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Ultrasound in diagnostics and therapy
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Ultrasound in diagnostics and therapy

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Ultrasound in diagnostics and therapy Ultrasound in diagnostics and therapy Presentation Transcript

  • Ultrasound in diagnostics and therapy
  • Ultrasonic waves in water approach an aluminium cylinder
  • Periodic motion causes pressure waves
  • Sound propagation parameters T Period (sec) Frequency = ff= 1/T Frequency = = 1/T Velocity = λ /T = λ *f λ Wavelength (mm) Velocity = λ /T = λ *f high pressure low pressure
  • Transducers produce sound: piezo-electric crystal - + + ++ ++ + ++ + - -- -- -- -- - - + - + + ++ ++ + + + ++ - + -- -- -- -- - - - + + + ++ ++ + + ++ - + -- -- -- -- - - - + + + ++ ++ + + ++ - + -- -- -- -- - - - + Applied voltage Applied voltage - + induces expansion. induces expansion.
  • Transducers detect sound: piezo-electric crystal + + + ++ ++ + ++ - + -- -- -- -- - - - + + + ++ ++ + - + ++ + - -- -- -- -- - - + - pressure + + ++ ++ + + ++ + - -- -- -- -- - - + - + + ++ ++ + + ++ + - -- -- -- -- - - + - Applied pressure Applied pressure + - induces voltage. induces voltage.
  • Pulse-echo principle D 2t t transducer target Delay time, T = 2t Delay time, T = 2t D=(v)(t) D=(v)(t) D = vT/2 D = vT/2
  • Ultrasound Transducers Can be used both to transmit & receive ultrasound Coaxial cable Transducer housing Acoustic absorber Backing block Electrodes Piezoelectric crystal Matching layer
  • Acoustic pulse production high-Q transducer electrical pulse low-Q transducer electrical pulse
  • Acoustic pulse production A medical transducer produces a “characteristic” frequency. For each electrical impulse, a pulse “train” that consists of N sinusiodal cycles is produced. The “Q” of a transducer is a measure of the number of cycles in a pulse train.
  • High- versus low-Q transducers High-Q transducers – High intensity – Long-duration pulse “train” Low-Q transducers – Lower intensity – Shorter-duration pulse train
  • Ultrasound definition Infrasound < 15 Hz 15 < Sound < 20 kHz Ultrasound> 20kHz 2 MHz < Medical ultrasound<20 MHz Internal local use about 50 MHz
  • Velocity of Sound Velocity of sound is an important parameter Two material qualities decide the velocity – bulk modulus, B and density, ρ Bulk modulus (compressibility) is defined as – ratio of increase in pressure to a change in volume – units are N/m2 » Air, B = 1.5×105 N m-2, ρ = 1.27 kg m-3 v = 345 m s-1 ( at room temperature & pressure) » Water, B = 2.05×109 N m-2, ρ = 1×103 kg m-3 v = 1432 m s-1 ( at room temperature & pressure)
  • Ultrasound propagation properties Velocity of sound in “soft tissue” is nearly constant = 1500 m/sec. Velocity of sound in bone and air differ greatly from soft tissue. Velocity = Frequency x Wavelength “Ultra”sound implies f > 1 MHz Wavelength = Velocity/Frequency Wavelength < 1.5 mm
  • Speed of sound in different materials dry Perspex air gelatine (10%) tooth brass steel natural rubber bone glass lung gall stone 0 1000 2000 3000 4000 5000 6000 speed of sound (ms-1) skin muscle brain saline water blood eye lens tendon fat
  • Sound Intensity & Attenuation Intensity of a wave: – Energy per unit time per unit area » Units: Wm-2; Symbol: I Sound is scattered & absorbed by matter – Reduction in intensity called attenuation – change in intensity ∝ distance × intensity ≈ µ = attenuation constant, dependent on material ∆I = −µI∆x
  • Attenuation of Sound − µx Io Integrating gives: Io is the original intensity I = I oe gµ Intensity a sin re D ec D istance
  • Attenuation Coefficient Attenuation of sound is usually expressed as decibel (dB) Change in decibels (dB) is defined as: 10 log10 ⎛ I ⎞ ⎜ ⎟ ⎝ Io ⎠ I = e − µx Io log(I/Io) = -µx * log(e) 10* log(I/Io) = -µx * 10 * log(e) = -µx *4.343 Attenuation coeff. in dB/m (α) = 4.343 µ (m-1)
  • Attenuation against Frequency 1000 Attenuation Coefficient (dBm-1) ng air 100 lu skin tis en tes n bi le sp 10 l o og m ae r H te wa 1. 0 0. 1 1.0 10 100 1000 Frequency (MHz)
  • Safety Issues High intensity ultrasound causes heating Could damage body tissues – Diagnostic ultrasound always used at low intensities 100 Intensity (W/cm2) 10 “Potentially harmful zone” 1 “Safe zone” 0.1 Diagnostic Ultrasound levels 0.01 Exposure time (seconds) 1 10 100 1,000 10,000 Time of exposure (s)
  • Lithotripsy - to remove kidney stones by ultrasound
  • Scattering of Ultrasound Attenuation made up from: absorption (heating) scattering depends on relative size of particle (a) wavelength (λ) Scale of Frequency Scattering Examples Interaction Dependence Strength a >> λ f 0=1 (no Diaphragm, large geometrical dependence) Strong vessels, soft region tissue/bone, cysts a~λ Predominates for Stochastic variable Moderate most structures region a << λ f4 Weak Blood
  • Reflection Z1 = ρ1v1 Z2 = ρ2v2 1 T =1-R R Z = acoustic impedance Z=ρv 2 R = [(Z1-Z2)/(Z1+Z2)]
  • Acoustic Impedances Material Impedance, Z (kg m-2 s-1) Air 0.0004 × 106 Blood 1.61× 106 Brain 1.58× 106 Fat 1.38× 106 Human soft tissue 1.63× 106 Kidney 1.62× 106 Liver 1.65× 106 Muscle 1.70× 106 Skull Bone 7.80× 106 Water 1.48× 106
  • Reflection: fat/kidney Zfat = 1.38 Zkidney = 1.62 1 .934 .064
  • Reflection: muscle/air Zmuscle = 1.70 Zair = 0.0004 1 .001 .999
  • Ultrasound reflection properties Acoustic energy is reflected at interfaces between tissues with differing acoustic impedances (Z). Acoustic impedance = product of velocity of sound (v) and physical density (ρ). The unit of acoustic impedance is the “Rayl.” Strength of acoustic reflection increases as difference in Z increases. For soft-tissue/air, soft-tissue/bone and bone/air interfaces, almost total reflection occurs.
  • Transmission velocity = v decreased velocity Frequency is unchanged during propagation. Therefore, wavelength must change as velocity of medium changes.
  • Transmission: muscle/fat vmuscle = 1585 m/s vfat = 1450 m/s 10% Change in wavelength
  • Refraction reflected refracted incident Angle of incidence = angle of reflection. Refracted wave changes direction.
  • Geometrical region (a>>λ) Sound reflected & refracted like light laws of reflection & refraction hold θi θ θi = θ r r sound velocity = v1 sound velocity = v2 sin θi v 1 = θt sin θr v 2
  • Doppler effect Stationary Source Moving Source Decreased wavelength Increased frequency
  • Doppler Ultrasound Waves reflected off moving surfaces have changed frequency – fractional change ∝ velocity » vsurface= velocity of surface » v = velocity of sound » fs = frequency of source » ∆f = change in frequency Measuring frequency of returned signal gives velocity
  • Doppler effect Moving source of sound changes perceived wavelength (frequency). Shift in frequency is termed “Doppler shift.” Change in frequency = 2f(S/v)cosθ. – f = frequency – S = source velocity – v = velocity of sound – θ = angle between “view” direction and direction of motion.
  • Doppler Ultrasound Used to monitor heartbeats, blood flow, etc. Can produce images showing motion – i.e. Imaging beating heart
  • Pulse-echo principle A short pulse is send out, and the time for the return pulses is measured – called A-scan transmitter/ Original pulse Echoes receiver Amplitude A B C A B Time ( depth ) C
  • Depth (axial) resolution 2d transducer tw d To resolve distance, d, To resolve distance, d, vtw<2d vtw<2d
  • Frequency and Resolution (axial resolution) This is for linear array transducers with parallel beams MHz Axial resolution Lateral resolution Wave length (mm) 3.0 1.1 mm 2.8 mm 0.5 4.0 0.8 mm 1.5 mm 0.375 5.0 0.6 mm 1.2 mm 0.3 7.5 0.4 mm 1.0 mm 0.2 10.0 0.3 mm 1.0 mm 0.15 For harmonic imaging the input frequency doubles the output frequency (it works just for low frequencies)
  • Axial resolution “Axial” resolution is defined as the ability to distinguish between two objects along the axis of the sound beam. For a given frequency, axial resolution improves as Q decreases. For a given Q, axial resolution improves with increasing transducer frequency.
  • Transducer beam shape 2r angle = λ/2r Fresnel Zone Fraunhoffer Zone r2/λ r2f/v
  • Small versus large transducer
  • High versus low frequency low frequency high frequency
  • Time-gain compensation transducer target Attenuation of soundwave (dB) Attenuation of soundwave (dB) is approximatley proportional to is approximatley proportional to distance (delay time). distance (delay time).
  • Focused transducer unfocused transducer focused transducer
  • Electronic focusing virtual transducer surface transducer array
  • B-mode scan target
  • Multi-element Transducers Ultrasound focused – time of arrival of pulse at each transducer gives direction. Called a B-scan Electrical pulse variable D D D D D D D D D delays 1 2 3 4 5 6 7 8 9 transducer array Focused Wavefront
  • Two Dimensional Imaging Using multi-element array, 2-D image can be constructed - called B mode imaging X B mode imaging system X Y Transducer array Y Computer display
  • 3D - Ultrasound
  • 3D - Ultrasound
  • 3D - Ultrasound
  • 3D - Ultrasound
  • 3D - Ultrasound
  • 3D - Ultrasound
  • Ultrasound and contrast Contrast agent A material which, when introduced into blood or tissue, causes one or more its acoustic properties to change significantly. The most common of these properties is backscatter coefficient. Intravascular contrast agents usually comprise microbubbles which increase the blood echo level and can hence enhance the detectability of blood flow. Microbubble contrast agents emits harmonics and can be disrupted by ultrasound, both of which phenomena form the basis of nonlinear imaging.
  • ARTIFACTS 2D1=v1*t1 2D2=v1*(D-d) + d*v2 =v1*D + d*(v2-v1) D-d v1 v1 v2 d
  • ARTIFACTS
  • ARTIFACTS
  • ARTIFACTS
  • ARTIFACTS Electrical pulse variable D D D D D D D D D delays 1 2 3 4 5 6 7 8 9 transducer array Focused Wavefront
  • ARTIFACTS To high pulse frequency Deep echo that take long time to return will interfere
  • Ultrasound/Doppler to look for thrombosis in the leg
  • Vascular Ultrasound Imaging technology Real time US Doppler – continuous wave spectral Doppler – pulsed wave spectral Doppler – Color Doppler flow imaging
  • Contrast and Resolution Boundaries make echos Structured materials make echos Motion/Doppler Shifts Resolution Resolution ~ λ = c/f 2 MHz: λ = 740 µ 10 MHz: λ = 150 µ
  • Doppler effect Stationary Source Moving Source Decreased wavelength Increased frequency
  • Doppler Techniques ∆f = v/c initial sound pulse 1.0 0.5 0.0 -0.5 -1.0 0.0 0.5 1.0 1.5 2.0 time ( µsec) moving blood cell recieves 1.0 0.5 0.0 -0.5 Reflected frequency 2v/c -1.0 0.0 0.5 1.0 1.5 2.0 time ( µsec) moving blood cell reflects 1.0 0.5 0.0 -0.5 -1.0 0.0 0.5 1.0 1.5 2.0 time ( µsec)
  • Doppler Techniques moving listener hears 1.0 1.0 0.5 0.5 0.0 0.0 -0.5 -0.5 -1.0 -1.0 0.0 0.5 1.0 1.5 2.0 0.0 0.5 1.0 1.5 2.0 time ( µsec) time ( µsec) moving listener hears 1.0 1.0 0.5 0.5 0.0 0.0 -0.5 -0.5 -1.0 -1.0 0.0 0.5 1.0 1.5 2.0 0.0 0.5 1.0 1.5 2.0 time ( µsec) time ( µsec) ∆f = v/c
  • Doppler Ultrasound Waves reflected off moving surfaces have changed frequency – fractional change ∝ velocity » vsurface= velocity of surface » v = velocity of sound » fs = frequency of source » ∆f = change in frequency Measuring frequency of returned signal gives velocity
  • Doppler effect Moving source of sound changes perceived wavelength (frequency). Shift in frequency is termed “Doppler shift.” Change in frequency = 2f(S/v)cosθ. – f = frequency – S = source velocity – v = velocity of sound − θ = angle between “view” direction and direction of motion.