direction is away from the
signal source (transducer).
227

Doppler equations cont..
• An increase in blood velocity produces an
increase in the Doppler shift.
• An increase in the transmitted frequency
produces an increase in the Doppler shift
• Doppler angle: The angle between the
direction of propagation of the ultrasound
wave and the direction of blood flow.
• Doppler angle between 45° and 60° are
recommended, larger angles should be
avoided
• A Doppler shift is not produced when the
Doppler angle is 90°.
• A Doppler angle of 0° produces the
maximum possible Doppler shift.
228

DOPPLER EQUATIONS:
2(v)(ft )(Cos θ)
• ∆F = (Fr – Ft) = [ C ]
Where by:
• ∆f = Doppler shift
• V = blood velocity
• ft = transmitted frequency
• Q = Doppler angle
• C = sound velocity in tissues (assumed to be
1540 m/sec).
• 2 = constant indicating that the Doppler must
travel to the main target and then back to the
transducer.
229

Blood velocity
• V = C ∆f
2 ft Cos θ
• A Doppler system is capable of measuring
only the Doppler shift.
• It calculates the blood velocity.
• An increase in the Doppler shift produces an
increase in the calculated blood velocity.
• ft = transmitted frequency
• ∆f = Doppler shift
• V = blood velocity
• fo = transmitted frequency
• Q = doppler angle
• C = sound velocity in tissues (assumed to be
1540 m/sec).
230

Doppler equations
2(v)(ft )(Cos θ)
• From ∆F = (Fr – Ft) = [ C ]
• The largest frequency shift i.e. the largest
Doppler signal is when the angle θ = 00
• No Doppler shift occurs when the Doppler
us beam is directly perpendicular to blood
flow (θ = 900 , cosine 900 = 0).
• Velocity estimates become less accurate at
angle exceeding 70 degree
• < 60 degree angle is recommended for
accurate estimation
• Note: Errors in Doppler angle setting can
cause big errors in velocity calculations.
231

Determinants of frequency shift ∆F:
• Velocity (V) of moving rbcs.
• Frequency (Ft) of the transmitted Doppler
us beam.
• The cosine of the angle between the
incident Doppler us beam and direction
of blood flow (angle θ/Insonation angle).
• N.B.These determinants are proportional
to Doppler frequency shift.
• Parameters which are ‘constant:
• The transmission frequency (Ft) is
determined by the transducer used in
the examination
• The speed of sound in human tissues is
assumed to be constant (C) which is
1540 m/sec
232

DOPPLER ULTRASOUND
MODES/INSTRUMENTATION
1. Continuous wave Doppler
2. Pulsed wave Doppler
• Color Doppler
• Power Doppler

CONTINUOUS WAVE DOPPLER
• It’s a simplest Form
• Uses separate transmit and receive
crystals that continuously transmit
and receive ultrasound.
• Able to detect the presence and
direction of flow,- unable to
distinguish signals arising from
vessels different depths.
• Portable and inexpensive
• Uses: Cardiac scanners (HIGH
VELOCITIES IN AORTA or VELOCITIES
IN STENOSED HEART VALVE),

PULSED-WAVE (PW DOPPLER
• Electrical signal is applied to the
transmitter element at regular
intervals as pulses.
• Uses brief pulses of ultrasound
energy using only one crystal.
• The echo delay time (Te) can be
converted into distance and the
depth of the echo source can be
determined.
• The sensitive volume from which
flow data are sampled can be
controlled in terms of shape,
depth and position.

Pulsed wave Doppler (PWD):
• Allows measurement of the
depth at which a returning signal
has originated; it is achieved by
emitting a pulse of sound i.e.
pulse wave Doppler.
• Only echoes received at a precise
time i.e. from a specific depth are
sampled
• Pulsed wave spectral Doppler is
more sensitive in determination
of flow characteristics than
colour doppler.
236

01.Spectral Doppler
(definitions and wave forms)
• Spectral: a Doppler category in which
Doppler shifts are detected along a
single line in order to produce a precise
graphic representation of the velocity of
blood flow.
• Peak: the highest Doppler shift
frequency at a moment in time or in an
individual Doppler spectrum. It
corresponds to the fastest moving target
in the sample.
• Mean: the average Doppler shift
frequency in an individual Doppler
spectrum.
237

Doppler spectral display:
• Analysis is done rapidly; it is
displayed in real time (live).
• Horizontal scale (x – axis) is
time in secs
• Vertical scale (y – axis)
represents blood flow
velocity (m/sec or cm/sec)
238

Spectral Doppler
• Gate: an electronically controlled
device that controls PW
transmission and reception
intervals; a mark superimposed
over the 2D image, which is used
during pulse-Doppler to select
the depth along Ultrasound
beam where shifts are to be
detected.
240

Spectral Doppler
• When B-mode imaging is used a
single line cursor and a gate are
position on 2D image to select
the position, where Doppler
shifts are to be detected.
• Allows the user to establish,
gate length (width) or sample
volume, which will be used to
produce audible sounds and
spectrum analyzer
representations.
241

spectral Doppler
• Sample volume: the selected region within
the gate sensitive to the presence of
Doppler shifted echoes.
• A larger sample volume, may produce
spectral broadening- it includes
information from rapid flow central area
and slow flow area at the peripheral of the
vessel.
• Spectral broadening can also be caused by
turbulence when stenosis is present.
242

Duplex ultrasound scanning
• Duplex ultrasound instruments:
are real-time B-mode scanners
with build in Doppler
capabilities
• B-mode imager (Outline
anatomic structures)
• Pulsed-Doppler (Flow and
movement)

Duplex spectral Doppler system
• Both B-mode imaging and Doppler
are possible with the same
transducers.
• True duplex system, is capable of
doing simultaneously both
Doppler and real time B-mode
imaging.
• Most flat sequenced linear array,
convex array, and electronically –
steered phased array imaging are
capable of duplex imaging. Same
piezoelectric element is used.
244

Doppler spectral display:
• FFT is a mathematical formula, used to rapidly analyze
the wide range of amplitudes, Doppler shifts and
display them as a function of time.
• Spectral Doppler is used when precise blood velocity
information is required.
• Returning (reflected) Doppler signals are processed
using a ‘Fast Fourier Transform Spectrum Analyzer’
• Range and mixture of Doppler frequency shifts (∆F)
are sorted into individual components and displayed
as a function of time on velocity (or frequency shift)
scales
• Spectral Doppler is used when precise blood velocity
information is required.
245

Spectrum analyzer
• Displays positive Doppler shifts
in the region above a zero-
reference base line and negative
Doppler shift in the region
below the zero-reference base
line.
• The escalation of the trace,
either above or below the base
line is an indication of the
magnitude of the measured
shift or calculated blood
velocity.
246

Components of a
spectrum
• Doppler signals occupying a
wide range of frequency shift,
result in ‘broadening’ of the
spectrum.
• Spectral broadening appears as
a decreased spectral window.

Spectrum analyzer
• Spectral broadening: the
increase in thickness of the
spectral display envelope
corresponding to the range of
Doppler shift frequencies
present at a given time.
• Spectral broadening appears as
a decreased spectral window.
• Spectral window: the clear space
beneath the outline of the
spectral display.
248

Normal peripheral arterial waveforms
patterns

Duplex analysis for peripheral arterial disease
• Waveform shape
• Peak systolic velocity
• Spectral window or Envelope
• Velocities:
-Femoral arteries 80-100cm/sec
-Popliteal arteries 60-80cm/sec
-Tibial arteries 40-60 cm/sec

Spectral waveforms:
• Blood vessels have unique flow
characteristics which are recognizable
by their Doppler spectral waveforms
(Doppler signature)
• Factors which affect Doppler spectral
waveforms include:
- Cardiac contraction
- vessel compliance
- Down stream vascular
resistance esp. in vascular bed
supplied by the vessel
252

02.COLOURDOPPLER
• Based on pulse Doppler
technique.
• Doppler shift: are converted
into color, and moving blood is
displayed in colors that
correspond to its velocity and
direction.
• Positive Doppler shift are
encoded as red, and negative
shifts are encoded is blue
• Blue Away, Red Towards “BART”

Colour images:
• Flow towards transducer is
usually red, where as flow away
from transducer is usually blue.
• It can be interchanged by the
sonographer
• Light colours, fast speed, where
as dark colours indicate slow
speed
• Colour shades depend on mean
velocities, as such colour
Doppler can not be used to
estimate peak velocities; spectral
Doppler is used instead
255

Velocity of the flow is represented in shades of
color-faster-brighter
• .

Color images:
• Change in colour within a blood
vessel can be due to:
- Change in Doppler angle
- Change in blood flow velocity
- Aliasing
- A diverging been from
transducer (sector or curved
array)
- A curving blood vessel
257

COLOR DOPPLER USES
• Color Doppler is capable of
showing the blood flow in
small vessels. (testes vessels)
• identify vessels, or to identify
focal areas of flow disturbance
• N.B waveforms from these areas
are obtained with pulsed
Doppler analysis.

Color Box
• Operator adjustable area within
the US image
• Affects the image resolution and
Quality
• Frame rate decreases with
increasing box size(Change in box
size)
• As small and superficial as possible
• Deep color box- slower PRF results
in ALIASING
• .

Spectral and Color Doppler comparison
SPECTRAL DOPPLER:
DEPICTION OF DOPPLER SHIFT INFORMATION IN
WAVEFORM
COLOR DOPPLER:
UTILIZE DOPPLER SHIFT INFORMATION TO SHOW BLOOD
FLOW IN COLOR

Color Doppler
• Advantages
- Provides an overall view of flow in
organs or structure
- Provides directional information
about flow
- Provides velocity information about
flow and shows turbulent type flow
• LIMITATIONS
- Semi quantitative
- Angle dependent aliasing
- Artifacts caused by the noise
- Poor Temporal resolution
Noise artifacts

Advantages of spectral and Color Doppler
SPECTRAL DOPPLER COLOR DOPPLER
01.Depicts quantitative flow at one site 01.Overal view of flow
02.Allows calculations of velocity and indices 02.Directional information of a flow
03.Good temporal resolution 03.Average velocity information about a flow

POWERDOPPLER
• Also known as Energy
Doppler/Amplitude Doppler
• In this mode color is assigned to the
power/strength/energy of the signal
rather than the Doppler frequency shift.
• Flow is usually displayed with one color,
The color and hue relate to the moving
blood volume rather than the direction
or the velocity of flow
• The Doppler detection sequence used in
power Doppler is identical to that used
in frequency-based color Doppler
imaging.

Color Doppler vs Power Doppler

266
Colour Doppler in same patient at the same area of study:
A: Colour flow Doppler imaging: it is based on frequency shift from a moving target
B: Power mode Doppler: shows distribution of power or amplitude of Doppler signal;
no direction or velocity information

267
Frequency and power mode
Colour mapping
A: Conventional colour doppler
shows differences in flow direc
tion and Doppler frequency shift
B: Power mode Doppler: indicates
amplitude of Doppler signal

Power Doppler
• USES:
1. To detect slow flow, in small
vessels.
2. Utilized where transducer
angling is awkward
- Power Doppler has no
Aliasing, because it has no
directional flow

Power Doppler
• Advantages
- Useful in localizing high flow
areas
- Angle independent
- No aliasing
- Improved signal to noise ratio
- More sensitive to detect low
flow
- Better able to define boundaries
• Limitations
- No directional information
- Poor temporal resolution due to
relative low PRF
- Sensitive to flash artifacts due to
low PRF

01.Aliasing
• Aliasing occur when the abnormal
velocity exceeds the rate at which the
pulsed wave system can record
properly
• Is an inacurate display of color or
spectral doppler velocity when the
velocity range exceeds the scale
available to display it
• Pulse repetition frequency is the
critical determinant of Nyquist limit
• Nyquist limit defines when will
aliasing occur
• Nyquist limit is the maximum doppler
shift frequency that can be correctly
measured without resulting in aliasing
in color or spectral doppler

ALIASING
• Pulse Repetition Frequency(PRF) Is the
number of pulses that an ultrasound
system transmits into the body per second.
• The PRF of the transmitted pulses is the
effective sampling frequency
PRF=1/T
• The muximum detectable frequency shift
(Nyquist Limit) is determined by the value
of one half of PRF
Nyquist limit (fmax) = PRF/2
so PRF=2fmax
• PRF must be at least twice the Nyquist Limit
(PRF>2fmax),other wise at high blood
velocities,ALIASING will occur.

Range ambiguity
• A high PRF may result in range
ambiguity, this decreases the
maximum depth from which the
location of a Doppler shift can
be accurately detected.
273

Reducing Aliasing
• Drop the baseline
• Increase the available velocity
range
• Decrease the doppler frequency
shift by using a lower insonating
frequency
• Increase the doppler angle

02.Blooming artifacts(Colour
bleed)
• Blooming artifacts or color bleed artifacts occur
when the color signal indicating blood flow
extends beyond its true boundaries spreading
into adjacent regions with no actual flow
• This artifacts mainly affects the portion of image
distal to the vessel
• Caused by abnormal high gain setting
• Causes the obscuration of thrombus or plaques
in the vessel.
• Also seen with ultrasound contrast
agents

03.Flash Artifact
• Represents a sporous (having
spores) flow signal arising due to
tissue /transducer motion
• Manifest as color signal due to
transducer motion/motion of an
anatomical structure secondary
to an external force or patient
motion
• Prevention:Motion
discrimination function

04.Miror Image artifact
• displays objects on both sides of
a strong reflector,
• The reflectors (diaphragm, pleural
surface and aortic wall) directs
some of the echoes to a second
reflector before it returns them to
the transducer resulting
multipath reflection.
• Eg Duplication of sub
clavian artery (pleura
reflector)
• .

Basics of Ultrasound Physics

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