Ultrasound Physics and Knobology Alan Macfarlane. Consultant Anaesthetist Glasgow Royal Infirmary

Transcription

1 Ultrasound Physics and Knobology Alan Macfarlane Consultant Anaesthetist Glasgow Royal Infirmary

2 RAPM 2009; 34: 40-46

3 Ultrasound Proficiency Understanding US image generation and device operation Image optimisation (non device related) Image Interpretation Needle insertion and injection

4 RAPM 2007; 32: RAPM 2007; 32:

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7 What is ultrasound?

8 What is ultrasound? Ultrasound wave is a form of acoustic energy

9 Generation of an US Image Pulser Transducer Receiver Microprocessor Display

10 Generation of US Image Courtesy of Vincent W.S. Chan MD, Toronto Western Hospital, Toronto, ON.

11 Transducers Piezoelectric crystal subject to A.C current Crystals in probe vibrate Waves transmitted to body via acoustic gel Thousands/second

12 Courtesy of Vincent W.S. Chan MD, Toronto Western Hospital, Toronto, ON. v=fλ = (1540m/s)

13 Courtesy of Vincent W.S. Chan MD, Toronto Western Hospital, Toronto, ON.

14 Pulse Echo Principle Speed of US in Tissue Air 300 ms -1 Soft Tissue 1500 ms -1 Bone 4000 ms -1 Time for signal return is proportional to depth (and the speed travelled through tissue) Medium Ultrasound Speed (m/sec) Air 300 Lung 500 Fat 1,450 Brain 1,520 Muscle 1,580 Liver 1,550 Kidney 1,560 Brain 1,560 Soft Tissue 1,540 Bone 4,000

15 US Interaction with Tissues Reflection Scatter Specular Refraction Transmission Absorption

16 US Interaction with Tissues Reflection Scatter Specular Refraction Transmission Absorption

17 US Interaction with Tissues Reflection Scatter Specular Refraction Transmission Absorption Attenuation

18 Refraction Courtesy of Vincent W.S. Chan MD, Toronto Western Hospital, Toronto, ON.

19 Bayonet artifact

20 Reflection Courtesy of Vincent W.S. Chan MD, Toronto Western Hospital, Toronto, ON.

21 Needle angle incidence

22 Reflection Returned to Probe Difference in Impedance (Z ) Between Tissue determines reflection Muscle/blood = 0.1% Muscle/Bone = 43% Tissue Z Air Lung 0.2 Fat 1.34 Liver 1.65 Blood 1.65 Kidney 1.63 Muscle 1.7 Bone 7.8

23 Attenuation Courtesy of Vincent W.S. Chan MD, Toronto Western Hospital, Toronto, ON.

24 Effect of Frequency on Attenuation Courtesy of Vincent W.S. Chan MD, Toronto Western Hospital, Toronto, ON. Increased frequency = Increased attenuation Increased attenuation= Decreased penetration

25 Fate of US Wave Degree of attenuation/reflection/absorption & return to probe= intensity of signal Arbitrary grey scale Black= Anechoic little reflection White= Hyperechoic strong reflection

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28 Transducers

29 Probe selection FREQUENCY SHAPE

30 Probe selection

31 Probe selection

32 Probe selection

33 Probe selection

34 Probe selection FREQUENCY Anticipated depth Axial resolution

35 Probe selection FREQUENCY Anticipated depth Axial resolution

36 Probe selection FREQUENCY Anticipated depth Axial resolution SHAPE Field of view

37 Probe selection FREQUENCY Anticipated depth Axial resolution SHAPE Access Field of view

38 Resolution: Ability to distinguish one object from another Axial: 2 objects Deep & superficial Lateral= 2 objects lateral & medial (Temporal) Courtesy of Vincent W.S. Chan MD, Toronto Western Hospital, Toronto, ON.

39 Probe selection

40 Probe selection

41 Probe selection High frequency = high spatial resolution, but less depth

42 Probe selection

43 Probe selection High frequency = high spatial resolution, but less depth All upper limb (+/- infraclavicular), popliteal, femoral, ankle

44 Probe selection High frequency = high spatial resolution, but less depth All upper limb (+/- infraclavicular), popliteal, femoral, ankle Low frequency = low spatial resolution, but greater depth of penetration

45 Probe selection High frequency = high spatial resolution, but less depth All upper limb (+/- infraclavicular), popliteal, femoral, ankle Low frequency = low spatial resolution, but greater depth of penetration Spine, sciatic, +/- infraclavicular

46 Courtesy of Vincent W.S. Chan MD, Toronto Western Hospital, Toronto, ON.

47 Supraclavicular

48 Ankle posterior tibial

49 Anterior Sciatic LT GMM

50 Parasaggital spine

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53 Depth Correct depth to include and optimise size of target Appropriate depth optimises temporal resolution

54 Lateral Resolution Courtesy of Vincent W.S. Chan MD, Toronto Western Hospital, Toronto, ON.

55 Lateral Resolution Courtesy of Vincent W.S. Chan MD, Toronto Western Hospital, Toronto, ON.

56 Lateral Resolution focal zone

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60 Gain Setting Echoes returning to probe are converted to electrical signal Gain increases intensity of overall electrical signal - Target - Background noise - Artefacts

61 Gain Setting Too Low Cannot See Nerve Cannot See Needle Courtesy of Vincent W.S. Chan MD, Toronto Western Hospital, Toronto, ON. Too dark

62 Gain Setting Too High Too bright White Out Courtesy of Vincent W.S. Chan MD, Toronto Western Hospital, Toronto, ON.

63 Gain Setting Just Right Can see Nerve Marbling of Muscle Courtesy of Vincent W.S. Chan MD, Toronto Western Hospital, Toronto, ON.

64 Time Gain Compensation

65 Time Gain Compensation Attenuation

66 Time Gain Compensation US beam attenuated with depth TGC Amplifies deeper part of screen Amplifies Noise Also

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71 Doppler effect if a sound wave is sent out (or reflected) by a moving object, the frequency of that sound wave will change as the object moves towards or away from a stationary listener

72 Doppler effect US Beam hits object moving away f US beam hits object moving towards f If at absolutely 90 0 to motion No Δ f NO FLOW DETECTED

73 Courtesy of Vincent W.S. Chan MD, Toronto Western Hospital, Toronto, ON.

74 Doppler effect Main use in regional anaesthesia - identify vessels as landmarks - Confirm absence of vessels in needle trajectory e.g. supraclavicular (Power Doppler)

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77 Compound Imaging SonoMB TM CrossXbeam TM SonoCT TM Spatial compound imaging Frequency compound imaging

78 Compound Imaging Reduces speckle Reduces artifact Improves needle visibility

79 Spatial Compound Imaging Probe Probe

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83 Drop Out

84 Acoustic Shadow

85 Acoustic Enhancement RAPM 2007; 32:

86 Reverberation Artifact RAPM 2007; 32:

87 NEEDLE LOCALISATION

88 Needle & Nerve Options

89 In plane Courtesy of Vincent W.S. Chan MD, Toronto Western Hospital, Toronto, ON.

90 Out of plane

91 Summary Choose appropriate frequency for depth Adjust depth and focus Adjust gain (+/- time gain compensation) Doppler for vessels Compound imaging Recognise artifacts

92 Questions?