Medical Ultrasound Imaging Systems

transcript

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)

Medical Ultrasound Imaging Systems-Part 1

Thomas L. SzaboAgilent Technologies (retired)

dszabo@massed.net

Outline

• Basic Imaging System Fundamentals• Tissue Characteristics• Wave Propagation & Attenuation• Transducers • Beamforming• Imaging Systems

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

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

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

Echo-ranging system

1echo 2echo 2forward

2object1boundary

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

Scan Methods

nTranslatio)(Linear

Angular)(Sector

Compound Contiguous

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)]

Basic Imaging System

Linear nTranslatio

ScanPlane

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)

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

Types of Scattering

Specularλ>>S

eDiffractivλ~S

Diffusiveλ<<S

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

Basic Electronic Imaging System

Linear nTranslatio

ScanPlane

Electronic

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

RF+−

=1Z 2Z

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

Acoustic Velocity of Tissues Normalized to Acoustic Velocity

of Blood

Acoustic Reflectivity of Tissues Normalized to Blood in dB

Plane Wave Propagation

• For a loss-less positive going plane wave traveling along the x axis:

)](exp[ tkxi ω−

wavenumbero

ock βω ==

AttenuationTissue attenuation =Absorption + Scattering

Absorption has a frequency power law form.

yff 0)( αα =

1)( 2 <<oβ

Propagation Factor

)]()([)()( ωβωβωαωγ EOi ++−=

OO C/ωβ =

( ) xexH γω =,

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−=

Medical Ultrasound Imaging Systems- Part 2

T. L. SzaboAgilent Technologies(retired)

dszabo@massed.net

Introducing the Piezoelectric Transducer as a Singing Capacitor

fo 3fo 5fo

|T(f)|

V+V−Voltage applied to electrodedsurfaces of piezoelectric materialresults in stresses at electrode locationsand a stress spectrum of odd harmonics

Simplified Model

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)

fo 3fo 5fo

|T(f)|

Simplified Transducer Model

Derive Radiation Impedance

PE = II&RA/2 = |I|2RA/2

RA(ζ)=RAOsinc2(ζ/2ζo)

nEquate electrical and acoustic powers

AA ZVhCZ

TP /|2sin|| 2

== ωπωω

AO πω=

( ) ( )2

ωπω

ωπωωπωω AA RX

Transducer Response Is Like A Bandpass Filter

-6 dB Transducer Bandwidth

frequencyfo

Center frequencyis mean of-6dB frequencies

Piezoelectric Acoustic Resonator

nResonances depend on the geometry and the loading of each face

nModes are interdependentnHigher frequencies (harmonic) resonant modes can be generated

fd=vd/2d fw=vw/2w

fL=vL/2L

Dicing the Sandwich

Backing

Crystal

MatchingLayer

Transducer Piezoelectric Materials

nPZTnPVDF CopolymernLeadmetaniobatenPZn

acousticport 2

acousticport 1

electricport 3

piezoelectric element

Force F1

Particle Velocity v1

Load Impedance Z1

Voltage V3

Current I3Impedance Z3= V3/I3

Transducer Model as 3 Port Device

acousticport 2

acousticport 1

electricport 3

piezoelectric element

matchinglayer 1

matchinglayer 2

electricalmatchingnetwork

source/receiver

backing

tissue

1D Transducer Model

matchingnetwork,cable

Electrical Loss EL Acoustic Loss AL

Transducer Loss TL=EL x AL

matching layers,lens

transducerimpedance

Transducer Models

nOne Dimensional Models:tMason*(W.P. Mason, 1948)tKLM*(Krimholtz, Leedom, Matthaei,1970)tSPICE*(Hutchens,1983)tInput:tOutput:

nThree Dimensional:tWeidlinger*tANSYS

Image Plane Scanning

Linear Format(Translation) Sector Format

(Angular)

Active Element Groups

Scan Line

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

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

Spatial Impulse Resolution

nAxial nAzimuthnElevation

Azimuth

Elevation-6 dB Ellipsoid

Front EndA/D

ReceiveBeamformer

SignalProcessors

Image Formation

TransmitBeamformer

CPU Controller

Output Device(Display)

InputDevice(Keyboard)

ImageStorage(Com/Link)

TransducerArray

Ultrasound Imaging System Block Diagram

Medical Ultrasound Imaging Systems

Documents