BMI 1 FS05 – Class 8, “US Instrumentation” Slide 1 Biomedical Imaging I Class 8 – Ultrasound...

BMI 1 FS05 – Class 8, “US Instrumentation” Slide 1

Biomedical Imaging IBiomedical Imaging I

Class 8 – Ultrasound Imaging II: Instrumentation and Applications

11/02/05

BMI 1 FS05 – Class 8, “US Instrumentation” Slide 2

Generation and Detection of Ultrasound

Generation and Detection of Ultrasound

BMI 1 FS05 – Class 8 “US Instrumentation” Slide 3

Piezoelectric effect IPiezoelectric effect I

Conversion of electric energy into mechanical energy and vice versa in materials with intrinsic el. dipole moments (structural anisotropy).

Electric field (~100 V) causes re-orientation of dipoles deformation

Deformation causes shift of dipoles induces Voltage

Examples of piezoelectric Materials:

Crystalline (quartz), Polycrystalline ceramic (PZT, lead zirconium titanate), Polymers (PVDF)

Crystalline: Quartz (SiO2)

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Piezoelectric effect IIPiezoelectric effect II

Polycrystalline (e.g., ferroelectric, PZT) Polymers (PVDF)

“ phase” (not p.e. active)

“ phase” (p.e. active)

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Transducer Q-factorTransducer Q-factor

Disc of piezoelectric material (usually PZT) shows mechanical resonance frequencies fres

Resonance curve (Q-factor

High Q: strong resonance(narrow curve)

Low Q: strongly damped, weak resonance (broad curve)

Tradeoff of high Q:

+ Efficient at fres (high signal-to-noise ratio)

– Pulse distortion

flo-Q

A (fres) = 0 dB

- 3 dB

fhi-Q

Amplitude

Frequency

0

3dB

fQ

f

020log

3 2

AdB A

db

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Transducer resonancesTransducer resonances

Transducer (disc) has mechanical resonances at frequencies

Lowest (fundamental) resonance frequency (standing wave):

Crystal

Len

gth

of

crys

tal,

L c

or 1,2,3,...2 2res c

c

ncf L n n

L

(c: speed of sound, : wavelength)

time

Transducer ends have 180 phase difference (= = /2)

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Transducer backingTransducer backing

Backing of transducer with impedance-matched, absorbing material reduces reflections from back damping of resonance

Reduces efficiency

Increases Bandwidth (lowers Q)

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Transducer–tissue mismatchTransducer–tissue mismatch

Impedance mismatch causes reflection, inefficient coupling of acoustical energy from transducer into tissue:

ZT 30 MRaylZL 1.5 MRayl It/Ii = 0.18

Solution: Matching layer(s)

increases coupling efficiency

damps crystal oscillations, increases bandwidth (reduces efficiency)

2

4t T l

i T l

I Z Z

I Z Z

Transducer Load (tissue)

ItIi

Ir

ZT

ZL

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Matching layersMatching layers

A layer between transducer and tissue with ZT > Zl > ZL creates stepwise transition

Ideally, 100 % coupling efficiency across a matching layer is possible because of destructive interference of back reflections if

layer thickness = /4

Zl chosen so that Ir,1 = Ir,2 :

Problems: Finding material with exact Zl value (~6.7 MRayl)

Dual-layer:

Mat

chin

gLa

yer

Loa

d (T

arge

t)

Tra

nsdu

cer

ZT

Zl

ZL

ItIiIt,LIr,1

Ir,lIr,2

l T LZ Z Z

= /4

= /23/ 4 1/ 4 1/ 4 3/ 4

,1 ,2;T L T Ll lZ Z Z Z Z Z

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Pulsed vs. C.W. modePulsed vs. C.W. mode

Low bandwidth:

No backing, matching possible

High efficiency (SNR)

High-Q

Strong “Pulse ringing” c.w. applications

Large Bandwidth:

Pulsed applications

Backing, matching

Low-Q

Lowered efficiency

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Axial beam profileAxial beam profile

Piston source: Oscillations of axial pressure in near-field (e.g. z0= (1 mm)2/0.3mm = 3 mm)

Caused by superposition of point wave sources across transducer (Huygens’ principle)

Function, see Webb Eq. (3.30)

2

0 NFB

rz z

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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

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Axial and lateral resolutionAxial and lateral resolution

Axial resolution = 0.5c, determined by spatial pulse length (= pulse duration). Pulse length determined by location of -3 dB point.

Lateral resolution determined by beam width (-3 dB beam width or - 6 dB width)

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Focusing of ultrasoundFocusing of ultrasound

Increased spatial resolution at specific depth

Self-focusing radiator or acoustic lens

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Transducer arraysTransducer arrays

Linear sequential array lateral scan

Linear phased array for beam steering, focusing

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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

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Array resolutionArray resolution

Lateral resolution determined by width of main 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

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Radiation patternRadiation pattern

Contributions of different terms to pattern:

Example for:

a =

g = 2

w = 32

a g w

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Ultrasound ImagingUltrasound Imaging

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A-mode (amplitude mode) IA-mode (amplitude mode) I

Oldest, simplest type

Display of the envelope of pulse-echoes vs. time, depth d = ct/2Pulse repetition rate ~ kHz (limited by penetration depth, c 1.5 mm/s 20 cm 270 s, plus additional wait time for reverberation and echoes)

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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

Time-Gain Compensation

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M-mode (“motion mode”)M-mode (“motion mode”)

Recording of variation in A scan over time

Cardiac imaging: wall thickness, valve function

see Fig. 3.17

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M-mode clinical exampleM-mode clinical example

B-Mode / M-Mode image of mitral valve

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B-mode (“brightness mode”)B-mode (“brightness mode”)

Lateral scan across tissue surface

Grayscale representation of echo amplitude

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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

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Linear switchedLinear switched

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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

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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]

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CW Doppler exampleCW Doppler example

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Pulsed Doppler – single volumePulsed Doppler – single volume

Time gate (range gate) is used to define depth location

Sample volume ~mm2

Center or carrier frequency 2-10 Mhz

Pulse repetition rate 1/T~ kHz

Red Box?

Demodulation of signal, see Webb, pp.138

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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)

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Modern US instrumentModern US instrument

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Duplex imaging example (c.w.)Duplex imaging example (c.w.)

www.medical.philips.com

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Duplex imaging (Pulsed Doppler)Duplex imaging (Pulsed Doppler)

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US imaging example (4D)US imaging example (4D)