Ultrasound. Sound waves Sounds are mechanical disturbances that propagate through the medium...
-
Upload
drusilla-marsh -
Category
Documents
-
view
214 -
download
0
Transcript of Ultrasound. Sound waves Sounds are mechanical disturbances that propagate through the medium...
![Page 1: Ultrasound. Sound waves Sounds are mechanical disturbances that propagate through the medium Frequencies](https://reader035.fdocuments.net/reader035/viewer/2022062717/56649e405503460f94b324b8/html5/thumbnails/1.jpg)
UltrasoundUltrasound
![Page 2: Ultrasound. Sound waves Sounds are mechanical disturbances that propagate through the medium Frequencies](https://reader035.fdocuments.net/reader035/viewer/2022062717/56649e405503460f94b324b8/html5/thumbnails/2.jpg)
Sound wavesSound waves
Sounds are mechanical disturbances that propagate through the medium
Frequencies <15Hz Infrasound
15Hz<Frequencies <20KHz Audible sound
Frequencies>20Khz Ultrasound
Medical Ultrasound frequency 2 -20MHz
Some experimental devices at 50MHz
![Page 3: Ultrasound. Sound waves Sounds are mechanical disturbances that propagate through the medium Frequencies](https://reader035.fdocuments.net/reader035/viewer/2022062717/56649e405503460f94b324b8/html5/thumbnails/3.jpg)
![Page 4: Ultrasound. Sound waves Sounds are mechanical disturbances that propagate through the medium Frequencies](https://reader035.fdocuments.net/reader035/viewer/2022062717/56649e405503460f94b324b8/html5/thumbnails/4.jpg)
Velocity and frequencyVelocity and frequency
For sound waves the relationship between frequency/velocity and wavelength is
c = f x
Speed of sound depends on the material sound travels
Velocity is inversely proportional to compressibility
the less compressible a material is the greater the velocity
Average velocity in tissue 1540 m/sec (air 331m/sec, fat 1450 m/sec)
The difference in speed of sound at the boundaries determines the contrast in US
![Page 5: Ultrasound. Sound waves Sounds are mechanical disturbances that propagate through the medium Frequencies](https://reader035.fdocuments.net/reader035/viewer/2022062717/56649e405503460f94b324b8/html5/thumbnails/5.jpg)
Wave SpeedWave Speed
cair= 331 m/s csalt water
= 1500 m/s
B = Bulk Modulus = density
Bulk modulus measures stiffness of a medium and its resistanceto being compressedSpeed of sound increases with stiffness of material
k = adiabatic bulk modulus = density
€
c =B
ρ
![Page 6: Ultrasound. Sound waves Sounds are mechanical disturbances that propagate through the medium Frequencies](https://reader035.fdocuments.net/reader035/viewer/2022062717/56649e405503460f94b324b8/html5/thumbnails/6.jpg)
Wave speed cntWave speed cnt
Changes in speed DO NOT affect the frequency so only the wavelength is dependent on the material.
What is the wavelength of a 2MHz beam traveling into tissue?
What is the wavelength of a 5MHz beam traveling into tissue?
![Page 7: Ultrasound. Sound waves Sounds are mechanical disturbances that propagate through the medium Frequencies](https://reader035.fdocuments.net/reader035/viewer/2022062717/56649e405503460f94b324b8/html5/thumbnails/7.jpg)
Wave speed cntWave speed cnt
Changes in speed DO NOT affect the frequency so only the wavelength is dependent on the material.
What is the wavelength of a 2MHz beam traveling into tissue? 0.77mm
What is the wavelength of a 10MHz beam traveling into tissue? 0.15mm
The wavelength determines the image resolution
Higher frequency -> higher resolution
Penetration is higher at smaller frequencies.
![Page 8: Ultrasound. Sound waves Sounds are mechanical disturbances that propagate through the medium Frequencies](https://reader035.fdocuments.net/reader035/viewer/2022062717/56649e405503460f94b324b8/html5/thumbnails/8.jpg)
Penetration and resolutionPenetration and resolution
Thick body parts (abdomen)
Low frequency ultrasound (3.5 - 5 Mhz)
Small body parts (thyroid, breat)
High frequency (7.5 - 10 Mhz)
![Page 9: Ultrasound. Sound waves Sounds are mechanical disturbances that propagate through the medium Frequencies](https://reader035.fdocuments.net/reader035/viewer/2022062717/56649e405503460f94b324b8/html5/thumbnails/9.jpg)
InterferenceInterference
Waves can constructively and destructively interfere
Constructive interference -> Increase in amplitude (waves in phase)
Destructive interference -> Null amplitude (waves out of phase)
![Page 10: Ultrasound. Sound waves Sounds are mechanical disturbances that propagate through the medium Frequencies](https://reader035.fdocuments.net/reader035/viewer/2022062717/56649e405503460f94b324b8/html5/thumbnails/10.jpg)
Acoustic ImpedanceAcoustic Impedance
Z= x c [kg/m2/sec] SI unit ([Rayl] =1 [kg/m2/sec])
Independent of frequency
Air -> Low Z
Bone -> High Z
Large difference in acoustic impedence in the body generate large reflections that translate in large US signals
Example going from soft tissue to air filled lunghs ->BIG REFLECTION
![Page 11: Ultrasound. Sound waves Sounds are mechanical disturbances that propagate through the medium Frequencies](https://reader035.fdocuments.net/reader035/viewer/2022062717/56649e405503460f94b324b8/html5/thumbnails/11.jpg)
Sound and pressureSound and pressure
Sound waves cause a change in local pressure in the media
Pressure (Pascal)=N/m2
Atmospheric pressure 100KPa
US will deliver 1 Mpa
Intensity I (amount of energy per unit time and area) is proportional to P2
This is the energy associated with the sound beam
Temporal and Spatial intensity when dealing with time or space
![Page 12: Ultrasound. Sound waves Sounds are mechanical disturbances that propagate through the medium Frequencies](https://reader035.fdocuments.net/reader035/viewer/2022062717/56649e405503460f94b324b8/html5/thumbnails/12.jpg)
Sound and pressureSound and pressure
Relative sound intensity (dB) (Bels => B, 1B=10dB)
Relative intensity dB= 10 log(I/Io) Io original intensity, and I measured intensity
Negative dB -> signal attenuation
-3dB -> signal attenuated of 50%
![Page 13: Ultrasound. Sound waves Sounds are mechanical disturbances that propagate through the medium Frequencies](https://reader035.fdocuments.net/reader035/viewer/2022062717/56649e405503460f94b324b8/html5/thumbnails/13.jpg)
AttenuationAttenuation
Loss by scatter or absorption
High frequency are attenuated more than low frequencies
Attenuation in homegeneous tissue is exponential
A 1Mhz attenuation in soft tissue is 1 dB/cm, 5 MHz -> 5dB/cm
Bone media attenuation increases as frequency squared.
Absorbed sound ->heat
![Page 14: Ultrasound. Sound waves Sounds are mechanical disturbances that propagate through the medium Frequencies](https://reader035.fdocuments.net/reader035/viewer/2022062717/56649e405503460f94b324b8/html5/thumbnails/14.jpg)
ReflectionReflection
Echo -> reflection of the sound beam
The percentage of US reflected depends on angle of incidence and ZSimilar to light
€
R =Z2 − Z1
Z2 + Z1
⎡
⎣ ⎢
⎤
⎦ ⎥
2
€
T =4 Z1 ⋅Z2[ ]
Z1 + Z2[ ]2
![Page 15: Ultrasound. Sound waves Sounds are mechanical disturbances that propagate through the medium Frequencies](https://reader035.fdocuments.net/reader035/viewer/2022062717/56649e405503460f94b324b8/html5/thumbnails/15.jpg)
ReflectionSnell’s LawReflectionSnell’s Law
i angle of incidence
t angle of transmittance
€
sin θ i( )
sin θ t( )=
v1
v2
![Page 16: Ultrasound. Sound waves Sounds are mechanical disturbances that propagate through the medium Frequencies](https://reader035.fdocuments.net/reader035/viewer/2022062717/56649e405503460f94b324b8/html5/thumbnails/16.jpg)
TransducerTransducer
Made of piezoelectric material
Crystals or ceramics
Stretching and compressing it generate V
Lead-zirconate-titanate (PZT)
• A high frequency voltage applied to PZT
generate high freq pressure waves
Are generators and detectorsAre generators and detectors
![Page 17: Ultrasound. Sound waves Sounds are mechanical disturbances that propagate through the medium Frequencies](https://reader035.fdocuments.net/reader035/viewer/2022062717/56649e405503460f94b324b8/html5/thumbnails/17.jpg)
Q factor Q factor
• Q factor is the frequency response of the piezoelectric crystal
• Determines purity of sound and for how long it will persist
• High Q transducers generate pure frequency spectrum (1 frequency)
• Q=operating frequency/BW
– BW bandwidth
– High Q -> narrow BW
– Low Q->broad BW
![Page 18: Ultrasound. Sound waves Sounds are mechanical disturbances that propagate through the medium Frequencies](https://reader035.fdocuments.net/reader035/viewer/2022062717/56649e405503460f94b324b8/html5/thumbnails/18.jpg)
Transducer backingTransducer backing
• Backing of transducer with impedance-matched, absorbing material reduces reflections from back damping of resonance
– Reduces efficiency
– Increases Bandwidth (lowers Q)
![Page 19: Ultrasound. Sound waves Sounds are mechanical disturbances that propagate through the medium Frequencies](https://reader035.fdocuments.net/reader035/viewer/2022062717/56649e405503460f94b324b8/html5/thumbnails/19.jpg)
Axial beam profileAxial beam profile
• Piston source: Oscillations of axial pressure in near-field (e.g. z0= (1 mm)2/0.3mm = 3 mm)
• NF Variation in pressure and amplitude
• Caused by superposition of point wave sources across transducer (Huygens’ principle)
• Side lobes = small beams of reduced intensity at an angle to the main beam
€
sin(θ ) = 1.22λ /(2r)
Near FieldFresnel Zone
Far FieldFraunhofer zone
US usually uses Fresnel Zone
€
z0 =r2
4
![Page 20: Ultrasound. Sound waves Sounds are mechanical disturbances that propagate through the medium Frequencies](https://reader035.fdocuments.net/reader035/viewer/2022062717/56649e405503460f94b324b8/html5/thumbnails/20.jpg)
Lateral beam profile Lateral beam profile
• Determined by Fraunhofer diffraction in the far field.
• Given by Fourier Transform of the aperture function
• Lateral resolution is defined by width of first lobe (angle of fist zero) in diffraction pattern
– For slit (width a):
– For disc (radius r, piston source):
sin 0.61 arcsin 0.61r r
⎛ ⎞= → = ⎜ ⎟⎝ ⎠
( ) 0
sin sinc
Minima at: sin
aI I
na
π θθ
λ
λθ
⎛ ⎞= ⎜ ⎟
⎝ ⎠
⇒ =
![Page 21: Ultrasound. Sound waves Sounds are mechanical disturbances that propagate through the medium Frequencies](https://reader035.fdocuments.net/reader035/viewer/2022062717/56649e405503460f94b324b8/html5/thumbnails/21.jpg)
Focused transducersFocused transducers
• Reduce beam width
• Concentrate beam intensity, increasing penetration and image quality
• All diagnostic transducers are focused
• Focal zone – Region where beam is focused
• Focal length – distance from the transducer and center focal zone
![Page 22: Ultrasound. Sound waves Sounds are mechanical disturbances that propagate through the medium Frequencies](https://reader035.fdocuments.net/reader035/viewer/2022062717/56649e405503460f94b324b8/html5/thumbnails/22.jpg)
Focusing of ultrasoundFocusing of ultrasound
• Increased spatial resolution at specific depth
• Self-focusing radiator or acoustic lens
![Page 23: Ultrasound. Sound waves Sounds are mechanical disturbances that propagate through the medium Frequencies](https://reader035.fdocuments.net/reader035/viewer/2022062717/56649e405503460f94b324b8/html5/thumbnails/23.jpg)
Array typesArray types
a) Linear Sequential (switched) ~1 cm 10-15 cm, up to 512 elements
b) Curvilinearsimilar to (a), wider field of view
c) Linear Phasedup to 128 elements, small footprint cardiac imaging
d) 1.5D Array3-9 elements in elevation allow for focusing
e) 2D PhasedFocusing, steering in both dimensions
![Page 24: Ultrasound. Sound waves Sounds are mechanical disturbances that propagate through the medium Frequencies](https://reader035.fdocuments.net/reader035/viewer/2022062717/56649e405503460f94b324b8/html5/thumbnails/24.jpg)
Array resolutionArray resolution
• Lateral resolution determined by width of main (w) lobe according to
Larger array dimension increased resolution
• Side lobes (“grating lobes”) reduce resolution and appear at
sinw
=
wa
g
sin 1, 2,3,...g
nn
g
= =
![Page 25: Ultrasound. Sound waves Sounds are mechanical disturbances that propagate through the medium Frequencies](https://reader035.fdocuments.net/reader035/viewer/2022062717/56649e405503460f94b324b8/html5/thumbnails/25.jpg)
Ultrasound ImagingUltrasound Imaging
![Page 26: Ultrasound. Sound waves Sounds are mechanical disturbances that propagate through the medium Frequencies](https://reader035.fdocuments.net/reader035/viewer/2022062717/56649e405503460f94b324b8/html5/thumbnails/26.jpg)
ImagingImaging
• Most ultrasound beam are brief pulses of 1 microsecond
• Wait time for returning echo
• Object must be large compared to wavelength
• Signal is amplified when returned (echo is small signal)
![Page 27: Ultrasound. Sound waves Sounds are mechanical disturbances that propagate through the medium Frequencies](https://reader035.fdocuments.net/reader035/viewer/2022062717/56649e405503460f94b324b8/html5/thumbnails/27.jpg)
A-mode (amplitude mode) IA-mode (amplitude mode) I
• Oldest, simplest type
• Display of the envelope of pulse-echoes vs. time, depth d = ct/2• Pulse repetition rate ~ kHz
(limited by penetration depth, c 1.5 mm/s 20 cm 270 s, plus additional wait time for reverberation and echoes)
![Page 28: Ultrasound. Sound waves Sounds are mechanical disturbances that propagate through the medium Frequencies](https://reader035.fdocuments.net/reader035/viewer/2022062717/56649e405503460f94b324b8/html5/thumbnails/28.jpg)
A-mode (amplitude mode) A-mode (amplitude mode)
• Or space! Also M mode!
depth
![Page 29: Ultrasound. Sound waves Sounds are mechanical disturbances that propagate through the medium Frequencies](https://reader035.fdocuments.net/reader035/viewer/2022062717/56649e405503460f94b324b8/html5/thumbnails/29.jpg)
A-mode IIA-mode II
• Frequencies: 2-5 MHz for abdominal, cardiac, brain; 5-15 MHz for ophthalmology, pediatrics, peripheral blood vessels
• Applications: ophthalmology (eye length, tumors), localization of brain midline, liver cirrhosis, myocardium infarction
• Logarithmic compression of echo amplitude (dynamic range of 70-80 dB)
Logarithmic compression of signals
![Page 30: Ultrasound. Sound waves Sounds are mechanical disturbances that propagate through the medium Frequencies](https://reader035.fdocuments.net/reader035/viewer/2022062717/56649e405503460f94b324b8/html5/thumbnails/30.jpg)
M mode or T-M modeM mode or T-M mode
• Time on horizontal axis and depth on vertical axis
• Time dependent motion
• Used to study rapid movement – cardiac valve motion
![Page 31: Ultrasound. Sound waves Sounds are mechanical disturbances that propagate through the medium Frequencies](https://reader035.fdocuments.net/reader035/viewer/2022062717/56649e405503460f94b324b8/html5/thumbnails/31.jpg)
B-mode clinical exampleB-mode clinical example
Static image of section of tissue
Brighter means intensity of echo
![Page 32: Ultrasound. Sound waves Sounds are mechanical disturbances that propagate through the medium Frequencies](https://reader035.fdocuments.net/reader035/viewer/2022062717/56649e405503460f94b324b8/html5/thumbnails/32.jpg)
B-mode (“brightness mode”)B-mode (“brightness mode”)
• Lateral scan across tissue surface
• Grayscale representation of echo amplitude
Add sense of direction to information-> where did echo come from
![Page 33: Ultrasound. Sound waves Sounds are mechanical disturbances that propagate through the medium Frequencies](https://reader035.fdocuments.net/reader035/viewer/2022062717/56649e405503460f94b324b8/html5/thumbnails/33.jpg)
Real-time B scannersReal-time B scanners
• Frame rate Rf ~30 Hz:
• Mechanical scan: Rocking or rotating transducer + no side lobes - mechanical action, motion artifacts
• Linear switched array
12
2acq f
d ct N R t
c d N−= × ⇒ = = d: depth
N: no. of lines
![Page 34: Ultrasound. Sound waves Sounds are mechanical disturbances that propagate through the medium Frequencies](https://reader035.fdocuments.net/reader035/viewer/2022062717/56649e405503460f94b324b8/html5/thumbnails/34.jpg)
Linear switchedLinear switched
![Page 35: Ultrasound. Sound waves Sounds are mechanical disturbances that propagate through the medium Frequencies](https://reader035.fdocuments.net/reader035/viewer/2022062717/56649e405503460f94b324b8/html5/thumbnails/35.jpg)
CW DopplerCW Doppler
• Doppler shift in detected frequency
• Separate transmitter and receiver
• Bandpass- filtering of Doppler signal:
– Clutter (Doppler signal from slow-moving tissue, mainly vessel walls) @ f<1 kHz
– LF (1/f) noise
– Blood flow signal @f < 15 kHz
• CW Doppler bears no depth information
2 cosshift
vf f
c
=
v: blood flow velocityc: speed of sound: angle between direction of blood flow and US beam
Frequency Counter
SpectrumAnalyzer
![Page 36: Ultrasound. Sound waves Sounds are mechanical disturbances that propagate through the medium Frequencies](https://reader035.fdocuments.net/reader035/viewer/2022062717/56649e405503460f94b324b8/html5/thumbnails/36.jpg)
CW Doppler clinical imagesCW Doppler clinical images
• CW ultrasonic flowmeter measurement (radial artery)
• Spectrasonogram:
Time-variation of Doppler Spectrum
t
f
t [0.2 s]
v [10cm/s]
![Page 37: Ultrasound. Sound waves Sounds are mechanical disturbances that propagate through the medium Frequencies](https://reader035.fdocuments.net/reader035/viewer/2022062717/56649e405503460f94b324b8/html5/thumbnails/37.jpg)
CW Doppler exampleCW Doppler example
![Page 38: Ultrasound. Sound waves Sounds are mechanical disturbances that propagate through the medium Frequencies](https://reader035.fdocuments.net/reader035/viewer/2022062717/56649e405503460f94b324b8/html5/thumbnails/38.jpg)
Duplex ImagingDuplex Imaging
• Combines real-time B-scan with US Doppler flowmetry
• B-Scan: linear or sector
• Doppler: C.W. or pulsed (fc = 2-5 MHz)
• Duplex Mode:
– Interlaced B-scan and color encoded Doppler images limits acquisition rate to 2 kHz (freezing of B-scan image possible)
– Variation of depth window (delay) allows 2D mapping (4-18 pulses per volume)
![Page 39: Ultrasound. Sound waves Sounds are mechanical disturbances that propagate through the medium Frequencies](https://reader035.fdocuments.net/reader035/viewer/2022062717/56649e405503460f94b324b8/html5/thumbnails/39.jpg)
Duplex imaging example (c.w.)Duplex imaging example (c.w.)
www.medical.philips.com
![Page 40: Ultrasound. Sound waves Sounds are mechanical disturbances that propagate through the medium Frequencies](https://reader035.fdocuments.net/reader035/viewer/2022062717/56649e405503460f94b324b8/html5/thumbnails/40.jpg)
Duplex imaging (Pulsed Doppler)Duplex imaging (Pulsed Doppler)
![Page 41: Ultrasound. Sound waves Sounds are mechanical disturbances that propagate through the medium Frequencies](https://reader035.fdocuments.net/reader035/viewer/2022062717/56649e405503460f94b324b8/html5/thumbnails/41.jpg)
US imaging example (4D)US imaging example (4D)