Introduction to Ultrasound Physics

Transcription

1 Introduction to Ultrasound Physics Vassilis Sboros Medical Physics and Cardiovascular Sciences University of Edinburgh

2 Transverse waves Water remains in position Disturbance traverse producing more wave along the path Disturbance travel at 90 o of water movement, hence transverse

3 Longitudinal wave Particles remains in position Disturbance travel at 0 o of particle movement, hence longitudinal

4 Sound-Mechanical wave Generated by piezoelectric crystals

5 Single reflection

6 Sound-Mechanical wave

7 Frequency 1Hz = 1 cycle per second Sound 20 Hz 20 khz Ultrasound > 20kHz Diagnostic Ultrasound 1-50 MHz Ultrasound Therapy 40kHz-1MHz

8 Wavelength Phase Velocity of sound Acoustic impedance Reflection Scattering Refraction Absorption Attenuation Some definitions

9 Wavelength λ λ = c ν For c tissue = 1540 m/s ν=1mhz, λ=1.54mm ν=3mhz, λ=0.51mm ν=10mhz, λ=0.15mm c air = 330 m/s λ=0.33mm c : velocity of sound (ms -1 ) ν : frequency (Hz)

10 Phase a) Angle of cycle rotation b) Phase difference between identical waves

11 Pressure Positive compression, negative rarefaction Units 1 Pa = N / m 2

12 Units W / m -2 Intensity (time)

13 Velocity of sound c c = κ ρ κ : stiffness (Pa) ρ : density (Kg/m 3 ) c air = 330 m/s c water = 1480 m/s c tissue = 1540 m/s c fat = 1450 m/s c blood = 1570 m/s c bone = 3500 m/s

14 Acoustic impedance Z Z = p u = ρ c p : pressure (Pa) u : particle velocity (m/s)

15 Reflection c c c c Z Z Z Z p p ρ ρ ρ ρ + = + = p muscle / p blood = 0.03 p fat / p muscle = 0.10 p bone / p muscle = 0.64 p muscle / p air = 0.99

16 Reflection a) Smooth surface b) Small particle c) Rough surface

17 Scattering General case for reflection λ >> particle size = Rayleigh scattering λ ~ particle size = Mie scattering λ << particle size = reflection

18 Refraction

19 Attenuation Attenuation = scattering + absorption Absorption = conversion to heat Intensity decays exponentially Frequency dependant

20 Interference Multiple ultrasound sources a) Constructive interference waves in phase b) Destructive interference waves in antiphase

21 Plane disk transducer

22 Intensity (space)

23 Frequency Spectrum a) Time domain b) Frequency domain (FFT)

24 Nonlinear propagation At high ultrasound pressure Time domain asymmetrical pattern Frequency domain (FFT) Harmonic frequencies

25 Bibliography McDicken W.N. Diagnostic Ultrasonics Churchill Livingstone New York Barnett E., Morley P. Clinical Diagnostic Ultrasound Blackwell Scientific Publications, Oxford Meire H.B., Cosgrove D.O., Dewbury K.C., Farrant P. Clinical Ultrasound a comprehensive text: Abdominal and General Ultrasound Vol.2 Churchill Livingstone New York 2001.

26 The Engineering of Ultrasound Imaging Vassilis Sboros Medical Physics and Cardiovascular Sciences University of Edinburgh

27 Transducer Engineering - Piezoelectric materials Positive Voltage = compression Synthetic ceramic - Lead Zirconate Titanate(PZT) High sensitivity High acoustic power Easy to micromachine Impedance 20x tissue Thickness = λ/2 -resonance Resonance due to internal reflection Determines transmit frequency

28 Transducer Engineering Backing layer PZT Impedance 20x tissue Duration of pulse difficult to control due to internal ringing Backing layer = absorber High impedance Reduces ringing

29 Transducer Engineering Matching layer PZT Impedance 20x tissue Only 20% of energy transmitted to tissue Matching layer = impedance matching Impedance lower than PZT and higher than tissue Remove some ringing 1 layer 2x sensitivity λ/4 thickness Constructive interference towards tissue Destructive interference towards PZT

30 Transducer Engineering Frequency bandwidth vssensitivity High sensitivity = specific dimensions for Backing, PZT and Matching layers Frequency band is narrow Resolution low >1 Matching layers Decreasing impedance Bandwidth 2x (60% to 120%) Little loss in sensitivity

31 1D Single Plane disk transducer

32 2D beams Array transducers a) Linear b) Curvilinear c) Trapezoidal d) Sector e) Radial

33 Transducer Engineering Lens Single element Focus has high sensitivity and resolution Linear Array Electronically in scan plane Only in elevation plane Phased Array Mild in scan plane Stronger in elevation plane

34 Linear Array Transducers 128 elements Binary processing Choice of frequency Penetration vsresolution or attenuation vs frequency Dimensions ~ 1/f ~1.3λwidth per element ~30λ height - elevation

35 Linear Array Transducers Active group of elements Finite beam per element Transmit fixed (~20) Receive (<20 to >20 as depth increases) Electronic focus

36 Linear Array Transducers Transmit Electronic Focus Transmission timing One focus Controllable

37 Linear Array Transducers Receive Electronic Focus Electronic delay Depth ~ element number Multiple foci Not controllable/automatic High resolution at all depths

38 Linear Array Transducers Transmit Multiple focus

39 Linear Array Transducers 1.5D array for improved elevation focus

40 Linear Array Transducers Transmit Apodization

41 Curvilinear Array Transducers Sector scanning Wider field Linear array structure Active element number reduced - Poorer resolution

42 Phased Array Transducers Sector scanning Narrow acoustic window Narrower elements All elements used (transmit and receive) Shorter near field per element Wider far field per element Beam steering ±45 o

43 Linear/Phased Array Transducers Compounding Reduction of noise Persistence Reduction of frame rate

44 Matrix Array Transducers

45 EndocavityArray Transducers a) Curvilinear transvaginal b) Curvilinear Transvaginal, transrectal c) Bi-plane Transrectal (prostate) d) Phased array Transoesophageal(heart)

46 Intravascular Array Transducers Curvilinear/convex 360 o High frequency (30MHz) Vessel wall

47 phantom

48 A-mode (transmission)

49 A-mode

50 Eye A-mode

51 B-mode scanning

52 Eye B-mode

53 B-mode Formation of B-mode image

54 B-mode

55 B-mode Transmit gain and power

56 B-mode Time gain compensation (TGC) Compensate for attenuation

57 B-mode Analogue to digital conversion limited values memory binary system sampling rate (40MHz) digital processing

58 B-mode Digital signal Rectification Enveloping

59 B-mode Compression Accommodate in the image low and high echoes

60 B-mode Image memory

61 B-mode Interpolation Linear?

62 B-mode Reading of image memory to form display Gray scale

63 Ultrasound Imaging Modes Real-time 2D imaging Good spatial resolution Good temporal resolution Good Penetration Heart scan

64 Ultrasound Imaging Modes 3D and 4D Good spatial resolution Poor temporal resolution OK Penetration Foetal scan Heart scan

65 Doppler Ultrasound Pete Hoskins and Vassilis Sboros Medical Physics and Cardiovascular Sciences University of Edinburgh

66 Doppler ultrasound Principles of Doppler CW/PW Doppler Doppler systems (spectral, duple, colour) and controls Principles of contrast imaging

67 Doppler effect

68 Controls Doppler system patient

69 Doppler effect Change in pitch is proportional to speed of source Change in pitch = f S -f O Doppler shift = f d = f S -f O Speed = v f d ~v

70 Doppler ultrasound Transducer Blood Transmission T Scattering R Reception R

71 Case 1. Blood stationary Transmission T Scattering R Reception R f r = f t

72 Case 2. Blood moving away from transducer Transmission T Scattering R Reception R f r < f t

73 Case 3. Blood moving towards transducer Transmission T Scattering R Reception R f r > f t

74 General case f t v f r = f t + f d f d = 2 f t v/c

75 Some values Transmit frequency Speed of sound Speed of blood 4 MHz 1540 m/s 1 m/s Doppler shift = 5194 Hz Hear Doppler signal

76 Doppler ultrasound Transmission Scattering Reception f t f r

77 Doppler ultrasound f t + f d f t θ v f d = 2 f t v cos θ/c

78 Cosine function Cosine Angle (degrees)

79 40 80 ο ο 60 ο

80 Some more values Transmit frequency Velocity Angle Speed of sound 3-5 MHz 0-3 m/s degrees 1540 m/s Doppler frequency shift 0-15 khz Audio range 0-20 khz Can hear Doppler shift frequencies

81 Doppler systems Spectral display Colour flow

82 Spectral display Frequency shift (khz) baseline Time (s)

83 Colour flow

84 Triplex display

85 Summary of systems and main controls 2 main types of system are Spectral Doppler Colour flow main controls for spectral Doppler adjust: position of sensitive region beam direction spectral Doppler display main controls for colour flow adjust: size and depth of colour box beam direction colour display

86 Spectral Doppler Frequency shift (khz) baseline Time (s)

87 Spectral Doppler -continuous wave (CW) Doppler signal processor Display T R Transducer Sensitive region Separate transmit and receive elements Emits ultrasound continuously Receives ultrasound continuously Doppler signals from sensitive region

88 Stand alone CW Doppler system: features No B-mode image No depth discrimination Use for vessels at defined location Use for vessels with characteristic waveform shapes Obstetric applications - umbilical arteries Peripheral vascular application - carotid, lower limb

89 CW spectral Doppler examples Arcuateartery External iliac Internal iliac Umbilical

90 2 vessels in beam

91 Pulsed wave (PW) Doppler systems Doppler signal processor Display Sensitive region Gate depth Emits ultrasound in pulses Gate length Depth discrimination Sensitive region depth and length set by user

92 Stand alone PW Doppler system -features No B-mode image Depth discrimination Use for vessels at defined location Use for vessels with characteristic waveform shapes Transcranial

93 Duplex system B-mode + PW Doppler = Duplex

94 Duplex system -features B-mode and PW Doppler depth discrimination all cardiovascular applications basis for all modern Doppler systems

95 System components and signal processing Doppler signal processor Display T R Tissue Blood Tissue Blood

96 Amplitude Received signal T R Blood Tissue From tissue (Clutter) From blood Tissue Frequency (MHz) Blood

97 Amplitude Demodulation Frequency (MHz) Frequency (Hz) Demodulation removes underlying transmit frequency

98 High pass filter Lost blood signal Frequency (Hz) Filter frequency thresholds Filtering removes the clutter signal

99 Amplitude 10ms Time Spectrum analysis Doppler frequency Time Spectrum analysis estimates all the frequencies present in the Doppler signal

100 Transducer Signal processor Received signal Frequency (MHz) Demodulator High pass filter Spectrum analysis Doppler signal Display Spectral display

101 Cut-off filter Filter low Filter high End diastolic flow Loss of end diastolic flow

102 Typical filter values Obstetrics Hz (little arterial movement) Vascular Hz (some arterial pulsation) cardiology 300Hz+ (valves and myocardium)

103 Pulsed wave (PW) Doppler Doppler signal processor Display Sensitive region Gate depth Gate length

104 Doppler signal CW PW

105 Aliasing Upper limit to detected velocity measured using PW Doppler Max Doppler frequency shift

106 CW Doppler signal PW Doppler signal (lots of samples) PW Doppler signal (2 samples/wavelength) PW Doppler signal (not enough samples) Aliasing

107 Aliasing Doppler frequency shift estimated correctly when: at least 2 samples per wavelength prf> 2 f d Maximum Doppler frequency shift which can be estimated is half the prf f d(max) = prf/2

108 Waveforms in disease Local disease (Atherosclerosis) Downstream disease (placental disease)

109 Atherosclerosis Jet Turbulence

110 Max velocity Quantification 1. Peak velocity

111 Measurement of blood velocity I. Transducer θ v v = c f d 2f t cos θ

112 Measurement of blood velocity II.

113 Measurement of blood velocity III.

114 Standard table Diameter Peak systolic stenosis (%) velocity (cm/s) 0 < < < > > 230

115 Downstream disease Uterine artery Fetus Placenta Spiral/arcuate arteries Abnormal placental development leads to increase in resistance to flow

116 Umbilical waveforms

117 Quantification 2. Waveform shape. Max Mean Min Resistance index (RI) = (max-min)/max Pulsatility index (PI) = (max-min)/mean

118 Estimation of RI Peak systolic marker End diastolic marker

119 Controls for CW, PW and duplex position of sensitive region (PW, duplex) gate length, gate depth beam direction (PW, duplex) Beam steering angle spectral Doppler display (CW, PW, duplex) gain Filter level Velocity scale Time scale Baseline Measurement (duplex) Beam-vessel angle

120 Colour flow

121

122

123 Colour flow image Display of 2D flow image superimposed on B-mode image

124 Colour boxes Sector Linear array Colour box Colour box Image built up line by line Each line consists of adjacent sample volumes

125 Colour flow system components Display B-scan processor Colour flow processor Spectral Doppler processor Beamformer Transducer Transmitters

126 Colour flow processor Demodulator Clutter filter Doppler statistic estimator Post processor Blood tissue discriminator

127 Clutter filter clutter blood Frequency (MHz) Frequency (MHz)

128 Frequency estimation Fast Fourier Transform ( data points) full frequency spectrum Autocorrelator(3 data points) mean frequency variance power

129 Post-processor Persistence or Frame-averaging Reduces noise lag in image High persistence Low persistence Value = 0.4 frame frame frame frame frame 5 Value = 0.6 frame frame 2

130 Colour image (mean Doppler frequency) Blood-tissue discriminator B-mode image

131 Colour image (mean Doppler frequency) Blood-tissue discriminator B-mode image

132 No blood tissue discriminator

133 With blood tissue discriminator

134 Colour modes Variance Colour processor Mean frequency Power Colour Doppler Power Doppler

135 Mean frequency: red-blue scale

136 Mean frequency + variance: red-blue + green

137 Power: no B-mode in colour box

138 Power: with B-mode in colour box

139 Angle dependence θ θ θ

140 Colour Doppler angle dependence

141 Power Doppler angle dependence

142 Angle dependence Doppler amplitude 40 o 90 o 60 o Doppler frequency Clutter filter

143 Angle dependence

144 Aliasing

145 Aliasing Doppler amplitude 3m/s 4m/s 1m/s 2m/s 3m/s Aliasing limit Doppler frequency Aliasing limit

146 Jet Recirculation