Basicphysicsoftransoesophagealechocardiography for the Workshop2 1207843509995849 8

8/6/2019 Basicphysicsoftransoesophagealechocardiography for the Workshop2 1207843509995849 8

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Department of Anaesthesiology, Narayana Hrudayalaya

Physics of

Echocardiography

Dr. Anil Kumar H.RJunior Consultant

Department of Anaesthesiology

Narayana Hrudayalaya


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History of Ultrasound Imaging

1760 - Abbe Lazzaro Spallanzani Father of ultrasound

1912 - First practical application for rather unsuccessful

search for Titanic

1942 - First used as diagnostic tool for localizing brain

tumors by Karl Dussik

1953 - First reflected Ultrasound to examine the heart, thebeginning of clinical echocardiography Dr.Helmut Hertz , a

Swedish Engineer and Dr. Inge Edler a cardiologist

1970s -Origin of TEE ,Lee Frazin, a cardiologist fromChicago mounts M-mode probe on a Transoesophageal

probe.


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I will be discussing about..

Ultrasound and its properties

Interactions of ultrasound with tissues

Instrumentation and Image formation by

ultrasoundDoppler effect and its applications


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Sound

Mechanical vibration transmitted through an elastic

medium

Pressure waves when propagate thro air at

appropriate frequency produce sensation of hearing

Surface Vibration Pressure Wave Ear

Vibration Propagation Perception


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As sound propagates through a medium the

particles of the medium vibrate

Air at equilibrium, in

the absence of a

sound wave

Compressions and

rarefactions that

constitute a sound

wave


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Compressions and rarefactions which constitute the

sound wave can be represented as Sine wave

Amplitude - maximal

compression of

particles above the

baseline

Wavelength - distance

between the two nearest points

of equal pressure and density


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Frequency No. of wavelenghths per unit time

1 cycle/ sec = 1 Hz

So, Frequency is inversely related to wavelength

Velocity Speed at which waves propagate

through a mediumDependent on physical properties of the medium

through which it travels

Directly proportional to stiffness of the material

Inversely proportional to density till a physiological limit

Velocity = frequency * Wavelength


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Sound velocity in different materials

Material Velocity ( m/s)

Air 330

Water 1497

Metal 3000 - 6000

Fat 1440

Blood 1570

Soft tissue 1540


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ULTRASOUND

Ultrasound is sound with a frequency over

20,000 Hz, which is the upper limit of human

hearing.

The basic principles and properties are same asthat of audible sound

Frequencies used for diagnostic ultrasound are

between 1 to 20 MHz


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Interaction of ultrasound wave with

tissues

1. Attenuation

2. Reflection

3.Scattering

4. Absorption


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Attenuation

Loss of intensity and amplitude of ultrasound wave asit travels through the tissues

Due to reflection, scattering and absorption

Proportional to Frequency and the distance the wavefront travels

Higher frequency , more attenuation

Longer the distance (Depth), more the attenuation

And also on the type of tissue through which the beamhas to pass

Expressed as Half power distance For most of soft tissues it is 0.5 1.0dB/cm/MHz


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Reflection

Basis of all ultrasound imagingFrom relatively large, regularly shaped objects with

smooth surfaces and lateral dimensions greater than one

wavelength Specular Echoes

These echoes are relatively intense and angle

dependent.

From endocardial and epicardial surfaces, valves and

pericardium

Amount of ultrasound beam that is reflected depends on

the difference inAcoustic impedance between themediums


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The resistance t

hat a material offers tothe passage of sound wave

Velocity of propagation v varies between

different tissues

Tissues also have differing densities

Acoustic impedance

Z = v

Soft tissue / bone and soft tissue / air

interfaces have large Acoustic Impedance

mismatch

Acoustic Impedance


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Scattering

Type of reflection that occurs whenultrasound wave strikes smaller(less than

one wavelength) , irregularly shapedobjects

- Rayleigh Scatterers ( e.g.. RBCs)

Are less angle dependantand less intense.Weaker than Specular echoes

Result in Speckle that produces the texture

within the tissues


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How is ultrasound imaging done?

From sound to image


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Pierre Curie

(1859-1906),

Nobel Prize in

Physics, 1903

Jacques Curie

(1856-1941)

PIEZOELECTRICEFFECT


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Crystals of tourmaline, quartz, topaz, cane sugar, and

Rochelle salt have the ability to generate an electriccharge in response to applied mechanical stress

Piezoelectricity" after the Greek word Piezein, which

means to squeeze or press.

Converse of this effect is also true


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Construction of a Transducer

Backing

Material

Electrodes

Piezoelectric

crystal


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ElectronicPhased Array

which uses the principle

of Electronic Delay

Phased Array Transducers


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Electronic Focusing

Electronic beam

steering


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Characteristics of ULTRASOUND BEAM


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Length of near field = ( radius)2/ wavelength of

emitted ultrasound


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1. Piezoelectric crystal

2. High frequency electrical signal withcontinuously changing polarity

3. Crystal resonates with high frequency

4. Producing ULTRASOUND

5. Directed towards the area to be imaged6. Crystal listens for the returning echoes for a

given period of time

7. Reflected waves converted to electric signals bythe crystal

8. processed and displayed


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Schematic representation of the recording and

display of the 2-D image


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Our TEE Work Station..


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Resolution

Ability to distinguish two points in space

Two components

Spatial Smallest distance that two targets

can be seperated for the system todistinguish between them.

Two components Axial and Lateral

Temporal


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Axial Resolution

The minimum separationbetween structures the

ultrasound beam can

distinguish parallel to its

path.

Determinants:

Wavelength smaller the

better

Pulse length shorter the

train of cycles greater the

resolution


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Lateral Resolution

Minimum separation betweenstructures the ultrasound

beam can distinguish in a

plane perpendicular to its

path.

Determinants:

Depends on beam width

smaller the better

Depth

Gain


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Temporal resolution

Ability of system to accurately track movingtargets over time

Anything that requires more time will

decrease temporal resolution

Determinants:Depth

Sweep angle

Line density

PRF


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The Trade off ..


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To visualise smaller objects shorter wavelengths

should be used which can be obtained by

increasing frequency ofU/S wave.

Drawbacks of high frequency

More scatter by insignificant inhomogeneity

More attenuation

Limited depth of penetration

For visualising deeper objects lower frequency is

useful, but will be at the cost of poor resolution

So..


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The reflected signal can be displayed

in four modes..

A- mode

B- mode

M- mode

2-Dimensional


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A mode

shows the

Amplitude of

reflected

energy at

certain depth

B- Brightness

mode shows

the energy asthe brightness

of the point

M- Motion modethe reflector is

moving so if the

depth is shown in a

time plot, the

motion will be seen

as a curve

A

B

C


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M- mode

Timed Motion display ; B Mode with timereference

A diagram that shows how the positions of thestructures along the path of the beam change

during the course of the cardiac cycle Strength of the returning echoes vertically and

temporal variation horizontally


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M Mode es..

Great temporal resolution- Updated1000/sec. Useful forprecise timing ofevents with in a cardiac cycle

Along with color flow Doppler for thetiming of abnormal flows

Quantitative measurements of size ,distance & velocity possible with out

sophisticated analyzing stations


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M-mode beam through Mitral Valve

M-ModeImaging


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2 D MODE

Provides more structural and functionalinformation

Rapid repetitive scanning along many different

radii with in an area in the shape of a fan

2-D image is built up by firing a beam , waiting

for the return echoes, maintaining the

information and then firing a new line from a

neighboring transducer along a neighboring line

in a sequence of B-mode lines.


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2-D imaging by steering the transducer over an area

that needs to be imaged


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Mechanical Steering of the Transducer


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Electronic Phased Array Transducers for 2-D imaging

Linear Array Curvilinear Array


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A single FRAME being formed

from one full sweep of beams

A CINE LOOP from multiple FRAMES


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Resembles an anatomic section easy

to interpret2-D imaging provides information about

the spatial relationships of different

parts of the heart to each other.

Updated 30- 60 times/sec ; lesser

temporal resolution compared to M-

mode


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Study of blood flow dynamics

Detects the direction and velocityof moving bloodwithin the heart.

Doppler Study


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Comparison between 2-D and Doppler

2-D Doppler

Ultrasound

target

Tissue Blood

Goal of

diagnosis

Anatomy Physiology

Type of

information

Structural Functional

So, both are complementary to each other


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Christian Andreas Doppler

(1803 1853)

DOPPLEREFFECT


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DOPPLER EFFECT-

Certain properties of light emitted from stars depend upon

the relative motion of the observer and the wave source.

Colored appearance of some stars as due to their motion

relative to the earth, the blue ones moving toward earth and

the redones moving away.


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OBSERVER 2

Long wavelength

Low frequency

OBSERVER1Small wavelength

High frequency


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Doppler Frequency Shift -Higher returned frequency if

RBCs are moving towards the and lower if the cells aremoving away

Doppler principle as applied in Echo..


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The Doppler equation

Velocity is given by Doppler equation..

V = c fd / 2 fo cos UV target velocity

C speed of sound in tissue

fdfrequency shift

fofrequency of emitted U/S

U - angle between U/S beam & direction oftarget velocity( received beam , not theemitted)


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Doppler Equation


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Doppler blood flow velocities aredisplayed as waveforms


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When flow is perpendicular to U/S beam angle of

incidence will be 900/2700 ;cosine of which is 0 no blood flow detected

Flow velocity measured most accurately when

beam is either parallel or anti parallel to bloodflow.

Diversion up to 200 can be tolerated( error of < or= to 6%)

Important consideration !


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Twin Paradoxes of Doppler

Best Doppler measurements are made when

the Doppler probe is aligned parallel to the

blood flow

High quality Doppler signals require low

Doppler frequencies( < 2MHz)


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Importance of being parallel to flow when

detecting flow through the aortic valve


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Velocity is directly proportional to

frequency shift and for clinical use it is

usual to discuss velocity rather than

frequency shift ( although either is

correct)

V E fd / cos UV = c fd / 2 fo cos UV E fd


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Applicationsof oppler - ifferent odes

to eas re lood elocities

Continuous wave

Pulsed waveColour Flow Mapping


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Modern echo scanners combine Doppler capabilites

with 2D imaging capabilities

Imaging mode is switced off(sometimes with theimage held in memory) while the Doppler modes are in

operation


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CONTINUOUS WAVE DOPPLER

Continuous generation of ultrasound waves coupled with

continuous ultrasound reception using a two crystaltransducer

CWD t LVOT i D TG A ti


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CWD at LVOT in Deep TG Aortic

Long axis view


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Can measure high velocity flows ( in

excess of 7m/sec)Lack of selectivity or depth discrimination

-Region where flow dynamics are being

measured cannot be precisely localized

Most common use Quantification of

pressure drop across a stenosis by applying

Bernoulli equation


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1/2 PV2 Pressure

KineticEnergy

PotentialEnergy

P= 4V2

Bernoulli EquationBalancing Kinetic and Potential energy

This goes down..As this goes up..


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PULSED WAVE DOPPLER

Doppler interrogation at a particular depthrather than across entire line ofU/S beam.

Ultrasound pulses at specific frequency - Pulse

Repetition Frequency (PRF) or Sampling rate

RANGEGATED -The instrument only listens fora very brief and fixed time after the transmission

of ultrasound pulse

Depth of sampling by varied by varying the time

delay for sampling


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Transducer alternately transmits and receives the

ultrasound data to a sample volume. Also known

as Range-gated Doppler.

PWD at LVOT in Deep TG aortic long


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PWD at LVOT in Deep TG aortic long

axis view


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PRF for a given transducer of a given frequency ata particular depth is fixed; But to measure higher

velocities higher PRFs are necessary

Drawback ambiguous information obtainedwhen flow velocity is high velocities (above 1.5 to

2 m/sec)

This effect is called Aliasing


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ALIASING

Aliasing will occur iflow pulse repetition frequenciesor velocity scalesare used and high velocities are

encountered

Abnormal velocity of sample volume exceeds therate at which the pulsed wave system can record it

properly.

Blood velocities appear in the direction opposite tothe conventional one


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Full spectral

display of a high

velocity profilefully recorded by

CW Doppler

PW display is

aliased, or cut

off, and the topis placed at the

bottom


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Aliasing occurs if the

frequency of the

sample volume is more

than the Nyquist limit

Nyquist limit = PRF/2


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To avoid Aliasing - PRF = 2 ( Doppler shift

frequency or Maximum velocity of Samplevolume)

Can be achieved by Decreasing the

frequency of transducer, decrease thedepth of interrogation by changing the view

( this increases the PRF)


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Color FlowDoppler

Displays flow data on 2-D Echocardiographicimage

Imparts more spatial information to Doppler

data

Displays real-time blood flow with in the heartas colors while showing 2D images in grayscale

Allows estimation of velocity, direction andpattern of blood flow

M lti t d PW D l i hi h bl d fl l iti


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Multigated, PW Doppler in whichblood flow velocities

are sampled at many locations along many lines

covering the entire imaging sector

E h d i d h h h l h


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Echo data is processed through two channels that

ultimately combine the image with the color flow data

in the final display.


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Color FlowDoppler..

Flow toward transducer redFlow awayfrom transducer blue

Faster the velocity more intense is thecolour

Flow velocity that changes by more thana preset value within a brief timeinterval (flow variance) green / flame


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CFM v/s Angiography

CFM AngiographyRecords velocity not flow; So

in MR, CFM jet area consists

of both atrial and ventricular

blood Billiard Ball Effect

Records flow

Larger regurgitant orifice

area there will be smaller jet

area

Larger regurgitant orifice

area there will be larger jet

area


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Instrumentation factors in ColorDoppler Imaging

Eccentric jets appear smaller than equivalently sized

central jets Coanda Effect

High pressure jetwill appear larger than a low-pressure

jet for the same amount of flow

As gain increases, jet appears larger

As ultrasound output power increases, jet area

increases

Lowering PRFmakes the jet larger

Increasing the transducer frequencymakes the jetappear larger

S


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To Summarise..

Knowledge of physics helps us appreciate

why we are seeing what we are seeing, And

what we can do to see it better

Echocardiography is based on the electrical

conversion of reflected ultrasound waves

from structures and blood flow within the

cardiovascular system.


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To Summarise..

Good quality image is a compromisebetween resolution and depth of interrogation

Doppler study complements 2-D echo

Aligning Doppler beam parallel to direction of

target velocity is key to obtaining accurate

measurements.


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Documents

Basicphysicsoftransoesophagealechocardiography for the Workshop2 1207843509995849 8