Page 1
11/15/2000 © copyright T. L. Szabo
Center for Subsurface Sensing and Imaging Systems (CenSSIS)
A National Science Foundation Engineering Research Center
Research and Industrial Collaboration ConferenceNovember 13-15, 2000
This work was supported in part by the Engineering Research Center Program of the National ScienceFoundation under award number EEC-9986821.
Medical Ultrasound Imaging Systems
Thomas Szabo, Agilent Technologies, Inc. (Retired)
Page 2
11/15/2000 © copyright T. L. Szabo
Medical Ultrasound Imaging Systems-Part 1
Thomas L. SzaboAgilent Technologies (retired)
[email protected]
Page 3
11/15/2000 © copyright T. L. Szabo
Outline
• Basic Imaging System Fundamentals• Tissue Characteristics• Wave Propagation & Attenuation• Transducers • Beamforming• Imaging Systems
Page 4
11/15/2000 © copyright T. L. Szabo
Origins of Echo-Ranging• Ancient Greeks- The Sounder (rope/weight)• 1880: Curies discover piezoelectricity• 1906:Lee De Forest invents triode amplifier• 1912: Titanic disaster• 1912: L.F. Richardson files for echo-ranging
patents within month of Titanic• 1918: Langevin & Chilowsky make echo-ranging
practical with piezoelectric quartz and vacuum tube amplifiers
Page 5
11/15/2000 © copyright T. L. Szabo
Ultrasound Transducers• Ultrasound transducers are reciprocal linear
electro-acoustic converters– They convert electrical signals into pressure or stress
waves– They convert returning echo (pressure or stress) waves
into electrical signals• Most often transducers are made of piezoelectric
materials which change their shape when electrical voltages are applied to them and which produce voltages when they are deformed
Page 6
11/15/2000 © copyright T. L. Szabo
Key Parts of an Echo ranging System
• Transducer (Piezoelectric)• Transmitter to excite transducer with pulses• Receiver to pick up returning echoes• Amplifier to increase amplitude of echoes• Display to show location and strength of
echoes
Page 7
11/15/2000 © copyright T. L. Szabo
Echo-ranging system
1echo 2echo 2forward
2object1boundary
xdcr
Page 8
11/15/2000 © copyright T. L. Szabo
Scan Fundamentals• Translation:Active transducer element(s) are slid
along a continuous path. (linear) • Angular: Active transducer element(s) are rotated
in angle (sector)• Compound scanning is a combination of both
translation and angular movement at each position• Contiguous scanning is a combination of pure
translation and pure angling with each type at different positions
Page 9
11/15/2000 © copyright T. L. Szabo
Scan Methods
nTranslatio)(Linear
Angular)(Sector
Compound Contiguous
Page 10
11/15/2000 © copyright T. L. Szabo
Key Parts of a Basic Imaging System
• Transducer to send and receive signals• Transducer position controller or sensor to move
or track position of transducer• Time base is reference for controller/sensor• Transmitter to excite transducer with pulses• Receiver to pick up returning echoes• Amplifier to increase amplitude of echoes• Display to show location and strength of echoes
[PPI (Plan Position Indicator)]
Page 11
11/15/2000 © copyright T. L. Szabo
Basic Imaging System
xdcr
Linear nTranslatio
ScanPlane
Page 12
11/15/2000 © copyright T. L. Szabo
Imaging Display• 2D Scanning consists of a pattern of sequenced
discrete scan lines in a scan plane• The displayed pattern of lines onscreen correspond
to the actual spatial pattern of scan lines • Each scan line is a pulse echo ranging time record • The relative amplitudes along each scan line are
translated into relative brightness levels for display (B-mode or brightness display)
Page 13
11/15/2000 © copyright T. L. Szabo
Scattering Fundamentals
• Most of the pulse echoes are caused by impedance discontinuities between dissimilar types of tissue
• Returning pulse echoes also depend on the size and smoothness of the object relative to the insonifying wavelength
• Echoes are also affected by the strength of focusing in the beam
Page 14
11/15/2000 © copyright T. L. Szabo
Types of Scattering
Specularλ>>S
eDiffractivλ~S
Diffusiveλ<<S
Page 15
11/15/2000 © copyright T. L. Szabo
Transducer Arrays• Arrays are groups of individually addressable
transducers (elements)• By sequencing and delaying the signals for each
array element the following can be done electronically:– Translation: changing the position of a group of active
elements– Steering: changing the angular direction of the beam
from a group of active elements– Focusing:changing the width of the beam along a
desired direction
Page 16
11/15/2000 © copyright T. L. Szabo
Basic Electronic Imaging System
Linear nTranslatio
ScanPlane
Electronic
xdcr
Page 17
11/15/2000 © copyright T. L. Szabo
Ultrasound Wave Propagation
• Acoustic material properties – Density ? (kg/m3)– Speed of sound (phase velocity) co (m/s)– Acoustic impedance Z=?co (Rayls)
• Acoustic reflectivity factor
c
12
12
ZZZZ
RF+−
=1Z 2Z
Page 18
Acoustic Tissue Characteristics• Most soft tissues are 75% water • The acoustic velocities of soft tissues vary at
most +/-10% of a mean tissue value of 1.54mm/µs
• Tissue attenuation is caused by absorption and scattering
• Absorption has a frequency power law form • Scattering is typically 10-15% of attenuation
Page 19
Acoustic Velocity of Tissues Normalized to Acoustic Velocity
of Blood
0
0.5
1
1.5
2
2.5B
one
Lun
g
Wat
er
Bre
ast
Bra
in
Fat
Kid
ney
Liv
er
Mus
cle
Sple
en
Page 20
Acoustic Reflectivity of Tissues Normalized to Blood in dB
-60
-50
-40
-30
-20
-10
0
10B
one
Lun
g
Wat
er
Bre
ast
Bra
in
Fat
Kid
ney
Liv
er
Mus
cle
Sple
en
Page 21
11/15/2000 © copyright T. L. Szabo
Plane Wave Propagation
• For a loss-less positive going plane wave traveling along the x axis:
)](exp[ tkxi ω−
wavenumbero
ock βω ==
Page 22
AttenuationTissue attenuation =Absorption + Scattering
Absorption has a frequency power law form.
yff 0)( αα =
1)( 2 <<oβ
α
Page 23
Propagation Factor
)]()([)()( ωβωβωαωγ EOi ++−=
OO C/ωβ =
( ) xexH γω =,
Page 24
Velocity Dispersion Relations• To obtain speed of sound
• For even or noninteger values of y
• For odd integer values of y
[ ])()(//)( ωβωβωβωω EOC +==
( ) ( ) 10 ||2/tan −= y
E y ωωπαωβ
( ) ( ) ||/2 0 ωωαπωβ nyE l−=
Page 25
11/15/2000 © copyright T. L. Szabo
Medical Ultrasound Imaging Systems- Part 2
T. L. SzaboAgilent Technologies(retired)
[email protected]
Page 26
11/15/2000 © copyright T. L. Szabo
Introducing the Piezoelectric Transducer as a Singing Capacitor
-d/2
d/2
tT(t)
fo 3fo 5fo
|T(f)|
f
d
V+V−Voltage applied to electrodedsurfaces of piezoelectric materialresults in stresses at electrode locationsand a stress spectrum of odd harmonics
Simplified Model
Page 27
11/15/2000 © copyright T. L. Szabo
nPiezoelectric effect T=hE,T=F/A, E=V/dnA simple model for symmetric loadingtStress time response
– T(t)=(hCoV/2A)[d(t-d/2)-d(t+d/2)]tStress frequency response fo=v/2d
– |T(f)|=(hCoV/2A)sin(πf/2fo)
-d/2
d/2
tT(t)
fo 3fo 5fo
|T(f)|
f
Simplified Transducer Model
Page 28
11/15/2000 © copyright T. L. Szabo
Derive Radiation Impedance
PE = II&RA/2 = |I|2RA/2
RA(ζ)=RAOsinc2(ζ/2ζo)
nEquate electrical and acoustic powers
( )A
AA ZVhCZ
TP /|2sin|| 2
00
2
== ωπωω
00
22C
kR T
AO πω=
( ) ( )2
0
000
22
//sin
−=
ωπω
ωπωωπωω AA RX
Page 29
11/15/2000 © copyright T. L. Szabo
Transducer Response Is Like A Bandpass Filter
-6 dB Transducer Bandwidth
frequencyfo
Center frequencyis mean of-6dB frequencies
Page 30
11/15/2000 © copyright T. L. Szabo
Piezoelectric Acoustic Resonator
nResonances depend on the geometry and the loading of each face
nModes are interdependentnHigher frequencies (harmonic) resonant modes can be generated
d
w
L
fd=vd/2d fw=vw/2w
fL=vL/2L
Page 31
11/15/2000 © copyright T. L. Szabo
Dicing the Sandwich
Backing
Crystal
MatchingLayer
Page 32
11/15/2000 © copyright T. L. Szabo
Transducer Piezoelectric Materials
nPZTnPVDF CopolymernLeadmetaniobatenPZn
Page 33
11/15/2000 © copyright T. L. Szabo
acousticport 2
acousticport 1
electricport 3
piezoelectric element
v2 v1
I3
F2 F1
V2
Force F1
Particle Velocity v1
Load Impedance Z1
Voltage V3
Current I3Impedance Z3= V3/I3
Transducer Model as 3 Port Device
Page 34
11/15/2000 © copyright T. L. Szabo
acousticport 2
acousticport 1
electricport 3
12
3
piezoelectric element
matchinglayer 1
matchinglayer 2
lens
electricalmatchingnetwork
source/receiver
backing
tissue
1D Transducer Model
Page 35
11/15/2000 © copyright T. L. Szabo
matchingnetwork,cable
Ra
Xa
Co
Rg
Vg
Zw
ZB
ZR
ZL
Electrical Loss EL Acoustic Loss AL
Transducer Loss TL=EL x AL
matching layers,lens
transducerimpedance
Page 36
11/15/2000 © copyright T. L. Szabo
Transducer Models
nOne Dimensional Models:tMason*(W.P. Mason, 1948)tKLM*(Krimholtz, Leedom, Matthaei,1970)tSPICE*(Hutchens,1983)tInput:tOutput:
nThree Dimensional:tWeidlinger*tANSYS
Page 37
11/15/2000 © copyright T. L. Szabo
Image Plane Scanning
Linear Format(Translation) Sector Format
(Angular)
Active Element Groups
Scan Line
Scan Line
Page 38
11/15/2000 © copyright T. L. Szabo
Beamforming
• Within the azimuth scan plane, focusing and steering are accomplished electronically with a one dimensional array
• In the orthogonal elevation plane, focusing for a 1D array is done mechanically with a fixed focal length lens
• For 1.5D arrays, crude elevation focusing is done electronically
• For 2D arrays, both azimuth and elevation focusing and steering can be done electronically
Page 39
11/15/2000 © copyright T. L. Szabo
Beamforming (Transmit and Dynamic Receive)
nTransmit–Steering and single focus @ one depth–Multiple splice zones
nReceive–Steering and dynamic focusing–Nearly continuous or many zones
Single Transmit
Multiple Zones
Page 40
11/15/2000 © copyright T. L. Szabo
Spatial Impulse Resolution
nAxial nAzimuthnElevation
Azimuth
Axial
Elevation-6 dB Ellipsoid
Page 41
11/15/2000 © copyright T. L. Szabo
Front EndA/D
ReceiveBeamformer
SignalProcessors
Image Formation
TransmitBeamformer
CPU Controller
Output Device(Display)
InputDevice(Keyboard)
ImageStorage(Com/Link)
TransducerArray
Ultrasound Imaging System Block Diagram