bym504e-mk-basics of ultrasound imaging

Basics of Ultrasound ImagingMustafa Karaman, PhD 1

Basics of Ultrasound Imaging

April 2014

Department of Electronics & Communications Engineering,

Istanbul Technical University, Istanbul, Turkey

Mustafa Karaman, Ph.D


Ultrasound B-Scan Image: Liver


Pulse-Echo Ultrasonic Imaging System


Ultrasonic Imaging

• Tissue-wave interaction

• Transducer design

• Analog and digital electronic design

• Array signal processing (beamforming)

• Signal and image processing


Acoustic Applications

– Medical Imaging (non-invasive diagnosis)

– Nondestructive testing (NDT)

– Underwater acoustics (sonar)


Medical Imaging

– non-invasive observation of internal structures

of human body

Based on interaction between tissue &

energy (x-rays, electric fields, ultrasound, etc.)

Different forms of energy/radiation

imaging of different characteristics of tissue


Medical Ultrasound

– represents mechanical properties of tissue

– has no harmful biological side effect

– allows real-time imaging

– offers small-size, low-cost systems

complementary diagnostic tool

+ (will) provide real-time 3D imaging


Medical Ultrasound Waves

– Longitudinal waves: 2-15 MHz

– Transverse waves

» high attenuation in tissue

not used


Ultrasound Parameters

water

fat

bone

liver

kidney

muscle

soft tissue

Speed

(m/s)

1480

1440

4080

1550

1560

1590

1540

Attenuation

(dB / MHz cm)

0.0025

0.56

12.0

0.95

1.1

1.8

0.81

Impedance

1.48

1.36

7.80

1.66

1.63

1.71

1.62

)/10( 26 smkg


Ultrasound Parameters


Frequency & Wavelength

f

c

In soft tissue: C = 1540 m/s

f

(MHz) (mm)

3.0 0.51

5.0 0.31


Reflection

2

12

122

ZZ

ZZPower reflection coefficient:

Reflection between X and Soft tissue Z = 1.62

X 2

water + 0.452 0.020

fat + 0.087 0.008

bone - 0.656 0.430

liver - 0.003 9.6e-6

Basics of Ultrasound ImagingMustafa Karaman, PhD

scattering

16


Reflection (Pulse-echo) & Transmission Modes

TX

Mode

(t>r/c)


A Sample Pulse-echo Signal

s(t)

S(w)

3.5Mhz, 40%BW


Attenuation in Pulse-Echo

ReII 2

0

F loss

(MHz) (dB/cm) (dB)

3.5 2.8 112

5.0 4.0 160

R

II 0

Medium Attenuation: Diffraction Attenuation:


Attenuation TGC

• Attenuation: compensated by TGC amplifier

• Gain: Operator controlled at discrete

range segments

range

gain


TGC Applied to B-Scan Image


Pulse-Echo Ultrasonic Imaging System


Transducers

23

PZT: Lead Zirconate Titanate


PZT: Lead Zirconate Titanate

24

ZPZT~30x105 g/cm2s ZSKIN=~1.7x105 g/cm2s


CMUT

25

vout

Vdc

~ vac

Generated Acoustic Wave

Incident Acoustic Wave


Trasnsducer Bandwidth

26


Trasnsducer Bandwidth

27


Beamforming

• Perhaps the most important building block.

• Probably the most expensive building block.

– 30 - 50% of parts & labor of a scanner

• Forming transmit/receive beams

Scan image plane (reconstruct image).

This slide is from K. Thomenius’ presentation.


Focusing

31


steering

32


Scan (Beam) Lines


Beamforming = steering + focusing


Tra

ns

du

ce

rs &

Sc

an

Fo

rma

ts

Transducer

Array

Active Subarray

Scan Line

Beam

Transducer Array = Active Subarray

Scan Line

Beam

Transducer

Array

Active

Subarray

Sc

an

Lin

e

Beam

Linear Array Curvi-Linear Array Phased Array


End of Session-1



Session 2


Transducer

Array

Active Subarray

Scan Line

Beam

Transducer Array = Active Subarray

Scan Line

Beam

Transducer

Array

Active

Subarray

Sc

an

Lin

e

Beam

Linear Array Curvi-Linear Array Phased Array

Recall From Session-1:

Transducers & Scan Formats


Recall From Session-1: Beamforming


PSF

• Point Spread Function

– Image of a point reflector (target)

• Also called LSF (line spread function)

– 2D cross-sectional image of line reflector (target)

• Represents the spatial impulse response of the (linear) imaging system.

• Used to characterize

– transducer response (radiation pattern, beam pattern)

– the image quality of the system.


r

(r,)

D/2x

dx

Observation

point

Geometry for driving PSF

-D/2

Excitation Pulse: p(t)

Aperture


PSF

( / )

Signal at the observation point:

( / )( , ) cos

Assume paraxial case: cos 1 &

1( , ) ( / )

For CW excitation: ( )

( , )

D

D

jwt

wj

jw t c jwt c

D D

p t cs t dx

r

s t p t r c dxr

p t e

s t e dx e e dx


PSF

2

( , )( , ) /

wj

jwt jwtcjwt

D

wjc

D

j

D

s th w e e dx e

e

e dx

e dx

Temporal frequency response


Near & Far Fields

2 2

2

2

2

2 sin

21 sin

[1 ( ) ( ) ...]

r x rx

x xr

r r

r O x O x

2

2

[1 ( ) ( )] Near-Field (Fresnel)

/ Far-Field (Fraunhofer)

[1 ( )]

sin

r O x O x

r D

r O x

r x

Distance from source point to observation point:

r

(r,)

x

dxAperture


One-Way PSF

2

22 / sin

2 / sin

( , )

(sin , )

(sin ) ( )

{ ( )}

sin / sin

/ sin

j

D

j r j x

D

j x

D

D

h w e dx

h w e e dx

h a x e dx

F a x

D

(r,)

dx

Aperture

Function

aD(x)


(sin ) (sin )t rh h

One-Way (Tx & Rx) PSF

• Transmit and receive responses of an

aperture are identical.

• Transmit and receive beam patterns (PSFs)

of a transducer are identical.


3dB width

main lobe

side lobes

1/ 2

PSF for D=16sin(8 sin )

(sin )0.5 sin

h

Sin


Aperture & Sampled Aperture (Array)

d

(r,

r

1 2 3 N

Continious

Aperture

Function

Sampled

Aperture

Function

d


PSF of Sampled Aperture (Array)

1

sin( sin )2

(sin ) exp( sin )

sin( sin )

N

n

Ndh j nd

d

• Note that: It is a periodic sinc()-like function with

a period of sin=2/d.

– Sampling in one FT domain corresponds to periodicity

in the other FT domain.

• This results in repeated main lobes, so called

grating lobes.

• To avoid grating lobes, choose d/2


Array PSF with Grating Lobes

main lobe 3dB beam width

first side lobe

side lobes

Grating lobe grating lobe


Two-Way PSF

2

(sin ) (sin ) (sin )

sin( sin )(sin ) (sin )

sin( sin )

sin( sin )(sin )

sin( sin )

t r

t r

TR

h h h

Ndh h

d

Ndh

d


sin 0.5 16 sin(sin )

sin 0.5 sinh

One-Way PSF


Two-Way PSF


Two-Way Pulsed PSFs

R: 1D 2D 3D 4D 5D 6D 7D

arr

ay (

N=

16

, d

=/2

)

-0.5

0.0

+0.5


Mesh Plot of Two-Way PSFs

1D

2D

3D

4D

5D

6D

7D


Effective Aperture

)(sin)(sin

)}()({

)}({)(sin

)()()(

rt

rt

e

rte

hh

xaxaF

xaFh

xaxaxa


Aperture Apodization

Array Channels


Apodized PSF

)(sin)(sin

]}[][{)(sin

hW

nanwFha


Effect of Apodization on PSF

Xtr & Rcv Hamming Window

Xtr & Rcv Uniform Window


Depth of Focal Zone

focal point

depth of focal zone

2

2

88 nofD

rr


F/number Apodization

RangeD 2D

f/number = Rfocal / Deffective, Deffective= D * cos

• Used for improved focal zone.


f/number apodization

63


Effective Aperture Size

r

(r,)

D

steering angle

focal point

D cos

effective aperture


Aperture Apodization

• increases depth of focal zone

•suppresses side lobes

• reduces point resolution

(increases main lobe width)

• reduces T/R power (SNR)


Pulse-Echo Ultrasound Imaging System


Output Signal of Beamformer

(Beamformed A-scan)


Beamformer Output

rN

n

nn crttptAts1

)/)(()()(

t rN

k

N

n

knnk crtttptAtAts1 1

)/2)()(()()()(

Receive Beamforming:

T/R Beamforming (synthetic):


Wavefronts With/out Focusing

Row-1: No focusing, Row-2: Xtr focusing, Row-3: Xtr & Rcv focusing

(N=64)


End of Session-2



Session 3


Sector Scan


Scan-Conversion


Scan-Conversion

/)(

/)(

/)(

121

1212

1211

CCCC

rdBBBC

rdAAAC


Scan-Conversion

sin

Ran

ge (r)

x

y


Logarithmic Compression

• To view regions with different contrast levels on the same display range

a = 10db_floor/20; % a=0.01 for db_floor=-40;

x = x / xmax;

if x()<a then x() = a

y = 20 log10(x)

• 40 dB < dB_floor < 60 dB


Image Resolution

• Axial (range) resolution

• Lateral (azimuth) resolution

• Point resolution

•Contrast resolution


Aixal & Lateral Resolution

R: 1D 2D 3D 4D 5D 6D 7D

arr

ay (

N=

16

, d

=/2

)

-0.5

0.0

+0.5


Resolution of B-Scan Image

axia

llateral


Axial & Lateral Resolution

• Axial (Range) Resolution

pulse shape

pulse width

Xducer bandwith

• Lateral (Azimuth) Resolution

array size

frequency & BW

apodization


Point Resolution

• capability of resolving point targets

main lobe width

3 dB resolution: FWHP of main lobe

6 dB resolution: FWHM of main lobe

nofD

rFWHM 22.122.1


Contrast Resolution

• capability of resolving regions with different

contrast levels

ratio of power in main lobe to power

in side lobes

Contrast-to-Nose-Ratio:2

2

2

1

21

CNR


Delay & Amplitude Quantization

• Delay & Amplitude Quantization side lobe level contrast resolution

• RMS Array/Delay Quantization Errors/ Main Lobe Amplitude

» m=fs/fo=32, N=128 - 59 dB

• RMS Signal-amplitude Quantization Sidelobe Level / Max Image

Amplitude

» N=128, B=8 bits -74 dB

(Peterson & Kino IEEE Trans. UFF, July 1984)

Nmd

6

NBa32

1


Real-Time Imaging Constraint

sfiring

cxRangex

frame

firingsx

s

frames1

/2

192

1/1540/20.0220

frame

firings

sfiring

smmxx

frame

firingsx

s

frames


Beam Space Sampling

maxmaxmax sin

/2

sin2

sin

sin2

e

e

NN

B

NBNNNNN ert

22

2/145sinsin 0

max

179128 BN


Issues

• volumetric scan hardware

• digital systems ADC cost

• flow imaging correlation proces.

• phase aberration resolution

• motion artifacts resolution


Dimension of a Transducer Array



3-D Imaging using 2-D Arrays


Why 2D?



Volumetric Imaging

# of array channels (NxN) Hardware

32x32 = 1K, 64x64 = 4K, 128x128 = 16K

• 2D sparse arrays with 256 channels currently

available for 3D/4D imaging.

• For ergonomic scanning, the number of cables should be limited by 256 – 512.

# of Beam lines Frame rate

New scanning methods needed.


Doppler Frequency

cos2 0fc

vff d

v: flow velocity (?)

f = fd: average Doppler frequency

c: ultrasound velocity

fo: ultrasound frequency

: angle between beam and velocity


Correlation Processing

• Differential phase/delay

• Phase of complex correlation coeff.

• Index of max of RF correlation func.

Estimation of motion, phase aberration, flow

k nnk nn

k

nn

nn

ksksksks

ksks

*

11

*

*

1

1,

)()()()(

)()(

k nnk nn

k nn

nn

mksmksksks

mksksm

)()()()(

)()()(

11

1

1,


RF Correlation


Non-Aberrated Wavefronts


Phase Aberration


Phase Aberration

Row-1: No aberration., Row-2: 1x aberration., Row-3: 2x aberration

(N=64)


Motion!

Maximum Velocity of Heart

Heart Valve cm/s =0.44mm 20 frames/s

– normal 30 cm/s 680 /s 34 /frame

– higher 60 cm/s 1360 /s 68 /frame

– abnormal 100 cm/s 2270 /s 134 /frame

Heart Wall 1-15 cm/s 20-340 /s 1-17 /frame


End of Session-3

bym504e-mk-basics of ultrasound imaging

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