Physics of echo i.tammi raju

Dr. I. tam m iraju
PHYSICS BEHIND
CONVENTIONAL AND
ADVANCED ECHO

overview
Introduction
Us-properties
Us-tissue interaction
Transducer
Us beam ,creation of image
Harmonics
Artifacts
Doppler
Spe
3d
TEE ,EPICARDIAC, IVUS,,INTRACARDIAC

INTRODUCTION
• Sound is a mechanical vibration
transmitted through an elastic medium.
• When it propagates through the air at
the appropriate frequency, sound may
produce the sensation of hearing.
• Ultrasound-- sound spectrum having a
frequency greater than 20,000 cycles
per second (20 KHz), which is
considerably above the audible range.

? ULTRA SOUND
• ultrasound can be directed as a beam and focused.
• it obeys the laws of reflection and refraction.
• targets of relatively small size reflect ultrasound and can
therefore be detected and characterized.
• A major disadvantage of ultrasound is that it is poorly
transmitted through a gaseous medium and attenuation
occurs rapidly, especially at higher frequencies.

Properties of us
Frequensy
Wavelength
Propagation speed
Amplitude
Power
Intensity

Thewavelengthis thedistancebetweentwoconsecutiveand
equivalent parts.
( 0 . 1 5 to 1 . 5 m m )
Thefrequencyis thenumberof compressions perunit of time
expressedinHertz. Theamplitudeis thepressuredifference
betweennadirandpeak.
Inechocardiography, waves areemittedas pulses consistingof
THE ANATOMY OF A WAVE

VELOCITY
• Velocity through a given medium
depends on the density and elastic
properties or stiffness of that
medium.
• Velocity is directly related to
stiffness and inversely related to
density.
• Ultrasound travels faster through
a stiff medium, such as bone.
• Velocity also varies with
temperature, but because body
temperature is maintained within a
relatively narrow range, this
phenomenon is of little significance
in medical imaging.

HOOKE'S LAW
F = -kx
F is the force,
k is the spring constant, and
x is the amount of particle
displacement
• Hooke's Law says force will be balanced by a force in the opposite
direction that is dependent on the amount of displacement and the
spring constant
• The negative sign indicates that the force is in the opposite direction.
Sound Propagation in Elastic Materials
•Hooke's Law, when used along with Newton's Second Law, can
explain about the speed of sound. within a material

AMPLITUDE
 Measure of the strength of the sound wave
It is defined as the difference between the peak pressure within the
medium and the average value, depicted as the height of the sine wave
above and below the baseline.
Amplitude is measured in decibels, a logarithmic unit that relates
acoustic pressure to some reference value.
The primary advantage of using a logarithmic scale to display
amplitude is that a very wide range of values can be accommodated
and weak signals can be displayed along side much stronger signals.

• Power
• defined as the rate of energy transfer to the medium(watts).
• Intensity :-
• power is usually represented over a given area (often the beam
area) and referred to as (watts per centimeter squared or W/cm2
).
• This is analogous to loudness.
• Intensity diminishes rapidly with propagation distance and has
important implications with respect to the biologic effects of
ultrasound, which are discussed later.

US-TISSUE INTERACTIONS
 These interactions between the ultrasound
beam with acoustic interfaces form the
basis for ultrasound imaging.
 The phenomena of reflection and
refraction obey the laws of optics and
depend on the angle of incidence between
the transmitted beam and the acoustic
interface as well as the acoustic mismatch,
i.e., the magnitude of the difference in
acoustic impedance.
 Small differences in velocity also
determine refraction.

Snell's Law describes the relationship between the angles and the
velocities of the waves. Snell's law equates the ratio of material
velocities V1 and V2 to the ratio of the sine's of incident (Q1) and
refracted (Q2) angles,
Snell's Law

??GEL
• Without the gel, the air-tissue interface at the skin surface results
in more than 99% of the US energy being reflected at this level.
• This is primarily due to the very high acoustic impedance 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, thereby allowing imaging to occur.

ATTENUATION
 The loss of ultrasound as it propagates
through a medium.
 three components: absorption,
scattering, and reflection
 Attenuation always
 increases with depth
 affected by the frequency
 the type of tissue.

HALF-VALUE LAYER OR THE HALF-
POWER DISTANCE
Attenuation may be expressed as which is a measure of
the distance that ultrasound travels before its amplitude is
attenuated to one half its original value.
As a rule of thumb, the attenuation of ultrasound in
tissue is between 0.5 and 1.0 dB/cm/MHz.
describes the expected loss of energy (in decibels) that
would occur over the round-trip distance that a beam
would travel after being emitted by a given transducer.

• The emission of sound
energy from the
transducer, followed by
a period of quiescence
during which the
transducer listens for
some of the transmitted
ultrasound energy to be
reflected back .
.
• The time required for
the ultrasound pulse to
make the round-trip
from transducer to
target and back again
allows calculation of the
distance between the
transducer and reflector.
DEAD TIME

A specular reflection occurs when
ultrasound encounters a target that
is large relative to the transmitted
wavelength. The amount of
ultrasound energy that is reflected to
the transducer by a specular target
SPECULAR REFLECTION

Targets that aresmallrelativetothetransmitted
wavelengthproduceascatteringof ultrasoundenergy,
resultinginasmallportionof energybeingreturnedto
thetransducer.
•This typeof interactionresults inspecklethat
produces thetexturewithintissues.
• forms thebasis forDopplerimaging
SCATTERED ECHOS

• Acoustic impedance (Z, measured in rayls)
• The velocity and direction of the ultrasound beam as it passes
through a medium are a function of the acoustic impedance
of that medium.
• velocity (m/s) X physical density (in Kg/cumm ). Z = Pv
Acoustic impedance is important in
the determination of acoustic transmission and reflection at the
boundary of two materials having different acoustic
impedances.
the design of ultrasonic transducers.
assessing absorption of sound in a medium.

•R = percent reflection of ultrasound signal
•Ultrasonic waves are reflected at boundaries where there is a difference in acoustic
impedances (Z) of the materials on each side of the boundary.
• This difference in Z is commonly referred to as the impedance mismatch.
• The greater the impedance mismatch, the greater the percentage of energy that will
be reflected at the interface or boundary between one medium and another.
ACOUSTIC IMPEDANCE MISMATCH

SIGNAL-TO-NOISE RATIO
 Is a measure of how the signal from the defect compares to
other background reflections (categorized as "noise").
 A signal-to-noise ratio of 3 to 1 is often required as a
minimum.

TRANSDUCER
•Consists of many small,
carefully arranged
piezoelectric elements
that are interconnected
electronically .
•The frequency of the
transducer is determined
by the thickness of these
elements.
 A piezoelectric crystal will vibrate when an electric current is
applied, resulting in the generation and transmission of
ultrasound energy.
 Conversely, when reflected energy encounters a piezoelectric
crystal, the crystal will change shape in response to this
interaction and produce an electrical impulse
 ceramics-ferroelectrics,barium titanate,and lead zirconate titanate

DAMPENING (O R BACKING) MATERIAL:- which sho rte ns the
ring ing re spo nse o f the pie zo e le ctric m ate rialafte r the brie f
e xcitatio n pulse .
•Abso rptio n o f backward and late rally e m m ite d e ne rg y
MATCHING LAYERS are applie d to pro vide aco ustic im pe dance
m atching be twe e n the pie z o e le ctric e le m e nts and the bo dy .
ACO USTIC LENS he lpe d in fo cussing

A phased-array ultrasound transducer
• phased-array transducers, which consist of a series
of small piezoelectric elements interconnected
electronically

• If the transducer face is round, the transmitted beam will remain
cylindrical for a distance, defined as the near field.
• After a particular distance of propagation, the beam will begin to
diverge and become cone shaped(far field).( decreased in intensity)
NEAR AND FAR FIELD

where
D = depth of near field
d = diameter of transducer
 = transmission frequency of transducer
c = speed of sound

Phased-arraytechnologypermits steeringof the
ultrasoundbeam. Byadjustingthetimingof
excitationof theindividual piezoelectric
crystals, thewavefront of ultrasoundenergy
canbedirected, as shown. Beamsteeringis a
BEAM STEERING,

Byadjustingthetimingof excitationof theindividual
crystals withinaphased-arraytransducer, thebeam
canbefocused.
Inthis example, theouterelements arefiredfirst,
followedsequentiallybythemorecentralelements.
Becausethespeedof soundis fixed, this manipulation
inthetimingof excitationresults inawavefront that
is curvedandfocused.
TRANSMIT FOCUSING

LINEAR ARRAY TECHNOLOGY
• Have a rectangular face with crystals aligned parallel to one
another along the length of the transducer face.
• Elements are excited simultaneously so the individual scan lines
are directed perpendicular to the face and remain parallel to
each other.
• This results in a rectangle-shaped beam that is unfocused.
Linear-array technology is often used for abdominal,
vascular, or obstetric applications.
• Alternatively, the face of a linear transducer can be curved to
create a sector scan.
• This innovative design is now being used in some hand held
ultrasound devices.

FOCUSING,INTENSITY AND
BEAM
• concentrating the acoustic energy into a smaller area—
• increased intensity at the point of focus.
• Intensity also varies across the lateral dimensions of the beam,
being greatest at the center and decreasing in intensity toward
the edges.

Theultrasoundbeamemittedbyatransducercanbeeither
unfocused(top) orcanbefocusedbyuseof anacoustic lens
(bottom). Focusingresults inanarrowerbeambut does not
changethelengthof thenearfield. Anundesirableeffect of

RESOLUTION
• Resolution is the ability to distinguish between two objects in
close proximity.
resolution has at least two components:
• spatial and temporal.

The Basis of Image Resolution (Part I)
SPL =spatial pulse length
¥» = wavelength
n = number of cycles in a pulse of ultrasound
c = speed of sound
µ = transmission frequency

. Withincreasingpulselength, thebandwidthbecomes narrower,
therebyreducingresolution. Therefore, to improveresolution, a
THERELATIONSHIPBETWEEN PULSEDURATION, OR
LENGTH, ANDBANDWIDTH

The Ying-Yang Relationship
between Resolution and
Penetration
• L = intensity attenuation loss (in decibels)
• µ = intensity attenuation coefficient (~0.8 dB/cm/MHz for soft tissue)
• = transducer frequency (in MHz)
• z = distance traveled in the medium by ultrasound wave (in cm)
intensity loss is greatest (or penetration is poorest) when using a transducer
with a higher frequency,
Thus, echocardiography requires a constant balancing act between
optimizing resolution without sacrificing penetration and vice versa

THE BASIS OF TEMPORAL
RESOLUTION
• where
• F = frame rate
• c = speed of sound
• D = sampling depth
• N = number of sampling lines per frame
• n = number of focal zones used to produce one image

To perform M-mode examinations, pulse repetition rates of
between 1,000 and 2,000 per second are used.
• For two-dimensional imaging, repetition rates of 3,000 to 5,000
per second are necessary to create the 90-degree sector scan.
• This does not mean, however, that temporal resolution is higher
for two-dimensional imaging. In fact, the opposite is true.
• Although the pulse repetition rate is lower for M-mode, because
all the pulses are devoted to a single raster line, the temporal
resolution is actually much higher for M-mode.
•

The center of the beam has higher intensity compared with the edges.
higher impedance (black dots) produce stronger echoes and can therefore be
detected even at the edges of the beam.
Weaker echo-producing targets (gray dots) only produce echoes when they are
located in the center of the beam.
B: The effect of beam width on target location is shown. Objects A and B are nearly
side by side with B slightly farther from the transducer. Because of the width of the
beam, both objects are recorded simultaneously. The resulting echoes suggest that
the two objects are directly behind each other (A' and B') rather than side by side.
Interrelationship between beam intensity and
acoustic impedance

A transaxial beam plot is demonstrated. The beam width or lateral
resolution is a function of the intensity of the ultrasonic beam. The
beam width is commonly measured at the half-intensity level, and, in
this case, the beam width would be reported as 6.2 mm

Returning sound waves
Low amplitude,high Voltage signals(rf data)
Logarithmic compression and filtering
envelope detection
polar video signal. (R-theta)
cartesian or rectangular format.
digital format or converted to analog data for videotape storage
and display
CREATING THE IMAGE
preprocessing
postprocessing
Curve fitting process

A block diagram shows the components of an echograph.

Ultrasoundenergyis usuallyemittedfromthetransducerina
series of pulses, eachonerepresentingacollectionof cycles.
Eachpulsehas adurationandis separatedfromthenext pulse
bythedeadtime. .
TRANSMITTING ULTRASOUND ENERGY
5microsec
(0.1%)

Ultrasound beam is directed through the heart at the level of the mitral valve.
A-mode:-The returning ultrasound information can be displayed in amplitude mode)
in which the amplitude of the spikes = strength of the returning signal.and depth.
B-mode:- Amplitude can be converted to brightness in which the strength of the
echoes at various depths is depicted as relative brightness.
M-mode:- Motion can be introduced by plotting the B-mode display against time.
This is the basis of echocardiography.
DISPLAY OPTIONS

DISPLAY OPTIONS
• M-mode presentation depicts anatomy along a single
dimension corresponding to the ultrasound beam creating
what has been called the ice-pick view of the heart.
• M-mode image provides an assessment of the dimensions of
the object and its motion relative to the ultrasound beam.

M-mode echocardiogram provides relative anatomic information along a
single line of information
ice-pick view of the heart

TRADEOFFS IN IMAGE CREATION
• The variables to consider include the
• desired depth of examination,
• the line density,
• PRF,
• the sweep angle, and
• the frame rate.
• Because the velocity of sound in the body is essentially fixed,
the time required to send and receive information is a
function of depth of view.
• Each pulse allows a single line of ultrasonic data to be
recorded.

SWEEP ;-To go from a single line of ultrasonic data to a two-
dimensional image, the beam must be swept through an angle that
typically varies from 30 to 90 degrees.
 The larger the angle is, the more lines are needed to fill the
sector with data.
 Because line density is an important determinant of image
quality, it is desirable to acquire as many ultrasonic lines as
possible.
 The term line density refers to the number of lines per degree of
sweep.
A line density of approximately two lines per degree is necessary
to construct a high-quality image.

• FRAME RATE :-
• Depending on the speed of motion of the structure of interest, a
higher or lower frame rate will be necessary to construct an
accurate and aesthetically pleasing movie of target motion.
• For example, the aortic valve can move from the closed to the open
position in less than 40 milliseconds. At an imaging rate 30 frames per
second, it is likely that the valve will appear closed in one frame and
open in the next, with no appearance of motion because intermediate
positions were not captured.
• If one wished to record the aortic valve in an intermediate
position, a very high frame rate must be employed.
• DISADV:- increasing the frame rate generally results in a decrease
in line density and degradation in image quality.

TELEVISION TECHNOLOGY
• Modern echocardiographic instruments -- images are displayed
on a television screen using the concept of fields and frames.
• Individual raster lines are thereby eliminated
• A field is the total ultrasonic data recorded during one complete
sweep of the beam.
• A frame is the total sum of all imaging data recorded and
generally implies that new information is superimposed on
previously recorded data.
• two fields are interlaced (to improve line density) to produce one
frame.
• Using this approach, the frame rate would be half of the
corresponding sweep rate.

Gain
• The amplitude, or
the degree of
amplification, of the
received signal.
• When gain is low,
weaker echoes from
the edge of the beam
may not be recorded
and the beam
appears relatively
narrow.
• If system gain is
increased, weaker

The amplitude of returning signals is plotted against distance, or
depth, from the transducer.
Time gain compensation can be used to enhance the amplitude of the
weaker signals returning from targets at greater depth .

GRAY SCALE IS A KEY CONCEPT IN THE CREATION OF A
TWO-DIMENSIONAL IMAGE.
The gray scale refers to the number of shades that can possibly be displayed
between the two extremes of white and black.
In the example, 256 shades are depicted. Each pixel is assigned one
of these shades. In a digital system in which imaging data are
stored as a binary code, eight bits are required to encode one of the
256 shades of gray

DYNAMIC RANGE
scattered echoes are by definition much weaker than specular
echoes, yet both are important in image construction. A mechanism
to accommodate both is necessary, and this is accomplished
through the use of a proper dynamic range.
EXTENT OF USEFUL ULTRASONIC SIGNALS THAT CAN BE PROCESSED

TISSUE HARMONIC IMAGING
• Propagation of the ultrasound
wave,, or fundamental, frequency
of the signal may be altered.
• new frequencies (integer multiples
of the original frequency)
harmonics.
• The returning signal contains both
fundamental and harmonic fr.
• By suppressing or eliminating the
fundamental component, an image
is created primarily from the
harmonic energy.
• the strength of the harmonic
frequency actually increases as the
wave penetrates the body

Fourier’s Theorem
 It states that any repetitive wave,
whatever shape, can be
constructed by adding together a
number of regular sine waves of
varying frequencies and amplitudes.
 The basic building block for a
particular complex wave is a sine
wave having the same frequency
and termed the “fundamental.”
 The detail that gives the complex
wave its shape is provided by
adding to the fundamental further
sine waves having frequencie sthat
are exact multiples of that of the
fundamental.
 These are called harmonics

Unlike fundamental frequencies, harmonic frequencies increase in strength as
the wave penetrates the body.
At the chest wall, where many artifacts are generated, very little harmonic
signal is present.
At useful imaging depths (4-8 cm), the relative strength of the harmonic
signal is at its maximal.

• The transducer sequentially
emits two pulses with similar
amplitude but with inverted
phase.
• When backscattered from a
linear reflector such as tissue,
and then summed, these pulses
cancel each other, resulting in
almost complete elimination of
the fundamental frequency
signal, called destructive
interference.
• The remaining harmonic energy
can then be selectively
amplified, producing a relatively
pure harmonic frequency
spectrum.
PULSE INVERSION TECHNOLOGY

ADVANTAGES
strong fundamental signals produce intense harmonics.
Reduces the artifact generation during harmonic imaging
Reduces near field clutter.
The signal-to-noise ratio is improved and
 Image quality is enhanced, especially in patients with poor sound
transmission due to obe-sity or dense muscle tissue, the harmonic
frequenciesmay make a better image than the fundamental fre-quency.
Improved endocardial border definition.

side effect :-
 strong specular echoes, such as those arising from valves, appear
thicker than they would on fundamental imaging.
particularly true in the far field --false-positive interpretations.
To avoid such pitfalls but to take advantage of the benefits of
harmonic imaging, most clinical studies should include both
fundamental and harmonic imaging in the course of the
examination.

ARTIFACTS
Side lobes
Reverberations
Shadowing
Near field clutter.

SIDE LOBES
• EDGE EFFECT.
• Side lobes occur because not all the energy produced by the
transducer remains within the single, central beam.
• Instead, a portion of the energy will concentrate off to the side of the
central beam and propagate radially.
• A side lobe may form where the propagation distance of waves
generated from opposite sides of a crystal differs by exactly one
wavelength.
• Side lobes are three-dimensional artifacts, and their intensity
diminishes with increasing angle.

A: The strong echoes produced by the
posterior mitral anulus and atrioventricular
groove produce a side lobe artifact that
appears as a mass within the left atrium.
B: Bright echoes within the pericardium

A: The source of the artifact is the posterior
pericardium, which is a very strong reflector.
In this case, the second line of echoes is twice the
distance from the transducer as the actual
Reverberatio
n artifacts
•The returning signal undergoes a second reflection at the near wall
interface leads to a weaker signal.
•In this case, the pulse required twice as long as the original pulse to
be detected and therefore incorrectly places the target at twice the
distance from the transducer.

A: A St. Jude mitral prosthesis (MV) is present.
The echo-free space beyond the sewing ring (*)
represents shadowing behind the strong echo-
reflecting sewing ring. The cascade of echoes
directly beyond the prosthetic valve itself that
extend into the left ventricle (LV) represent
shadowing is
compared with
reverberations
Shadowing results in the absence of echoes
directly behind the target

NEAR FIELD CLUTTER
• This problem, also referred to as
ringdown artifact, arises from high-
amplitude oscillations of the
piezoelectric elements.
• This only involves the near field
and has been greatly reduced in
modern-day systems.
• The artifact is troublesome when
trying to identify structures that are
particularly close to the transducer,
such as the right ventricular free
wall or left ventricular apex.

INTRODUCTION TO DOPPLER
Physical principles were
outlined by Johann Christian
Doppler in his seminal work
titled
‘ On the colored light of
Double stars and some other
Heavenly Bodies’ in 1842
Postulated that properties of
light emitted from stars
depend on the relative
motion of the observer and
wave source

DOPPLER
ECHOCARDIOGRAPHY
 Doppler echocardiography depends on measurement of the
relative change in the returned ultrasound frequency when
compared to the transmitted frequency
 Depending on the relative changes of the returning
frequencies, Doppler Echocardiographic systems measure
the characteristics of flow:
 direction
 , velocity and
 turbulence

Doppler Equation --determination velocity

The Doppler Display
AUDIO OUTPUT
 The changing velocities (frequencies) are converted into audible
sounds and, after some processing, are emitted from speakers
placed within the machine.
High pitched sounds result from large Doppler shifts and indicate
the presence of high velocities, while low pitched sounds result
from lesser Doppler shifts.
Laminar flow produces a smooth, pleasant sound, whereas
turbulent flows result in harsh high pitched sounds

spectral velocity recordings
 The display of the spectrum of the various velocities
encountered by the Doppler beam is accomplished by
microcomputers that decode the returning complex Doppler
signal and process it into its various velocity components.
There are two basic methods for accomplishing this
 The most popular is Fast Fourier Transform.
 Chirp-Z transform

SPECTRAL ANALYSIS
 Spectral recording is made
up of a series of bins
( vertical axis) that are
recorded over time
( horizontal axis)
 The brightness of the signal
at a given bin reflects the
number of RBCs at that
velocity

Turbulence, results in the spectral broadening
(display of velocities that are low, mid and
high) and an increase in peak velocity as seen in
disease states.

Five basic types of Doppler techniques:
continuous wave Doppler,
pulsed wave Doppler,
color flow imaging,
tissue Doppler, and
 duplex scanning.

PULSED WAVE DOPPLER
Range gating is accomplished
through timing circuits that
only samples Doppler shift
from a given region
Sample volume is a three
dimensional , tear drop
shaped portion of ultrasound
beam
Its width is determined by the
width of the ultrasound beam
at the selected depth and by
the length of each transmitted
impulse

Pulsed wave doppler
Disadvantages
1. Inability to measure high blood flow velocities as seen in
certain types of congenital and valvular diseases.
2. This is attributed to Aliasing and results in inability of
pulsed wave Doppler to record velocities above 1.5-2m/sec

Occurs when the abnormal velocity exceeds the rate at which
the pulsed wave system can record properly
ALIASING

Nyquist limit
Pulse repetition frequency is
the critical determinant of
Nyquist limit
Nyquist limit defines when
aliasing will occur.
PRF is limited by the distance
the sample volume is placed
into the heart and frequency
of transducer
The closer the sample volume
is located to the transducer
higher PRF can be used and
vice versa

Control of aliasing
Lower frequency transducer increases the ability of the PW
system to record high velocities but at the cost of signal to
noise ratio
A frequency of around 2.5 Mhz is preferred
The more practical way is to shift the baseline to make use of
the velocity in the opposite channel.
 This si called zero shift or zero-offset in some sytems

Continuous wave Doppler
 Simultaneously transmits and receives
ultrasound signals continuously.
 This can be accomplished in one of two
ways. One type of transducer employs two
distinct elements: one to transmit and the
other to receive.
 Alternatively, with phased-array technology,
one crystal within the array is dedicated to
transmitting while another is simultaneously
receiving.

Anonimaging, orPedoff, continuous wave
DopplertransducerThetransducercontains
twoelements: onefortransmittingandonefor
receiving.

Under physiologic conditions, most examples of blood flow
in the cardiovascular system are laminar,meaning that
individual blood cells are traveling at approximately the same
speed in approximately the same direction parallel to the
walls of the chamber or vessel.

VARIOUS TYPES OF FLOW PATTERNS ARE SHOWN

COLOR FLOW IMAGING
 Uses multiple sample volumes along
multiple raster lines to record the
Doppler shift.
 By overlaying this information on a
two-dimensional or M-mode
template, the color flow image is
created.
 Each pixel represents a region of
interest in which the flow
characteristics must be measured.
 Rather than analyzing the entire
velocity spectrum within one of
these small ,only mean frequencies
and frequency spreads (variance) are
calculated.
 the direction of flow can be
displayed using red (toward) and
blue (away).

As a first step, for each pixel, the strength of the returning
echo is determined.
 If it is above a predetermined threshold, it is painted a shade
of gray and displayed as a two-dimensional echocardiographic
data point.
If it is below the threshold, it is analyzed as Doppler
information.
By repetitive sampling, an average value for mean velocity
and variance is determined with greater accuracy.
 The flow velocity, direction, and a measure of variance are
then integrated and displayed as a color value.

The brightness of the primary colors encodes the magnitude
of the mean velocity.
High variance, or turbulence, is coded green, which, when
mixed with red or blue, yields yellow or cyan, respectively,
often with a mosaic appearance.

TECHNICAL LIMITATIONS OF
COLOR DOPPLER IMAGING
Because no two manufacturers approach in exactly the same
way, one of the fundamental problems of color Doppler
imaging is the difficulty in comparing images from different
ultrasound systems.

BILLIARD BALL EFFECT
 Mitral regurgitation is demonstrated
by some of the triangles moving
through the orifice into the left
atrium.
 The effect of those cells on the left
atrial blood (circles) is shown.
 Because of the increase in velocity,
some of the triangles and some circles
are encoded and displayed by the color
Doppler signal (filled triangles and
circles).
 It cannot distinguish whether the
moving left atrial blood originated in
the ventricle (the filled triangles) or
atrium (the filled circles), simply that
it has sufficient velocity to be detected

 As the ROA increases, flow
rate increases and more blood
enters the chamber and is
detected by the Doppler method
(middle panel).
 However, because velocity is
inversely related to orifice area,
as the ROA increases, the
velocity of the regurgitant jet
may decrease (if the pressure
gradient is less).
 Because Doppler imaging
records velocity, this larger (but
lower velocity) flow may be
recorded as a smaller color jet
(lower panel).

aliasing
mirror imaging, also called
crosstalk.
As the name suggests, this is the
appearance of a symmetric spectral
image on the opposite side of the
baseline from the true signal.
Such mirror images are usually less
intense but similar in most other
features to the actual signal.
 These artifacts can be reduced by
decreasing the power output and
optimizing the alignment of the
Doppler beam with the flow
direction.
Doppler Artifacts
A.descendingaortic flow.
B: A stenotic porcinemitral valveis
recordedwithpulsedwaveDoppler
imaging.

BEAM WIDTH ARTIFACTS
With pulsed Doppler imaging, it must be remembered that the
sample volume(s) has finite dimensions that tend to increase with
depth.
 A sample volume placed in the far field is large enough to
straddle more than one flow jet.
For example, a large sample volume may hinder one's ability to
distinguish aortic stenosis from mitral regurgitation.

Shadowing:-
may occur, masking color flow information beyond strong
reflectors.
 Ghosting:-
 is a phenomenon in which brief swathes of color are painted
over large regions of the image.
Ghosts are usually a solid color (either red or blue) and bleed
into the tissue area of the image.
These are produced by the motion of strong reflectors such
as prosthetic valves.

Ghosting occurs when brief displays of color are painted over
regions of tissue, as shown in the illustration

GAIN SETTING ARTIFACTS
color Doppler imaging is very gain dependent.
Too much gain can create a mosaic distribution of color
signals throughout the image.
Too little gain eliminates all but the strongest Doppler signals
and may lead to significant underestimation.
With experience, the operator learns to adjust the gain settings
to eliminate background noise, without oversuppression of
actual flow information.

Tissue Doppler Imaging
A unique and relatively recent application of the Doppler
principle is tissue Doppler imaging.
The Doppler technique can be used to record the motion of the
myocardium rather than the blood within it.
To apply Doppler imaging to tissue, two important differences
must be recognized.
 First, because the velocity of the tissue is <<than blood
flow, the machine must be adjusted to record a much lower
range of velocities.
 Second, because the tissue is a much stronger reflector of
the Doppler signal compared with blood, additional adjustments
are required to avoid oversaturation.

One obvious limitation is that the incident angle between
the beam and the direction of target motion varies from
region to region.
This limits the ability of the technique to provide absolute
velocity information, although direction and relative changes
in tissue velocity are displayed.
Myocardial velocity mapping can be obtained.
Subepicardial layers > subendocardial vel
Red for thickening/blue for thinning.

STRAIN RATE IMAGING
Strain is a measure of the deformation that occurs when
force is applied to tissue.
 Strain rate is simply its temporal derivative.
By measuring instantaneous velocity at two closely
positioned points within the myocardium and knowing the
initial distance between two points, both strain and strain
rate can be determined.
The Doppler tissue imaging technique has been used
successfully to derive the velocity information needed to
calculate strain.

SPECKLE TRACKING
Speckles are unique natural
acoustic markers resulting from
the interaction of ultrasound
energy with tissue and are
generated by the refected
ultrasound beam
Their size is 20–40 pixels.
The speckle pattern is unique
for each myocardial region and
is relatively stable throughout
the cardiac cycle.
 Each speckle can be identifed
and followed accurately over a
number of consecutive frames.

 When tracking a defned region of
speckles, a software algorithm
extracts displacement, velocity,
strain, and strain rate.
 Low image frame rate may not be
adequate for strain rate
calculations.
 Using apical views, longitudinal
velocity, longitudinal strain, and
longitudinal displacement can be
calcu-lated as well as transverse
displacement.
 In short-axis images radial strain
radial displacement,circumferential
strain, rotational velocity, and rota-
tional strain can be calculated for all
myocardial seg-ment.
SPECKLE TRACKING

Involve incorporating an ultrasound
transducer on the tip of a
gastroscope-like device
The physics are identical to that for
transthoracic echocardiography.
 The transducer face is obviously
smaller than a transthoracic probe,
and hence there are fewer channels
available for transmission.
The current-generation probes are
all multiplane devices.
 phased-array technology to steer the
ultrasound examining plane through
a 180-degree arc. This effectively
provides 360-degree panoramic
image of the heart
TEE
•Operate at multiple frequencies, typically 3.5 to
7.0 MHz
•Benefit of tissue harmonic imaging is probably less
than that of transthoracic imaging

Three-Dimensional
Echocardiography
Three-dimensional echocardiography is a technique currently
in evolution.
The eventual goal is a real-time, three-dimensional display of
cardiac anatomy including all the modalities mentioned
above including Doppler flow imaging, M-mode
echocardiography, and Doppler tissue imaging.
Several approaches have been taken to acquire and display
three-dimensional information.

Acquisition of a three-
dimensional data
There have been two basic approaches
 The first has been to acquire a series of two-dimensional
images that are then stored, typically encoded with
registration of the precise angle at which the heart is
interrogated as well as electrocardiographic and respiratory
gating information.
 An entire sequence of images is thus obtained, through
either a transthoracic or transesophageal route, that is stored
and then reconstructed into a three-dimensional data set

Acquisition of a three-
dimensional data
 In this schematic, three separate
imaging planes have been collected,
timed to the QRS, and
subsequently merged into a three-
dimensional data set
 The methodology involved in
three-dimensional reconstruction
from the transesophageal
echocardiographic approach using a
multiplane rotational image.
 Six representative images are
demonstrated. These images are
then merged into a three-
dimensional data set for subsequent
reconstruction

VOLUMETRIC IMAGING.
The second and more promising
method of acquisition of three-
dimensional echocardiographic
data .
 In this technique, a transducer
is constructed with a rectangular
(vs. linear) ultrasound array of
transmit/receive crystals .
This allows collection of a
pyramidal image data set,
representing real-time, three-
dimensional acquisition of data.

ANALYSIS OF 3D DATA
The first is simply to display the
entire three-dimensional data set on a
grid with several intersecting
interrogation lines.
The two-dimensional image along any
of the interrogation lines can then be
displayed
Although not providing a true three-
dimensional image, this methodology
allows precise location of a two-
dimensional plane along either typical
or atypical orientations to maximize
accuracy for measurement of
complex structures and so on.

CROPPING
A second method of display is to
slice the full-volume data set to
expose the interior.
A 3D data set consists of bricks of
pixels called volume elements or
voxels
A process known as cropping can
be used to cut into the volume and
make some voxels invisible
3D data sets of voxels are turned
into 2D images in a process known
as volume rendering
This provides a three-dimensional
perspective of what is otherwise a
two-dimensional image .

Epicardial Imaging
ultrasound probe directly to the cardiac structures provides a
high-resolution, nonobstructive view of cardiac structures.
Because the very near field of most ultrasound transducers is
subject to distortion, probes have been designed specifically
for epicardial application.
Imaging from these probes provides remarkably high-
resolution, high-fidelity images of cardiac structures.
 They are infrequently used because of the limited
applicability.

Intracardiac
Echocardiography
This technology involves a single-
plane, high-frequency transducer
(typically 10 MHz) on the tip of a
steerable intravascular catheter,
typically 9 to 13 French in size.
The catheter can be steered in
both directions laterally and can
be retroflexed and anteflexed.
greatest use for monitoring
complex interventional procedures
such as percutaneous atrial septal
defect closure, atrial
septostomy, and pulmonary vein
isolation for treatment of atrial
fibrillation

•Intracardiac two-dimensional imaging device recorded at the time
of atrial septal puncture for performance of an electrophysiologic
procedure.
•Greatest use for monitoring complex interventional procedures
such as percutaneous atrial septal defect closure, atrial
septostomy, and pulmonary vein isolation for treatment of atrial
fibrillation
Intracardiac Echocardiography

Intravascular Ultrasound
(IVUS)
Intracoronary ultrasound was developed before the
intracardiac probes noted previously.
Typically, these are ultraminiaturized ultrasound transducers
mounted on modified intracoronary catheters.
These devices operate at frequencies of 10 to 30 MHz and
provide circumferential 360-degree imaging.
Because of the high frequencies, the depth of penetration is
relatively low and the field of depth is limited to usually less
than 1 cm.
Most devices provide only anatomic imaging and do not
provide Doppler information

INTRACORONARY
ULTRASOUND
performed in the cardiac
catheterization laboratory.
 The technique provides high-
resolution images of the proximal
coronary arteries and can
identify calcification and
characterize plaque
true anatomic severity of
intermediate coronary lesions
noted on angiography or
 for highly precise evaluation of
coronary arterial wall
anatomy.

BIOLOGIC EFFECTS
depend on the total energy applied to a given region.
 Thus, both the intensity of the ultrasound beam and the
duration of exposure are important factors
Thermal index (TI) is the ratio of output power emitted by
the transducer to the power needed to raise tissue
temperature by 1°C.

heat
The biologic effects of ultrasound energy are related primarily to
the production of heat. (a goal of ultrasonic therapy).
Heat is generated whenever ultrasound energy is absorbed, and
the amount of heat produced depends on the intensity of the
ultrasound, the time of exposure, and the specific absorption
characteristics of the tissue.
 It should also be noted that the flow of blood and specifically the
perfusion of tissue have a dampening effect on heat generation
and physically allow heat to be carried away from the point of
energy transfer

Mechanical Index (MI)
 intended to convey the likelihood of cavitation
resulting from the ultra-sonic energy during the exam.
 Cavitation refers to activity of created or pre-existing gas
bubbles within the tissues due to ultrasound energy.
Because of the relatively high viscosity of blood and soft
tissue, significant cavitation is unlikely.
An important aspect of cavitation concerns its effect
during the injection or infusion of contrast
microbubbles.

Despite considerable study, virtually no clinically important
biologic effects attributable to ultrasound at diagnostic power
levels have been demonstrated.

Physics of echo i.tammi raju

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