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Ultrasound physics and image optimization1 (1)

  2. 2. Overview  Basic principles  Instrumentation  Image optimization  Bioeffects of Ultrasound
  3. 3. What is ultrasound?  Ultrasound or ultrasonography is a medical imaging technique that uses high frequency sound waves to obtain cross sectional images of the body.  Also known as ‘pulse echo’ technique
  4. 4. Audible sound 15 – 20 000 Hz < 15 infrasound > 20 000 ultrasound Medical USG 1-20 MHz
  5. 5. Sound  Sound is a mechanical energy that travels through matter, as a result of vibration of the particles of the medium through which the sound wave is moving.  Vibration of particles  Transport the energy through medium  pressure wave  Change in pressure  sinusoidal waveform
  6. 6. compression compression compression compression rarefaction rarefaction rarefaction rarefaction compression rarefaction
  7. 7. Waves Transverse waves Longitudinal waves waves parallel to the direction of energy transfer Waves perpendicular to the direction of energy transfer
  8. 8. Sound waves longitudinal waves (air) Compression Space where density and pressure is elevated (positive) Rarefaction Space where density and pressure is depressed (negative)
  9. 9.  Unit of acoustic frequency(number of cycles in a unit of time) - Hertz (Hz)  1 Hz = 1 cycle per second  High frequencies expressed in kHz and MHz  1 kHz = 1000Hz  1 MHz = 1,000kHz
  10. 10. Speed of sound  Determined by properties of medium  i.e. Its resistance to compression  C α-1 COMPRESSIBILITY • FASTER IN SOLID • SLOWER IN GAS  Propagation velocity of sound (c) = f x λ
  12. 12. REFRACTIONREFLECTION ECHO ABSORPTION Interaction of ultrasound with matter
  13. 13. Reflection  Reflection occurs at a boundary/interface between two adjacent tissues  Z = ρc
  14. 14. Z1 Z2 The difference in acoustic impedance (z) between the two tissues causes reflection of the sound wave
  15. 15.  angle of incidence α-1 amount of reflected sound
  16. 16. soft tissue - air interface reflects almost the entire beam, transducer must be directly coupled to the patient skin without an air gap bone - tissue interface Bone transmits about 70% of sound energy due to its impedance difference - hence not possible to image through it air/gas interface causes total reflection - hence they cast a shadow and structures underneath cannot be imaged E.g Bowel wall can be imaged and not its lumen
  17. 17. Specular reflectors are smooth so when incident beam strikes, the reflected beam leaves in the same angle as the incident beam (bright fibrous structures like) Diaphragm, Bladder wall Tendons Diffuse reflectors tend to be rough so when the incident beam strikes, echoes from the interfaces are scattered in all directions kidney, liver, small blood vessels walls of heart chambers Specular reflectors Diffuse reflectors
  18. 18. Interference If two sound waves of the same wavelength cross each other, the pressure waves combine. Constructive interference waves in step/phase, their amplitudes add up - Destructive interference waves out of phase, they tend to cancel out each other
  19. 19. Refraction  When sound passes from a tissue with one acoustic propagation velocity to a tissue with a higher or lower sound velocity, there is a change in the direction of the sound wave  Causes artefacts
  20. 20. Absorption  As the ultrasound pulse moves through matter, it continuously loses energy - attenuation  There is absorption of the ultrasound energy by the material and gets converted into HEAT.  Scattering and refraction interactions also remove some of the energy from the pulse and contribute to its overall attenuation, but absorption is the most significant  Reduction of intensity of beam as it traverses through matter.
  21. 21. Attenuation  Attenuation of sound energy influences the depth in tissue, from which useful information can be obtained  Affects transducer selection and operator controlled instrument settings  The deeper the wave travels in the body, the weaker it becomes  The amplitude/strength of the wave decreases with increasing depth  Decibel
  22. 22. TISSUE absorption + reflection + scattering Attenuation
  23. 23.  frequencies a attenuation  Attenuation determines the efficiency with which ultrasound penetrates a specific tissue
  24. 24. Instrumentation 1. Transmitter - energize transducer 2. Transducer 3. Receiver and processor - detect and amplify backscattered energy 4. Display – presents the ultrasound image/data 5. Recording/Storage
  25. 25. Transmitter  Pulsed ultrasound  Produces precisely timed high amplitude voltage as pulses to energize the transducer  Controls PRF  Pulse repetition frequency – rate of pulses emitted by transducer
  26. 26. Pulse repetition frequency  Determines time interval between pulses  Determines depth  PRF – 1 to 10 kHz for diagnostic imaging  Pulse should travel to depth and return before next pulse is send  Highest USG frequency permitting penetration to depth of interest should be selected
  27. 27. Pulse echo principle  To determine the depth of the tissues.  T = 2D / c ---- principle of pulse echo  Generation of sound pulse  Reflection of pulse from targets  Detection of reflected echoes  Calculation and display of range of targets
  28. 28. Transducer  Device that converts one form of energy into another  Ultrasonic transducers convert an electric signal  ultrasonic energy  reflected back from the tissues into electrical signal  Transducer functions as both transmitter and receiver  Used in both pulsed and continuous wave modes
  29. 29.  Piezoelectric element located near the face of transducer.  Front and back faces coated with thin conducting film  Surfaces of crystal coated with gold or silver electrodes  Outside electrode grounded to protect the patient and its surface is coated with water tight electric insulator  Inside electrode rest against a thick backing block that absorbs sound waves transmitted back into transducer  Housing – strong plastic  An acoustic insulator of rubber or cork prevents the sound from passing into the housing
  30. 30. Piezoelectricity  Discovered by Pierre and Jacques Curie in 1880  Greek word “piezein” - to press  Piezoelectric effect – application of an electric field to certain materials causes a change in their physical dimensions  Piezoelectric material - innumerable dipoles arranged in a geometric pattern  Each dipole – positive charge at one end and negative charge at other
  31. 31. Generates ultrasound waves Crystal expands and contracts Alignment of dipoles Electric field Generation of voltage When crystal gets compressed
  32. 32. Serves as ultrasonic signal for display Voltage amplified Induces voltage between the electrodes Forces dipoles to change orientation Physical compression of crystal element Transmits their energy to transducer Echoes reflect back to transducer from each tissue interface Sound waves pass though the body
  33. 33.  Compression force and associated voltage – piezoelectric effect  In response to a compressive force on a piezoelectric slab, a net electric dipole moment  detection of a voltage in response to this strain  Reverse piezoelectricity - In response to the application of an electric field, a deformation of the piezoelectric slab detected.
  34. 34. Piezoelectric material  Quartz – original transducer material  Ferroelectrics ( artificial piezoelectric material ) – barium titanate, lead zirconate titanate  To produce polarization the ceramic is heated at high temperature in a strong electric field  At high temperature, the dipoles are free to move, and the electric field brings them into the desired alignment  The crystal is then gradually cooled while subjected to a constant high voltage
  35. 35. Curie temperature  It is the temperature above which this polarization is lost  PZT - 365˚C  Quartz - 573˚C
  36. 36. Resonant frequency  Frequency at which the wavelength = 2 x thickness of piezoelectric disc  Constructive interference / resonance of waves emitted from forwards and back face of disc occurs  Transducer most efficient as transmitter and receiver of sound  Depends on thickness of disc and material of disc (determines velocity)
  37. 37. • The thicker the piezoelectric element, the lower the resonant frequency • Conversely thinner the element higher the operating frequency • Range of frequencies produced by a transducer – bandwidth • Broad band technology produces medical transducers that contain more than one operating frequencies
  38. 38. 2.5 MHz deep abdomen, obstetric and gynaecological imaging 3.5MHz general abdomen, obstetric and gynaecological imaging 5.0MHz vascular, breast, pelvic imaging 7.5MHz breast, thyroid 10.0MHz breast, thyroid, superficial veins, superficial masses, musculoskeletal imaging. 15.0MHz superficial structures, musculoskeletal imaging.
  39. 39. Pulse duration/length  Transducer continues to vibrate for a short time after it is stimulated  ultrasound pulse will be several cycles long  Damping block is used to stop the crystal vibration so that transducer is ready to receive the reflected waves from tissue interfaces within the body  Damping materials used – tungsten and rubber powder suspended in epoxy resin(backing block)
  40. 40. Q factor  Mechanical coefficient ( Q ) – mean frequency : bandwidth  Greater Q –  narrower bandwidth,  lesser damping,  longer ring down time/ pulse duration  Lesser Q –  broader bandwidth,  heavier damping,  smaller ring down time/ pulse duration
  41. 41. Coupling agents  Commonly known as GEL  Fluid medium needed to provide a link between the transducer and the patient  Composition –  Carbomer – 10gm  Propylene glycol – 75gm  Trolamine – 12.5gm  EDTA – 0.25gm  Distilled water – upto 550ml
  42. 42. Ultrasound beam characteristics  Unfocused beam – An ultrasound beam leaving a flat crystal element has an initial cylindrical segment, followed by a diverging conical portion  Frequency –  Higher the frequency, longer will be the cylindrical segment or near field (Fresnel zone) and far field (Fraunhofer zone) becomes less divergent  Lateral resolution – junction of near and far fields  Depth resolution improves at higher frequencies  As frequency is increased, greater absorption of sound energy  weakens beam intensity
  43. 43.  Near field / Fresnel zone  Interference of pressure waves near the transducer results in great amplitude variation  Far field / Fraunhofer zone  Farther from the transducer, the waves diverge and amplitude decreases at steady state  Higher frequency transducer, larger diameter - increases length of near zone and decreases divergence of far zone, hence beam becomes more directional Crystal diameter –Increase in crystal diameter increases the near zone length but worsens lateral and depth resolution
  44. 44. Focused beam  Improves both lateral and depth resolution  Concave crystal face – Greater the concavity, nearer the beam will focus relative to the crystal  Acoustic lenses – Shorter the focal length, the nearer the lens will focus the beam relative to the lens face
  45. 45. Receiver  Detects and amplifies weak signals  Compensates the differences in echo strength, which results from attenuation by different tissue thickness  Compression of wider range of amplitudes returning to transducer into a range that can be displayed to the user
  46. 46. Time gain compensation  Amplitude of sound pulse diminishes as it travels through body due to attenuation and echo pulse similarly attenuated as it travels back to transducer  Selective amplification of signals received from greater depth  TGC is under operator control
  47. 47. Dynamic range  Equivalent to number of shades of gray  Ratio of the largest to the smallest echoes processed by components of ultrasound device  Dynamic range decreases as signals pass through the imaging system as operations such as TGC eliminate small and large signals  smaller change in echo amplitude that will effect the gray scale brightness  image contrast is enhanced for smaller dynamic range  Echoes returning from tissues can have dynamic ranges of 100 to 150dB  Increasing dynamic range yields more shades of gray in ultrasound signals
  48. 48. Modes of display  A mode  B mode  M mode
  49. 49. A mode  ‘A’ - amplitude  Earliest incarnation of ultrasound system  Shows signals as spikes and height of the spike represents the amplitude of the reflected signal  Only shows position of tissue interfaces  Ophthalmology studies – to detect optic nerve
  50. 50. M mode  ‘M’ – motion  Detection of motion of a structure or structures over time along scan line of interest  A B mode image is frozen on the screen and used to direct the beam from a stationary transducer along a line of interest  Echoes are displayed as a line of moving bright dots  Displays echo amplitude and shows position of moving reflectors  Evaluation of rapid motion of cardiac valves and of cardiac chamber and vessel walls
  51. 51. B mode  ‘B’ – brightness  Involves the pulse echo sequence with the buildup of an image from innumerable echo blips  Brightness corresponds to received echo amplitude  Greatest intensity signal – white  Absence of signal – black  Signs of intermediate intensity – gray
  52. 52.  Static B mode ultrasound -  Image is compiled as sound beam is scanned across patient  Larger field of view, restricted transducer movements and single-depth focusing • Real time ultrasound -  Image is automatically scanned in a succession of frames sufficiently rapidly to demonstrate the motion of tissues  Frame rate of 15-60/second  Assessment of both anatomy and motion
  53. 53. Gray scale imaging  Display great amplitudes of echoes arising from tissues as varying shades of gray on monitor  Scan convertor –  Improve grayscale images  Enable freeze frame  Manipulate data and archive if necessary  Write, read and erase  Analog and digital scan convertors
  54. 54. 3D Ultrasound  Images obtained from a set of 2D scans  3D scanners used for fetal, gynaecological and cardiac scanning  4D ultrasound is the term used for real time 3D imaging
  55. 55. To produce real-time images Mechanical scanners Conventional /group of single element transducers is mechanically moved to form images in real time Oscillating Rotating electronic scanners Other uses an array of transducers, which do not move but are activated electronically – Linear Curvilinear Phased
  56. 56. Mechanical – Oscillating transducers With unenclosed crystal – single element caused to oscillate through an angle, which defines the field of view; Sector scan With enclosed crystal – transducer enclosed within a fluid filled container; use of an electromagnet and trapezoidal image produced
  57. 57. Mechanical – Rotating transducers  Employs 3 or 4 crystals mounted symmetrically on a rotating wheel  The wheel is driven by an electric motor to perform circular motion in one direction only  The crystal elements are excited one at a time to provide the ultrasound beam  Replaced by annular array transducers
  58. 58. Linear Array  Array element is pulsed sequentially to produce a rectangular scan pattern  At any time, only one element or subset of elements is in operation. The echoes are received before the next element is excited  Used for small parts, vascular and obstetric applications
  59. 59. Curvilinear array  One element or a subset of elements at a time scanned sequentially  Produces trapezoid scan pattern
  60. 60. Phased array  All elements are scanned at the same time  Ultrasound beam is caused to sweep back and forth across the patient by electronically controlled steering and focusing  Ideal for cardiac imaging
  61. 61. Transducer selection  For general purpose – convex with 3.5MHz  For obstetric purpose – convex or linear with 3.5MHz  For superficial structures – linear with 5MHz  For paediatric or thin people – 5MHz
  62. 62. Tissue harmonics imaging  Based on nonlinear propagation of ultrasound wave propagating through tissue causing distortion and generating harmonics  It decreases “clutter” noise  It yields sharper overall images with improved contrast, axial and lateral resolution  Used in obstetrics, cardiology and abdominal applications
  63. 63. Spatial Resolution  Ability to distinguish between two closely situated structures from one another  Axial or lateral resolution
  64. 64. Axial (depth) resolution  Minimum reflector resolution along axis of ultrasound beam  Determined by pulse length  Higher frequencies  lesser pulse length  higher axial resolution
  65. 65. Lateral (horizontal) resolution  Minimal reflector resolution perpendicular to beam and parallel to transducer  Determined by beam width  Beam width is a function of wavelength, piezoelectric transducer diameter, distance from transducer  Can be controlled by focusing the beam
  66. 66. Image optimization  Gain and TGC - Sensitivity controls  Focus – Lateral resolution  Frequency – Axial resoultion  High  good axial resolution but poor penetration  Low  less axial resolution but better penetration  Depth – decrease in depth  increase in frame rate (temporal resolution)  Sector width  increase in frame rate
  67. 67. TGC gain freeze depth focus
  68. 68.  Gain –  Overall brightness of the image  Amplification of returning ultrasound signal received from all depths  High gain  high amplitude  differentiation lost  Adjust to minimum required to obtain a complete range from low (dark gray) to high (white) amplitude signals  Time Gain compensation –  Adjustment of image brightness at a selective depth  Corrects varying depths of intensities  Adjust TGC to decrease in near field and increase in far field (‘’ shape)
  69. 69.  Focus –  Allows focus of ultrasound beam to area of interest  Optimizes ultrasound intensity in near and far field  improved spatial resolution  Depth –  Adjustment of the depth of field of view  Greater the depth, lesser resolution and frame rate  Set the depth to minimum required to visualize all structures of interest  Frequency –  Increased frequency  increase resolution  Sector width –  Wide sector  frame rate and temporal resolution
  70. 70. • Dynamic range – • 100dB of ultrasound information available but monitor can display a much smaller range • Allows compression of wide spectrum of amplitudes • Compressed signals then displayed on monitor varying shades of gray
  71. 71. Linear array 18-5MHz Linear array 12-3MHz Curved array 5-1MHz Tightly curved array 10-3MHz
  72. 72. Bioeffects  Thermal  Mechanical
  73. 73. Thermal bioeffect  Mechanism - with sound wave attenuation, energy is lost as heat  Longer use of ultrasound on the tissue  longer the ultrasound has attenuated in the tissue  More the tissue attenuates ultrasound  more the bioeffects  More ultrasound attenuates at the surface and focal zone  Higher the frequency/power/PRF/pulse duration , more the attenuation and thus the heating
  74. 74. Thermal indices  NCRP guidelines – in ultrasound examinations, no temperature rise > 1°C  TI – indicator of relative potential for increasing tissue temperature  TIS – Thermal index for soft tissue  TIB – Thermal index for bone, at the focus  TIC – Thermal index for bone, at surface
  75. 75. Mechanical bioeffects  Current USG systems can produce cavitation  Determines potential for bubble formation in vivo  Cavitation -  Stable  Inertial / Transient
  76. 76. Stable cavitation  A bubble forms as the peak low pressure region of an ultrasound pulse passes through a nucleation site  This bubble grows and shrinks with the high pressure and low pressure region of the ultrasound pulses  This growing and shrinking of the bubble causes fluid to flow around the bubble - micro-streaming  Causes mechanical damage, sometimes cell lysis
  77. 77. Inertial/transient cavitation  Here, the bubbles formed in a similar way but under high pressure or intensities, they expand very quickly following which they collapse violently  This collapse produces high temperature and pressure, which damages cells in the region of the collapse
  78. 78. Mechanical index  It is an approximation of mechanical effects  It is a calculated number based on ultrasound parameters and facts about the machine that the manufacturer built into the programming  Mechanical index less than 1.9, no mechanical effects is seen

Notas del editor

  • In diagnostic ultrasound, only the longitudinal waves are important.
  • Reflection
    Though its not a EMR
    V = SPEED
    Z = RAYLS (Kg / m3.s )
  • Reflection
    Though its not a EMR
  • Amount of reflection determined by angle of incidence between sound beam and reflecting surface 
  • Water produces by far the least attenuation. This means that water is a very good conductor of ultrasound. Water within the body, such as in cysts and the bladder, forms "windows" through which underlying structures can be easily imaged.  
    Most of the soft tissues of the body have attenuation coefficient values of approximately 1 dB per cm per MHz, with the exception of fat and muscle.  
    Muscle has a range of values that depends on the direction of the ultrasound with respect to the muscle fibers.  
    Lung has a much higher attenuation rate than either air or soft tissue. This is because the small pockets of air in the alveoli are very effective in scattering ultrasound energy. Because of this, the normal lung structure is extremely difficult to penetrate with ultrasound.  
    Compared to the soft tissues of the body, bone has a relatively high attenuation rate. Bone, in effect, shields some parts of the body against easy access by ultrasound.