Basics Physics of ultrasound

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Training Material inherited form Philips Basics of Ultrasonography. Covers the fundamentals of Ultrasound Waveform, Piezoelectric Effect, Phased Echo Concept, Goal of Ultrasound, Ultrasound Image Construction process, Types of Resolution, Probe Internals, The Doppler Effect, Spectrum Waveform and concept, Color Doppler, Components of Ultrasound.

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  • V = velocity of blood flow
    F = transmit frequency of transducer
    cos ø = angle of incidence
    C = speed of sound in soft tissue
  • Basics Physics of ultrasound

    1. 1. Physics (basics) of Ultrasound Ravindran Padmanabhan Aarthi Scans
    2. 2. Applications Training for Service – Ravindran Padmanabhan 2 What is Sound ? • Mechanical and Longitudinal waves wave that can transfer a distance using a media. • Cannot travel throughVacuum.
    3. 3. Applications Training for Service – Ravindran Padmanabhan 3 What is Ultrasound? • Ultrasound is a mechanical, longitudinal wave with a frequency exceeding the upper limit of human hearing, which is 20,000 Hz or 20 kHz. • Typically at 2 – 20 Mhz.
    4. 4. Applications Training for Service – Ravindran Padmanabhan 4 Basic Ultrasound Physics Velocity Frequency Amplitude Wavelength
    5. 5. Applications Training for Service – Ravindran Padmanabhan 5 Velocity • Speed at which a sound wave travels through a medium(cm/sec) • Determined by density and stiffness of media – Slowest in air/gas – Fastest in solids • Average speed of ultrasound in body is 1540m/sec
    6. 6. Applications Training for Service – Ravindran Padmanabhan 6 Velocity Near Field Imaging Far Field Imaging Tissues closer appear on top and faster the waves return Tissues further appear at the bottom & slower the waves return
    7. 7. Applications Training for Service – Ravindran Padmanabhan 7 Frequency • Number of cycles per second • Units are Hertz • Ultrasound imaging frequency range 2-20Mhz
    8. 8. Applications Training for Service – Ravindran Padmanabhan 8 Frequency Higher the freq Lower the penetration and Higher the resolution Low the freq higher the penetration and lower the resolution
    9. 9. Applications Training for Service – Ravindran Padmanabhan 9 Wavelength • Distance over which one cycle occurs
    10. 10. Applications Training for Service – Ravindran Padmanabhan 10 Velocity (v), Frequency (ƒ), & Wavelength ( )λ • Given a constant velocity, as frequency increases wavelength decreases V = ƒ λ
    11. 11. Applications Training for Service – Ravindran Padmanabhan 11 Amplitude • The strength/intensity of a sound wave at any given time • Represented as height of the wave • Decreases with increasing depth
    12. 12. Applications Training for Service – Ravindran Padmanabhan 12 Amplitude Defines the Brightness of the image Irrespective of the Freq the Amp remains constant The Higher the Amp the brighter the image and the lower the more darker the images Returning Waves
    13. 13. Applications Training for Service – Ravindran Padmanabhan 13 How Ultrasound Works… • How does an ultrasound machine make an image ?
    14. 14. Applications Training for Service – Ravindran Padmanabhan 14 Piezoelectric Effect of Ultrasound 1. Electrical Energy converted to Sound waves 2. The Sound waves are reflected by tissues 3. Reflected Sound waves are converted to electrical signals and later to Image
    15. 15. Applications Training for Service – Ravindran Padmanabhan 15 Pulse-Echo Method • Ultrasound transducer produces “pulses” of ultrasound waves • These waves travel within the body and interact with various tissues • The reflected waves return to the transducer and are processed by the ultrasound machine • An image which represents these reflections is formed on the monitor
    16. 16. Applications Training for Service – Ravindran Padmanabhan 16 Interactions of Ultrasound with tissue • Reflection • Transmission • Attenuation • Scattering
    17. 17. Applications Training for Service – Ravindran Padmanabhan 17 Reflection – Occurs at a boundary between 2 adjacent tissues or media – The amount of reflection depends on differences in acoustic impedance (z) between media – The ultrasound image is formed from reflected echoes TransducerTransducer Z = Density x Velocity
    18. 18. Applications Training for Service – Ravindran Padmanabhan 18 Scattering • Redirection of sound in several directions • Caused by interaction with small reflector or rough surface • Only portion of sound wave returns to transducer
    19. 19. Applications Training for Service – Ravindran Padmanabhan 19 • Not all the sound wave is reflected, some continues deeper into the body • These waves will reflect from deeper tissue structures TransducerTransducer Transmission
    20. 20. Applications Training for Service – Ravindran Padmanabhan 20 • The deeper the wave travels in the body, the weaker it becomes • The amplitude of the wave decreases with increasing depth Attenuation
    21. 21. Applications Training for Service – Ravindran Padmanabhan 21 Attenuation High Freq TheLongerittravelstheTheLongerittravelsthe morelessResolutionmorelessResolution Low Freq Absorption Reflection TheSignalbehindtheboneTheSignalbehindthebone isAne-echoicisAne-echoic Attenuation Co-efficient •Gas | Fluid – Low AC – allow Wave to passGas | Fluid – Low AC – allow Wave to pass •Bones – High AC – will blockBones – High AC – will block
    22. 22. Applications Training for Service – Ravindran Padmanabhan 22 Summary of Ultrasound Attributes
    23. 23. Applications Training for Service – Ravindran Padmanabhan 23 Echogenicity (caused by Reflection) Ane-Echoic Hypeo-Echoic Hyper-Echoic
    24. 24. Applications Training for Service – Ravindran Padmanabhan 24 Time Gain Compensation (Post Processing) • The Returned signal is amplified for better imaging at depths. TGC too low far field TGC corrected
    25. 25. Applications Training for Service – Ravindran Padmanabhan 25 Goal of an Ultrasound System • The ultimate goal of any ultrasound system is to make like tissues look alike and unlike tissues look different.
    26. 26. Applications Training for Service – Ravindran Padmanabhan 26 • Acoustic Impedance • Resolving capability of the system – axial/lateral resolution – spatial resolution – contrast resolution – temporal resolution • Beamformation – send and receive • Processing Power – ability to capture, preserve and display the information Accomplishing this goal depends upon...
    27. 27. Applications Training for Service – Ravindran Padmanabhan 27 Acoustic Impedance • The product of the tissue’s density and the sound velocity within the tissue • Amplitude of returning echo is proportional to the difference in acoustic impedance between the two tissues • Velocities: – Soft tissues = 1400-1600m/sec – Bone = 4080 – Air = 330 • Thus, when an ultrasound beam encounters two regions of very different acoustic impedances, the beam is reflected or absorbed – Cannot penetrate – Example: soft tissue – bone interface
    28. 28. Applications Training for Service – Ravindran Padmanabhan 28 Acoustic Impedance • Two regions of very different acoustic impedances, the beam is reflected or absorbed
    29. 29. Applications Training for Service – Ravindran Padmanabhan 29 Types of Resolution • Axial Resolution – specifies how close together two objects can be along the axis of the beam, yet still be detected as two separate objects – frequency (wavelength) affects axial resolution
    30. 30. Applications Training for Service – Ravindran Padmanabhan 30 Types of Resolution • Lateral Resolution – the ability to resolve two adjacent objects that are perpendicular to the beam axis as separate objects – beamwidth affects lateral resolution
    31. 31. Applications Training for Service – Ravindran Padmanabhan 31 Types of Resolution • Spatial Resolution – also called DetailDetail Resolution – the combination of AXIAL and LATERAL resolution – some customers may use this term
    32. 32. Applications Training for Service – Ravindran Padmanabhan 32 Types of Resolution • Contrast Resolution – the ability to resolve two adjacent objects of similar intensity/reflective properties as separate objects
    33. 33. Applications Training for Service – Ravindran Padmanabhan 33 Types of Resolution • Temporal Resolution – the ability to accurately locate the position of moving structures at particular instants in time – also known as frame rate • VERY IMPORTANT IN CARDIOLOGY
    34. 34. Applications Training for Service – Ravindran Padmanabhan 34 What determines how far ultrasound waves can travel? • The FREQUENCY of the transducer – The HIGHER the frequency, the LESS it can penetrate – The LOWER the frequency, the DEEPER it can penetrate – Attenuation is directly related to frequency • The frequency of a transducer is labeled in Megahertz (MHz)
    35. 35. Applications Training for Service – Ravindran Padmanabhan 35 Frequency vs. Resolution • The frequency also affects the QUALITY of the ultrasound image – The HIGHERHIGHER the frequency, the BETTERBETTER the resolution – The LOWERLOWER the frequency, the LESSLESS the resolution
    36. 36. Applications Training for Service – Ravindran Padmanabhan 36 Frequency vs. Resolution • A 12 MHz transducer has very good resolution, but cannot penetrate very deep into the body • A 3 MHz transducer can penetrate deep into the body, but the resolution is not as good as the 12 MHz
    37. 37. Applications Training for Service – Ravindran Padmanabhan 37 How is an image formed on the monitor? • The amplitude of each reflected wave is represented by a dot • The position of the dot represents the depth from which the echo is received • The brightness of the dot represents the strength of the returning echo • These dots are combined to form a complete image
    38. 38. Applications Training for Service – Ravindran Padmanabhan 38 Position of Reflected Echoes • How does the system know the depth of the reflection? • TIMING – The system calculates how long it takes for the echo to return to the transducer – The velocity in tissue is assumed constant at 1540m/sec Velocity = Distance x Time 2
    39. 39. Applications Training for Service – Ravindran Padmanabhan 39 Reflected Echoes • Strong Reflections = White dots – Pericardium, calcified structures,diaphragm • Weaker Reflections = Grey dots – Myocardium, valve tissue, vessel walls,liver • No Reflections = Black dots – Intra-cardiac cavities,gall bladder
    40. 40. Applications Training for Service – Ravindran Padmanabhan 40 Ultrasound - Internals
    41. 41. Applications Training for Service – Ravindran Padmanabhan 41 How Ultrasound Image is constructed 1. Electrical Energy converted to Sound waves 2. The Sound waves are reflected by tissues 3. Reflected Sound waves are converted to electrical signals and later to Image  Acoustic ImpedanceAcoustic Impedance  VelocityVelocity  FrequencyFrequency  ReflectionReflection  AmplificationAmplification
    42. 42. Applications Training for Service – Ravindran Padmanabhan 45 Probes (Explain) Phased Array or Cardiac Linear or Vascular Curvilinear or Abdomen Range is ideal to switch between General, High and low ResolutionRange is ideal to switch between General, High and low Resolution
    43. 43. Applications Training for Service – Ravindran Padmanabhan 46 Probe Orientation
    44. 44. Applications Training for Service – Ravindran Padmanabhan 47 Imaging Planes • Transverse or Axial • Longitudinal or Saggital • Coronal
    45. 45. Basic Doppler Principles
    46. 46. Applications Training for Service – Ravindran Padmanabhan 49 The Doppler Effect • Apparent change in received frequency due to a relative motion between a sound source and sound receiver – Sound TOWARD receiver = frequency – Sound AWAY from receiver = frequency
    47. 47. Applications Training for Service – Ravindran Padmanabhan 50 Doppler in Ultrasound • Used to evaluate and quantify blood flow – Transducer is the sound source and receiver – Flow is in motion relative to the transducer • Doppler produces an audible signal as well as a graphical representation of flow = Spectral Waveform
    48. 48. Applications Training for Service – Ravindran Padmanabhan 51 Doppler in Ultrasound • The Doppler shift produced by moving blood flow is calculated by the ultrasound system using the following equation: Doppler Frequency Shift = 2vf cos ø C
    49. 49. Applications Training for Service – Ravindran Padmanabhan 52 Doppler Display • The spectral waveform represents the audible signal and provides information about: – the direction of the flow – how fast the flow is traveling (velocity) – the quality of the flow (normal vs. abnormal)
    50. 50. Applications Training for Service – Ravindran Padmanabhan 53 The Direction of Flow • Flow coming TOWARD the transducer is represented above the baseline • Flow traveling AWAY from the transducer is represented below the baseline Zero Baseline
    51. 51. Applications Training for Service – Ravindran Padmanabhan 54 The Direction of Flow ø < 60 degreesø
    52. 52. Applications Training for Service – Ravindran Padmanabhan 55 The Direction of Flow Cos 900 = 0 no Doppler shift
    53. 53. Applications Training for Service – Ravindran Padmanabhan 56 The Direction of Flow ø Cos 600 = 1
    54. 54. Applications Training for Service – Ravindran Padmanabhan 57 Velocity of Flow • Measuring the spectral trace provides information about velocity of flow Freq/Velocity Time
    55. 55. Applications Training for Service – Ravindran Padmanabhan 58 Velocity of Flow Faster Flow Slower Flow cm/scm/s
    56. 56. Applications Training for Service – Ravindran Padmanabhan 59 What if the velocity is too high to display? • This effect is called ALIASING (wraparound) • The Doppler sample rate is not adequate for high velocity shifts • The ‘peaks’ are cut off and displayed below baseline cm/s
    57. 57. Applications Training for Service – Ravindran Padmanabhan 60 Quality of Flow • Some examples of common measurements of the trace provide values such as: – Maximum and mean velocity – Resistance Index (RI) – Pulsatility Index (PI) – Acceleration and Deceleration Times – Volume flows, shunts, pressure gradients
    58. 58. Applications Training for Service – Ravindran Padmanabhan 61 Spectral Analysis • Each of these measurements has a normal range of values for specific clinical applications • The amount of disease present is based on these calculated values • Also, the ENVELOPE or WINDOW provides information about the quality of flow
    59. 59. Applications Training for Service – Ravindran Padmanabhan 62 Spectral Window cm/s Spectral Analysis
    60. 60. Applications Training for Service – Ravindran Padmanabhan 63 Laminar Flow • Layers of flow (normal) • Slowest at vessel wall • Fastest within center of vessel
    61. 61. Applications Training for Service – Ravindran Padmanabhan 64 Laminar Flow • Disease states disrupt laminar flow
    62. 62. Applications Training for Service – Ravindran Padmanabhan 65 Types of Doppler • Continuous Wave (CW) – Uses different crystals to send and receive the signal – One crystal constantly sends a sound wave of a single frequency, the other constantly receives the reflected signal
    63. 63. Applications Training for Service – Ravindran Padmanabhan 67 Types of Doppler • Pulsed Wave (PW) – Produces short bursts/pulses of sound – Uses the same crystals to send and receive the signal – This follows the same pulse-echo technique used in 2D image formation
    64. 64. Applications Training for Service – Ravindran Padmanabhan 69 Types of Doppler • Continuous Wave is widely used in Cardiology Applications – Requires ability to display very high velocities without aliasing • Pulsed Wave is used in all clinical applications.
    65. 65. Applications Training for Service – Ravindran Padmanabhan 70 What defines a good Doppler display? • No background noise • Clean window/envelope in normal flow states • Clear audible signal • Accurate display of velocities
    66. 66. Applications Training for Service – Ravindran Padmanabhan 71 Clinical Challenges with Doppler • Obtaining the appropriate Doppler angle • Doppler in deep vessels/large patients • Displaying hwithout aliasing, found in abnormal flow states (PW Doppler) • Exact site for sampling
    67. 67. Colour Doppler Imaging
    68. 68. Applications Training for Service – Ravindran Padmanabhan 73 Why use Colour? • To visualise blood flow and differentiate it from surrounding tissue • Provides a ‘map’ for placing a Doppler sample volume • The basic questions are – Is there flow present? – What direction is it traveling? – How fast is it traveling?
    69. 69. Applications Training for Service – Ravindran Padmanabhan 74 What is Colour Doppler? • Utilizes pulse-echo Doppler flow principles to generate a colour image • This image is superimposed on the 2D image • The red and blue display provides information regarding DIRECTION andVELOCITY of flow
    70. 70. Applications Training for Service – Ravindran Padmanabhan 75 • Regardless of colour, the top of the bar represents flow coming towards the transducer and the bottom of the bar represents flow away from the transducer Flow Toward Flow Away Direction of Flow with Colour
    71. 71. Applications Training for Service – Ravindran Padmanabhan 76 Direction of Flow with Colour • Flow direction is determined by the orientation of the transducer to the vessel or chamber
    72. 72. Applications Training for Service – Ravindran Padmanabhan 77 Direction of Flow with Colour • Linear array transducers display a square shaped colour box which STEERS from left to right to provide a Doppler angle • All other transducers display a sector shaped colour box which is adjusted in position by using the trackball
    73. 73. Applications Training for Service – Ravindran Padmanabhan 78 ROI. • Position: – The box must approach the vessel/heart chamber at an angle other than 90 degrees – Following the Doppler principle, there will be little or no colour displayed at perpendicular incidence • Size – The larger the ROI the lower the frame rate
    74. 74. Applications Training for Service – Ravindran Padmanabhan 79 Velocity of Flow with Colour • Unlike PW or CW Doppler, colour estimates a mean (average) velocity using the autocorrelation technique – each echo is correlated with the corresponding echo from the previous pulse, thus determining the motion that has occurred during each pulse
    75. 75. Applications Training for Service – Ravindran Padmanabhan 80 • The SHADING of the colour provides information about theVELOCITY of the flow – The DEEPER the shade of red or blue, the slower the flow – The LIGHTER the shade, the faster the flow Velocity of Flow with Colour
    76. 76. Applications Training for Service – Ravindran Padmanabhan 81 Velocity of Flow with Colour Q What if the flow velocity is traveling faster than the lightest shade represented on the map? A ALIASING occurs – the colour “wraps” to the opposite color – this aids in placement of sample volume
    77. 77. Applications Training for Service – Ravindran Padmanabhan 88 What is Power Motion Imaging ? Doppler technique that looks at high amplitude low velocity signals.
    78. 78. Applications Training for Service – Ravindran Padmanabhan 89 Power Motion Imaging TM
    79. 79. Applications Training for Service – Ravindran Padmanabhan 91 Power Motion Imaging • Enhanced Doppler imaging technology • Less angle dependent • Improves visualisation of wall motion • Excels on technically difficult patients
    80. 80. Applications Training for Service – Ravindran Padmanabhan 92 Power Motion Imaging • Helps to visualise the endocardium and segmental wall motion – more reliable identification of lateral left ventricular wall – enhances stress echo studies
    81. 81. Applications Training for Service – Ravindran Padmanabhan 93 Components of Ultrasound

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