Physics of ultrasound

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Physics 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
  • Physics of ultrasound

    1. 1. Physics of Ultrasound Karen Lamble Jennifer Green European Training Managers
    2. 2. Applications Training for Service – Jennifer Green&Karen Lamble 2 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.
    3. 3. Applications Training for Service – Jennifer Green&Karen Lamble 3 Basic Ultrasound Physics Phase Frequency Amplitude Wavelength
    4. 4. Applications Training for Service – Jennifer Green&Karen Lamble 4 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
    5. 5. Applications Training for Service – Jennifer Green&Karen Lamble 5 Frequency • Number of cycles per second • Units are Hertz • Ultrasound imaging frequency range 2-20Mhz
    6. 6. Applications Training for Service – Jennifer Green&Karen Lamble 6 Wavelength • Distance over which one cycle occurs
    7. 7. Applications Training for Service – Jennifer Green&Karen Lamble 7 Velocity (v), Frequency (ƒ), & Wavelength ( )λ • Given a constant velocity, as frequency increases wavelength decreases V = ƒ λ
    8. 8. Applications Training for Service – Jennifer Green&Karen Lamble 8 Amplitude • The strength/intensity of a sound wave at any given time • Represented as height of the wave • Decreases with increasing depth
    9. 9. Applications Training for Service – Jennifer Green&Karen Lamble 10 Interactions of Ultrasound with tissue • Reflection • Transmission • Attenuation • Scattering
    10. 10. Applications Training for Service – Jennifer Green&Karen Lamble 11 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
    11. 11. Applications Training for Service – Jennifer Green&Karen Lamble 12 • Not all the sound wave is reflected, some continues deeper into the body • These waves will reflect from deeper tissue structures TransducerTransducer Transmission
    12. 12. Applications Training for Service – Jennifer Green&Karen Lamble 13 • The deeper the wave travels in the body, the weaker it becomes • The amplitude of the wave decreases with increasing depth Attenuation
    13. 13. Applications Training for Service – Jennifer Green&Karen Lamble 14 Scattering • Redirection of sound in several directions • Caused by interaction with small reflector or rough surface • Only portion of sound wave returns to transducer
    14. 14. Applications Training for Service – Jennifer Green&Karen Lamble 15 Goal of an Ultrasound System • The ultimate goal of any ultrasound system is to make like tissues look alike and unlike tissues look different. • An echocardiography system has to sample and display structural information at a reasonable frame rate
    15. 15. Applications Training for Service – Jennifer Green&Karen Lamble 16 Accomplishing this goal depends upon... • 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
    16. 16. Applications Training for Service – Jennifer Green&Karen Lamble 17 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
    17. 17. Applications Training for Service – Jennifer Green&Karen Lamble 18 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
    18. 18. Applications Training for Service – Jennifer Green&Karen Lamble 19 Types of Resolution • Spatial Resolution – also called DetailDetail Resolution – the combination of AXIAL and LATERAL resolution – some customers may use this term
    19. 19. Applications Training for Service – Jennifer Green&Karen Lamble 20 Types of Resolution • Contrast Resolution – the ability to resolve two adjacent objects of similar intensity/reflective properties as separate objects
    20. 20. Applications Training for Service – Jennifer Green&Karen Lamble 21 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
    21. 21. Applications Training for Service – Jennifer Green&Karen Lamble 22 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)
    22. 22. Applications Training for Service – Jennifer Green&Karen Lamble 23 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
    23. 23. Applications Training for Service – Jennifer Green&Karen Lamble 25 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
    24. 24. Applications Training for Service – Jennifer Green&Karen Lamble 26 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
    25. 25. Applications Training for Service – Jennifer Green&Karen Lamble 27 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
    26. 26. Applications Training for Service – Jennifer Green&Karen Lamble 28 Tissue Harmonics
    27. 27. Applications Training for Service – Jennifer Green&Karen Lamble 29 Tissue Signature
    28. 28. Applications Training for Service – Jennifer Green&Karen Lamble 30 Tissue Harmonic Imaging
    29. 29. Basic Doppler Principles
    30. 30. Applications Training for Service – Jennifer Green&Karen Lamble 32 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
    31. 31. Applications Training for Service – Jennifer Green&Karen Lamble 33 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
    32. 32. Applications Training for Service – Jennifer Green&Karen Lamble 34 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
    33. 33. Applications Training for Service – Jennifer Green&Karen Lamble 35 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)
    34. 34. Applications Training for Service – Jennifer Green&Karen Lamble 36 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
    35. 35. Applications Training for Service – Jennifer Green&Karen Lamble 37 The Direction of Flow ø < 60 degreesø
    36. 36. Applications Training for Service – Jennifer Green&Karen Lamble 38 The Direction of Flow Cos 900 = 0 no Doppler shift
    37. 37. Applications Training for Service – Jennifer Green&Karen Lamble 40 Velocity of Flow • Measuring the spectral trace provides information about velocity of flow Freq/Velocity Time
    38. 38. Applications Training for Service – Jennifer Green&Karen Lamble 42 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
    39. 39. Applications Training for Service – Jennifer Green&Karen Lamble 43 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
    40. 40. Applications Training for Service – Jennifer Green&Karen Lamble 44 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
    41. 41. Applications Training for Service – Jennifer Green&Karen Lamble 45 Spectral Window cm/s Spectral Analysis
    42. 42. Applications Training for Service – Jennifer Green&Karen Lamble 46 Laminar Flow • Layers of flow (normal) • Slowest at vessel wall • Fastest within center of vessel
    43. 43. Applications Training for Service – Jennifer Green&Karen Lamble 47 Laminar Flow • Disease states disrupt laminar flow
    44. 44. Applications Training for Service – Jennifer Green&Karen Lamble 48 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
    45. 45. Applications Training for Service – Jennifer Green&Karen Lamble 49 Types of Doppler • Advantages of CW – Can accurately display flow of any velocity without aliasing • Disadvantages of CW – Samples everything along the Doppler line – Cannot position the Doppler to listen at a specific area along it’s path
    46. 46. Applications Training for Service – Jennifer Green&Karen Lamble 50 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
    47. 47. Applications Training for Service – Jennifer Green&Karen Lamble 51 Types of Doppler • Advantages of PW – Can sample at a specific site along the Doppler line. The location of the sample is represented by the Sample Volume • Disadvantages of PW – The maximum velocity which can be displayed is limited. The signal will always alias at a given point, based on the transducer frequency.
    48. 48. Applications Training for Service – Jennifer Green&Karen Lamble 52 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.
    49. 49. Applications Training for Service – Jennifer Green&Karen Lamble 53 What defines a good Doppler display? • No background noise • Clean window/envelope in normal flow states • Clear audible signal • Accurate display of velocities
    50. 50. Colour Doppler Imaging
    51. 51. Applications Training for Service – Jennifer Green&Karen Lamble 56 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?
    52. 52. Applications Training for Service – Jennifer Green&Karen Lamble 57 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 and VELOCITY of flow
    53. 53. Applications Training for Service – Jennifer Green&Karen Lamble 58 • 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
    54. 54. Applications Training for Service – Jennifer Green&Karen Lamble 59 Direction of Flow with Colour • Flow direction is determined by the orientation of the transducer to the vessel or chamber
    55. 55. Applications Training for Service – Jennifer Green&Karen Lamble 60 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
    56. 56. Applications Training for Service – Jennifer Green&Karen Lamble 61 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
    57. 57. Applications Training for Service – Jennifer Green&Karen Lamble 62 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
    58. 58. Applications Training for Service – Jennifer Green&Karen Lamble 63 • The SHADING of the colour provides information about the VELOCITY 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
    59. 59. Applications Training for Service – Jennifer Green&Karen Lamble 64 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
    60. 60. Applications Training for Service – Jennifer Green&Karen Lamble 65 Variance Mapping • Ultrasound systems allow the user to utilise variance mapping. – The system analyses each pixel of colour information. Using a certain algorithm it will highlight with a certain colour (green) flow which is turbulent. – Useful visual indicator. – Most machines allow you you to take this mapping off your image post acquisition.
    61. 61. Applications Training for Service – Jennifer Green&Karen Lamble 66 Colour vs. CPA (Colour Power Angio) • Flow information is generated based on the AMPLITUDE or STRENGTH of blood cell motion • CPA image is superimposed on the 2D greyscale image
    62. 62. Applications Training for Service – Jennifer Green&Karen Lamble 67 CPA Display • The colour maps for CPA are represented by a single continuous colour • CPA does not provide directional information • CPA does not display aliasing
    63. 63. Applications Training for Service – Jennifer Green&Karen Lamble 68 CPA vs. Colour • CPA has – Better sensitivity to flow states – Less angle dependency than traditional colour • However – It is more sensitive to motion artefacts
    64. 64. Applications Training for Service – Jennifer Green&Karen Lamble 71 What is PMI ? Proprietary Doppler technique that looks at high amplitude low velocity signals. (Available only on HDI systems)
    65. 65. Applications Training for Service – Jennifer Green&Karen Lamble 72 What is PMI ? PMI uses the same principles as CPA only the reverse! In CPA we are looking at low amplitude, relatively high velocity signals (blood pools). Whereas in PMI we are looking for high amplitude, low velocity signals (valve and wall structures).
    66. 66. Applications Training for Service – Jennifer Green&Karen Lamble 74 Power Motion Imaging • Enhanced Doppler imaging technology • Less angle dependent • Improves visualisation of wall motion • Excels on technically difficult patients

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