Ultrasound Physics & Knobology

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Ultrasound Physics & Knobology

  1. 1. Ultrasound Physics & Knobology Hong Kong College of Emergency Medicine 09/2006 HKCEM 1
  2. 2. What is Ultrasound? Human sensitivity: 20 - 20,000Hz Ultrasound: > 20,000Hz Diagnostic Ultrasound: 2.5 - 14 MHz 09/2006 HKCEM 2
  3. 3. Definition/ Terminology Cycle Frequency: cycles per second Wave length Period Amplitude Compression - area of high density Rarefication - area of low density velocity = λ x ƒ = constant for a given medium 09/2006 HKCEM 3
  4. 4. Variation in particle density at one point as a function of time 09/2006 HKCEM 4
  5. 5. A, Initial observation. B, After a short time, regions of high & low density are displaced to the right 09/2006 HKCEM 5
  6. 6. Ultrasound Production Piezoelectric effect: – Crystals vibrate at given frequency when an alternating current is applied – Crystal acts as speaker and microphone Continuous mode: – therapeutic systems – continuous-wave Doppler (CW) Pulsed-echo mode: – all other applications 09/2006 HKCEM 6
  7. 7. Continuous-wave output 09/2006 HKCEM 7
  8. 8. Pulsed Echo Signal generation = only ~1% of the entire pulse cycle On times; off times Time of signal return proportional to distance traveled 09/2006 HKCEM 8
  9. 9. Pulsed wave output 09/2006 HKCEM 9
  10. 10. Pulsed Echo PRF: pulse repetition frequency – Very important in Color Doppler SPL: spatial pulse length = wavelength x no. of cycles Maximal resolution = 0.5 SPL – The smallest distance between 2 points that USG can delineate 09/2006 HKCEM 10
  11. 11. SPL with maximal resolution. Two objects (vertical lines) are separated by 0.5 SPL. The echo from each interface is shown by dashed lines. The objects are just resolvable. 09/2006 HKCEM 11
  12. 12. Pulse length shortened by increasing the frequency. A The four-cycle pulse from a low-frequency transducer includes both objects within the SPL. B The four-cycle pulse from a high-frequency transducer has a shorter spatial pulse length and can resolve objects located more closely together. 09/2006 HKCEM 12
  13. 13. Ultrasound Transmission 1540 m/s – Velocity assumed the same for all tissue in calculation (which is not totally true) Acoustic impedance Attenuation Reflection Refraction Scatter: objects irregular or smaller than the ultrasound beam 09/2006 HKCEM 13
  14. 14. Windows Getting a good acoustic window is important for scanning accuracy and image quality Solid organ or fluid filled area – Liver – Urinary bladder – Spleen – Fluid filled stomach (for pancreatic scan) 09/2006 HKCEM 14
  15. 15. Resolution Ability to delineate between two different objects Axial resolution: – high frequency = shorter SPL = better axial resolution but lower penetration 09/2006 HKCEM 15
  16. 16. Resolution Lateral Resolution – Sound beam: width of the crystal – Compressed down the pathway – Near field – Focal zone: best lateral resolution – Far field – Dead zone: distance between the transducer face and the first identifiable echo 09/2006 HKCEM 16
  17. 17. Resolution Temporal Resolution – Frame per second – Multiple focal zones • decreases frame rate • decrease temporal resolution 09/2006 HKCEM 17
  18. 18. Diagram illustrating Axial Resolution 09/2006 HKCEM 18
  19. 19. Diagram illustrating Lateral Resolution 09/2006 HKCEM 19
  20. 20. Modes A mode: amplitude B mode: brightness Real time (frames/sec) M mode: motion 09/2006 HKCEM 20
  21. 21. Doppler Ultrasound Continuous wave (CW) – Seldom used, not a/v in most AED machines Pulsed wave (PW) Color Doppler Duplex Doppler – Putting Color Doppler on top of Grey-scale B mode Power Doppler 09/2006 HKCEM 21
  22. 22. Transducers Formats – linear - rectangular field of view – sector - pie-shaped field of view 09/2006 HKCEM 22
  23. 23. In sector scanning, the resolution becomes poorer at increasing depth. 09/2006 HKCEM 23
  24. 24. Transducers Mechanical Probe: seldom used now Electronic Probe: – Linear array transducers • piezoelectric elements linearly arranged • sequentially activated to produce an image – Phased array transducers • smaller scanning surface (foot print) • good for echocardiography • more expensive • elements are activated with phase differences to allow steering of the ultrasound signal 09/2006 HKCEM 24
  25. 25. Linear array image of testicle Linear array transducer 09/2006 HKCEM 25
  26. 26. Convex array image of RUQ Convex array transducer 09/2006 HKCEM 26
  27. 27. Phased array (cardiac) image Phased array transducer 09/2006 HKCEM 27
  28. 28. Scanning Skills Try different windows A lot of gel Transducer movements: – Rotate – Angle (Tip-toe) – Pivot (fan-shaped movement) Transducer placement basics: – longitudinal view – transverse view – coronal view 09/2006 HKCEM 28
  29. 29. Knobology Power Gain TGC; DGC; STC Dynamic range Depth Zoom/magnification Freeze Calculations Print 09/2006 HKCEM 29
  30. 30. Image Optimization Adjust the following in order: Depth Focus TGC Zoom Gain Dynamic Range (contrast) 09/2006 HKCEM 30
  31. 31. Depth Start with higher “Depth” Decrease “Depth” to put area of interest at ¾ depth of screen Leave a small area behind to observe useful artifacts e.g. shadowing, enhancement 09/2006 HKCEM 31
  32. 32. Depth (too much !) 09/2006 HKCEM 32
  33. 33. Depth (too little !) 09/2006 HKCEM 33
  34. 34. Depth (just right !) 09/2006 HKCEM 34
  35. 35. Focus Focal zone = best lateral resolution Adjust focus to the level of the point of interest Most modern machines can have multiple focal zones Increase number of focal zone → decrease frame rate 09/2006 HKCEM 35
  36. 36. Focus (out focus !) Focus 09/2006 HKCEM 36
  37. 37. Focus (good !) 09/2006 HKCEM 37
  38. 38. Focus (multiple !) 09/2006 HKCEM 38
  39. 39. TGC Adjust TGC to obtain a smooth grey-scale picture Transit of TGC knobs should be gradual TGC = DGC = STC – TGC: time gain compensation – DGC: distance gain compensation – STC: spatial time compensation 09/2006 HKCEM 39
  40. 40. Zoom Zoom in to magnify point of interest Write-zoom: improve image quality Read-zoom: only magnification Useful situation: – Measurement of CBD – M-mode documentation of foetal pulsation 09/2006 HKCEM 40
  41. 41. Zoom 09/2006 HKCEM 41
  42. 42. Gain Optimize gain to obtain most details Most beginners use too high gain High gain can obscure many details 09/2006 HKCEM 42
  43. 43. Gain (too high !) 09/2006 HKCEM 43
  44. 44. Gain (too low !) 09/2006 HKCEM 44
  45. 45. Gain (that’s right !) 09/2006 HKCEM 45
  46. 46. Dynamic Range How white is white and how black is black? Adjust Dynamic Range for the contrast High “Dynamic Range” = Low “Contrast” 09/2006 HKCEM 46
  47. 47. Dynamic Range (too low !) 09/2006 HKCEM 47
  48. 48. Dynamic Range (too high !) 09/2006 HKCEM 48
  49. 49. Image Descriptions Anechoic Echogenic Hyperechoic Vs Isoechoic Vs Hypoechoic Interface Noise Mirror image Shadow Cystic Vs Sonolucent 09/2006 HKCEM 49
  50. 50. Artifacts Shadowing Enhancement Edge artifact Side-lobe artifact Reverberation Comet tail 09/2006 HKCEM 50
  51. 51. Shadowing and Enhancement Longitudinal image of the gallbladder. Shadowing artifacts are caused by gallstones. Enhancement is seen as a result of the fluid-filled gallbladder. 09/2006 HKCEM 51
  52. 52. Edge Artifact Refraction artifacts at the edges of ruptured patellar tendon. 09/2006 HKCEM 52
  53. 53. Side-lobe Artifact Side lobes (shaded regions) at various angles with respect to the main beam. 09/2006 HKCEM 53
  54. 54. Reverberation Artifact 09/2006 HKCEM 54
  55. 55. Reverberation Artifact Reverberation artifact in the bladder 09/2006 HKCEM 55
  56. 56. Comet Tail Artifact FB Comet tail artifact (arrow) at the back of a metallic FB. 09/2006 HKCEM 56
  57. 57. Guide to Image Acquisition Relearn anatomy Correct machine and transducer Master the gain, power, and TGC buttons Maximize the gain before increasing the power TGC should be smooth Boundaries of organs should be visualized Go from wider view and zoom in Acquire images in at least two planes 09/2006 HKCEM 57
  58. 58. Infection Control Wipe off and disinfect the transducer after use Wear glove on the scan hand Cover TVS probe with condom Dispose used condom appropiately 09/2006 HKCEM 58
  59. 59. Maintenance DO – switch off machine after use – ‘freeze’ when not scanning – handle transducers with care – clean transducers after use with soft towel or tissue paper – disinfect transducers after use 09/2006 HKCEM 59
  60. 60. Maintenance Don’t – Drop, shock or knock transducers – Disconnect or connect transducers when power is ‘on’ (Hot plug is OK on some new machine) – Leave extension cord on the floor to be rolled over by the machine – Leave extension cord in tension or in acute angles. The connecting points will be disrupted. 09/2006 HKCEM 60
  61. 61. Biological Effects Cavitation – rarefication phase – liquid to gas – stable Vs collapse Heat production There is so far no firm evidence of Bio- hazard of Diagnostic Ultrasound to human 09/2006 HKCEM 61
  62. 62. Biological Effects AIUM statement on clinical safety 1983 No confirmed biological effects on patients or instrument operators caused by exposure at intensities typical of present diagnostic ultrasound instruments have ever been reported. Although the possibility exists that such biological effects may be identified in the future, current data indicate that the benefits to patients of the prudent use of diagnostic ultrasound outweigh the risks, if any, that may be present. 09/2006 HKCEM 62
  63. 63. Biological Effects ALARA principle: As Low As Reasonably Achievable 09/2006 HKCEM 63
  64. 64. The End 09/2006 HKCEM 64

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