Physics of ultrasound and echocardiography


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  • Velocity on the ordinate and time on abscissa, Flow towards the transucer above the baseline and flow away from the transducer below the baseline
  • Increase in kinetic energy as blood accelerates through a stenosis must be accompanied by a concomitant fall in potential energy represented by pressure across that stenosis
  • Physics of ultrasound and echocardiography

    1. 1. Physics Of Ultrasound And Echocardiography
    2. 2. 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 mounted M-mode probe on a Transoesophageal probe.
    3. 3. Topic outline 1. Echo basics • Tips on Ultrasound waves / interaction with tissues • Ultrasound transducers /probes • Image Resolution 2. Imaging modes • 2-D Imaging & Imaging planes (normal 2D Echo) • M-Mode (Normal M-Mode Echo) 3. Doppler Echo • Basic principles • Doppler Imaging Modalities • CW Doppler • Pulsed Doppler • CF Doppler • Relationship between Doppler velocity and pressure gradient
    4. 4. Sound Mechanical vibration transmitted through an elastic medium Pressure waves when propagate thro’ air at appropriate frequency produce sensation of hearing Vibration Propagation Surface Vibration Pressure Wave Ear
    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. “Sine wave”  Amplitude - maximal compression of particles above the baseline  Wavelength - distance between the two nearest points of equal pressure and density One Compression and rarefaction constitute one sound wave . It can be represented as “Sine wave”.
    7. 7. Velocity = frequency x Wavelength  Frequency – No. of wavelengths 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 within a physiological limit
    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 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
    10. 10.  Medical ultrasound imaging typically uses sound waves at frequencies of 1,000,000 to 20,000,000 Hz (1.0 to 20 MHz). In contrast, the human auditory spectrum (between 20 and 20,000 Hz)  Frequency and wavelength are mathematically related to the velocity of the ultrasound beam within the tissue: Velocity = Wavelength (mm) x frequency (Hz)  The speed with which an acoustic wave moves through a medium is dependent upon the density and resistance of the medium.  Media that are dense will transmit a mechanical wave with greater speed than those that are less dense.  The resolution of a recording, ie, the ability to distinguish two objects that are spatially close together, varies directly with the frequency and inversely with the wavelength High frequency, short wavelength ultrasound can separate objects that are less than 1 mm apart.
    11. 11.  Imaging with higher frequency (and lower wavelength) transducers permits enhanced spatial resolution  However, because of attenuation, the depth of tissue penetration or the ability to transmit sufficient ultrasonic energy into the chest is directly related to wavelength and therefore inversely related to transducer frequency  As a result, the trade-off for use of higher frequency transducers is reduced tissue penetration  The trade-off between tissue resolution and penetration guides the choice of transducer frequency for clinical imaging.  As an example, higher frequency transducers can be used in echocardiography for imaging of structures close to the transducer.
    12. 12. Interaction of ultrasound wave with tissues 1. Attenuation 2. Reflection 3. Scattering 4. Absorption
    13. 13. 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
    14. 14. 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
    15. 15.  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 = ρv”  Soft tissue / bone and soft tissue / air interfaces have large “Acoustic Impedance mismatch” Acoustic Impedance
    16. 16. 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
    17. 17. Interaction Of Ultrasound Waves With Tissues When an ultrasonic wave travels through a homogeneous medium, its path is a straight line. However, when the medium is not homogeneous or when the wave travels through a medium with two or more interfaces, its path is altered; either of the ff:  Scattering:  Small structures, eg, less than 1 wavelength in lateral dimension, result in scattering of the ultrasound signal  Unlike a reflected beam, scattering results in the US beam being radiated in all directions, with minimal signal returning to the transducer  Refraction:  Attenuation:  Signal strength is progressively reduced due to absorption of the US energy by conversion to heat (frequency and, wavelength dependent)  The depth of penetration:  30 cm for a 1 MHz transducer,  12 cm for 2.5 MHz transducer, and  6 cm for a 5 MHz transducer  Air has a very high acoustic impedance, resulting in significant signal attenuation when imaging through lung tissue, especially emphysematous lung, or pathologic conditions such as pneumomediastinum or subcutaneous emphysema  In contrast, filling of the pleural space with fluid, generally enhances ultrasound imaging
    18. 18. How is ultrasound imaging done? “From sound to image”
    19. 19. Pierre Curie (1859-1906), Nobel Prize in Physics, 1903 Jacques Curie (1856-1941) PIEZOELECTRIC EFFECT
    20. 20. 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 “Piezoelectricity" after the Greek word Piezein, which means to squeeze or press. “Converse” of this effect is also true
    21. 21. Construction of a Transducer Backing Material Electrodes Piezoelectric crystal
    22. 22. US transducers use a piezoelectric crystal to generate and receive ultrasound waves Image formation: is related to the distance of a structure from the transducer, based upon the time interval between ultrasound transmission and arrival of the reflected signal The amplitude is proportional to the incident angle and acoustic impedance, and timing is proportional to the distance from the transducer Ultrasound Transducers
    23. 23. Production of ultrasound 1. Piezoelectric crystal 2. High frequency electrical signal with continuously changing polarity 3. Crystal resonates with high frequency 4. Producing ULTRASOUND 5. Directed towards the area to be imaged 6. Crystal “listens” for the returning echoes for a given period of time 7. Reflected waves converted to electric signals by the crystal 8. processed and displayed
    24. 24. Schematic representation of the recording and display of the 2-D image
    25. 25. Electronic Phased Array which uses the principle of Electronic Delay Phased Array Transducers
    26. 26. Electronic Focusing Electronic beam steering
    27. 27. Characteristics of ULTRASOUND BEAM
    28. 28. Length of near field = ( radius)2 / wavelength of emitted ultrasound
    29. 29. TEE workstation
    30. 30. Resolution Ability to distinguish two points in space Two components – Spatial – Smallest distance that two targets can be separated for the system to distinguish between them. Two components – Axial and Lateral Temporal
    31. 31. • 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
    32. 32. • 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
    33. 33. 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
    34. 34. The Trade off ..
    35. 35. To visualize 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..
    36. 36. The reflected signal can be displayed in four modes.. A- mode B- mode M- mode 2-Dimensional
    37. 37. A. Twodimensional (2-D) imaging : – A 2D image is generated from data obtained mechanically (mechanical transducer) or electronically (phased-array transducer) – The signal received undergoes a complex manipulation to form the final image displayed on the monitor including signal amplification, time-gain compensation, filtering, compression and rectification. B. M-mode:  Motion or "M"-mode echocardiography is among the earliest forms of cardiac ultrasound  The very high temporal resolution by M-mode imaging permits: – identification of subtle abnormalities such as fluttering of the anterior mitral leaflet due to aortic insufficiency or movement of a vegetation. – dimensional measurements or changes, such as chamber size and endocardial thickening, can be readily appreciated 2-D & M Mode
    38. 38. 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
    39. 39. M- mode • Timed Motion 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
    40. 40. 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
    41. 41. 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.
    42. 42. 2-D imaging by steering the transducer over an area that needs to be imaged
    43. 43. Mechanical Steering of the Transducer
    44. 44. Electronic Phased Array Transducers for 2-D imaging Linear Array Curvilinear Array
    45. 45. A single ‘FRAME’ being formed from one full sweep of beams A ‘CINE LOOP’ from multiple FRAMES
    46. 46. 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
    47. 47. OPTIMIZATION OF 2-D IMAGES Technical Factors I  TRANSDUCER:  High frequency increases backscatter and resolution but lacks depth penetration  Low-frequency transducers permit good penetration but reduced image resolution  DEPTH:  The deeper the field of the image, the slower the frame rate  The smallest depth that permits display of the region of interest should be employed  FOCUS:  Indicates the region of the image in which the ultrasound beam is narrowest  Resolution is greatest in this region  GAIN:  This function adjusts the displayed amplitude of all received signals
    48. 48. Study of blood flow dynamics Detects the direction and velocity of moving blood within the heart. Doppler Study
    49. 49. Comparison between 2-D and Doppler 2-D Doppler Ultrasound target Tissue Blood Goal of diagnosis Anatomy Physiology Type of information Structural Functional So, both are complementary to each other
    50. 50. Christian Andreas Doppler (1803 – 1853) DOPPLER EFFECT
    51. 51. 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.
    52. 52. OBSERVER 2 Long wavelength Low frequency OBSERVER 1 Small wavelength High frequency
    53. 53. 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..
    54. 54. The Doppler equation Velocity is given by Doppler equation.. V = c fd / 2 fo cos  V – target velocity C – speed of sound in tissue fd –frequency shift fo –frequency of emitted U/S  - angle between U/S beam & direction of target velocity( received beam , not the emitted)
    55. 55. Doppler Equation
    56. 56. Doppler blood flow velocities are displayed as waveforms
    57. 57. When flow is perpendicular to U/S beam angle of incidence will be 900/2700 ; 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 200 can be tolerated( error of < or = to 6%) Important consideration !
    58. 58. “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)
    59. 59. Importance of being parallel to flow when detecting flow through the aortic valve
    60. 60. 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 a fd / cos V = c fd / 2 fo cos V a fd
    61. 61. BASIC PRINCIPLES:  utilizes ultrasound to record blood flow within the cardiovascular system (While M-mode and 2D echo create ultrasonic images of the heart)  is based upon the changes in frequency of the backscatter signal from small moving structures, ie, red blood cells, intercepted by the ultrasound beam
    62. 62.  A moving target will backscatter an ultrasound beam to the transducer so that the frequency observed when the target is moving toward the transducer is higher and the frequency observed when the target is moving away from the transducer is lower than the original transmitter frequency  This Doppler phenomenon is familiar to us as the sound of a train whistle as it moves toward (higher frequency) or away (lower frequency) from the observer  This difference in frequency between the transmitted frequency (F[t]) and received frequency (F[r]) is the Doppler shift:  Doppler shift (F[d]) = F[r] - F[t]
    63. 63. Doppler effect(Pairs of transmitting (T) and receiving (R) transducers): • With a stationary target (panel A): the carrier frequency [f(t)] from the transmitting transducer strikes the target and is reflected back to the receiving transducer at the reflected frequency [f(r)], which is unaltered • with a target moving toward the transducer (panel B): An increase in f(r) is seen • with a target moving away from the transducer (panel C): f(r) is reduced •In all cases, the extent to which f(t) is increased or reduced is proportional to the velocity of the target
    64. 64. A flow moving toward the transducer has a higher observed frequency than a flow moving away from the transducer.
    65. 65.  Blood flow velocity (V) is related to the Doppler shift by the speed of sound in blood (C) and ø (the intercept angle between the ultrasound beam and the direction of blood flow)  A factor of 2 is used to correct for the "round-trip" transit time to and from the transducer.  F[d] = 2 x F[t] x [(V x cos ø)] ÷ C  This equation can be solved for V, by substituting (F[r] - F[t]) for F[d]:  V = [(F[r] -F[t]) x C] ÷ (2 x F[t] x cos ø)
    66. 66.  the angle of the ultrasound beam and the direction of blood flow are critically important in the calculation  For ø of 0º and 180º (parallel with blood flow), cosine ø = 1  For ø of 90º (perpendicular to blood flow), cosine ø = 0 and the Doppler shift is 0  For ø up to 20º, cos ø results in a minimal (<10 percent) change in the Doppler shift  For ø of 60º, cosine ø = 0.50  The value of ø is particularly important for accurate assessment of high velocity jets, which occur in aortic stenosis or pulmonary artery hypertension  It is generally assumed that ø is 0º and cos ø is therefore 1 •Ideally, the beam should be placed parallel to blood flow When the beam does not lie parallel, it is possible to introduce a correction into the calculation of flow velocity by measuring the cosine of the angle of interrogation and introducing this value into the Doppler equation
    67. 67. SPECTRAL ANALYSIS  When the backscattered signal is received by the transducer, the difference between the transmitted and backscattered signal is determined by comparing the two waveforms with the frequency content analyzed by: fast Fourier transform (FFT)  The display generated by this frequency analysis is termed spectral analysis  By convention, time is displayed on the x axis and frequency shift on the y axis  Shifts toward the transducer are represented as "positive" deflections from the "zero" baseline, and shifts away from the transducer are displayed as "negative" deflections
    68. 68. • Spectral information can be displayed in real time (Doppler figure) The Doppler signal portrays the entire period of flow, ie: acceleration (a), peak flow (pf), and deceleration (d).
    69. 69. Applications of Doppler - Different modes to measure blood velocities Continuous wave Pulsed wave Colour Flow Mapping
    70. 70. Modern echo scanners combine Doppler capabilities with 2D imaging capabilities Imaging mode is switched off (sometimes with the image held in memory) while the Doppler modes are in operation
    71. 71. CONTINUOUS WAVE DOPPLER Continuous generation of ultrasound waves coupled with continuous ultrasound reception using a two crystal transducer
    72. 72. CWD at LVOT in Deep TG Aortic Long axis view
    73. 73. 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
    74. 74. 1/2 PV2 Pressure Kinetic Energy Potential Energy P = 4V2 Bernoulli Equation Balancing Kinetic and Potential energy This goes down..As this goes up..
    75. 75. Doppler Velocity And Pressure Gradient  Doppler echo can estimate the pressure difference across a stenotic valve or between two chambers  This r/n ship is defined by the Bernoulli equation and is dependent on :  velocity proximal to a stenosis (V1)  velocity in the stenotic jet (V2)  density of blood (p), acceleration of blood through the orifice (dv/dt), and viscous losses (R[v]):  The pressure gradient (Δ P) can be calculated from:  Δ P = [0.5 x p x (V2 x V2 - V1 x V1)] + [p x (dv/dt)] + R[v] (If one assumes that the last two terms (acceleration and viscous losses) are small, and then enter the constants, the formula is simplified to):  Δ P (mmHg) = 4 x (V2 x V2 - V1 x V1)  Thus, the Bernoulli formula may be further simplified:  Δ P (mmHg) = 4V2
    76. 76. 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
    77. 77. Transducer alternately transmits and receives the ultrasound data to a sample volume. Also known as Range-gated Doppler.
    78. 78. PWD at LVOT in Deep TG aortic long axis view
    79. 79. 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
    80. 80. 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
    81. 81. 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
    82. 82. Aliasing occurs if the frequency of the sample volume is more than the Nyquist limit Nyquist limit = PRF/2
    83. 83. 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)
    84. 84. 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
    85. 85. Multigated, PW Doppler in which blood flow velocities are sampled at many locations along many lines covering the entire imaging sector
    86. 86. Echo data is processed through two channels that ultimately combine the image with the color flow data in the final display.
    87. 87. 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
    88. 88. 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
    89. 89. 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
    90. 90. Advantages & disadvantages Doppler methods used for cardiac evaluation : A. continuous wave doppler B. Pulsed wave doppler C. color flow doppler
    91. 91. CONTINUOUS WAVE DOPPLER  employs two dedicated ultrasound crystals, one for continuous transmission and a second for continuous reception  This permits measurement of very high frequency Doppler shifts or velocities  Limitations of this technique:  It receives a continuous signal along the entire length of the US beam  Thus, there may be overlap in certain settings, such as:  stenoses in series (eg, left ventricular outflow tract gradient and aortic stenosis) or  flows that are in close proximity/alignment (eg, AS and MR)
    92. 92. PULSED DOPPLER  permits sampling of blood flow velocities from a specific region  In contrast to continuous wave Doppler which records signal along the entire length of the ultrasound beam  is always performed with 2D guidance to determine the sample volume position  Particularly useful for assessing the relatively low velocity flows associated with: 1) transmitral or transtricuspid blood flow, 2) pulmonary venous flow, 3) left atrial appendage flow, or 4) for confirming the location of eccentric jets of aortic insufficiency or mitral regurgitation
    93. 93. COLOR FLOW IMAGING • With CF imaging, velocities are displayed using a color scale:  with flow toward the transducer displayed in orange/red  flow away from the transducer displayed as blue
    94. 94. SECOND HARMONIC IMAGING (Improving Resolution)  An ultrasound wave traveling through tissue becomes distorted, which generates additional sound frequencies that are harmonics of the original or fundamental frequency  produces more harmonics the further it travels through tissue  uses broadband transducers that receive double the transmitted frequency  When compared to conventional imaging, it reduces variations in ultrasound intensity along endocardial and myocardial surfaces, enhancing these structures  of particular benefit for patients in whom optimal echocardiographic images are technically difficult to obtain  harmonic imaging improves interphase definition