prof. dr. philippecattin: ultrasound contents ultrasound

Ultrasound

Principles of MedicalImaging

Prof. Dr. Philippe Cattin

MIAC, University of Basel

Oct 17th, 2016

Oct 17th, 2016Principles of Medical Imaging

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Prof. Dr. Philippe Cattin: Ultrasound

Contents

Abstract

1 Image Generation

Echography

A-Mode

B-Mode

M-Mode

2.5D Ultrasound

3D Ultrasound

4D Ultrasound

2 Ultrasound Transducers

Ultrasound Transducers

2.1 Single Element Transducer

Transducer Design

Piezoeletric Materials

Impedance Matching

Impedance Matching (2)




Pulse Geometry

Pulse Repetition FrequencyOct 17th, 2016Principles of Medical Imaging

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Images of Transducers

2.2 Linear Sequenced Transducer

Linear Sequenced Transducer

Linear Sequenced Transducer (2)



Linear Sequenced Transducer:Examples

2.3 Phased Array Transducer

Phased Array Principle

Phased Array Example: Mitral Valve

Phased Array Example: IVUS

2.4 Annular Array Transducer

Annular Array Transducer

3 The Ultrasonic Field

The Ultrasonic Field

The Ultrasonic Field (2)

Axial Resolution

Resolution

4 Doppler Imaging

Doppler Imaging

Basics of Doppler Shift

Continuous vs Pulsed Doppler

Continuous vs Pulsed Doppler (2)

Duplex and Colour Velocity Imaging

5 Imaging Artefacts Oct 17th, 2016Principles of Medical Imaging

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5.1 Beam Artefacts

Beam Artefacts

Beam Artefacts (2)

5.2 Multiple Echo Artefacts

Multiple Echo Artefacts

Multiple Echo Artefacts (2)

Multiple Echo Artefacts (3)

5.3 Velocity Artefacts

Velocity Artefacts

Velocity Artefacts (2)

5.4 Attenuation Artefact

Attenuation Artefacts

Attenuation Artefacts (2)

5.5 Speckle

Speckle

Properties of Speckle

Speckle Tracking

Speckle Tracking (2)


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(2)


Abstract

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Image Generation


(4)Echography

Imaging Principle

Emission of Ultrasound

waves

Reflection on tissue

boundaries

Imaging Frequency

depending on

application

for Intra-Vascular

US (IVUS)

Fig. 5.1: Principle of

Ultrasound imaging

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Image Generation

(5)


A-Mode

Fig. 5.2: A-mode Ultrasound

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Image Generation

(6)


B-Mode

The B-Mode orBrightness-Modeencodes the reflectedecho strength asgrey-values (correctedfor the image depth).

Interfaces

between different

tissues are seen as

bright regions

The B-Mode

picture shows a

section (slice)

through the body

who's image depth

depends on

transducer

parameters

(frequency,

focusing,...)

The image display

is constructed

from scan lines

(depends on the

transducer design)

Fig. 5.3: B-mode Ultrasound showing the

four chambers of the human heart

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Image Generation

(7)


M-Mode

If the framerate is highenoughM-Modemovies canbeproduced.

Fig. 5.4: M-Mode 4-chamber view of the heart

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Image Generation

(8)


2.5D Ultrasound

A recent development inImage-Guided Therapy(IGT) is the 2.5D Ultrasoundit requires a

tracked 2D Ultrasound

probe, that is

manually pivoted or

translated over the

patient.

The captured 2D slices arethen assembled into asparse 3D data set → thus2.5D.

Fig. 5.5: Principle of 2.5D

Ultrasound acquisitions

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Image Generation

(9)


3D Ultrasound

Recent developments in computation equipment allowsthe visualisation of 3D image sequences in real-time.

Fig. 5.6: Surface renderings of 3D Ultrasound data sets

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Image Generation

(10)


4D Ultrasound

Recentdevelopmentsincomputationequipmenteven allow tovisualise 4Dmoviesequences.

Fig. 5.7: 4-dimensional movie of a fetus (week 31)

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UltrasoundTransducers


(12)Ultrasound Transducers

The mechanically scanned Ultrasound probes have almostentirely been replaced by electronically scanned multi-element array transducers. There exist two basic types ofelectronically scanned transducers:

Sequenced (switched) transducer arrays

linear or

curvilinear

Phased transducer arrays

linear

annular

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Single ElementTransducer


(14)Transducer Design

The device that converts theelectrical energy into soundwaves is called Transducer.

Today's transducers usepiezoelectric crystals such asceramic lead zirconate titanate( ) to convert theelectric into mechanicalenergy.

Fig. 5.8: Example of an US transducer

Fig. 5.9: Basic design of a

single transducer Ultrasound

head

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Single Element Transducer

(15)


Piezoeletric Materials

Piezoelectric materials have two niceproperties:

Piezoelectric materials change

their shape upon the application

of an electric field as the

orientation of the dipoles

changes.

1.

Conversely, if a mechanical

forces is applied to the cristal a

the electric field is changed

producing a small voltage signal.

2.

→ The piezoelectric crystalsthus function as thetransmitter as well as thereceiver!

Fig. 5.10: Basic design

of a single transducer

Ultrasound head

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(16)


Impedance Matching

There is a large impedancedifference ( )between the piezoelectriccristal and the skin of thepatient → only a minor partof the energy penetrates thepatient's skin.

Fig. 5.11: Large impedance

difference between the transducer

cristal and the patients skin (gel)

Example:

For a transducer impedance of and a

tissue acoustic impedance of the

amount of reflected sound energy is given by Eq → 3.23[FundamentalsOfUltrasound.html#(41)] and yields a reflection ratio

of , thus roughly of the acoustic energy is

reflected.

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(17)



The impedance adaption issolved by attaching atransmission layer ormatching layer to thepiezoelectric crystal face →quarter-wave matching

What is the optimalimpedance andlayer thickness toget the maximumenergy into apatients body?

The gel coupling mediumbetween the skin andmatching layer avoidsfurther signal loss byremoving air bubbles.

Fig. 5.12: US head with a quarter

wavelength matching layer for

impedance adaption

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(18)



Principle of the QuarterWavelength Layer

The matching layer isoptimal when

(5.1)

From energy preservation itdirectly follows

(5.2)

Fig. 5.13: Reflectance model of the

quarter wavelength layer

We know that both and are non-zero. Eq 5.1 can thus

only be satisfied if

they have a phase-shift of and1.

both amplitudes are equal .2.

→ they then cancel out each other thanks to destructiveinterference.

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(19)



Requirement (1) is straight forward and valid as long asthe layer thickness satisfies

(5.3)

Due to a range of frequencies in the ultrasound pulse thematching layer can never be exactly for all

wavelengths → less than efficiency.

Multiple matching layers are sometimes used to furtherimprove efficiency.

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(20)



Requirement (2) states and yields with Eq → 3.23[FundamentalsOfUltrasound.html#(41)]

(5.4)

this can be simplified to

(5.5)

For our practical example (see this [@]) with

and a tissue acoustic impedance of

Eq 5.5 yields

(5.6)

as the optimal impedance for the matching layer.

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(21)


Pulse Geometry

Ideally, the pulse wave wouldraise and fall very sharply andcontain only one wavelength,but a pulse usually containsseveral oscillations, see Fig5.14. The pulse packet can becharacterised by

The pulse wavelength

Its amplitude

The Spatial pulse length

The Pulse duration

The Pulse repetition period

and the Pulse

repetition frequency

Example:

A and a leaves a period of

between pulses. Thetransducer is thus of thetime in receive mode.

Fig. 5.14: A typical pulse shape

Fig. 5.15: Pulses at two

different frequencies

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(22)


Pulse RepetitionFrequency

The time between pulses ( must be higher than the

return trip which is equivalent to twice the image depth.

The maximum isthus defined by

(5.7)

for an image depth of and the

maximum is thus.

A typical value for in practice is .

Fig. 5.16: Maximum pulse repetition

frequency

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(23)


Images of Transducers

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Linear SequencedTransducer


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(25)Linear SequencedTransducer

The Linear sequenced array transducerconsists of many (up to 128) individualtransducer elements arranged ingroups.

As the near-field of a very narrow

single element beam would be very

small, groups of elements are

grouped and pulsed simultaneously

(usually 8 to 32 elements) → wider

beam with improved resolution at

depth

A scanning motion is obtained by

shifting an element one at a time

As only a small number oftransducer elements are activeat a time (8 to 32) theelectronics is rather simple,compared to phased arraydesigns.

Fig. 5.17: Commonly

used linear array

designs in diagnostic

imaging

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(26)


Linear SequencedTransducer (2)

By adapting the delays(shifting the phases)of the individualelements linearsteering is possible.

Fig. 5.18: Phased linear steering

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(27)



By delaying orphasing the excitationpulses, linear arrayscan be focused.

It is even possible toswitch between multiple focal points.The frame rate isthen, however,reduced . Fig. 5.19: Beam is

focused by adapting

the delays

Fig. 5.20: Focal

point can be

changed

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(28)



The position of the narrow section ofthe beam is controlled by the aperturesize (number of elements in the group).

The number of scan lines can bevirtually doubled if two groups havingdifferent sizes are used.

Fig. 5.21: Different

aperture size

depending on

number of active

elements

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(29)


Linear SequencedTransducer: Examples

Fig. 5.22: Thoracic diaphragm wall

(Provided by GE Healthcare)

Fig. 5.23: Liver image

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Phased ArrayTransducer


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(31)Phased Array Principle

In Phased array transducers, thetransmit pulses are applied to allelements via an elementindividual delay allowing aswiveled wavefront.

A wavefront angle of requires very narrow elements ofabout dimensions.

Fewer number of elements (48

to 128) compared to linear

arrays → smaller footprint

Transmit, receive and delay

electronics for each element

separately

In receive mode individual

element delays are introduced

that enable the transducer to

be direction sensitive

Phased arrays allow for

miniaturised probe designs →

tiny catheter sized ultrasound

probes for intra-luminal

inspection

Fig. 5.24: Phased array

switching can produce

either planar or focused

beams

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Phased Array Transducer

(32)


Phased Array Example:Mitral Valve

Fig. 5.25: Phased array transducer

head

Fig. 5.26: Example image captured

with a phased array transducer

(Mitral valve stenosis)

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Phased Array Transducer

(33)


Phased Array Example:IVUS

Fig. 5.27: Catheter

sized Ultrasound

device

Fig. 5.28: Example

image of a coronary

artery

Fig. 5.29: Fluoroscopic

contrast image of the

cardio-vascular tree

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Annular ArrayTransducer


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(35)Annular ArrayTransducer

The Annular array transducerconsists of concentrictransducers operated as aphased array, see Fig 5.30.

↑ Excellent image quality,

since lateral resolution at

depth can be controlled by

signal phasing

↑ The overall depth of

focus can be controlled by

the delay between pulsing

signals

↓ Can not be steered

electronically → mechanical

wobbling

↓ Doppler imaging is not

possible due to the

mechanical wobbling

producing interfering

signals

Annular array transducers areused when fine detail isimportant such as in fetalexaminations (obstetrics).

Fig. 5.30: (a) Design of an

annular array transducer, (b)

scan pattern achieved by the

mechanical scan head

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The UltrasonicField


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(37)The Ultrasonic Field

The shape of a single flatelement transducer is split intwo zones:

the near-field or → Fresnel

zone [http://en.wikipedia.org

/wiki/Fresnel_diffraction] and

the far-field or →

Fraunhofer zone

[http://en.wikipedia.org

/wiki/Fraunhofer_diffraction],

see Fig 5.31.

The near-field retainsthe width of thetransducer, the beamthen spreads out in thefar-field → decreasingthe lateral resolution.

Fig. 5.31: Beam profile of a

single transducer

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(38)


The Ultrasonic Field (2)

The length of the near-field isgoverned by

(5.8)

and the divergence of thefar-field by

(5.9)

where is the radius of thetransducer and thewavelength.

Resolution at depth isbest with a widetransducer at highfrequency

Fig. 5.32: Examples of various

US fields

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(39)


Axial Resolution

Axial resolution defines the ability to resolve two closelyplaced surfaces parallel to the direction of the beam andis determined by the spatial pulse length (SPL):

the higher the frequency, the shorter the SPL the

better the axial resolution

BUT the higher the frequency, the lower the depth.

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(40)


Resolution

Lateral Resolution:

The Lateral Resolution

depends on the beam

focusing

Aperture ↑ Resolution ↑

Axial Resolution:

Frequency ↑ Resolution ↑

Frequency ↑ Attenuation ↑

→ find an optimum betweenresolution and penetrationdepth.

Fig. 5.33: Lateral resolution

depends on size of focal point

In general: Ultrasound devices have better axialthan lateral resolution!

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Doppler Imaging


(42)Doppler Imaging

For simple cases wherethe transducer is in linewith the flowing medium(blood) the observedfrequency is given by

(5.10)

where is the velocity ofsound in the medium, the velocity of the bloodand the Ultrasoundfrequency.

Fig. 5.34: Doppler effect as we know it

from emergency siren

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Doppler Imaging

(43)


Basics of Doppler Shift

For a transducer with anincident angle of , theobserved frequency isgiven by

(5.11)

where is the Dopplershift (frequency change)and the angle between thesound beam and thedirection of the blood flow.

Note that

Doppler shift increases

as transducer is aligned

with the vessel axis (

gets smaller)

Doppler shift can be

positive or negative

Relative Doppler shift is

small for blood flow

rates

Fig. 5.35: Basic Doppler geometry

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Doppler Imaging

(44)


Continuous vs PulsedDoppler

Continuous Wave Doppler

Simple design

The transmitted and received signals are often →

electronically mixed [http://en.wikipedia.org

/wiki/Electronic_mixer] (additive) and low-pass filtered to

form an audible signal

Fig. 5.36: Continuous wave

Doppler principleFig. 5.37: The transmitted and

received signals are mixed

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Doppler Imaging

(45)


Continuous vs PulsedDoppler (2)

Pulsed Wave Doppler

Allows to select the

tissue depth by limiting

the frequency analysis

to echo pulses that are

received at specific time

intervals after pulse

generation → gated

Analysis at multiple

depths is possible

Fig. 5.38: Pulsed Doppler principle

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Doppler Imaging

(46)


Duplex and ColourVelocity Imaging

The design on the right allowsto combine a pulsed Dopplerwith a real-time M-ModeUltrasound.

Blood flowing towards thetransducer is coded red andblood flowing away is coded inblue.

Fig. 5.39: B-Mode image with Doppler

information

Fig. 5.40: Linear array with

Doppler transducer used for

combining flow in duplex

imaging

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Imaging Artefacts

Beam Artefacts


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(49)Beam Artefacts

Problem:

Ultrasound processing assumes thatthe echos originated from within themain beam.

Notes:

US beam has a complex 3D shape

with low-energy off-axis lobes

Strong reflectors outsize the main

beam might generate a detectable

signal → will be displayed as coming

from within the main beam!

Best recognised in regions expected

to be anechoic

Fig. 5.41: US beam with the side lobes and

grating lobes

Fig. 5.42: Multiple

copies of the same

structure are caused

by the side lobes

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Beam Artefacts

(50)


Beam Artefacts (2)

Problem:

A high echogenic object in thefar-field may produce a signalstrong enough to be detected.The object then appears asoriginating from within themain beam.

Note 1:

Beam width artefacts are bestrecognised when a structurethat should be anechoic - suchas the bladder - containsperipheral echos

Note 2:

By adjusting the focal zone tothe level of interest improvesimage quality

Fig. 5.43: High echogenic

objects in the far-field appear

as originating from the main

beam, (e) US image of a

partially filled bladder that

shows echoes (arrow) in the

expected anechoic urine, (f)

Same anatomical structure

after adjusting the focal zone

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Multiple EchoArtefacts


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(52)Multiple Echo Artefacts

Problem:

US assumes that anecho returns to thetransducer after asingle reflection andthat the depth of anobjects is related tothe time for this roundtrip.

Notes:

Two highly

reflective parallel

surfaces reflect

the beam forth and

back → multiple

echoes are

recorded and

displayed

(Reverberation

artefact)

Only the first

reflection is

properly

positioned

Comet tail artefact

is a special form of

reverberation at

Fig. 5.44: Reverberation artefacts

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closely spaced

parallel structures

→ they have a

triangular shape

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(53)


Multiple Echo Artefacts(2)

Problem:

Liquids trapped in atetrahedron of airbubbles create acontinuous soundwave that istransmitted back tothe transducer

Notes:

Ring-down

artefacts are

displayed as a line

or series of

parallel bands

extending

posterior to a gas

collections

Fig. 5.45: Oscillating air bubbles

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(54)


Multiple Echo Artefacts(3)

Problem:

Mirror artefacts are causedby structures indirectly(from the backside) hit bythe Ultrasound beam.

Notes:

The display shows an

imaginary object

mirrored and

equidistant from the

highly reflective

interface (e.g.

diaphragm)

The true object is always

closer (proximal) to the

transducer

Fig. 5.46: The black arrows show

the beam path producing mirror

images. The crosses mark the real

structure, whereas the arrow

points to the mirrored structure.

White marks the diaphragm.

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Velocity Artefacts


(56)Velocity Artefacts

Problem:

Ultrasound imagingassumes a constantspeed of sound in humantissue of .

Depending on the type oftissue it can, however,travel faster or slowerthan this.

Notes:

As adjacent beams

not necessarily travel

through the same

tissues, speed

displacements can

occur

Fig. 5.47: Speed displacement artefact

caused by different tissue speeds

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Velocity Artefacts

(57)


Velocity Artefacts (2)

Problem:

The Ultrasound beam may undergorefraction while traveling through tissues→ Snell's law. The Ultrasound devicesassume that the acoustic waves travel on astraight line.

Notes:

Structures can appear wider than they

actually are

Structures can be duplicated

Fig. 5.48:

Refraction

artefact

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AttenuationArtefact


(59)Attenuation Artefacts

Problem:

As an Ultrasound beamtravels through the bodyits energy becomesattenuated. Ultrasoundequipment compensatesthis effect duringamplification (Time gaincompensation). Echoesthat take longer to returnare more amplified. Theimage thus appears moreuniform.

Fig. 5.49: Shadowing artefact caused

by a strong attenuator

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Attenuation Artefact

(60)


Attenuation Artefacts(2)

Similar to the strongattenuator artefact, theartefact caused by aweak attenuatorbrightens the imagedistally to the transducer.

Fig. 5.50: Artefact caused by a weak

attenuator

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Speckle


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(62)Speckle

Most tissues appear inUltrasound as being filled withtiny scatter like structures →Speckle.

Speckle is a result of

interference between

multiple scattered echoes

produced within the

volume of the incident

Ultrasound pulse

In fact most of the signal

intensity seen in

Ultrasound images results

from scatter interactions

Fig. 5.51: Irregular interference

pattern caused by multiple scatterers

somewhat randomly distributed. The

speckle pattern thus appears random

too

Fig. 5.52: Ultrasound pulse

scattered off tiny

reflectors

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Speckle

(63)


Properties of Speckle

The appearance of speckle isnot completely random, butfollows physical principles:

The size of the speckle

cells depends on the lateral

dimension and axial pulse

length → see regions "a"

and "b"

The orientation of the

speckle cells reflects the

orientation of the acoustic

wave, thus the direction of

the beam lines → see

regions "c" and "d"

The speckle pattern does

not change with time but

with varying transducer

position/orientation and

organ configuration

Fig. 5.53: Liver Ultrasound

image with varying speckle

patterns

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Speckle

(64)


Speckle Tracking

Speckle patterns are quasirandom.

Fig. 5.54: Speckle pattern

comparison in the myocardium

Speckle patterns stayreasonably stable even withorgan motion.

Fig. 5.55: M-mode speckle pattern

in the septum of the myocardium

of Fig 5.54 that nicely follows the

septal motion

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Speckle

(65)


Speckle Tracking (2)

As the speckle patterns stay reasonablystable, simple template matchingallows to accurately track regions inthe image, e.g. the myocardium.

Note:

As the speckle pattern will not

repeat perfectly, the search should

be done from frame to frame →

danger of drift

Reverberations will also degrade

tracking performance

The lower lateral resolution will

result in a smeared speckle patterns

→ tracking is less effective.

If the frame rate is too low → poor

tracking because of large changes

If the frame rate is too high → poor

tracking because of the reduced

lateral resolution

If multiple regions aresimultaneously tracked,deformations of organs e.g. themyocardium (heart muscle) canbe measured.

Fig. 5.56: Template

tracking

Fig. 5.57: Motion of

the myocardium

tracked using

multiple regions

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