1.
Physics of Echocardiography Dr. Anil Kumar H.R Junior Consultant Department of Anaesthesiology Narayana Hrudayalaya

2.
<ul><li>History of Ultrasound Imaging </li></ul><ul><ul><li>1760 - Abbe Lazzaro Spallanzani – Father of ultrasound </li></ul></ul><ul><ul><li>1912 - First practical application for rather unsuccessful search for Titanic </li></ul></ul><ul><ul><li>1942 - First used as diagnostic tool for localizing brain tumors by Karl Dussik </li></ul></ul><ul><ul><li>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 </li></ul></ul><ul><ul><li>1970s - Origin of TEE , Lee Frazin, a cardiologist from Chicago mounts M-mode probe on a Transoesophageal probe. </li></ul></ul>

3.
I will be discussing about.. <ul><li>Ultrasound and its properties </li></ul><ul><li>Interactions of ultrasound with tissues </li></ul><ul><li>Instrumentation and Image formation by ultrasound </li></ul><ul><li>Doppler effect and its applications </li></ul>

4.
Sound <ul><li>Mechanical vibration transmitted through an elastic medium </li></ul><ul><li>Pressure waves when propagate thro’ air at appropriate frequency produce sensation of hearing </li></ul>Surface Vibration Pressure Wave Ear Vibration Propagation Perception

5.
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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<ul><li>Frequency – No. of wavelenghths per unit time </li></ul><ul><li>1 cycle/ sec = 1 Hz </li></ul><ul><li>So, Frequency is inversely related to wavelength </li></ul><ul><li>Velocity – Speed at which waves propagate through a medium </li></ul><ul><ul><li>Dependent on physical properties of the medium through which it travels </li></ul></ul><ul><ul><li>Directly proportional to stiffness of the material </li></ul></ul><ul><ul><li>Inversely proportional to density till a physiological limit </li></ul></ul><ul><ul><li>Velocity = frequency * Wavelength </li></ul></ul>

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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 <ul><li>Ultrasound is sound with a frequency over 20,000 Hz, which is the upper limit of human hearing. </li></ul><ul><li>The basic principles and properties are same as that of audible sound </li></ul><ul><li>Frequencies used for diagnostic ultrasound are between 1 to 20 MHz </li></ul>

10.
Interaction of ultrasound wave with tissues <ul><li>Attenuation </li></ul><ul><li>Reflection </li></ul><ul><li>Scattering </li></ul><ul><li>Absorption </li></ul>

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Attenuation <ul><li>Loss of intensity and amplitude of ultrasound wave as it travels through the tissues </li></ul><ul><li>Due to reflection, scattering and absorption </li></ul><ul><li>Proportional to Frequency and the distance the wave front travels – </li></ul><ul><ul><li>Higher frequency , more attenuation </li></ul></ul><ul><ul><li>Longer the distance (Depth), more the attenuation </li></ul></ul><ul><li>And also on the type of tissue through which the beam has to pass </li></ul><ul><li>Expressed as “Half – power distance” </li></ul><ul><li>For most of soft tissues it is 0.5 – 1.0 dB/cm/MHz </li></ul>

12.
Reflection <ul><li>Basis of all ultrasound imaging </li></ul><ul><li>From relatively large, regularly shaped objects with smooth surfaces and lateral dimensions greater than one wavelength – Specular Echoes </li></ul><ul><li>These echoes are relatively intense and angle dependent. </li></ul><ul><li>From endocardial and epicardial surfaces, valves and pericardium </li></ul><ul><li>Amount of ultrasound beam that is reflected depends on the difference in Acoustic impedance between the mediums </li></ul>

13.
<ul><li>The resistance that a material offers to the passage of sound wave </li></ul><ul><li>Velocity of propagation “ v ” varies between different tissues </li></ul><ul><li>Tissues also have differing densities “ ρ ” </li></ul><ul><li>Acoustic impedance </li></ul><ul><li>“ Z = ρ v” </li></ul><ul><li>Soft tissue / bone and soft tissue / air interfaces have large “Acoustic Impedance mismatch” </li></ul>Acoustic Impedance

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

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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) PIEZOELECTRIC EFFECT

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<ul><li>Crystals of tourmaline, quartz, topaz, cane sugar, and Rochelle salt have the ability to generate an electric charge in response to applied mechanical stress </li></ul><ul><li>“ P iezoelectricity &quot; after the Greek word Piezein , which means to squeeze or press. </li></ul><ul><li>“ Converse” of this effect is also true </li></ul>

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Construction of a Transducer Backing Material Electrodes Piezoelectric crystal

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Electronic Phased 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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<ul><li>Piezoelectric crystal </li></ul><ul><li>High frequency electrical signal with continuously changing polarity </li></ul><ul><li>Crystal resonates with high frequency </li></ul><ul><li>Producing ULTRASOUND </li></ul><ul><li>Directed towards the area to be imaged </li></ul><ul><li>Crystal “listens” for the returning echoes for a given period of time </li></ul><ul><li>Reflected waves converted to electric signals by the crystal </li></ul><ul><li>processed and displayed </li></ul>

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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 <ul><li>Ability to distinguish two points in space </li></ul><ul><li>Two components – </li></ul><ul><ul><li>Spatial – Smallest distance that two targets can be seperated for the system to distinguish between them. </li></ul></ul><ul><ul><ul><li>Two components – Axial and Lateral </li></ul></ul></ul><ul><ul><li>Temporal </li></ul></ul>

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<ul><li>Axial Resolution </li></ul><ul><ul><li>The minimum separation between structures the ultrasound beam can distinguish parallel to its path. </li></ul></ul><ul><ul><li>Determinants: </li></ul></ul><ul><ul><ul><li>Wavelength – smaller the better </li></ul></ul></ul><ul><ul><ul><li>Pulse length – shorter the train of cycles greater the resolution </li></ul></ul></ul>

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<ul><li>Lateral Resolution </li></ul><ul><ul><li>Minimum separation between structures the ultrasound beam can distinguish in a plane perpendicular to its path. </li></ul></ul><ul><ul><li>Determinants: </li></ul></ul><ul><ul><ul><li>Depends on beam width – smaller the better </li></ul></ul></ul><ul><ul><ul><li>Depth </li></ul></ul></ul><ul><ul><ul><li>Gain </li></ul></ul></ul>

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Temporal resolution <ul><li>Ability of system to accurately track moving targets over time </li></ul><ul><li>Anything that requires more time will decrease temporal resolution </li></ul><ul><li>Determinants: </li></ul><ul><ul><li>Depth </li></ul></ul><ul><ul><li>Sweep angle </li></ul></ul><ul><ul><li>Line density </li></ul></ul><ul><ul><li>PRF </li></ul></ul>

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The Trade off ..

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<ul><li>To visualise smaller objects shorter wavelengths should be used which can be obtained by increasing frequency of U/S wave. </li></ul><ul><li>Drawbacks of high frequency – </li></ul><ul><ul><li>More scatter by insignificant inhomogeneity </li></ul></ul><ul><ul><li>More attenuation </li></ul></ul><ul><ul><li>Limited depth of penetration </li></ul></ul><ul><li>For visualising deeper objects lower frequency is useful, but will be at the cost of poor resolution </li></ul>So..

33.
<ul><li>The reflected signal can be displayed in four modes.. </li></ul><ul><ul><li>A- mode </li></ul></ul><ul><ul><li>B- mode </li></ul></ul><ul><ul><li>M- mode </li></ul></ul><ul><ul><li>2-Dimensional </li></ul></ul>

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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

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M - mode <ul><li>Timed M otion display ; B – Mode with time reference </li></ul><ul><li>A diagram that shows how the positions of the structures along the path of the beam change during the course of the cardiac cycle </li></ul><ul><li>Strength of the returning echoes vertically and temporal variation horizontally </li></ul>

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M – Mode uses.. <ul><li>Great temporal resolution- Updated 1000/sec. Useful for precise timing of events with in a cardiac cycle </li></ul><ul><li>Along with color flow Doppler – for the timing of abnormal flows </li></ul><ul><li>Quantitative measurements of size , distance & velocity possible with out sophisticated analyzing stations </li></ul>

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M-mode beam through Mitral Valve M-Mode Imaging

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2 – D MODE <ul><li>Provides more structural and functional information </li></ul><ul><li>Rapid repetitive scanning along many different radii with in an area in the shape of a fan </li></ul><ul><li>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. </li></ul>

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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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<ul><li>Resembles an anatomic section – easy to interpret </li></ul><ul><li>2-D imaging provides information about the spatial relationships of different parts of the heart to each other. </li></ul><ul><li>Updated 30- 60 times/sec ; lesser temporal resolution compared to M-mode </li></ul>

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<ul><li>Study of blood flow dynamics </li></ul><ul><li>Detects the direction and velocity of moving blood within the heart. </li></ul>Doppler Study

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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

46.
Christian Andreas Doppler (1803 – 1853) DOPPLER EFFECT

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DOPPLER EFFECT- <ul><li>Certain properties of light emitted from stars depend upon the relative motion of the observer and the wave source. </li></ul><ul><li>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. </li></ul>

48.
OBSERVER 2 Long wavelength Low frequency OBSERVER 1 Small wavelength High frequency

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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..

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The Doppler equation <ul><li>Velocity is given by Doppler equation.. </li></ul><ul><li>V = c f d / 2 f o cos  </li></ul><ul><ul><ul><li>V – target velocity </li></ul></ul></ul><ul><ul><ul><li>C – speed of sound in tissue </li></ul></ul></ul><ul><ul><ul><li>f d –frequency shift </li></ul></ul></ul><ul><ul><ul><li>f o –frequency of emitted U/S </li></ul></ul></ul><ul><ul><ul><li> - angle between U/S beam & direction of target velocity( received beam , not the emitted) </li></ul></ul></ul>

51.
Doppler Equation

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<ul><li>Doppler blood flow velocities are displayed as waveforms </li></ul>

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<ul><li>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 </li></ul><ul><li>Flow velocity measured most accurately when beam is either parallel or anti parallel to blood flow. </li></ul><ul><li>Diversion up to 20 0 can be tolerated( error of < or = to 6%) </li></ul>Important consideration !

54.
“ Twin Paradoxes of Doppler” <ul><li>Best Doppler measurements are made when the Doppler probe is aligned parallel to the blood flow </li></ul><ul><li>High quality Doppler signals require low Doppler frequencies( < 2MHz) </li></ul>

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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   f d / cos  V = c f d / 2 f o cos  V   f d

57.
Applications of Doppler - Different modes to measure blood velocities <ul><ul><li>Continuous wave </li></ul></ul><ul><ul><li>Pulsed wave </li></ul></ul><ul><ul><li>Colour Flow Mapping </li></ul></ul>

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<ul><li>Modern echo scanners combine Doppler capabilites with 2 D imaging capabilities </li></ul><ul><li>Imaging mode is switced off (sometimes with the image held in memory) while the Doppler modes are in operation </li></ul>

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CONTINUOUS WAVE DOPPLER <ul><li>Continuous generation of ultrasound waves coupled with continuous ultrasound reception using a two crystal transducer </li></ul>

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CWD at LVOT in Deep TG Aortic Long axis view

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<ul><li>Can measure high velocity flows ( in excess of 7m/sec) </li></ul><ul><li>Lack of selectivity or depth discrimination - Region where flow dynamics are being measured cannot be precisely localized </li></ul><ul><li>Most common use – Quantification of pressure drop across a stenosis by applying Bernoulli equation </li></ul>

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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..

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PULSED WAVE DOPPLER <ul><li>Doppler interrogation at a particular depth rather than across entire line of U/S beam. </li></ul><ul><li>Ultrasound pulses at specific frequency - Pulse Repetition Frequency (PRF) or Sampling rate </li></ul><ul><li>RANGE GATED - The instrument only listens for a very brief and fixed time after the transmission of ultrasound pulse </li></ul><ul><li>Depth of sampling by varied by varying the time delay for sampling </li></ul>

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Transducer alternately transmits and receives the ultrasound data to a sample volume. Also known as Range-gated Doppler.

65.
PWD at LVOT in Deep TG aortic long axis view

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<ul><li>PRF for a given transducer of a given frequency at a particular depth is fixed ; But to measure higher velocities higher PRFs are necessary </li></ul><ul><li>Drawback – ambiguous information obtained when flow velocity is high velocities (above 1.5 to 2 m/sec) </li></ul><ul><li>This effect is called Aliasing </li></ul>

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ALIASING <ul><li>Aliasing will occur if low pulse repetition frequencies or velocity scales are used and high velocities are encountered </li></ul><ul><li>Abnormal velocity of sample volume exceeds the rate at which the pulsed wave system can record it properly. </li></ul><ul><li>Blood velocities appear in the direction opposite to the conventional one </li></ul>

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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

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<ul><li>Aliasing occurs if the frequency of the sample volume is more than the Nyquist limit </li></ul><ul><li>Nyquist limit = PRF/2 </li></ul>

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<ul><li>To avoid Aliasing - PRF = 2 ( Doppler shift frequency or Maximum velocity of Sample volume) </li></ul><ul><li>Can be achieved by – Decreasing the frequency of transducer , decrease the depth of interrogation by changing the view ( this increases the PRF) </li></ul>

71.
Color Flow Doppler <ul><li>Displays flow data on 2-D Echocardiographic image </li></ul><ul><li>Imparts more spatial information to Doppler data </li></ul><ul><li>Displays real-time blood flow with in the heart as colors while showing 2D images in gray scale </li></ul><ul><li>Allows estimation of velocity, direction and pattern of blood flow </li></ul>

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

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

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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

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Instrumentation factors in Color Doppler Imaging <ul><li>Eccentric jets appear smaller than equivalently sized central jets – Coanda Effect </li></ul><ul><li>High pressure jet will appear larger than a low-pressure jet for the same amount of flow </li></ul><ul><li>As gain increases , jet appears larger </li></ul><ul><li>As ultrasound output power increases , jet area increases </li></ul><ul><li>Lowering PRF makes the jet larger </li></ul><ul><li>Increasing the transducer frequency makes the jet appear larger </li></ul>

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To Summarise.. <ul><li>Knowledge of physics helps us appreciate “ why we are seeing what we are seeing, And what we can do to see it better” </li></ul><ul><li>Echocardiography is based on the electrical conversion of reflected ultrasound waves from structures and blood flow within the cardiovascular system. </li></ul>

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To Summarise.. <ul><li>Good quality image is a compromise between resolution and depth of interrogation </li></ul><ul><li>Doppler study complements 2-D echo </li></ul><ul><li>Aligning Doppler beam parallel to direction of target velocity is key to obtaining accurate measurements. </li></ul>