Prof. Dr. Philippe Cattin: Basics of Ultrasound Contents ... · Basics of Ultrasound Principles of Medical Imaging Prof. Dr. PhilippeCattin MIAC, University of Basel Oct 10th/17th,

Basics ofUltrasound

Principles of MedicalImaging

Prof. Dr. Philippe Cattin

MIAC, University of Basel

Oct 10th/17th, 2016

Oct 10th/17th, 2016Principles of Medical Imaging

1 of 60 26.09.2016 08:35

Contents

2

4

5

6

7

8

9

10

11

12

13

15

16

18

19

20

22

23

24

Prof. Dr. Philippe Cattin: Basics of Ultrasound

Contents

Abstract

1 Introduction

Why Ultrasound

Applications in Obstetrics

Applications in Cardiology

Applications in Inner Medicine

Applications in Musculoskeletal System

Applications of Ultrasound Elastography

Advantages and Disadvantages of Ultrasound

Challenges for Image Analysis

Challenges for Image Analysis (2)

Challenges for Image Analysis (3)

2 Research at MIAC using US

Myocardial Contrast Echocardiography

Myocardial Contrast Enhancement (2)

3 History

History

History (2)

History (3)

4 Properties of Ultrasound

Properties of Ultrasound

Common Sound Frequencies

Physical Principles of UltrasoundOct 10th/17th, 2016Principles of Medical Imaging

2 of 60 26.09.2016 08:35

25

26

27

28

29

30

31

32

33

34

35

36

37

39

40

41

42

43

44

45

46

47

48

49

50

52

Acquisition Principle

Propagation

Propagation (2)

Propagation (3)

Ultrasound Characteristics

Ultrasound Wavelength Change

Ultrasound Characteristics (2)

Modulus of Elasticity

Acoustic Impedance

Acoustic Impedance (2)

Properties of Different Materials

Power and Intensity

Symbols often used to Characterise Ultrasound

5 Interaction with Matter

Interaction with Matter

Reflection and Refraction

Reflection (2)

Reflection (3)

Reflection Example: Air - Fat

Reflection Example: Liver - Kidney

Diffraction

Attenuation or Absorption

Attenuation or Absorption (2)


Decibel Scale


6 Safety of Diagnostic Ultrasound

Safety of Diagnostic Ultrasound Oct 10th/17th, 2016Principles of Medical Imaging

3 of 60 26.09.2016 08:35

53

54

55

56

Bioeffects

Thermal Bioeffects

Non-thermal Bioeffects

Output Display Standard Using Thermal andMechanical Indices


4 of 60 26.09.2016 08:35


(2)


Abstract

5 of 60 26.09.2016 08:35

Introduction


(4)Why Ultrasound

Ultrasound (US) is the most widely used imaging

technology worldwide

Popular due to availability, speed, low cost, patient-

friendliness (no radiation)

Applied in obstetrics, cardiology, inner medicine,

urology,...

Ongoing research to improve image quality, speed and

new application areas such a intra-operative navigation,

tumour therapy,...

6 of 60 26.09.2016 08:35


Introduction

(5)


Applications in Obstetrics

Follow fetal development

Detect pathologies

Fig. 4.1: Two-dimensional B-mode

Ultrasound image of a fetus

Fig. 4.2: Three-dimensional image of

the same fetus ~5 months after

conception

7 of 60 26.09.2016 08:35


Introduction

(6)


Applications in Cardiology

Blood flow in vessels (Doppler US)

Contraction, Rhythm

Blood flow in the heart (defects on wall muscle, valve

defects)

Assessment of cardiac perfusion

8 of 60 26.09.2016 08:35


Fig. 4.3: Myocardial contrast echocardiography

(MCE)

Fig. 4.4: Prenatal

diagnostic of the Fallot-

Tetralogie

9 of 60 26.09.2016 08:35


Introduction

(7)


Applications in InnerMedicine

Gallstone

Perfusion of renal transplant

Fig. 4.5: Gallstone (red arrow) within

the gallbladder produces a bright

surface echo and causes a dark

acoustic shadow (S)

Fig. 4.6: Perfusion Doppler image of

a renal transplant

10 of 60 26.09.2016 08:35


Introduction

(8)


Applications inMusculoskeletal System

Visualisation of tendons, ligaments

Investigation under movement is possible → simplifies the

detection of ruptures, obstructions, ...

Fig. 4.7: The arrows show the large

gap of the rupture Achilles tendon

Fig. 4.8: US image of ISS astronaut

Mike Fincke's biceps tendon, where

"D" denotes the deltoid muscle and

"T" is the proximal intracapsular end

of the long biceps tendon

11 of 60 26.09.2016 08:35


Introduction

(9)


Applications of UltrasoundElastography

US Elastography is often used to classify tumours.

Malignant tumours are 10 to 100 times stiffer than the

normal soft tissue around.

Fig. 4.9: Elastogram (of a breast)

indication a mass with a high

probability of being malignant

tumour

12 of 60 26.09.2016 08:35


Introduction

(10)


Advantages andDisadvantages of Ultrasound

Advantages

No ionising radiation

Real-time imaging (functional

examinations)

Small, transportable

equipment

Comparatively low cost

Disadvantages

Moderate to poor image

quality

Image interpretation is

difficult and operator

dependent

Acoustic window

Limited penetration depth

(attenuation, total reflection)

Shadows by dense objects

Fig. 4.10: SmartPhone with

Ultrasound capability

Fig. 4.11: Application scenario

13 of 60 26.09.2016 08:35


Introduction

(11)


Challenges for ImageAnalysis

Artefacts: speckle pattern, attenuation, shadows,

contours parallel to the beam direction

Lateral resolution depends on the depth

Fig. 4.12: US image of the heart

chambers

Fig. 4.13: US image of a gallstone

14 of 60 26.09.2016 08:35


Introduction

(12)


Challenges for ImageAnalysis (2)

Sound propagates at differentspeed in different tissues

Estimation of distances

difficult (

assumed in Ultrasound

devices)

Distortion of contours in

Ultrasound images

Fig. 4.14: Different sound speeds

distort distance measurements

Material

Water

Blood

Brain

Fat

Muscle

Bone

AirTab. 4.1: Different important sound

speed in tissue

15 of 60 26.09.2016 08:35


Introduction

(13)


Challenges for ImageAnalysis (3)

Segmentation in the presence of noise (speckle patterns arein a strict sense not noise but interferences of reflections) andartefacts.

Fig. 4.15: Segmentation

example 1

Fig. 4.16: Segmentation

example 2

16 of 60 26.09.2016 08:35

Research at MIACusing US


17 of 60 26.09.2016 08:35


(15)Myocardial ContrastEchocardiography

In MyocardialContrastEchocardiography(MCE) a contrastagent microbubbles is injectedinto thecirculation. Thesebubbles produce ahigh-contrastresponse visible asbright sport in themovie.

Procedure

Contrast agent

is injected

1.

Heart is

imaged with

the Ultrasound

system

2.

By increasing

the Ultrasound

energy the

bubbles in the

Ultrasound

plane are

destroyed

(blue frame

3.

18 of 60 26.09.2016 08:35


around the US

image)

The time it

takes to refill

the myocard

with bubbles is

a measure of

muscle

perfusion

(health of the

coronary

arteries)

4.

Problem

Analysing andevaluating thesemovies by hand isa time consumingand error pronetask.

19 of 60 26.09.2016 08:35


Research at MIAC using US

(16)


Myocardial ContrastEnhancement (2)

The developed software allows to analyse a US moviesequence in a matter of minutes rather than one to two hours.

Fig. 4.17: Interface for semi-automatic evaluation of the data sets

20 of 60 26.09.2016 08:35

History


(18)History

1877: Lord Raleigh - "Theory

of Sound"

1880: Pierre & Jacques Curie

- Piezoelectric effect

1914: Langevin - First

Ultrasound generator using

piezoelectric effect

1928: Solokov - Ultrasound

for material testing

1942: Dussik - First

application of Ultrasound in

medical diagnostics

... different medical

applications (gall stones,

tumours)

End of 1960's: Boom of

Ultrasound in medical

diagnostics

Fig 4.18: The bat use Ultrasound

for navigation

Fig 4.19: Edlet and Hertz's

echocardiographic trace of the

mitral valve (1950)

21 of 60 26.09.2016 08:35


History

(19)


History (2)

Fig 4.20: Pan-Scanner -

The transducer rotated

in a semicircular arc

around the patient

(1957)

Fig 4.21: Scan converter

allowed for the first time

to use the upcoming

computer technology to

improve US

Fig 4.22: A state-

of-the-art Ultrasound

console

22 of 60 26.09.2016 08:35


History

(20)


History (3)

Fig 4.23: Development of the B-mode Ultrasound image quality

23 of 60 26.09.2016 08:35

Properties ofUltrasound


(22)Properties of Ultrasound

The frequencies of medical Ultrasound waves are severalmagnitudes higher than the upper limit of → human hearing[http://en.wikipedia.org/wiki/Human_hearing].

Fig. 4.24: Approximate frequency ranges of sound

24 of 60 26.09.2016 08:35



(23)


Common SoundFrequencies

Sound Frequency

Adult audible range

Range for children'shearing

Male speaking voice

Female speaking voice

Standard pitch (ConcertA)

Bat

Medical Ultrasound

Maximum soundfrequency

Tab. 4.2: Common sound frequencies and frequency

ranges

25 of 60 26.09.2016 08:35



(24)


Physical Principles ofUltrasound

Ultrasound imaging differs in three fundamental aspects fromother imaging methods:

Ultrasound is a non-ionising longitudinal wave1.

The reflected signal is recorded rather than the

transmitted part (unlike e.g. X-ray)

2.

Ultrasound images tissue boundaries instead of e.g.

density information, see Fig. 4.25

3.

US CT MRI

Fig. 4.25: Comparison of US, CT and MRI

Comparing Ultrasound with other imaging modalities,such as CT or MRI, thus requires image processing.

26 of 60 26.09.2016 08:35



(25)


Acquisition Principle

Ultrasoundmeasures the time apulse takes to travelfrom the transducerto the reflectingsurface and back,see Fig 4.26 →time-of-flight

The depth can be

calculated, if the sound

velocity of the tissue

is known

The magnitude of the

echo is coded as gray

value on the display

Fig. 4.26: Two sound waves reflected

from two surfaces. Echo arrival is

and respectively, assuming a speed

of sound of

27 of 60 26.09.2016 08:35



(26)


Propagation

Ultrasound is a cyclic longitudinal pressure wave andrequires a medium (gas, liquid, solid)

Fig. 4.27: Three-dimensional

visualisation of a propagating

pressure wave

Fig. 4.28: Propagation of sound

energy as elongation about a center

of equilibrium (particle velocity

about its center is )

Ultrasound waves behave according to theconventional laws associated for light waves exceptthat they require a medium.

28 of 60 26.09.2016 08:35



(27)


Propagation (2)

The soundwave applies a

force from the left to

particle '1'

1.

Particle '1' acquires a

speed of about its center

and moves to the right by

compressing and

stretching its connecting

bonds

2.

Particle '2' now receives

that force from '1' and is

displaced to the right

3.

Now that particle '1' has

transferred its force it

moves back to its

equilibrium position

4.

The displacement of

particle '2' is slightly

smaller than '1' due to

energy loss (heat) within

the molecular bond

causing the soundwave to

slowly fade out

5.

This vibration process ofcompression and rarefactionis repeated for the remainingparticles.

Fig. 4.29: Propagation of sound

energy as elongation about a center

of equilibrium (particle velocity about

its center is )

29 of 60 26.09.2016 08:35


30 of 60 26.09.2016 08:35



(28)


Propagation (3)

The propagation of a soundwave canbe visualised, Fig 4.30, as a sinewave with a

Wavelength ,

Frequency , and

and an Amplitude

that characterises the transmittedUltrasound wave in the tissue.

Fig. 4.30: The Ultrasound

waves with different

amplitudes

31 of 60 26.09.2016 08:35



(29)


Ultrasound Characteristics

Sound particle velocity

Sound particle velocity is the velocity of the

material particles as they oscillate about their

equilibrium.

Acoustic Pressure

The Acoustic Pressure is caused by the pressure

changes and measured in Pascal

Frequency and Wavelength

The Frequency and the Wavelength are related

with , where is the speed of sound (propagation

velocity) in the respective tissue.

Propagation Velocity

The Propagation Velocity is the speed with which the

soundwave travels through the medium.

This velocity is tissue specific and often only

approximately known. The stiffer the springs and the

smaller the particle masses, the higher to propagation

velocity.

Propagation velocity, wavelength and frequency are

related by

(4.1)

The propagation velocity increases from gases to liquids

32 of 60 26.09.2016 08:35


and is highest in solids.

33 of 60 26.09.2016 08:35



(30)


Ultrasound WavelengthChange

As we have seen, the Wavelength, Frequency and Propagation

Velocity are related by

(4.2)

When the Ultrasoundwave travels from onemedium to the other, thevelocity changes.

This change of velocity thusinduces a change of theWavelength and not theFrequency of the Ultrasoundwave, see Fig 4.31

Fig. 4.31: If the speed of sound

changes in a medium, the

wavelength changes as well

34 of 60 26.09.2016 08:35



(31)


Ultrasound Characteristics(2)

Amplitude

The maximum Amplitude coincides with the compression

peak, see Fig 4.30. Reducing the power level results in a

smaller amplitude.

Power

The Power is the rate of sound energy transfer into the

tissue and is measured in watts

Intensity

The Intensity is a measure of power per unit area and is

commonly measured as .

35 of 60 26.09.2016 08:35



(32)


Modulus of Elasticity

The rate of transfer from one molecule to the other dependson the molecule's reluctance to motion and the density. Thewave velocity (speed of sound) are connected throughYoung's Modulus of Elasticity .

(4.3)

where is the speed of sound, the material density and Young's Modulus.

The higher the modulus (stiffer springs) and lower

molecule masses, the higher the velocity

The modulus of elasticity is inversely proportional to the

compressibility

The modulus is measured in pressure units e.g.

Fat

Soft tissue

Bone

36 of 60 26.09.2016 08:35



(33)


Acoustic Impedance

When pressure is applied to a molecule it will moveexerting a pressure on an adjacent molecule. Acousticpressure increases with particle velocity but is also dependson properties of the medium. The equation is similar toelectrical resistance:

Acoustic Impedance Electronic Resistance

(4.4) (4.5)

(4.6) (4.7)

(4.8) (4.9)

The acoustic impedance is measured in often

shortened to rayl.

37 of 60 26.09.2016 08:35



(34)


Acoustic Impedance (2)

The Acoustic Impedance is also related to the Modulus ofElasticity .

The stiffer the bonding, the greater the pressure exerted

by a molecule moving at a particular velocity, so

(4.10)

A material having a great springiness (low value) has highmolecular motion and will absorb sound energy and less willbe transferred to the next molecule, so

(4.11)

combining Eq 4.10, 4.11 and 4.3 yields

(4.12)

38 of 60 26.09.2016 08:35



(35)


Properties of DifferentMaterials

MaterialVelocity

[ ]

Density

[ ]

Acoustic Impedance

[

]

Air 330 1.3 0.00043

Fat 1470 970 1.42

Castor oil 1500 933 1.40

Water 1492 1000 1.48

Soft tissue 1500 <1000 ~1.45

Brain 1530 1020 1.56

Blood 1570 1020 1.60

Kidney 1561 1030 1.61

Liver 1549 1060 1.64

Muscle 1568 1040 1.63

Eye lens 1620 1130 1.83

Bone(compact)

4080 1700 6.12

Bone (porous) 2.5

Bone marrow 1700 970 1.65

The different speeds of sound in the tissue makes itdifficult to measure accurate depths and distances inthe image.

39 of 60 26.09.2016 08:35



(36)


Power and Intensity

Acoustics Electronics

Power (4.13) (4.14)

Intensity (4.15)

Energy (4.16)

where is the area.

Increasing the energy drives the compression bands

closer together → more energy in the tissue and higher

oscillation speed

During travel, energy is lost as heat decreasing the wave

amplitude → frequency and wavelength remain for the

same tissue

40 of 60 26.09.2016 08:35



(37)


Symbols often used toCharacterise Ultrasound

Measurement Symbol Unit Clinical Range

Velocity (speed ofsound)

(for soft

tissue)

Wavelength (for

soft tissue)

Frequency

Elastic modulus (in bone)

Acousticimpedance

Density (for

water)

Pressure

Elongationvelocity

41 of 60 26.09.2016 08:35

Interaction withMatter


(39)Interaction with Matter

When Ultrasound interacts with matter, they exhibit the samephenomena as visible light:

Reflection (specular and non-specular)

Refraction

Diffraction

Attenuation or absorption

42 of 60 26.09.2016 08:35



(40)


Reflection and Refraction

The laws of wave optics also applyfor Ultrasound waves. From energypreservation we know that(neglecting absorption)

(4.17)

the law of reflection states

(4.18)

and → Snell's law [http://en.wikipedia.org

/wiki/Snell_law]

(4.19)

gives the relationship between thesound speeds and angles.

Fig. 4.32: Sound wave at an

angle to the surface

43 of 60 26.09.2016 08:35



(41)


Reflection (2)

Given the Impedances of thetwo tissues and the angles andthe incidence intensity , the reflected and transmitted intensitiescan be calculated with

(4.20)

and

(4.21)

from Eq 4.17 we know that

(4.22)

also holds.

Fig. 4.33: Amount of fraction

reflected depending on the

impedance difference

44 of 60 26.09.2016 08:35



(42)


Reflection (3)

For a sound wave perpendicular to asmooth surface ( ), Eq 4.20 canbe simplified and yields for thereflected intensity

(4.23)

and similarly the transmitted waveintensity is given by

(4.24)

Fig. 4.34: Sound wave

perpendicular to a smooth

surface

The amount of reflection depends on impedance

differences .

Reflection is greatest when the impedance

difference is large.

45 of 60 26.09.2016 08:35



(43)


Reflection Example: Air -Fat

Given:

A sound wave hits an air-fat interface perpendicular to thesurface, thus . The impedance of air is and of fat .

Result:

The amount of reflected sound intensity can then becalculated using Eq 4.23

(4.25)

yielding an Ultrasound reflection ratio of .

Conclusion:

Almost all energy is reflected at a tissue-air boundary →

Ultrasound is not usable for air-filled cavities such as the

lungs,...

46 of 60 26.09.2016 08:35


47 of 60 26.09.2016 08:35



(44)


Reflection Example: Liver -Kidney

Given:

A sound wave hits a liver-kidney interface perpendicular tothe surface, thus . The impedance of liver is

and of kidney .

Result:

The amount of reflected sound intensity can be calculated asin the previous example and yields . The rate

of transmitted intensity (Eq 4.24) is then given by

(4.26)

which yields

Conclusion:

Only very little energy is reflected when the impedances

closely match → organ boundary not well visible in

Ultrasound

48 of 60 26.09.2016 08:35



(45)


Diffraction

Diffraction is the bending ofUltrasound into the shadow of astrong absorber.

It takes place at the edges of astrong absorber. Although theultrasonic intensity in the shadowis less than in the incident field,it is not reduced to zero, due todiffraction around the edges ofthe dense material → imageartefacts. Fig. 4.35: Diffraction occurs

behind the gallstone

49 of 60 26.09.2016 08:35



(46)


Attenuation or Absorption

The intensity of a sound wave decreases along its direction ofpropagation. There are two physical processes involved:

Absorption: responsible for of energy loss

Beam divergence

50 of 60 26.09.2016 08:35



(47)


Attenuation or Absorption(2)

The decrease in the ultrasonicintensity with increasing depth

can be characterised by anexponential law

(4.27)

where is the initial soundintensity at and is atissue dependent Absorptioncoefficient, see Fig 4.36.

The rate of energy loss depends on the type tissue andthe Ultrasound frequency.Maximum loss is at the relaxationfrequency .

Fig. 4.36: Dependence of the

absorption coefficient with

frequency

51 of 60 26.09.2016 08:35



(48)



Instead of the absorptioncoefficient a measure of powerloss is often given:

(4.28)

where is the attenuation in

per unit length.

MaterialAbsorption

Fat 0.6

Blood 0.18

Brain 0.85

Soft tissue 1.0

Liver 0.9

Muscle (along

fibers)1.2

Muscle (across

fibers)3.3

Eye lens 2.0

Bone 20.0

Tab. 4.3: Sound absorption of

various biological tissues

52 of 60 26.09.2016 08:35



(49)


Decibel Scale

Sound power or intensity variations are compared usingthe Decibel Scale

(4.29)

Example:

The power loss of an incident source with and

an echo power of would be

(4.30)

The half power distance or a reduction of is thedistance over which the power is reduced by derived from .

53 of 60 26.09.2016 08:35



(50)



Higher Ultrasoundfrequencies areattenuated more thanlower frequencies

As a consequence, the requireddepth (absorption) determinesthe order of magnitude of thefrequencies used in Ultrasounddiagnostics, see Fig 4.37.

High frequencies (short

wavelengths) → high spatial

resolution but only limited

penetration depth

Low frequencies (long

wavelengths) → lower spatial

resolution but wider depth

range.

Frequencies between are thus used for superficialstructures and frequenciesaround are used fordeeper-lying details e.g.

Fig. 4.37: Tissue depth for 50%

loss of intensity for a typical

frequency range assuming

54 of 60 26.09.2016 08:35

Safety of DiagnosticUltrasound


(52)Safety of DiagnosticUltrasound

"Diagnostic ultrasound has proven to be a valuabletool in medical practice. An excellent safety recordexists in that, after decades of clinical use, there is noknown instance of human injury as a result ofexposure to diagnostic ultrasound. Evidence exists,however, to indicate that at least a hypothetical riskfor clinical diagnostic ultrasound must be presumed."

1990 Radiological Health Bulletin.

The above report explicitly talks of diagnostic Ultrasound.Ultrasound such as HIFU (High-Frequency FocusedUltrasound) does "harm" the patient by destroying tissue.This is of course intended for cancerous tissue or kidneystones.

55 of 60 26.09.2016 08:35


Safety of Diagnostic Ultrasound

(53)

Prof. Dr. Philippe Cattin: Basics of

Ultrasound

Bioeffects

Two mechanisms are known to alter biological systems withUltrasound, the

thermal mechanism, and

mechanical mechanism associated with cavitation.

56 of 60 26.09.2016 08:35



(54)


Ultrasound

Thermal Bioeffects

A major part of the Ultrasound energy emitted duringexamination is absorbed by the tissue. This absorption resultsin the generation of heat. If the absorbed energy exceeds thebody's ability to dissipate heat, the local temperature will rise.If the temperature is too high, tissue is destroyed.

Measuring the absorption of Ultrasound energy is difficult. Itcan, however, be estimated solving the differential equationaka bio-heat equation:

(4.31)

where

Density of tissue and blood

Specific heat of tissue and blood

Temperature of tissue and capillary blood

Volumetric flow rate of blood per unit mass of tissue

Rate of metabolic heat generation

Local specific absorption rate

57 of 60 26.09.2016 08:35



(55)


Ultrasound

Non-thermal Bioeffects

It is known since the 1930's thatbiological cells can be damaged in theabsence of significant heating. Thismechanism is linked to the phenomenaof acoustic cavitation. Cavitationformation is connected to negativepressure peaks. In the rangea minimum pressure of isrequired to cause cavitation.

Fig. 4.38: Cavitating

propeller model in a

water tunnel experiment

[→ wikipedia[http://en.wikipedia.org

/wiki/Cavitation]]

Fig. 4.39: Cavitation

bubble imploding close to

surface (e.g. cell)

generating a jet (4) of the

surrounding liquid [→

wikipedia[http://en.wikipedia.org

/wiki/Cavitation]]

58 of 60 26.09.2016 08:35



(56)


Ultrasound

Output Display StandardUsing Thermal andMechanical Indices

The Ultrasound devices provide astandardised display of real-timeindices relating to the potentialfor Ultrasound-inducedbioeffects. Two types of indicesare displayed:

the thermal index (TI) to

estimate the temperature

increases and

the mechanical index (MI)

estimating non-termal

bioeffects such as cavitation.

The thermal index (TI) is furthersubdivided into a (TIs) forsoft-tissue, (TIb) for bone, and(TIc) for cranial bone. A value of1 means a temperature increaseof . These estimates aremaximum numbers and the realtemperature increase is typicallysignificantly lower.

Fig. 4.40: The thermal index (TI)

and mechanical index (MI) can b

nicely seen in the top right corner

59 of 60 26.09.2016 08:35


60 of 60 26.09.2016 08:35

Prof. Dr. Philippe Cattin: Basics of Ultrasound Contents ... · Basics of Ultrasound Principles of Medical Imaging Prof. Dr. PhilippeCattin MIAC, University of Basel Oct 10th/17th,

Documents