Hunting Bats. Diagnostic Ultrasound. Ultrasound Real-time modality

(1)

Diagnostic Ultrasound - Per Åke Olofsson MTA / UMAS

Ultralyd kursus

F Føøtal Medicintal Medicin

5

5 --7 oktober 20057 oktober 2005 Rigshospitalet K Rigshospitalet Køøbenhavnbenhavn

Per Åke Olofsson

Dpt

Dptof of BiomedicalBiomedicalEngineering, Engineering,

Malmö University Hospital, Sweden

Diagnostik Ultrasound

Basic physics, image reconstruction and signal processing

17

17--WEEK FETAL PROFILEWEEK FETAL PROFILE

Introduction Diagnostic Ultrasound - Per Åke Olofsson MTA / UMAS

Ultrasound

–

Real

-

time modality

2-D Imaging

Severe hypertrophy

Diagnostic Ultrasound

z

Basic Physics

z

Pulse Echo Method

z

2-D Acquisition and Image Reconstruction

z

Doppler Principle

z

CW and PW Doppler

z

Colour Doppler

Physics Diagnostic Ultrasound - Per Åke Olofsson MTA / UMAS

Hunting

Bats

(2)

Sound propagation

in a media

Classification of sound

Infrasound

< 20 Hz

Audible sound

20 – 20 000 Hz

Ultrasound

>20 000 Hz

Bats

30

30 –

–

80

80 kHz

kHz

Surgical ultrasound

25

25 –

–

55

55 kHz

kHz

Diagnostic ultrasound

2

2 –

–

15

15 MHz

MHz

Physics

Diagnostic Ultrasound -Physics Per Åke Olofsson MTA / UMAS Diagnostic Ultrasound - Per Åke Olofsson MTA / UMAS

Longitudinal sound wave

propagation

wavelength

λ

(3)

Acoustic properties

Medium

Speed of sound

(m/s)

Blood

1566

Brain

1505

-

1612

Fat

1446

Kidney

1567

Liver

1566

Muscle

1542

-

1656

Bone

2070

-

5350

Distilled water

1480

Air

333

333 Perpex

Perpex

2670

Aluminium

6260

Brass

4430

Bio

lo

g

ica

l

Bio

lo

g

ica

l

Non Non

bi

ol

og

ic

al

bi

ol

og

ic

al

Wave equation

λ

= c / f

Physics

λ = wavelength

f = frequency

c = speed of sound

3 MHz

⇔

λ

= 0,52 mm

6 MHz

⇔

λ

= 0,26 mm

10 MHz

⇔

λ

= 0,15 mm

Acoustic

Impedance

Characteristic

Impedance

Z =

ρ

c

ρ = density

c = propagation speed of sound

Reflection

Physics 2 1 1 2 2 1 1 2 2 2 1 2 1 2

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

+

−

=

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

+

−

=

c

Z

I

reflected incident

_ρ

ρ

(4)

Different types of reflections

Scattering

Physics

Speckle or texture

Echo interference

Constructive interference

Destructive interference

Physics

(5)

Diagnostic Ultrasound -Physics Per Åke Olofsson MTA / UMAS

Attenuation

DGC Depth Gain Compensation

TGC Time Gain Compensation

Physics

Pulse

Echo

Principle

time Amplitude

t = d

x x

2/c

t = d

x

2/c or d = t

x

c/2

c = 1540 m/s

time/depth Skin

Tissue 1 Tissue 2 Tissue 3

Transducer

t2 t1

d1 d2

1 cm

⇔13 •

10

-6

_s

Pulse Echo Method

z

Reflection at

tissue

boundaries

z

Distance

proportional

to time

z

In B-mode

amplitude

controls

intensity

Distance A-mode B-mode M-mode Time S lo w sw ee p Physics

(6)

Diagnostic Ultrasound -Physics Per Åke Olofsson MTA / UMAS

Piezo

electric effect

Longitudinal / axial resolution

2-D Imaging

Effect

of

beamwidth

on lateral resolution

2-D Imaging Diagnostic Ultrasound - Per Åke Olofsson MTA / UMAS

Increased

frequency

B

decreased

lateral

width

but

..

Increased

frequency

B

more

attenuation

(7)

Linear Array Transducer

z

Multiple

identical

crystal

elements

Image Acquisition

z

Multi element transducer array

z

Sequential excitation of crystal elements

2-D Imaging

Curved linear array

Transmit

focusing

(8)

Receive

focusing

2

2 -

-

D sector

z

Transmit one

line

z

Receive along

the same line

z

Steer to second

line

z

Sequentially

scans the

complete sector

2-D Imaging

Phased Array

z

Steering controlled by

time difference

between excitation

signals for adjacent

elements

z

Steering both in

transmit and receive

mode

Digital Time delay Array Object Transmitter An al og Digital A co u st ic s ign a ls E le ct ri c si g n a ls

Computer for control of tranmit and recieve

Excitation pulses Pu ls e a m pli tude

Phased Array Transducer

z

Variable steering

angle

z

Small footprint

(9)

Diagnostic Ultrasound -Harmonic Imaging Per Åke Olofsson MTA / UMAS

+

∆

c

+

∆

c

--

∆

c

--

∆

c

Summation of frequencies

z

Frequencies

could be

added

z

and separated

according to

Fourier

theory

+

=

Harmonic Imaging

Tissue Second

Harmonics

generation

p

ac

ρ

ν

f

A

B

P

2

d

= nonlinerar parameter for media = second harmonic intensity

= frequency of insonification

= density = acoustic velocity = distance = applied acoustic pressure

P

2 _A

B

+ 2

π f

2 ρ ν

3 d p

ac

2

(

)

Water 4,2 - 6,1 Water 30C 1 atm 5,5 Blood 6,3 Liver 7,6 Spleen 7,8 Fat 11,1

≈

Harmonic Imaging

A

B

_parameter

Second

harmonic

intensity

in tissue

(10)

Non

-

linear

relation

Harmonics

Applied

Appliedpressurepressure

Harmonic Imaging Diagnostic Ultrasound - Per Åke Olofsson MTA / UMAS

Reverberation

Fundamental

Harmonic

Harmonic Imaging

Grating

lobes

fundamental vs

harmonic

Main

lobe

Grating

Gratinglobeslobesfundamentalfundamental

Grating

Gratinglobeslobesharmonicharmonic

Fundamental

–

Tissue

Harmonic

Fundamental

beamprofile

Harmonic

beamprofile

Harmonic Harmonic beamprofile beamprofile after after amplification amplification Harmonic Imaging