basic ultrasound 2013 cachard archamps

Cachard ESMP 2013 Archamps 1

Basic principles of ultrasound

Christian CACHARD Christian.cachard@creatis.univ-lyon1.fr

CREATIS www.creatis.insa-lyon.fr

Université Lyon 1

Teresa Robinson Consultant Clinical Scientist

Head of the Vascular Studies Unit, United Bristol Healthcare NHS Trust

Université de Lyon

CREATIS is a key european laboratory

for biological and medical imaging

3D modelling of human heart based on MRI

Simulation of dose distribution for radiotherapy

at the interface between

engineering, computer sciences

and living sciences

Elastography

About 200 persons

Development of

imaging methods,

new algorithms,

and instrumental systems to answer medical questions

3D augmented reality

3D imaging of cardiac muscular fibers

In vivo quantification of metabolite concentration (MR spectroscopy at 4.7T)

1 - Imaging of the Heart-Vessels-Lungs

2 - Images et models

3 – Ultrasound Imaging

4 - Tomographic imaging and therapy with radiation

5 - MRI and Optics : Methods and Systems

6 – Brain imaging

MR spectroscopy

Multi-organs segmentation Dynamic model of the heart

Diffusion Tensor Imaging of the brain

Microarchitecture and micro-vasculature of trabecular bone(1 voxel=1,4µm)

6 research teams

Segmentation and tracking of carotid artery wall in US

Ultrasound imaging platform

3 research ultrasound scanners, motorized and automated acquisition system Imaging biological deformation in vivo

2D ultrasound elastography ultrasonore Breast cancer, Coll. HCL and

Institute of Cancer Research, Londres

Medical applications

Cardio-vascular: atherosclerosis and ischemia

3D dynamic model of the heart including fiber orientation. Coll. Auckland Bioengineering Institute

3D tomographic reconstruction of a stent, Coll. GEHealthcare

MRI sequences for cardiac function quantification

Real-time quantification of Carotide wall movement Coll. HCL et Hopital Univ. Sydney Clinical protocol SARD

High resolution simulation of diffusion tensor imaging Coll. Harbin Institute of Technology

Basic principles of ultrasound

1: Overview

2: Sound waves

3: Ultrasound generation

4: Ultrasound in tissue

Ultrasound scanning

Scanner

Probe Ultrasound imaging is Non Destructive Testing

The place of ultrasound in medical imaging

Public Hospitals in Lyon (2012)

– 9 MRI

– 11 scanners

– 8 gamma (scintillation) cameras

– 1 PET (Positron Emission Tomographie)

– More than 100 ultrasound scanners

Real time imaging

10 to 60 frames/s

The place of ultrasound in medical imaging

Ultrasound has the last ten years been the fastest growing imaging modality for non-invasive medical diagnosis.

Of all the various kinds of diagnostic produced in the world, one of four is an ultrasound scan.

Reasons for this are the ability to image soft tissue and blood flow • the real time imaging capabilities,

• the harmlessness for the patient and the physician (no radiation)

• the low cost of the equipment.

• no special building requirements as for X-ray, Nuclear, and Magnetic Resonance imaging.

Limitations are that ultrasound imaging cannot be done through bone or air (limitations on chest imaging).

Mechanical wave

Sound is a mechanical wave

Created by a vibrating object

Propagated through a medium

Vacuum chamber

Knocking the bell inside the vacuum chamber

no sound propagates

The acoustic pressure

The acoustic pressure is the change of pressure around the

static (ambient) pressure

Acoustic pressure amplitude

p (Pa)

105 Ambient pressure

• Ultrasound energy is exactly like

sound energy, it is a variation in the

pressure within a medium.

• Sound is a pressure wave

Ultrasound

Frequency of Sound

20 000 000 Hz

2 000 000 Hz

20 000 Hz Audible Sound

Diagnostic Medical

Ultrasound

(3-7 MHz)

Infrasound (earthquake) 20 Hz

20 MHz

20 kHz

The probe: transmitter and receiver

• The same probe is used first as transmitter,

second as receiver

• Probe Loudspeaker + Microphone

Ultrasound scanner and sonar

• Ultrasound scanner works as sonar

Diagnostic Ultrasound

Echoes

return

Processed into

picture

Pictures

analysed

Sound waves

directed into

patient

Which pulse(s)?

Which processing?

c= 1500 m/s

or time

Probe Time

tf time of flight

Range of depth and time of flight

0.75 cm < depth < 15 cm

10 µs < tf: time of flight < 200 µs

Target

(sound velocity in water)

Reflection and transmission

One transmitted pulse gives rise to a train of

received echoes. Time

We can calculate where the echoes have come

from by timing how long they take to get back.

The probe includes

128 to 512

transducer elements

… … … ....

Depth /

Time of frame acquisition

Amplitude

… … … ....

128 to 512

transducer

elements

Depth /

0.75 cm < depth < 15 cm

10 µs < time of flight < 200 µs

TPRF >> maximum time of flight

PRF: Pulse Repetition Frequency

PRF = 1/ TPRF

TPRF > 5 x (maximum time of flight)

1 kHZ < PRF < 20 kHZ

Frequency 1 MHz < frequency< 10 MHz

Time of flight 10 µs < time of flight < 200 µs

Pulse Repetion Frequency 1 kHZ < PRF < 20 kHZ

Echoes from ONE pulse

The echo

amplitudes are

converted to

shades of grey

A-Mode

(amplitude)

B-Mode

(brightness)

Amplitude

B (Brightness) Mode Image

64 to 512

transducer elements

10 to 60 frames/s

Visible scan

lines (48)

1970’

B mode image 1970’

Number of elements 64 <Nelt < 512

Frame acquisition (PRF = 5kHz) 10 frame/s < Nframe <80 frame/s

Shapes of the images: linear or sectorial

32 Thyroid mass

B (Brightness) Mode Images

Ovarian Cyst

Popliteal artery

Gall bladder & stone

Linear probe Sectorial probe

Shapes of the images: linear or sectorial

Scanning of pressure beam with

a sectorial probe

B mode and M mode

B-mode

M-mode

(or TM mode,

Time Motion)

t1 t2 t3

Imaging modes

A mode

TM mode B mode

Envelope Signal

Time (s)

Scan plane Distance (cm)

th (cm

One frame: 20 to 50 ms One continuous image of 5 s

Doppler modes: velocity measurements

37 Spectral Doppler

“Ultrasonic Doppler Modes”

Piero Tortoli

Colour Doppler

1: Overview

2: Sound Waves

3: Ultrasound generation

4: Ultrasound in Tissue

Basic Principles of Ultrasound

Wave Motion: transverse wave

Up and down

Particle movement

Wave propagation

Wave Motion: transverse wave

Stadium wave

Direction of energy transport

Wave motion: Longitudinal Wave

Direction of energy transport

Transverse Wave Longitudinal Wave

Particle movement

Wave propagation

Particle movement

Wave propagation

Wave motion in tissue

Direction of energy transport Direction of energy transport

Elastic deformable medium

– gas, liquid or solid.

Molecules do not travel from one

end of the medium to the other.

No flow of particles

Pressure amplitude

wave velocity c

At each spatial position , the material points are oscillating around their

equilibrium position with a particle velocity v

if u is the displacement of the material point, v = du/dt

Molecules do not travel from one end of the medium to the other.

wave velocity c

Valeur de u pour une pression donnée

Sound is a mechanical wave

• Created by a vibrating object

• Propagated through a medium

Sound is a pressure wave

• Consists of repeating pattern of high and low pressure regions

Sound is a longitudinal wave

• Motion of particles is in a direction parallel to direction of energy transport

The Nature of a Sound Wave in tissue (liquids)

The Frequency of a wave

T = 1 / f

Peak excess pressure

amplitude A0 of wave

ftAA 2sin0

Wave equation

Wavelength and frequency of the wave

Wavelength, = c T = c / f

Wavelength,

Spatial periodicity

Period, T=1/f

Temporal periodicity

Depends on source

Depends on velocity of sound, c

(depends on material)

Distance

Pressure rare

Velocity of the wave

Air c = 330 m/s

Water c = 1500 m/s

If source is 3 MHz frequency

= 1500/ 3 .106 = 0.5mm

= 330/ 3.3 .103 = 0.5 m

If source is 3.3 kHz frequency

On ultrasound scanner the ultrasound wave is emitted as pulses (not a continuous sine )

Ultrasound Pulse

Pressure

Length of pulse is about 3 to 5 periods

f = 3 MHz

1 µs < Tp < 1.66 µs

Time of pulse (length 3 periods) 0.3 µs < TP< 3 µs

Number of elements 64 <Nelt < 512

Frame rate (PRF = 5kHz) 10 frame/s < Nframe <80 frame/s

Time of frame(PRF = 5kHz) 12 ms < Time of frame< 100 ms

Range Equation

It is possible to predict the distance (d) of a reflecting surface from the transducer if the time (t) between transmission and reception of the pulse is measured and the velocity (c) of the ultrasound along the path is known

Pulse Echo

Intensity

The intensity associated with the wave is

defined as the power flowing through a

unit area (measured in W/m2 or mW/cm2 )

Ii (t,r) p (t,r) 2

Intensity for a sinusoidal wave

• Time averaged intensity (I) for

a sinusoidal wave (where P0 is

the peak-pressure amplitude)

Ii (t,r) p (t,r) 2

Pressure

Intensity

Intensity for a pulsed wave

PRFsppa

Ispta (Temporal

Average)

Isptp (Temporal

Isppa (Pulse

Average)

≈ 1/200

Intensity Pressure

Intensity

True time axis

1: Overview and History

2: Sound Waves

3: Ultrasound Generation

Sources of Sound Waves

Sound production requires

a vibrating object

Vocal chords

Collision!

Audio speaker system

Piezoelectric element

Ultrasound generation and detection

Pierre and Jacques Currie

Piezoelectric effect discovered in 1880

• Piezoelectric materials

• Quartz

• PZT (Lead, Zirconate,Titanate- PbZrTi)

Quartz

- Expansion

- Contraction

• Apply a voltage between the two faces of a piezoelectric material: result is deformation (inverse piezoelectric effect)

Ultrasound generation

Ultrasound detection

Apply force to piezoelectric material: result is electrical charge proportional to force (direct piezoelectric effect)

The frequency of the force applied will affect the frequency with which a voltage is generated

Piezoelectric materials and transducers

“Ultrasound transducers”

Hervé Liebgott

Beam Shape

Beam width lateral resolution

Beam Shape

W<<λ Small point source

Diffraction

directional

Spherical wave Plane wave

Plane Disc Source: Intensity of the field

Near field Far field

Non-uniform beam Uniform beam

Intensity

Distance from probe

Intensity on axis

propagation

Avoid measurements in the near field

Ultrasound beam from a plane disc source

Intensity

Distance from probe

Spatial Resolution

Temporal Resolution

Contrast Resolution

Imaging Resolution

Spatial Resolution

Spatial (in space)

axial (along the beam)

lateral (across the beam)

• azimuth (in the scan plane)

• elevation or slice thickness (perpendicular to the scan plane)

Axial Resolution

The minimum reflector spacing along the axis of the ultrasound beam that results in separate, distinguishable echoes on the display.

Axial Resolution

Example 10 MHz transducer

000,000,3

000,000,10

ra = number of cycles of the transmitted pulse x 3

ra =3 x 0.5 = 1.5 mm

ra =3 x 0.15 = 0.45 mm

Example 3 MHz transducer

First definition

Resolution is length at half size

Second definition

Lateral Resolution

The ability to distinguish two closely spaced reflectors that are positioned perpendicular to the axis of the ultrasound beam.

Elevational Resolution (slice thickness)

Works in a direction perpendicular to the image plane.

Dictates the thickness of the section of tissue that contributes to echoes visualized on the image.

The time interval between pulses

• limits the temporal resolution

• it is usually set so that there is sufficient time for the most distant echo to return to the transducer before the next pulse is launched

Temporal Resolution

Interval to allow

echoes to return

Sequence of pulses from transducer

Typical PRF = 5kHz

TPRF = 0.2 ms

TPRF can be chosen as the delay of five times the maximum

observed distance

Contrast Resolution

The ability to display regions of differing echo size

Cachard ESMP 2013 Archamps 75 75

“Q-assurance of US equipment”

Jean Martial Mari

1: Overview and History

2: Sound Waves

3: Ultrasound Generation

Speed of Sound

Bulk modulus

Density

Air 330m/s

Water 1480m/s

Fat 1460m/s

Blood 1560m/s

Muscle 1600m/s

Bone 4060m/s

Speed of Sound

Average soft tissue

value = 1540m/s

Programme the

ultrasound

machine with...

This can lead to small errors in the estimated distance travelled because of the

variation in the speed of sound in different tissues.

Acoustic Impedance

Bulk modulus Density x

Acoustic Impedance

Acoustic impedance analogous to electrical impedance

P = local pressure

v = local particle velocity

U : Potential

I : Intensity

Similar Values

Air 0.0004 x 106 rayls

Lung 0.18 x 106

Fat 1.34 x 106

Water 1.48 x 106

Blood 1.65 x 106

Muscle 1.71 x 106

Skull Bone 7.80 x 106

Acoustic Impedance

Reflection at boundaries

At the boundary between tissues ultrasound is partially reflected

The relative proportions of the energy reflected and transmitted depend on the acoustic impedance between the two materials

Incident

Transmitted wave

Reflected

The laws of optics apply to ultrasound

Reflection at interface perpendicular to the wave

Pi , vi

Pt , vt

Pr , vr Replace v with P/z, the

reflexion coefficient is obtained

Reflection at interface perpendicular to the wave

Pi , vi

Pt , vt

Pr , vr

• z2 z1 1r

complete transmission

complete reflexion

Specular Reflection

Non-perpendicular

Incidence

Perpendicular

Incidence

Reflected beam

travels off at an angle.

No wave go back to

the probe Strong orientation dependence

Diffuse Reflection

Reflected waves

travel in various

directions away

from the interface

Diffuse Reflection

Reflected waves travel in

various directions away from

the interface

Some orientation dependence

Rayleigh Scattering

Particles size <<

Waves are scattered and

travel off in all directions

Energy loss f 4

Rayleigh Scattering

Particles size <<

Little orientation dependence

Waves are scattered and

travel off in all directions

Energy loss f 4

The amplitude of the echoes (image grey level)

does not have a simple relationship with the tissue (unlike X-ray CT [Hounsfield numbers]).

Echo size depends on

• size of structure compared with λ

• relative acoustic impedances across boundary

• shape and orientation of boundary

Echo Amplitude - Beware!

In tissue: Specular Reflection, Diffuse Reflection or Rayleigh Scattering?

Particles size <<

Specular reflection from large flat boundaries

Diffuse reflection

from small structures

Rayleigh Scattering from very small structures

Strong Weak Very weak

Reflection or scattering?

Coupling gel

Strong

Reflection

Very weak

Reflection

Calcification

Attenuation

The energy of the ultrasound beam is reduced with distance

Energy is lost from the beam by:

Absorption (conversion into heat)

Scattering (reflection out of beam confines,

refraction, divergence)

Attenuation in an image

Bright deep to the cyst Dark

deep to

the defect

in the

phantom!

Dark deep to the cyst

Attenuation Coefficient

z eII 0

The intensity, Iz, of an ultrasound beam is related

to distance from the source, z, thus:

Where I0 is the intensity at z = 0, the transducer face.

Attenuation Coefficient

Attenuation is approximately exponential,

the slope of the logarithmic graph is constant.

Attenuation coefficient is quoted in dB/cm

In addition, for soft tissue,

attenuation is proportional to frequency.

The attenuation coefficient for soft tissue is

0.5 - 0.7 dB/cm/MHz

Attenuation – compensation: TGC

Average echo

Amplification

factor

Echo train

after compensation

(distance)

For deeper distance noisy is amplified

TGC: Time Gain Compensation

Nonlinear propagation

Nonlinear coefficient

Nonlinear parameter

𝜌𝜕𝑣

𝜕𝑡+ 𝛻𝑝 = 0 The motion equation

The pressure is expanded using the Taylor series

The celerity

linear

Nonlinear propagation: celerity

Nonlinear coefficient

Nonlinear parameter

f0 + 2 f0+ 3 f0 + .... f0

After propagation

𝑣 =𝑝

𝜌 𝑐

Non-Linear propagation

Under conditions of relatively high pressure amplitude the speed of

sound is NOT CONSTANT but varies over the propagation path (z)

Material B/A

water (30°C) 5.2

blood 6.3

liver 7.6

spleen 7.8

fat 11.1

𝑐 𝑧 = 𝑐0 + 1 + 𝐵

2 𝐴𝑣(𝑧)

After a sufficient distance, the faster

moving high-pressure parts of the wave

catch up to the slower low-pressure parts.

The result is a

sawtooth wave

The distorted wave has many harmonic frequencies

Higher

Nonlinear Propagation

Harmonics

Fundamental

Frequency

Amplitude

The amplitude decreases is about 20 dB per harmonic

Harmonic Imaging

A relatively recent innovation in

diagnostic ultrasound imaging is

Tissue Harmonic Imaging

Discovered by accident it uses the

effects of non-linear propagation.

Ultrasound contrast agent

“Contrast agents in US”

Thierry Bettinger

Native harmonic frequencies are used to improve images

• By tuning the receiver to the harmonic frequency (2Fo) rather than the transmitted frequency Fo

Benefits

• Reduces clutter (noise), increases resolution at depth, improves sensitivity

117 117

Thermal

• tissue heating, cell death for T > 42oC

Mechanical

• cavitation bubbles for pressures > threshold o unfortunately threshold is frequency dependent

Safety

Possible damage from ultrasound:

• Thermal index

– relates to temperature

– potential for heating effects (metabolic rate)

• Mechanical Index

– relates to pressure

– potential for bubble effects (cavitation)

Mechanical and Thermal Indices

Thermal Index

• TI is the ratio between:

– the power exposing the tissue, W

– the power required to cause a 1oC temperature

rise, Wdeg

• MI describes the likelihood of the negative

pressure causing bubble activity

MI P-d

P-d is the ‘derated’ pressure at the site in the body

f is the frequency of the pulse

megapascals

(megahertz) 1/2

Mechanical Index

Medical Index (MI)

Ultrasound medical scanner: 0.01 < MI < 2

The transmitted pressure amplitude (Pascal) and

Medical Index (MI)

Example: f = 2 MHz

P (MPa) 0,0 0,1 0,1 0,2 0,5 1,0

MI 0,01 0,04 0,07 0,14 0,35 0,71

MI > 3 Possibility of minor damage to neonatal lung or intestine

MI 2 Limitation of medical scanner

MI >0.7 Theoretical risk of cavitation.

TI>0.7 Restrict exposure time of a fetus

TI>1.0 Eye scanning not recommended

TI>3.0 Fetal scanning not recommeded

Guidelines

Cachard ESMP 2013 Archamps 125 125

“Therapeutic US principles”

Jean Martial Mari

Scanner settings

•Fixed settings : MI, TCG, gain

•Adjustement: focus, sector

PHILIPS

Ultrasound Imaging, Bjorn A. J. Angelsen, ISBN 82-995811-0-9, Emantec AS, Trondheim, Norway, www.ultrasoundbook.com

Ultrasound in Medecine, Institute of Physics, Publishing Bristol and Philadelphia

References

basic ultrasound 2013 cachard archamps

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