22 23 24 us basics interaction transducer imaging safteywork2007/22 23 24 us basics...

Prof. Ed X. Wu

Medical Imaging

Ultrasound Imaging

Overview

• History• Physics of Ultrasound

wave propagation,attenuation, scattering, and reflection

• Generation and Detection of UltrasoundPiezoelectric Transducers

• Ultrasound Imaging ModesA-Mode, B-Mode, M-Mode,Doppler flow imaging

• Imaging Artifacts• Bioeffects and Safety

Overview






History of Ultrasound Imaging

• 1822 Swiss physicist Daniel Colladen used underwater bell in an attempt to calculate the speed of sound in the waters of Lake Geneva.

1435 meters/second,

History of Ultrasound Imaging

• 1822 Swiss physicist Daniel Colladen used underwater bell in an attempt to calculate the speed of sound in the waters of Lake Geneva.

• 1877 Lord Rayleigh (England) published famous treatise "The Theory of Sound" in which the fundamental physics of sound vibrations (waves), transmission and refraction were clearly delineated.

• 1880 Pierre and Jacques Curie discovered in Paris, France, the piezoelectric effect incertain crystals, which brings breakthroughin echo-sounding techniques.

History of Ultrasound Imaging• 1912 (one month after sinking of Titanic) British Patent

Office: LF Richardson filed first patent for an under-water echo ranging SONAR(Sound Navigation Ranging )

• 1914 Canadian Reginald A Fessenden designed and built the first working sonar system. It was used to detect icebergs up to two miles away, and signaling and detection of submarines.

• 1942 Karl Theodore Dussik, a neurologist/psychiatrist at the University of Vienna, Austria, published paper on "Hyperphonography of the Brain”. He is regarded as the first physician to employed ultrasound in medical diagnosis.

History of Ultrasound Imaging• 1952 Wild and Reid publish first two-dimensional clinical

ultrasound images.

• 1958 Ian Donald, Professor at the University of Glasgow, Scotland, Department of Midwifery, publishes first paper concerning ultrasound in Obstetrics and Gynecology ('Investigation of Abdominal Masses by Pulsed Ultrasound’, The Lancet)

• 1961 Use of first ultrasound system to image fetus.

History of Ultrasound Imaging1822 Colladen used underwater bell to calculate the speed of sound in waters of Lake Geneva.1830 Savart developed large, toothed wheel to generate very high frequencies.1842 Magnetostrictive effect discovered by Joule.1845 Stokes investigated effect of viscosity on attenuation.1860 Tyndall developed the sensitive flame to detect high frequency waves.1866 Kundt used dust figures in a tube to measure sound velocity.1876 Galton invented the ultrasonic whistle.1877 Rayleigh's "Theory of Sound" laid foundation for modern acoustics.1880 Curie brothers discovered the piezoelectric effect.

1890 Koenig, studying audibility limits, produced vibrations up to 90,000 Hz.1903 Lebedev and coworkers developed complete ultrasonic system to study absorption of waves.1912 Sinking of Titanic led to proposals on use of acoustic waves to detect icebergs.1912 Richardson files first patent for an underwater echo ranging sonar.1914 Fessenden built first working sonar system in the United States which could detect icebergs two miles away.1915 Langevin originated modern science of ultrasonics through work on the"Hydrophone" for submarine detection.1921 Cady discovered the quartz stabilized oscillator.1922 Hartmann developed the air-jet ultrasonic generator.1925 Pierce developed the ultrasonic interferometer.1926 Boyle and Lehmann discovered the effect of bubbles and cavitation in liquids by ultrasound.

1927 Wood and Loomis described effects of intense ultrasound.1928 Pierce developed the magnetostrictive transducer.1928 Herzfeld and Rice developed molecular theory for dispersion and absorption of sound in gases.1928 Sokolov proposed use of ultrasound for flaw detection.1930 Debye and Sears and Lucas and Biquard discover diffraction of light by ultrasound.1930 Harvey reported on the physical, chemical, and biological effects of ultrasound in macromolecules, microorganisms and cells.1937 Sokolov invented an ultrasonic image tube.

1937 Dussik brothers made first attempt at medical imaging with ultrasound.1938 Pierce and Griffin detect the ultrasonic cries of bats.1939 Pohlman investigated the therapeutic uses of ultrasonics.1940 Firestone, in the United States and Sproule, in Britain, discovered ultrasonic pulse-echo metal-flaw detection.1940 Sonar extensively developed and used to detect submarines.

1941 "Reflectoscopes" extensively developed for non-destructive metal testing.1944 Lynn and Putnam successfully used ultrasound waves to destroy brain tissue of animals.1945 Newer piezoelectric ceramics such as barium titanate discovered.1945 Start of the development of power ultrasonic processes.1948 Start of extensive study of ultrasonic medical imaging in the United States and Japan.

1954 Jaffe discovered the new piezoelectric ceramics lead titanate-zirconate.

Overview






Electromagnetic Spectrum

Is Ultrasound part of EM spectrum?

• Sound is not part of EM spectrum.

• Sound needs medium to propagate.

• Sound consists of traveling pressure waves.

Physics of Ultrasound


uniform distribution of molecules in medium

movement of piston to right produces zone of compression

withdrawal of piston to left produces zone of rarefaction

alternate movement of piston establishes longitudinal wave

piston = “transducer”

• Wave Equation in 1 and 3 dimensions, (with p := pressure, c:= speed of sound):


∂ 2 p

∂x2=

1

c 2∂ 2 p

∂t 2 or ∇2 p =

1

c 2∂ 2 p

∂t 2

p(x, t) = p0e−i(kx−ωt) or p(r, t) = p01re−i(kr−ωt)

Solutions in 1D or 3D (point source)

K=2π/λ ω = 2πf λ=c/f

20 Hz 20 kHz 1 MHz 20 MHz

Frequency f (Hz)101 102 103 104 105 106 107

infra-sound

audible sound ULTRASOUND

DiagnosticUltrasound

Ultrasound Frequencies

ω = 2πfflight ~ 1015 fxray ~ 1018

Speed of Sound

Needs a medium to travel !Independent of frequency !

W.R. Hendee, E.R. Ritenour, Medical Imaging Physics, Mosby-Year Book, St. Louis, 1992, p. 484

Wavelength =propagation speed

frequency

λ = cf

c = 1.54 mm/µs= 1540 m/s

For example:

f: 1.0 kHz 1.0 MHz 10 MHzλ: 1.54 m 1.54 mm 0.154 mm

ULTRASOUND

Ultrasound Wavelength

Overview






Ultrasound Attenuation

Conversion

Definition of decibel:

dB = 10 log10(I/Io)

where Io is reference intensity (The unit of ultrasound intensity is energy/s/cm2).

Use of Decibel

Example: For audible sound I0 ≡ 10-16 W/cm2.

=> What is Intensity I for 100 dB sound?

10(100/10) • I0 = 10-6 W/cm2

or 1010 times louder (in intensity) than reference!

Definition of decibel:

dB = 10 log10(I/Io)

where Io is reference intensity.

Use of Decibel

Example II:For ultrasound no standard reference is used.

“The reflected ultrasound signal is 20dB below transmitted signal.”

=> Reflected signal is 102 times smaller (in intensity) than transmitted signal!


W.R. Hendee, E.R. Ritenour, Medical Imaging Physics, Mosby-Year Book, St. Louis, 1992, Ch. 20

1( / ) ( )

200( ) 10

dB cm z cmp z p

α− × ×= ×

Ultrasound attenuation increases as frequency increases; thus the penetration reduces as the frequency increases.




Diagnostic Ultrasound

Overview






Ultrasound Scattering

Z = ρc

where ρ is the medium density;c is the speed of sound.

Acoustic Impedance Z

Speed & Acoustic Impedance

Tissue speed (m/s) Acoustic Impedance Z(kg/m2/s) ×106

air 330 0.0004fat 1460 1.34

water 1480 1.48liver 1555 1.65

blood 1560 1.65muscle 1600 1.71skull bone 4080 7.80

Acoustic Impedance


10-6

Reflection & Transmission

impedance

tissue 1 tissue 2Ii

Ir

It

R =IrI i

=Z2 cosθi − Z1 cosθ t

Z2 cosθi + Z1 cosθ t

⎛

⎝ ⎜ ⎜

⎞

⎠ ⎟ ⎟

2reflection coefficient:

T =I tI i

=4Z2 Z1 cos2 θ i

Z2 cosθi + Z1 cosθt( )2

transmission coefficient:

Snell’s law?

Reflection & Transmission @ 00

impedance

tissue 1 tissue 2Ii

Ir

It

R =IrI i

=Z2 − Z1

Z2 + Z1

⎛

⎝ ⎜ ⎜

⎞

⎠ ⎟ ⎟

2T =

I tI i

=4Z2 Z1

Z2 + Z1( )2

reflection coefficient: transmission coefficient:

R + T =1

Reflection & Transmission

Interface Intensity Reflection Intensity Transmission@ 00 Coefficient R Coefficient T

muscle/liver 0.0004 0.9996fat/muscle 0.014 0.986muscle/bone 0.64 0.36muscle/air 0.999 0.001

Magnitude of Echo (R)

total reflection R=1

G. Kossogg et al, Ultrasound Med Biol 1976; 2:90& W.R. Hendee, E.R. Ritenour, Medical Imaging Physics, Mosby-Year Book, St. Louis, 1992, Ch. 20

Overview






Ultrasound Transducer


dPZT = n λ/2 (where n is an odd integer.)

Fundamental frequency when n=1;Third harmonic frequency when n=3.

Transducer Resonance

Continuous Wave Excitation

The resonance frequency is determined by the element thickness dPZT. Resonance occurs at frequencies corresponding to half the wavelength λ/2, 3λ/2, 5λ/2,…

dPZT = λ n/2

where n is a positive odd integers.

Transducer Materials

Near and Far Field

Near Field Far Field

Measurement of Pressure Amplitude

Pressure Amplitude Field

Near Field

Far Field

transducer

|p|2

Acoustic pressure along z-axis for a disc transducer

Near Field Far Field

D2/4λ

Near and Far Field

Acoustic pressure along lateral direction for a disc transducer in far field

Lateral Field Properties

Piezoelectric Element

Matching Layer

Acoustic Insulator

Backing Material

ElectricConnector

Basic Transducer Design

Q Factor of Transducer

Transducers for diagnostic imaging typically end up with a broad bandwidth which can be described by the Q factor:

Q = f0/∆f

where f0 is the center frequency and ∆f is 50% frequency bandwidth.

∆f

f0


Ultrasound transducer convert electric energy to acoustic energy and acoustic energy to electric signals.

+ + + + + + + + +

- - - - - - - - -

Piezoelectric Material

The piezoelectric effect: a force applied to opposite faces of piezoelectric materials results in an electrical signal and vice versa. This was discovered by Pierre and Jacques Curie in 1880s.

+ + + + + + + + +

- - - - - - - - -

Piezoelectric Material

Ultrasound Detection

Overview






Three primary ways to display echo information:

• A - mode

• B - mode

• M - mode

A - Mode Scan

Amplitude - Mode Scan

A - Mode Scan

e.g. eye examination


A - Mode Scan

another example


Information in A - Mode

• reflector distance

• relative amplitude of echoes

• whether a structure is echogenic or anechoic

TISSUE

Echo

Ultrasound Pulse

D = c • t/2

Range Equation

c = speed of soundt = time between

sending pulseand receiving signal

Range Equation: Example

If the speed of sound is 1540 m/s and a reflector is positioned 1 cm from the transducer, how long does it take a pulse of sound to travel to the reflector and an echo to return to the transducer?

t = 2D/c = 2*0.01(m) / 1540(m/s)

t = 0.000013 s

t = 13 µs.

Information in A - Mode

• reflector distance

• relative amplitude of echoes

• whether a structure is echogenic or anechoic

echogenicmass

anechoicmass

Advantages of A-Mode:• precise information on structure

dimensions;• inexpensive, easy to produce;

Disadvantages of A-Mode:• only one dimensional (distance from the

transducer)• no recording of motion patterns;

A-mode and B-mode Display

A-modeamplitude display

B-modeBrightness display(Gray-scale)

B - Mode Scan

e.g. fetal examination Brightness - Mode Scan

B - Mode Scan

Frame Rate Limitation

The image frame rate indicates the number of times per second a sweep of the ultrasound beam is done. The higher the frame rate, the better is the ability to image fast moving structures. This ability is referred to as the temporal resolution.

The frame rate is limited by the time needed for echo collections because for each acoustic line, a time delay is required to wait for the echoes from the maximum depth. This time delay is determined by the speed of sound and the setting on the maximum visualization depth.

Frame Rate Limitation

If the maximum visualization depth is D and the speed of sound is c, the minimum time interval Tline for each acoustic line is:

Tline = 2D/c

If each image frame consists of N acoustic lines, then the minimum time interval Tframe for a sweep of the ultrasound beam is:

Tframe = N Tline = 2ND/c

The maximum allowable frame rate FRmax is:

FRmax = 1/ Tframe (typical ~20/s)

Typical Spatial Resolutions• Axial: 0.1 mm - 1 mm

• Lateral: 1 mm - to around 5 mm

• Slice thickness: 2 mm - to around 12 mm

• Effect of frequency?

Advantages

• Depiction of anatomical cross-sections

• Depiction of motion in two dimensions

Disadvantages

• temporal resolution limited by frame rates (usually 20-30/s)

• Relative costly, complex to produce.

B - Mode Scanning

M - Mode Scan

e.g. heart valve examination

M - Mode Scan

e.g. heart valve examination

Time

Depth

The M-mode displays reflector depth on one axis and time on an orthogonal axis. (Note: the “time” here should not be confused with the time delay we talked before between pulse emission and echo reception.) Reflector moving velocity can be estimated by measuring the slope on an M-mode display.

∆d

∆t

0.5 sec

0.5 cm

M - Mode Imaging

Echocardiographic Tracing

Time

Dep

th

Information provided by M-Mode

• Reflector movement patterns

• Reflector distance from the transducer

• Excellent temporal resolution

• Precise information on reflector motion

• Precise information on structure dimensions

• Inexpensive, simple to produce

Advantages of M-Mode

• Only one dimension (distance from the transducer)

• No cross-sectional imaging

• More recent technologies such as Doppler and color flow image displays are relegating M-mode to a display of less importance in echocardiography

Disadvantages of M-Mode

Overview






DOPPLER EFFECT

Change in observed frequency of a sound wave when either source or listener are moving relative to

one another.

λ

Stationary source and listener:

SourceListener or Receiver

ft = fr and λt = λr

where ft and λt are the transmittedfrequency and wavelength

and fr and λr are the received frequency and wavelength.

DOPPLER EFFECT

λ

Stationary listener, source moving towards the listener:

Source Listener

The wavelength of the sound heard is shortened:λr = λt - ∆λ

where λt is the transmitted wavelength and λr is the wavelength heard. ∆λ is the distance traveled by the source in one period. Use vs for the source velocity:

∆λ = vs/ft

DOPPLER EFFECT

λ

Stationary source, listener moving towards the source:

Source Listener

The wavelength of the sound heard is shortened:λr = λt - ∆λ

where λt is the transmitted wavelength and λr is the wavelength heard. ∆λ is the distance traveled by the listener in one period. Use vl for the listener velocity:

∆λ = vl/fr

DOPPLER EFFECT

DOPPLER SHIFT

The difference between the frequency of the returning echo and the frequency of the transmitted beam

fd = fr - fo = 2 • fo • v/c • cosθ

θ

frf0

v := speed of particles

c:= speed of sound

fd = 2 fo v cos(θ)/c= 6.5 kHz (for θ = 00)= 5.6 kHz (for θ = 300)= 3.3 kHz (for θ = 600)= 0.0 kHz (for θ = 900)

Doppler Frequency vs. Doppler Angle

Color Flow Imaging

Overview


wave propagationattenuation, scattering, and reflection


• Ultrasound Imaging Systems• Imaging Artifacts• Doppler Flow Imaging• Bioeffects and Safety

Today’s ultrasound scanner , such as Acuson Sequoia (left), support a number of different transducers, operating modes and image display devices. Images can be transferred through network for remote readings.

Multiple Frequency transducers

Ultrasound Imaging Systems

Electronics – Block Diagram

Time-Gain-Compensation

• TGC compensates for tissue attenuation

• Rate of TGC is often called the “slope”

• Higher attenuating tissue needs steeper slopes

• Higher frequencies need steeper slopes

Radio frequency echo signals

Demodulation to yield envelopes

TGC compensation

Logarithmic compression

Elimination of signals below threshold setting

ThresholdAfter compression

After TGC

After demodulation

Unprocessed signals

After elimination of below-threshold signals

Time

Signalvoltage

Overview






Structures and features in an image that do not correspond to the object being imaged.

Object

artifact

Imaging Artifacts

Reverberation Artifacts

Reverberation Artifacts

Multiple echoes from same boundary appear at different depth.

real boundary

imaginary boundaries

Mirror Artifact

Not real !

Acoustic Shadowing

Stones in gallbladder reduce the transmission of sound and “cast” shadow.

Overview






Heating

• When sound is absorbed, energy in the wave is converted to heat.

• This is the basis for ultrasound physical therapy and ultrasound hyperthermia.

• In some cases, possibilities of heating from diagnostic exposures can’t be ruled out.

Cavitation

• Refers to the generation, growth and interaction of small gas bubbles in a sound field.

• Bubbles are more easily compressed than tissues; leads to greater stresses on cells.

1. Widespread clinical use over 35 years has not established any adverse effect arising from exposure to diagnostic ultrasound

2. Studies using this method show no evidence of an effect on birth weight of humans.

3. Studies have shown no causal association of diagnostic ultrasound with the adverse fetal outcomes studied.

Epidemiology

•• Second mostly used imaging modality Second mostly used imaging modality clinically. clinically.

•• NonNon--invasively visualize the internal invasively visualize the internal structures.structures.

•• Medical ultrasound uses sound waves at Medical ultrasound uses sound waves at 2 2 -- 15 MHz.15 MHz.

•• It is widely used in cardiology, OB/GYN, It is widely used in cardiology, OB/GYN, urology, vascular diagnosis, urology, vascular diagnosis, renal/hepatic imaging, etc.renal/hepatic imaging, etc.

http:// www.philips.com

http:// fotosearch.com/

US Images of a baby! (from Lei Sun)

Phase Array Method in Ultrasound Imaging

- Limitations of traditional single-transducer method?

1D Transducer Array – Sequential electronic sweep

Phase Array Transmitting (Electronic steering of ultrasound waves)- resolution factor

Phase Array Transmitting (Electronic steering of ultrasound waves)

Angular steering of continuous waves by linear phase variation across the array

Phase Array Transmitting (Electronic focusing)

- application in high-intensity focused ultrasound (HIFU surgery)

Single Transducer

Vs Phase Array

Transmitting

1D Transducer Array – Phase array reception of reflected signals (Phase array ultrasound)

Phase Array Reception

Digital delay & analog delay

2D Transducer Array

2D Transducer Array for Direct 3D Imaging

Intravascular Ultrasound Imaging (IVUS)

Ultrasound Harmonic Imaging

Optoacuostic Imaging

EXTRAS

Tomography of ultrasound attenuation ?

EXTRAS- Micro gas bubbles (2-10um) to create more reflected signal in US

EXTRAS- High intensity Focused Ultrasound for Tumor/tissue Removal

EXTRAS- High intensity Focused Ultrasound for Tumor/tissue Removal

22 23 24 us basics interaction transducer imaging safteywork2007/22 23 24 us basics...

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