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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)
Impedance Matching (3)
Impedance Matching (4)
Impedance Matching (5)
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 (3)
Linear Sequenced Transducer (4)
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)
Oct 17th, 2016Principles of Medical Imaging
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Oct 17th, 2016Principles of Medical Imaging
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Prof. Dr. Philippe Cattin: Ultrasound
Abstract
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Image Generation
Oct 17th, 2016Principles of Medical Imaging
(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
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Prof. Dr. Philippe Cattin: Ultrasound
A-Mode
Fig. 5.2: A-mode Ultrasound
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Image Generation
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Prof. Dr. Philippe Cattin: Ultrasound
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
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Prof. Dr. Philippe Cattin: Ultrasound
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
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Prof. Dr. Philippe Cattin: Ultrasound
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
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Prof. Dr. Philippe Cattin: Ultrasound
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
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Prof. Dr. Philippe Cattin: Ultrasound
4D Ultrasound
Recentdevelopmentsincomputationequipmenteven allow tovisualise 4Dmoviesequences.
Fig. 5.7: 4-dimensional movie of a fetus (week 31)
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UltrasoundTransducers
Oct 17th, 2016Principles of Medical Imaging
(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
Oct 17th, 2016Principles of Medical Imaging
(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
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Prof. Dr. Philippe Cattin: Ultrasound
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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Single Element Transducer
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Prof. Dr. Philippe Cattin: Ultrasound
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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Single Element Transducer
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Prof. Dr. Philippe Cattin: Ultrasound
Impedance Matching (2)
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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Single Element Transducer
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Prof. Dr. Philippe Cattin: Ultrasound
Impedance Matching (3)
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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Single Element Transducer
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Prof. Dr. Philippe Cattin: Ultrasound
Impedance Matching (4)
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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Single Element Transducer
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Prof. Dr. Philippe Cattin: Ultrasound
Impedance Matching (5)
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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Single Element Transducer
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Prof. Dr. Philippe Cattin: Ultrasound
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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Single Element Transducer
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Prof. Dr. Philippe Cattin: Ultrasound
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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Single Element Transducer
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Prof. Dr. Philippe Cattin: Ultrasound
Images of Transducers
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Linear SequencedTransducer
Oct 17th, 2016Principles of Medical Imaging
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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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Linear Sequenced Transducer
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Prof. Dr. Philippe Cattin: Ultrasound
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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Linear Sequenced Transducer
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Prof. Dr. Philippe Cattin: Ultrasound
Linear SequencedTransducer (3)
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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Linear Sequenced Transducer
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Prof. Dr. Philippe Cattin: Ultrasound
Linear SequencedTransducer (4)
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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Linear Sequenced Transducer
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Prof. Dr. Philippe Cattin: Ultrasound
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
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Prof. Dr. Philippe Cattin: Ultrasound
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
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Prof. Dr. Philippe Cattin: Ultrasound
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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The Ultrasonic Field
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Prof. Dr. Philippe Cattin: Ultrasound
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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The Ultrasonic Field
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Prof. Dr. Philippe Cattin: Ultrasound
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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The Ultrasonic Field
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Prof. Dr. Philippe Cattin: Ultrasound
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
Oct 17th, 2016Principles of Medical 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
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Prof. Dr. Philippe Cattin: Ultrasound
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
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Prof. Dr. Philippe Cattin: Ultrasound
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
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Prof. Dr. Philippe Cattin: Ultrasound
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
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Prof. Dr. Philippe Cattin: Ultrasound
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
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Prof. Dr. Philippe Cattin: Ultrasound
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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Multiple Echo Artefacts
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Prof. Dr. Philippe Cattin: Ultrasound
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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Multiple Echo Artefacts
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Prof. Dr. Philippe Cattin: Ultrasound
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
Oct 17th, 2016Principles of Medical Imaging
(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
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Prof. Dr. Philippe Cattin: Ultrasound
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
Oct 17th, 2016Principles of Medical Imaging
(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
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Prof. Dr. Philippe Cattin: Ultrasound
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
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Prof. Dr. Philippe Cattin: Ultrasound
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)
Prof. Dr. Philippe Cattin: Ultrasound
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)
Prof. Dr. Philippe Cattin: Ultrasound
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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68 of 68 26.09.2016 08:35
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