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Basic Physics Of Transoesophageal Echocardiography For The Workshop2

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Basics Physics of ultrasound
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Basic Physics Of Transoesophageal Echocardiography For The Workshop2

  1. 1. Physics of Echocardiography Dr. Anil Kumar H.R Junior Consultant Department of Anaesthesiology Narayana Hrudayalaya
  2. 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. 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. 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. 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
  6. 6. 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
  7. 7. <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>
  8. 8. Sound velocity in different materials Material Velocity ( m/s) Air 330 Water 1497 Metal 3000 - 6000 Fat 1440 Blood 1570 Soft tissue 1540
  9. 9. 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. 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>
  11. 11. 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. 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. 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
  14. 14. 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>
  15. 16. How is ultrasound imaging done? “ From sound to image”
  16. 17. Pierre Curie (1859-1906), Nobel Prize in Physics, 1903 Jacques Curie (1856-1941) PIEZOELECTRIC EFFECT
  17. 18. <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>
  18. 19. Construction of a Transducer Backing Material Electrodes Piezoelectric crystal
  19. 20. Electronic Phased Array which uses the principle of Electronic Delay Phased Array Transducers
  20. 21. Electronic Focusing Electronic beam steering
  21. 22. Characteristics of ULTRASOUND BEAM
  22. 23. Length of near field = ( radius) 2 / wavelength of emitted ultrasound
  23. 24. <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>
  24. 25. Schematic representation of the recording and display of the 2-D image
  25. 26. Our TEE Work Station..
  26. 27. 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>
  27. 28. <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>
  28. 29. <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>
  29. 30. 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>
  30. 31. The Trade off ..
  31. 32. <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..
  32. 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>
  33. 34. 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
  34. 35. 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>
  35. 36. 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>
  36. 37. M-mode beam through Mitral Valve M-Mode Imaging
  37. 38. 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>
  38. 39. 2-D imaging by steering the transducer over an area that needs to be imaged
  39. 40. Mechanical Steering of the Transducer
  40. 41. Electronic Phased Array Transducers for 2-D imaging Linear Array Curvilinear Array
  41. 42. A single ‘FRAME’ being formed from one full sweep of beams A ‘CINE LOOP’ from multiple FRAMES
  42. 43. <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>
  43. 44. <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
  44. 45. 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
  45. 46. Christian Andreas Doppler (1803 – 1853) DOPPLER EFFECT
  46. 47. 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>
  47. 48. OBSERVER 2 Long wavelength Low frequency OBSERVER 1 Small wavelength High frequency
  48. 49. 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..
  49. 50. 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>
  50. 51. Doppler Equation
  51. 52. <ul><li>Doppler blood flow velocities are displayed as waveforms </li></ul>
  52. 53. <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 !
  53. 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>
  54. 55. Importance of being parallel to flow when detecting flow through the aortic valve
  55. 56. 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
  56. 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>
  57. 58. <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>
  58. 59. CONTINUOUS WAVE DOPPLER <ul><li>Continuous generation of ultrasound waves coupled with continuous ultrasound reception using a two crystal transducer </li></ul>
  59. 60. CWD at LVOT in Deep TG Aortic Long axis view
  60. 61. <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>
  61. 62. 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..
  62. 63. 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>
  63. 64. Transducer alternately transmits and receives the ultrasound data to a sample volume. Also known as Range-gated Doppler.
  64. 65. PWD at LVOT in Deep TG aortic long axis view
  65. 66. <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>
  66. 67. 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>
  67. 68. 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
  68. 69. <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>
  69. 70. <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>
  70. 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>
  71. 72. Multigated, PW Doppler in which blood flow velocities are sampled at many locations along many lines covering the entire imaging sector
  72. 73. Echo data is processed through two channels that ultimately combine the image with the color flow data in the final display.
  73. 74. 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>
  74. 75. 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
  75. 76. 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>
  76. 77. 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>
  77. 78. 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>

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