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Doppler Physics

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Beginners guide to understand the concepts and techniques of Doppler USG imaging -types , technical info,subtypes ,artefacts and interpretation

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Doppler Physics

  1. 1. DOPPLER PHYSICS •  Dr.Sahil  Chaudhry,  AJIMS
  2. 2. OUTLINE •  Doppler  Principles   •  Pulsed  and  Con:nuous  Doppler     •  Aliasing  and  Nyquist  Criteria   •  Spectral  Analysis   •  Colour  flow  imaging   •  Power  Doppler   •  Doppler  Ar:facts  
  3. 3. Waves from a stationary source Wave  peaks  evenly  spaced  around  the  source  at  1  wavelength  intervals  
  4. 4. Waves from a moving source Source  moving  this  way   Old  posi7ons  of  source  
  5. 5. Doppler Effect v Change  in  the  perceived  frequency  of  sound   emiGed  by  a  moving  source.   v The  basis  of  Doppler  ultrasonography  is  the  fact   that  refected/scaGered  ultrasonic  waves  from  a   moving  interface  will  undergo  a  frequency  shiK.    
  6. 6. RECEIVED FREQUENCY TRANSMITTED FREQUENCY DOPPLER SHIFT
  7. 7. •  In  diagnos:c  ultrasound,  the  Doppler  effect  is  used   to  measure  blood  flow  velocity.     •  When  the  emiGed  ultrasound  beam  strikes  moving   blood  cells,  the  laGer  reflect  the  pulse  with  a   specific  Doppler  shiK  frequency  that  depends  on   the  velocity  and  direc:on  of  blood  flow.  
  8. 8. •  IF RECEIVED FREQUENCY = TRANSMITTED FREQUENCY, NO DOPPLER SHIFT ¨  Posi:ve  shiK     Ø     Received  freq  >   transmiGed    freq   Ø  Flow  towards  the   transducer   ¨  Nega:ve  shiK   Ø  TransmiGed  freq  >   received  freq   Ø  Flow  away  from  the   transducer  
  9. 9. •  Angle     •             Cos  (a)     Doppler  shiK  depends  on  the  cosine   of  the  angle  between  the  sound   beam  and  the  direc:on  of  the   mo:on                V  =          Fd    ×  C                                2  fₒ  ×  cos  ᶱ     Op:mal  angle              30°  -­‐  60°       Angle   Cos  theta   0   1   45   0.7   60   0.5   90   0  
  10. 10. Angle  to  Flow   Angle Cosine 0 1.00 15 0.97 45 0.71 60 0.50 75 0.26 90 0.00
  11. 11. The size of the Doppler signal is dependent on:       •  Blood  velocity:  as  velocity   increases,  so  does  the  Doppler   frequency   •  Ultrasound  frequency:  higher   ultrasound  frequencies  give   increased  Doppler  frequency.   •  Angle  of  Insona:on  
  12. 12. Continuous Doppler •  Uses  two  crystals,  one  to  send   and  one  to  receive.   •  Uses  con:nuous  transmission   and  recep:on  of  ultrasound.     •  Doppler  signals  are  obtained   from  all  vessels  in  the  path  of   the  ultrasound  beam  (un:l  the   ultrasound  beam  becomes   sufficiently  aGenuated  due  to   depth).     •  Unable  to  determine  the  specific   loca:on  of  veloci:es  within  the   beam  and  cannot  be  used  to   produce  color  flow  images.     •  Used  in  adult  cardiac  scanners   to  inves:gate  the  high  veloci:es   in  the  aorta.     AUDI O   AMP LIFIE R   FIL TE R   DEMOD ULATO R   OSCILLAT OR,   TRANSMI T   AMPLIFIE R   RECEIV ER,   AMPLI FIER  
  13. 13. CW  DOPPLER   •  Doppler shift can be located at any depth in the flow sensitive zone of beam. •  The Doppler receiver is unable to determine the exact location of the Doppler shift. •  Thus CW lacks range resolution. •  Because it is continuously sample returning echoes it have no limitations on measuring high flow velocities.
  14. 14. Directional Doppler ü  Quadrature  detec:on  helps  in  determining  flow  direc:on.       ü  Received  echo  signals  are  amplified  àsplit  into  two  iden:cal   channels  for  demodula:on.   ü  The  reference  signals  from  the  transmiGer  sent  to  the  two   demodulators  are  90  degrees  out  of  phase.       ü  Two  separate  Doppler  signals  are  produced.  They  are  iden:cal  except   for  a  small  phase  difference  between  them,  and  this  phase  difference   can  be  used  to  determine  whether  the  Doppler  shiK  is  posi:ve  or   nega:ve.  
  15. 15. Pulsed Doppler •  The  transducer  both  sends  and   receives  the  signal.   •  The  returned  signal  is  gated  so  that   only  informa:on  about  the  desired   depth  is  computed   •  Pulses  –  just  like  real  :me  scanning   •  Need  to  “gate”  analysis  of  received   pulse,  so  we  know  where  the  moving   objects  are.   •  This  allows  measurement  of  the   depth  (or  range)  of  the  flow  site.   Addi:onally,  the  size  of  the  sample   volume  (or  range  gate)  can  be   changed.  Pulsed  wave  ultrasound  is   used  to  provide  data  for  Doppler   sonograms  and  color  flow  images   Sam ple   Demod ulator   Gate   size   and   depth   Master   Oscillat or   Rec eive r   Gated   Trasmi Ger  
  16. 16. Continuous doppler Pulsed doppler Ø       Separate    crystal  for          transmiqng  &  receiving   Ø     Can  measure  high  veloci:es   Ø     Range  ambiguity     Ø     Single  crystal  transmits  &   receives.   Ø     Range  resolu:on   Ø     Can’t  measure  very  high  veloci:es  
  17. 17. Doppler Modes Colour                                                        Power                                              Spectral  
  18. 18. Physics of Spectral Flow Vascular  Flow   •  Blood  flow  is  normally  laminar  with  velocity  decreasing   from  the  center  outward  to  the  vessel  walls  
  19. 19. Hemodynamic Principles Laminar  Flow   •  Con:nuous  or  laminar  flow  is  characterized  by  a   constant  velocity  over  :me.   •  The  flow  profile  of  laminar  flow  is  determined  by   iner:al  and  fric:onal  forces.  Fric:on  produces  a   laminar,  or,  in  the  three-­‐dimensional  model,  parabolic   flow  profile.  Flow  is  fastest  toward  the  center  of  a   vessel  and  decreases  toward  the  wall,  where  it   approximates  zero.   •  Color  duplex  ultrasound  reflects  this  flow  profile  by   lighter  color  shades  in  the  center  (fast  flow)  and  darker   shades  near  the  wall  (slow  flow)  
  20. 20. Typical triphasic Doppler waveform of the popliteal artery.     color duplex scan depicts laminar flow with lighter coloring in the center darker colors toward the margins.
  21. 21. Pulsatile Flow •  In  contrast  to  laminar  flow,  pulsa:le  flow  changes   periodically  over  :me.  Phases  of  accelera:on  and   decelera:on  vary  in  rela:on  to  changes  in  pressure.     •  The  pressure  amplitude  generated  by  the  leK  ventricle   is  reduced  by  the  compliance  of  the  aorta  and  other   large  vessels  (windkessel  effect),  resul:ng  in  a  more   steady  flow.     •  Another  factor  affec:ng  the  flow  profile  is  the   peripheral  resistance   •  As  the  peripheral  resistance  is  a  crucial  factor  affec:ng   the  waveform,  a  dis:nc:on  is  made  between  low-­‐ resistance  flow  and  high-­‐resistance  flow.  
  22. 22. Low Resistance Flow •  Arteries  supplying  parenchymal  organs   and  the  brain  are  characterized  by  a  fairly   steady  blood  flow  as  a  result  of  low   peripheral  resistance.  In  these  arteries,  a   moderate  systolic  rise  is  followed  by  a   steady  flow  that  persists  throughout   diastole.  This  flow  profile  is  typical  of  the   renal,  hepa:c,  splenic,  internal  caro:d,   and  vertebral  arteries   •  The  windkessel  effect  thus  ensures  a   more  con:nuous  flow  than  would  result   from  the  ac:on  of  the  leK  ventricle  and   aor:c  valve  alone.  As  a  result,  flow  will   become  more  pulsa:le  when  this  effect   and  the  normal  elas:city  of  the  vessels   are  lost.  
  23. 23. High Resistance Flow •  A  high  peripheral  resistance  results   in  a  more  pulsa:le  flow  with  a   steep  systolic  upslope  during  the   accelera:on  phase,  followed  by   decelera:on  and  a  significant   reflux  in  early  diastole  and  short   backward  flow  in  mid-­‐diastole.   Zero  flow  is  typically  seen  in  end   diastole.  This  paGern  is  referred  to   as  triphasic  flow.   •  High-­‐resistance  flow  is  typical  of   the  arteries  supplying  Muscles  and   the  skin  
  24. 24. Transition from laminar to turbulent flow    Ø Turbulent  flow  occurs  when   laminar  flow  breaks  down  and  the   par:cles  in  the  fluid  move   randomly  in  all  direc:ons  with   variable  speeds   Ø   Turbulent  flow  is  more  likely  to   occur  at  high  veloci:es  (V),  and  the   cri:cal  velocity  at  which  flow   becomes  turbulent  depends  on  the   viscosity,  the  density  of  the  fluid   and  the  diameter  of  the  vessel  (d).   Reynolds  described  this   rela:onship,  which  defines  a  value   called  the  Reynolds  number  (Re)  
  25. 25. Color  Flow  Imaging   •   Doppler data evaluated using autocorrelation. •  Autocorrelation is a technique that compare the echo from each pulse with the echo from the previous pulse. •  Autocorrelation requires a minimum of 3 pulses per scan line.
  26. 26. Color  Flow  Imaging   •  This technique can only produce an estimate of the mean frequency shift and mean velocity. •  Increasing the line per frame provides an image with more resolution at the expense of the frame rate.
  27. 27. Color  Flow  Imaging   Color  Resolu7on   Frame  rate   Number  of  lines  in   Gray  scale  imaging  
  28. 28. Color  Flow  Imaging   •  To produce the color flow image, the mean Doppler shift is encoded according to a preset color map. •  This color information is superimposed on the gray scale anatomic scan in real time.
  29. 29. Color  Flow  Maps   •  Velocity color map •  Variance color map
  30. 30. .  
  31. 31. Velocity  Color  Bar   Increasing flow velocity away from the transducer Zero flow Increasing flow velocity toward the transducer
  32. 32. Variance  Color  Bar   Variance  Color  
  33. 33.        Near  occlusion                      Total  occlusion  
  34. 34. Colour Doppler Limita:ons  :   Ø Semi  quan:ta:ve     Ø Angle  dependence   Ø Aliasing   Ø Ar:facts  caused  by  the  noise   Ø Poor  temporal  resolu:on.    
  35. 35. Colour Box Color box is an operator-adjustable area within US image in which all color Doppler information is displayed. Because frame rate decreases as box size increases, image resolution & quality are affected by box size and width. Box should be as small & superficial as possible while still providing necessary information. A deep color box will result in a slower PRF, which may produce aliasing of depicted color flow.
  36. 36. Colour Box
  37. 37. Aliasing •  Aliasing  is  produc:on   of  ar:ficial  low   frequency  signals   when  the  sampling   rate  is  less  than  twice   the  doppler  signal   frequency.  When  the   Doppler  shiKs  exceed  a   value  Nyquist   frequency,  veloci:es   are  perceived  as  going   in  opposite  direc:on.   Aliasing    occurs  when    Doppler  shi2  >  Nyquist   frequency   Nyquist    freq  -­‐  Pulse  Repe<<on  Frequency                              2  
  38. 38. Nyquist  Sampling  Limit   •  The  Maximum  Doppler  frequency  that  can  be   sampled  is  ½  the  PRF   •  Example,  if  PRF  =  8  kHz   – Max  Doppler  frequency  is  4  kHz   •  Example,  if  PRF  =  4  kHz   – Max  Doppler  frequency  is  2  kHz  
  39. 39. Adjustments to be made to avoid aliasing •  Increasing  the  PRF   •  Moving  color  or  spectral  baseline  up  or  down.   •  Decreasing  Doppler  shiK  frequency  (changing   angle  of  insona:on).     •  Using  a  lower-­‐frequency  transducer.    
  40. 40.  Doppler  Spectrum  Assessment   Assess the following: 1. Presence of flow 2. Direction of flow 3. Amplitude 4. Window 5. Pulsatility
  41. 41.  Doppler  Spectrum  Assessment   Check  for  Flow   Flow     Detected   No  Flow     Detected   Check     SV  Placement   Sensi7ve   Decreased   Sensi7vity   Improve   Sensi7vity   Check     Sensi7vity   Check  Beam-­‐   flow  angle    
  42. 42.  Doppler  Spectrum  Assessment   Sensitivity can be improved by: •  Increasing power or gain. •  Decreasing the velocity scale. •  Decreasing the reject or filter. •  Slowly increasing the SV size.
  43. 43.  Doppler  Spectrum  Assessment   Direction of Flow Pulsed Doppler use quadrature phase detection to provide bidirectional Doppler information.
  44. 44.  Doppler  Spectrum  Assessment   Flow can either be: •  Mono-phasic •  Bi-phasic •  Tri-phasic •  Bidirectional
  45. 45.  Spectral    Display   Frequency   Time   Mono-­‐phasic  Flow     Flow  on  just  on  side     of  the  Baseline.  
  46. 46.  Spectral    Display   Frequency   Time   Bi-­‐phasic  Flow     Flow  start  on  one   side  of  the  Baseline     and  then  crosses  to     the  other.  
  47. 47.  Spectral    Display   Frequency   Time   Tri-­‐phasic  Flow     Flow  start  on  one  side     of    the    baseline  side,   then  crosses  to  the     other,  then  returns  to     the  original  side.  
  48. 48.  Spectral    Display   Time   Frequency   Bidirec7onal  Flow     Flow  which  occurs   simultaneously  on     both  sides  of  the     baseline.    
  49. 49.  Doppler  Spectrum  Assessment   Amplitude The spectrum displays echo amplitude by varying the brightness of the display. The amplitude of the echoes are determined by: •  Echo intensity •  Power •  Gain •  Dynamic range
  50. 50.  Doppler  Spectrum  Assessment   Window •  Received Doppler shift consist of a range of frequencies. •  Narrow range of frequencies will result in a narrow display line. •  The clear area underneath the spectrum is called the window.
  51. 51.  Spectral    Display   Sonic  Window   Velocity   Time   A  narrow  range  of  frequencies   results  in  large  clear  window.  
  52. 52.  Spectral    Display   Sonic  Window   Velocity   Time   A  broad  range  of  frequencies   results  in  diminished  window.  
  53. 53. Spectrum  Broadening   Loss of the Spectral window is called Spectral Broadening.
  54. 54. Spectrum  Broadening   Occurs usually: •  As the blood decelerates in diastole •  If sample volume is placed to close to the vessel wall •  In small vessels (parabolic velocity profile)
  55. 55. Spectrum  Broadening   •  Tortuous vessels. •  Low flow states.. •  Excessive gain/power/dynamic range
  56. 56. Spectrum  Broadening   It is hallmark of disturbed and/or turbulent flow.
  57. 57. Spectrum  Broadening   Pulsatility •  Measures the difference between the maximum and minimum velocities within the cardiac cycle. •  Indices are unit less. •  All increase in value as flow pulsatility increases. •  Can be measured without knowledge of the Doppler angle.
  58. 58. Spectral analysis          sharp  systolic  peak  +   reversed  diastolic  flow              (e.g.)  extremity  artery  in   res:ng  stage.            Broad  systolic  peak  +   forward  flow  in  diastole              (e.g.)   ICA,renal,vertebral,celiac.                Sharp  systolic  peak  +  forward   flow  in  diastole.            (e.g.)    ECA  &  SMA  (during   fas:ng)    
  59. 59. Spectral changes in disturbed flow
  60. 60. •  Doppler indices are : Ø PI Ø RI Ø SYSTOLIC / DIASTOLIC RATIO Ø Acceleration time(AT) and acceleration index(AI) Ø SPECTRAL BROADENING •  These indices can thus serve as a semiquantitative parameter for the evaluation of stenoses  
  61. 61. Pulsatility Index §  It is defined as the maximum height of the waveform, S, minus the minimum diastolic, D (which may be negative), divided by the mean height, M, •  Stenoses or occlusions in arteries will alter the Doppler waveform and the pulsatility index.
  62. 62. Pourcelot’s Resistance index (RI) §  The resistance indices, in particular the Pourcelot index, reflect wall elasticity as well as the peripheral resistance of the organ supplied •  In vessels with greater peripheral resistance, the Pourcelot index is higher and end-diastolic velocity decreases. §  It is defined as follows where E is end diastolic velocity. The value of RI can be calculated by the scanner and displayed on the screen.
  63. 63. Acceleration Time and Index
  64. 64. Spectral Broadening §  There have been several definitions of spectral broadening (SB) described over the years in an attempt to quantify the spread of frequencies present within a spectrum. One such definition is as follows: §  Increased SB indicates the presence of arterial disease  
  65. 65. •  SPECTRAL  DOPPLER   •  COLOUR  DOPPLER   Depic7on  of  Doppler  shiQ   informa7on  in  waveform   U7lize  the  Doppler  shiQ   informa7on  to  show  blood  flow  in   color  
  66. 66. •  SPECTRAL  DOPPLER   Advantages  :   •  Depicts    quan:ta:ve   flow  at  one  site   •  Allows  calcula:ons   of  velocity  and   indices     •  Good  temporal   resolu:on   •  COLOUR  DOPPLER   Advantages  :   Ø Overall  view  of  flow   Ø   Direc:onal   informa:on  about   flow   Ø   Averaged  velocity   informa:on  about   flow  
  67. 67. Power Doppler ¨  Power  or  intensity  of   Doppler  signal  is   measured  rather   than  Doppler  shiK.     Limita:ons  :   ¨  No  direc:on  /   velocity  informa:on   ¨  Slow  frame  rate      
  68. 68. Power Doppler Ø A  color-­‐coded  map  of  Doppler  shiKs  superimposed   onto  a  B-­‐mode  ultrasound  image     Ø Color  flow  imaging  have  to  produce  several   thousand  color  points  of  flow  informa:on  for  each   frame  superimposed  on  the  B-­‐mode  image.     Ø Color  flow  imaging  uses  fewer,  shorter  pulses  along   each  color  scan  line  of  the  image  to  give  a  mean   frequency  shiK  and  a  variance  at  each  small  area  of   measurement.  This  frequency  shiK  is  displayed  as  a   color  pixel.  
  69. 69. Power Doppler   Ø The  transducer  elements  are  switched  rapidly  between  B-­‐ mode  and  color  flow  imaging  to  give  an  impression  of  a   combined  simultaneous  image.   Ø   The  pulses  used  for  color  flow  imaging  are  typically  three   to  four  :mes  longer  than  those  for  the  B-­‐mode  image,  with   a  corresponding  loss  of  axial  resolu:on.     Ø Assignment  of  color  to  frequency  shiKs  is  usually  based  on   direc:on  (for  example,  red  for  Doppler  shiKs  towards  the   ultrasound  beam  and  blue  for  shiKs  away  from  it)  and   magnitude  (different  color  hues  or  lighter  satura:on  for   higher  frequency  shiKs).    
  70. 70. Power Doppler Advantages  :   Ø  Increased  sensi:vity   of  flow  detec:on   Ø  Less  angle   dependence   Ø  No  aliasing   Ø  Noise  –  a   homogenous   background  
  71. 71. Spectral Velocity Scale Scale  change  adjustment  
  72. 72. Colour Velocity Scale
  73. 73. Spectral Baseline
  74. 74. Colour Baseline
  75. 75. Wall Filter ¨  Filters  eliminate   typically  low-­‐ frequency  high-­‐ intensity  noise  that   may  arise  from   vessel  wall  mo:on    
  76. 76. Spectral Filter  Very  High  filter                                                     Color  duplex  US  image  obtained  with  a   high  wall  filter  seVng  shows  loss  of   the  low-­‐velocity-­‐flow  component  of  the   spectral  waveform  immediately  above  the   baseline.  Higher-­‐velocity  flow  is   well  depicted,  and  accurate  flow   quan:fica:on  can  s:ll  occur.  In  the   evalua:on  of  the  liver  vasculature,  this  is   likely  to  become  relevant  only  when  flow   velocity  is  very  low  and  falls  within  the   range  of  veloci:es  that  are  filtered  out.  
  77. 77. Spectral Filter   Color  duplex  US   image  demonstrates   how  the  spectral   waveform   progressively  fills  in   toward  the  baseline.    Op:mal  filter  50-­‐100  Hz  
  78. 78. Colour gain Amplifica:on  of  the  sampled   informa:on  
  79. 79. Spectral gain
  80. 80. Angle Correction •  Angle   correc:on   refers  to   adjustment   of  Doppler   angle  &  is   used  to   calibrate   velocity  scale   for  the  angle   between  US   beam  and   blood  flow   being   measured  
  81. 81. 45’’   0’   >90’   • The  angle  of  insona7on  should  also  be  between  45°-­‐  60°.     • Flow  may  appear  to  be  reversed  when  the  beam-­‐flow  angle  changes  about  90°.     • Complete  loss  of  flow  may  be  evident  when  the  beam-­‐flow  angle  is  90°.    
  82. 82. Beam Steering                              45°   ’       >90°  
  83. 83. Gate Size Represents the area of flow assessed with Doppler.
  84. 84. Sample Volume Sample volume size should be 1/3 of the diameter of the vessel.
  85. 85. Inversion ¨  Ability  to  manually     invert  the  spectral   wave  or    color   seqngs.      
  86. 86. Colour Inversion
  87. 87. Spectral Inversion
  88. 88. Doppler artifacts •  Aliasing   •  Mirror  image   •  Blooming   •  Color  in  non  vascular  structures   •  Twinkle  ar:facts  
  89. 89. Mirror  image  ar:fact   •  any  vessel  adjacent  to  a   highly  reflec:ve  surface,   such  as  the  lung,   subdiaphragma:c  region   of  the  liver  and  the   supraclavicular  region  
  90. 90. Blooming artifact
  91. 91. Twinkling artifact      Rapidly   fluctua:ng   mixture  of   Doppler   signals  (red   and  blue   pixels)  that   imitate   turbulent   flow  
  92. 92. Colour in non vascular structures (Colour flash artifact) •   Manifests  as  a  colour   signal  due  to  transducer   or  pa:ent  mo:on   •  Hypoechoic  areas  such  as   a  cyst  or  a  duct  are   suscep:ble  to  colour  flash   ar:fact.  

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