Basic Physics Of Transoesophageal Echocardiography For The Workshop2

Physics of Echocardiography Dr. Anil Kumar H.R Junior 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.

I will be discussing about..
Ultrasound and its properties
Interactions of ultrasound with tissues
Instrumentation and Image formation by ultrasound
Doppler effect and its applications

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

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 material
Inversely 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
Attenuation
Reflection
Scattering
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 imaging
From 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 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 = ρ v”
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 echoes
Result 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
“ P iezoelectricity " 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

Piezoelectric crystal
High frequency electrical signal with continuously changing polarity
Crystal resonates with high frequency
Producing ULTRASOUND
Directed towards the area to be imaged
Crystal “listens” for the returning echoes for a given period of time
Reflected waves converted to electric signals by the crystal
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 space
Two 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 time
Anything that requires more time will decrease temporal resolution
Determinants:
Depth
Sweep angle
Line density
PRF

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 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..

The reflected signal can be displayed in four modes..
A- mode
B- mode
M- mode
2-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 M otion 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 cycle
Along 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 information
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.

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 interpret
2-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 So, both are complementary to each other 2-D Doppler Ultrasound target Tissue Blood Goal of diagnosis Anatomy Physiology Type of information Structural Functional

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 1 Small wavelength High 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 f d / 2 f o cos 
V – target velocity
C – speed of sound in tissue
f d –frequency shift
f o –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 90 0 /270 0 ; 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 20 0 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   f d / cos  V = c f d / 2 f o cos  V   f d

Applications of Doppler - Different modes to measure blood velocities
Continuous wave
Pulsed wave
Colour Flow Mapping

Modern echo scanners combine Doppler capabilites with 2 D 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 localized
Most common use – Quantification of pressure drop across a stenosis by applying Bernoulli equation

1/2 PV 2 Pressure Kinetic Energy Potential Energy P = 4V 2 Bernoulli Equation Balancing 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 rate
RANGE GATED - The instrument only listens for a very brief and fixed time after the transmission of ultrasound pulse
Depth 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 image
Imparts more spatial information to Doppler data
Displays real-time blood flow with in the heart as colors while showing 2D images in gray scale
Allows 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 Effect
High pressure jet will 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 PRF makes the jet larger
Increasing 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

by aniyahr on Apr 10, 2008

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Basic Physics Of Transoesophageal Echocardiography For The Workshop2 — Presentation Transcript

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