Basic Physics Of Transoesophageal Echocardiography For The Workshop2

Department of Anaesthesiology, Narayana Hrudayalaya

Physics of Echocardiography

Dr. Anil Kumar H.RJunior Consultant

Department of Anaesthesiology Narayana Hrudayalaya


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, the

beginning of clinical echocardiography – Dr.Helmut Hertz , a Swedish Engineer and Dr. Inge Edler a cardiologist

▫ 1970s - Origin of TEE ,Lee Frazin, a cardiologist from Chicago mounts M-mode probe on a Transoesophageal probe.

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I will be discussing about..

Ultrasound and its propertiesInteractions of ultrasound with tissuesInstrumentation and Image formation by ultrasoundDoppler effect and its applications


Sound Mechanical vibration transmitted through an elastic mediumPressure waves when propagate thro’ air at appropriate frequency produce sensation of hearing

Surface Vibration Pressure Wave Ear

Vibration Propagation Perception


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


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


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 medium

Dependent on physical properties of the medium through which it travels Directly proportional to stiffness of the materialInversely proportional to density till a physiological limit

Velocity = frequency * Wavelength


Sound velocity in different materials

Material Velocity ( m/s)

Air 330

Water 1497

Metal 3000 - 6000

Fat 1440

Blood 1570

Soft tissue 1540


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 as that of audible sound

Frequencies used for diagnostic ultrasound are between 1 to 20 MHz


Interaction of ultrasound wave with tissues

1. Attenuation2. Reflection 3. Scattering 4. Absorption


Attenuation

Loss of intensity and amplitude of ultrasound wave as it travels through the tissues

Due to reflection, scattering and absorption Proportional to Frequency and the distance the wave

front travels – Higher frequency , more attenuation Longer the distance (Depth), more the attenuation

And also on the type of tissue through which the beam has to pass

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


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 pericardiumAmount of ultrasound beam that is reflected depends on the difference in Acoustic impedance between the mediums


The resistance that a material offers to the passage of sound wave Velocity of propagation “v” varies between different tissues

Tissues also have differing densities “ρ”Acoustic impedance

“Z = ρv” Soft tissue / bone and soft tissue / air interfaces have large “Acoustic Impedance mismatch”

Acoustic Impedance


Scattering

Type of reflection that occurs when ultrasound wave strikes smaller(less than one wavelength) , irregularly shaped objects - Rayleigh Scatterers ( e.g.. RBCs)Are less angle dependant and less intense. Weaker than Specular echoesResult in “Speckle” that produces the texture within the tissues


How is ultrasound imaging done?

“From sound to image”


Pierre Curie (1859-1906),Nobel Prize in Physics, 1903

Jacques Curie (1856-1941)

PIEZOELECTRIC EFFECT


Crystals of tourmaline, quartz, topaz, cane sugar, and Rochelle salt have the ability to generate an electric charge 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


Construction of a Transducer

Backing Material

Electrodes

Piezoelectric crystal


Electronic Phased Array which uses the principle of Electronic Delay

Phased Array Transducers


Electronic Focusing

Electronic beam steering


Characteristics of ULTRASOUND BEAM


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


2. Piezoelectric crystal3. High frequency electrical signal with continuously

changing polarity 4. Crystal resonates with high frequency 5. Producing ULTRASOUND 6. Directed towards the area to be imaged 7. Crystal “listens” for the returning echoes for a

given period of time 8. Reflected waves converted to electric signals by the

crystal9. processed and displayed


Schematic representation of the recording and display of the 2-D image


Our TEE Work Station..


Resolution

Ability to distinguish two points in spaceTwo components –

Spatial – Smallest distance that two targets can be seperated for the system to distinguish between them.

Two components – Axial and Lateral

Temporal


• Axial Resolution ▫ The minimum separation

between 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


• Lateral Resolution▫ Minimum separation between

structures the ultrasound beam can distinguish in a plane perpendicular to its path.

▫ Determinants: ▫ Depends on beam width –

smaller the better ▫ Depth ▫ Gain


Temporal resolution

Ability of system to accurately track moving targets over timeAnything that requires more time will decrease temporal resolutionDeterminants:

DepthSweep angleLine densityPRF


The Trade off ..


To visualise smaller objects shorter wavelengths should be used which can be obtained by increasing frequency of U/S wave.Drawbacks of high frequency –

More scatter by insignificant inhomogeneityMore attenuation Limited depth of penetration

For visualising deeper objects lower frequency is useful, but will be at the cost of poor resolution

So..


The reflected signal can be displayed in four modes..

A- modeB- mode M- mode2-Dimensional


A –mode shows the

Amplitude of reflected energy at

certain depth

B- Brightness mode shows the energy as the brightness

of the point

M- Motion mode the reflector is moving so if the

depth is shown in a time plot, the

motion will be seen as a curve

A

B

C


M- mode

• Timed Motion display ; B – Mode with time reference

• A diagram that shows how the positions of the structures along the path of the beam change during the course of the cardiac cycle

• Strength of the returning echoes vertically and temporal variation horizontally


M – Mode uses..

Great temporal resolution- Updated 1000/sec. Useful for precise timing of events with in a cardiac cycleAlong with color flow Doppler – for the timing of abnormal flows Quantitative measurements of size , distance & velocity possible with out sophisticated analyzing stations


M-mode beam through Mitral Valve

M-Mode Imaging


2 – D MODE

Provides more structural and functional informationRapid repetitive scanning along many different radii with in an area in the shape of a fan2-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.


2-D imaging by steering the transducer over an area that needs to be imaged


Mechanical Steering of the Transducer


Electronic Phased Array Transducers for 2-D imaging

Linear Array Curvilinear Array


A single ‘FRAME’ being formed from one full sweep of beams

A ‘CINE LOOP’ from multiple FRAMES


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


Study of blood flow dynamics

Detects the direction and velocity of moving blood within the heart.

Doppler Study


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


Christian Andreas Doppler (1803 – 1853)

DOPPLER EFFECT


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 red ones moving away.


OBSERVER 2 Long wavelength Low frequency

OBSERVER 1Small wavelengthHigh frequency


Doppler Frequency Shift - Higher returned frequency if RBCs are moving towards the and lower if the cells are moving away

Doppler principle as applied in Echo..


The Doppler equation

Velocity is given by Doppler equation..

V = c fd / 2 fo cos V – target velocity C – speed of sound in tissue fd –frequency shift fo –frequency of emitted U/S

- angle between U/S beam & direction of target velocity( received beam , not the emitted)


Doppler Equation


Doppler blood flow velocities are

displayed as waveforms


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 blood flow.

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

Important consideration !


“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)


Importance of being parallel to flow when detecting flow through the aortic valve


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


Applications of Doppler - Different modes to measure blood velocities

Continuous wavePulsed waveColour Flow Mapping


Modern echo scanners combine Doppler capabilites

with 2D imaging capabilities

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


CONTINUOUS WAVE DOPPLER Continuous generation of ultrasound waves coupled with

continuous ultrasound reception using a two crystal transducer


CWD at LVOT in Deep TG Aortic Long axis view


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 localizedMost common use – Quantification of pressure drop across a stenosis by applying Bernoulli equation


1/2 PV2 Pressure

Kinetic Energy

Potential Energy

P = 4V2

Bernoulli EquationBalancing Kinetic and Potential energy

This goes down..As this goes up..


PULSED WAVE DOPPLER

Doppler interrogation at a particular depth rather than across entire line of U/S beam.Ultrasound pulses at specific frequency - Pulse Repetition Frequency (PRF) or Sampling rateRANGE GATED - The instrument only listens for a very brief and fixed time after the transmission of ultrasound pulseDepth of sampling by varied by varying the time delay for sampling


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 axis view


PRF for a given transducer of a given frequency at a particular depth is fixed; But to measure higher velocities higher PRFs are necessary

Drawback – ambiguous information obtained when flow velocity is high velocities (above 1.5 to 2 m/sec)

This effect is called Aliasing


ALIASING

Aliasing will occur if low pulse repetition frequencies or velocity scales are used and high velocities are encountered

Abnormal velocity of sample volume exceeds the rate at which the pulsed wave system can record it properly.

Blood velocities appear in the direction opposite to the conventional one


Full spectral display of a high velocity profile fully recorded by CW Doppler

PW display is aliased, or cut off, and the top is placed at the bottom


Aliasing occurs if the frequency of the sample volume is more than the Nyquist limit

Nyquist limit = PRF/2


To avoid Aliasing - PRF = 2 ( Doppler shift frequency or Maximum velocity of Sample volume)

Can be achieved by – Decreasing the frequency of transducer, decrease the depth of interrogation by changing the view ( this increases the PRF)


Color Flow Doppler

Displays flow data on 2-D Echocardiographic imageImparts more spatial information to Doppler data Displays real-time blood flow with in the heart as colors while showing 2D images in gray scaleAllows estimation of velocity, direction and pattern of blood flow


Multigated, PW Doppler in which blood flow velocities are sampled at many locations along many lines covering the entire imaging sector


Echo data is processed through two channels that ultimately combine the image with the color flow data in the final display.


Color Flow Doppler..Flow toward transducer – red Flow away from transducer – blue Faster the velocity – more intense is the colour Flow velocity that changes by more than a preset value within a brief time interval (flow variance) – green / flame


CFM v/s Angiography

CFM Angiography

Records 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


Instrumentation factors in Color Doppler Imaging

Eccentric jets appear smaller than equivalently sized central jets – Coanda EffectHigh pressure jet will appear larger than a low-pressure jet for the same amount of flowAs gain increases, jet appears largerAs ultrasound output power increases, jet area increasesLowering PRF makes the jet largerIncreasing the transducer frequency makes the jet appear larger


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.


To Summarise..

Good quality image is a compromise between 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.

Basic Physics Of Transoesophageal Echocardiography For The Workshop2

Health & Medicine

Transcript of Basic Physics Of Transoesophageal Echocardiography For The Workshop2