Transcript Slide 1

Unit II. Image formation and acquisition
principles.
Part II Major imaging modalities
Dr. Felipe Orihuela-Espina
Outline




Fundamental models of image formation
 Kinds of radiation and imaged properties
The imaging system
 Point spread function
 Imaging filters: Monochromatic, colour, multi-spectral and hyperspectral images
 Resolution (pixel, spatial, radiometric/magnitude,spectral, temporal, superresolution)
Image quality and uncertainties in image formation (digitization, quantum efficiency,
metamerism, calibration, CNR, SNR)
Major imaging modalities
 Magnetic Resonance Imaging
 Optical Imaging
 X-Ray




(X-Ray) CT
(X-Ray) Fluoroscopy
 Coherent Tomography (OCT)
 Diffuse Optical Imaging (NIRS)
 Microscopy
 Confocal imaging
 One and two-photon imaging
Electrical and magnetic imaging
 EEG/MEG
 EMG
 ECG
Ultrasound
© 2015. Dr. Felipe Orihuela-Espina
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Some references
 Very nice slides on image acquisition
systems including CT, MRI, Ultrasound,
PET, SPECT
 http://webpages.uncc.edu/krs/courses/6010/m
edvis/imaq1a.pdf
 You may find some good MRI examples
at:
 http://www.mrtip.com/serv1.php?type=db1&dbs=t2%20weig
hted%20image
© 2015. Dr. Felipe Orihuela-Espina
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MAJOR IMAGING MODALITIES
© 2015. Dr. Felipe Orihuela-Espina
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ELECTRICAL AND MAGNETIC
IMAGING
© 2015. Dr. Felipe Orihuela-Espina
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Electrochemistry
 Electrochemistry is the study of reactions
in which charged particles (ions or
electrons) cross the interface between two
phases of matter*
 typically a metallic phase (the electrode) and
a conductive solution, or electrolyte.
 Source:
[http://www.chem1.com/acad/webtext/elchem/
ec1.html]
*A phase of matter is a region of space (a thermodynamic system), throughout
which all physical properties of a material are essentially uniform. Source:
[Wikipedia:Phase_(matter)]
© 2015. Dr. Felipe Orihuela-Espina
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Electrodes
 An electrode is a
collector or emitter of
electric charge or of
electric-charge carriers.
 Often, just a metal strip
 When an electrode is
placed in a solution
(electrolyte), the
electrode acquires either
a positive or negative
charge with respect to the
solution. This potential
difference is called
electrode potential.
Figure from: [http://www.askiitians.com/iit-jeechemistry/physical-chemistry/electrodepotential.aspx]
© 2015. Dr. Felipe Orihuela-Espina
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Electrodes
 An electrode reaction or electrode
process refers to the net oxidation
or reduction process that takes
place at an electrode. In the case of
oxidation:
1. Anions in the electrolyte will flow to
2.
3.
the interface boundary.
Cations in the electrolyte will flow
away from the interface boundary.
To counteract this, electrons in the
electrode will flow away from the
interface boundary creating a
current in the electrode.

Oxidation: Ions pass from the
electrode into solution leaving a
negative charge on the electrode.

Reduction: Ions pass from the
solution to the electrode leaving a
positive charge on the electrode
Figure from:
[http://soundlab.cs.princeton.edu/learni
ng/tutorials/sensors/node10.html]
© 2015. Dr. Felipe Orihuela-Espina
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Electrodes
 Any electrode may act as anode or cathode
depending on the direction of the flow of ions.
 When electrode is negatively charged with
respect to solution, i.e., it acts as anode.
Oxidation occurs.
 The anode is the electrode where oxidation reactions
take place
 When electrode is positively charged with respect
to solution, i.e., it acts as cathode. Reduction
occurs.
 The cathode is the electrode where reduction reactions
take place
© 2015. Dr. Felipe Orihuela-Espina
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Electrodes
 To measure a
potential difference
(voltage) we need 2
electrodes; one acting
as a reference.
Figure from:
[http://web.nmsu.edu/~kburke/Instrume
ntation/IS_Electrod.html]
© 2015. Dr. Felipe Orihuela-Espina
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Electrophysiology
 Electrophysiology is the study of electric activity in
biological tissues and bodies.
 It involves measurements of voltage change or electric
current on a wide variety of scales from single ion channel
proteins to whole organs
 In electrophysiology an electrode acts transducer
between the ionic transport of the nerve and the
electron flow in copper wire.
 Its variants are standard in several procedures e.g.
 ECG or EKG for cardiac activity monitoring,
 EEG for temporal resolution in neuroimage,
 EMG for muscle activity.
© 2015. Dr. Felipe Orihuela-Espina
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Electrophysiology
 Electric and magnetic imaging senses the
electromagnetic activity that the body uses to
transmit information through the nervous
system (NS).
 Since it monitors activity of neurons is a
direct measure of NS activity including the
brain.
 Since the source is the body itself, there is no
external irradiation.
© 2015. Dr. Felipe Orihuela-Espina
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Nervous system
 Tissues are groups of cells that work
together to carry out a certain task within
an organism
 An organ is a group of tissues that work
together
 An organ system is a group of organs
that work together to perform one or more
functions.
Definitions from wikipedia (several pages)
© 2015. Dr. Felipe Orihuela-Espina
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Nervous System
 The nervous system is
the organ system
responsible for the
transmission and
reception of signals
between different parts of
a body.
 The system is composed
mainly of two types of
cells:
 Neurons – responsible for
information transmission
 Glial cells – responsible for
homeostasis and protection
© 2015. Dr. Felipe Orihuela-Espina
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Nervous system
 The neuron is the
basic cell of the NS*.
 Neurons specialized
in information
transmission and they
do this in two forms:
 Chemical
 Electrical Figure from: [Wikipedia: Complete_neuron_cell_diagram_en.svg]
*There are as many as 10,000 specific types of neurons responsible for different tasks in the human brain.
Mainlly they can be coarsely classified in: motor neurons (for conveying motor information), sensory neurons
(for conveying sensory information), and interneurons (which convey information between different types of
neurons). Source: [Stufflebeam R “Neurons, Synapses, Action Potentials, and Neurotransmission”
http://www.mind.ilstu.edu/curriculum/neurons_intro/neurons_intro.php]
© 2015. Dr. Felipe Orihuela-Espina
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Electrophysiology
 The cell membrane of the
axon and soma contain
gated ion channels that
allow the neuron to
generate and propagate
an electrical signal (an
action potential).
 These signals are
generated and
propagated by chargecarrying ions.
 Ions, not electrons, are the
carriers of current in the
nervous system.
Depolarization and resting state
Figures from: [science.education.nih.gov]
© 2015. Dr. Felipe Orihuela-Espina
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Electrophysiology
 There are four main types of
gated channel.
 Voltage-gated channels
Figure from:
[http://7e.biopsychology.com/step03.01.
html]
(open/close in response to a
change in membrane potential)
 Chemically-gated ion channels
(open/close in response to
binding with an extracellular
chemical messenger
(neurotransmitter))
 Mechanically-gated ion
channels (open/close in
response to physical stimuli)
 Thermally-gated channels
(open/close in response to
temperature changes)
 We won’t see them in detail
© 2015. Dr. Felipe Orihuela-Espina
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Electrophysiology
 ☞ Our knowledge of ion channels is still limited
 They are too tiny to be seen in detail, even with the

electron microscope. Nanoscopy may soon sort out
this limitation.
We do not know for sure if there are separate
channels for the different ions (Na, K), or one channel
with different permeability.
 To know more:
 The origin of the resting membrane potential
 An interactive presentation. A bit tough on chemistry.
 http://www.st-andrews.ac.uk/~wjh/neurotut/mempot.html
© 2015. Dr. Felipe Orihuela-Espina
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Electrophysiology
 Action potentials are brief,
rapid, large changes in the
membrane potential in
which the potential actually
reverses.
 Action potentials are
propagated/conducted
along the axon of a neuron
without losing their strength
(nondecremental
conduction)
Source and figure from:
[https://dundeemedstudentnotes.wordpress.
com/category/nervoussystem/physiology/page/2/]
© 2015. Dr. Felipe Orihuela-Espina
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Electrophysiology
In the resting state,
potassium is
concentrated on the
inside of cells.
Depolarization and resting state
Figure from: [http://peer.tamu.edu/curriculum_modules/OrganSystems/module_5/whatweknow2.htm]
© 2015. Dr. Felipe Orihuela-Espina
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Electrophysiology
Step 1. A resting (diffusion)
membrane potential normally
exists through the efflux of
positive-charged (potassium)
ions maintaining
an electrochemical equilibrium
of –75 mV.
Depolarization and resting state
Figure from: [http://peer.tamu.edu/curriculum_modules/OrganSystems/module_5/whatweknow2.htm]
© 2015. Dr. Felipe Orihuela-Espina
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Electrophysiology
Step 2. When a neuron
is stimulated,
molecular receptors in
its membrane open.
Depolarization and resting state
Figure from: [http://peer.tamu.edu/curriculum_modules/OrganSystems/module_5/whatweknow2.htm]
© 2015. Dr. Felipe Orihuela-Espina
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Electrophysiology
Step 3. Sodium ions move in
because their is an electrical
pulling force (inside is negative
and sodium ions are positive) and
an osmotic force (sodium is more
concentrated on the outside).
Depolarization and resting state
Figure from: [http://peer.tamu.edu/curriculum_modules/OrganSystems/module_5/whatweknow2.htm]
© 2015. Dr. Felipe Orihuela-Espina
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Electrophysiology
Step 4. As sodium
rushes in, the inside
becomes positive, and
that forces out
potassium.
Depolarization and resting state
Figure from: [http://peer.tamu.edu/curriculum_modules/OrganSystems/module_5/whatweknow2.htm]
© 2015. Dr. Felipe Orihuela-Espina
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Electrophysiology
Figure from: [https://dundeemedstudentnotes.wordpress.com/category/nervoussystem/physiology/page/2/]
© 2015. Dr. Felipe Orihuela-Espina
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Electrophysiology
 ☞ The transmission of the action potentials
along the axon is a complicated process.
 It may occur by means of contiguous conduction
(in non-myelinated fibers) or by salitatory
conduction (in myelinated fibers).
 After an action potential has occurred, there is a
transient negative shift, called the
afterhyperpolarization or refractory period. This
mechanism prevents an action potential from
traveling back the way it just came.
 To know more:
 Grace and Bunney (1983) Neuroscience
10(2):317-331
© 2015. Dr. Felipe Orihuela-Espina
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Electrophysiology
 Neurotransmission (or
synaptic transmission)
is communication
between neurons as
accomplished by the
movement of chemicals
or electrical signals
across a synapse.
 Source: [Stufflebeam R
“Neurons, Synapses,
Action Potentials, and
Neurotransmission”
http://www.mind.ilstu.edu
/curriculum/neurons_intr
o/neurons_intro.php]
Figure from: [imgarcade.com]
© 2015. Dr. Felipe Orihuela-Espina
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Electrophysiology
Figure from: [https://www.studyblue.com/notes/note/n/exam-2/deck/1245341]
© 2015. Dr. Felipe Orihuela-Espina
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 To be able to measure
electrical activity of the
body “non-invasively”*
from the outside;
electrodes are placed
over the skin.
decrease the skin
impedance (mainly from
the stratum corneum) to
facilitate the transduction
of the ionic currents
 This may be achieved by
using conductive gels
Figure from:
[http://soundlab.cs.princeton.
edu/learning/tutorials/sensors
/node10.html]
 It is necessary to
Figure from: [imgarcade.com]
Electrophysiology through the skin
* Electrolyte conductive gels are often abrasive; they remove dead cells. No the
so called dry electrodes operate gel-free. [Lopez-Gordo et al (2014) Sensors 14,
© 2015. Dr. Felipe Orihuela-Espina
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12847-12870]
EEG
 Electroencephalography (EEG) is the recording
of electrical activity along the scalp.
 EEG is the oldest of neuroimaging modalities
dating back to 1924 when Hans Berger described
the alpha waves of the human brain*.
 Regarded as the gold standard when it comes to
temporal resolution.
* Berger was not the first to notice electrical activity through the skull; that occurred
even earlier with the work of Richard Caton in 1875 who work with dogs and apes and
possibly was recording spontaneous activity). Note that Caton’s work is even earlier
than X-Rays discovery!
© 2015. Dr. Felipe Orihuela-Espina
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EEG
 EEG activity reflects the
summation of the synchronous
activity of thousands or
millions of neurons that have
similar spatial orientation.
 The electric potential generated
by an individual neuron is far
too small to be picked up by
EEG.
 The EEG signal originates
mainly from cerebral cortex
pyramidal cells because of
their orientation relative to the
cortical surface
 If the cells do not have similar
spatial orientation, their ions do
not line up and create waves to
be detected.
Schema of electric dipole, ionic currents and differential measure
Figure from [Lopez-Gordo et al (2014) Sensors 14:12847-12870]
© 2015. Dr. Felipe Orihuela-Espina
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EEG
 “The origin of cerebral potentials is based
upon the intrinsic electrophysiological
properties of the nervous system. Identifying
the generator source(s) and electrical field(s)
of propagation are the basis for recognizing
electrographic patterns that underly the
expression of the “brain waves” as normal or
abnormal.”
 Source: [Tatum IV, W. O.; Aatif, M. H.; Benbadis,
S. R. & Kaplan, P. W. Handbook of EEG
interpretation. Demos Medical Publishing, 2008,
276 pp.]
© 2015. Dr. Felipe Orihuela-Espina
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EEG
© 2015. Dr. Felipe Orihuela-Espina
Figure from: [www.ecvv.com]
Figure from: [www.brainmaster.com]
33
EEG
The right-leg driver (DRL) circuit eliminate
electromagnetic interference noise
originated by the body acting as an antenna.
If this is not present; a Faraday cage may also
be used.
© 2015. Dr. Felipe Orihuela-Espina
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EEG
Figure from: [http://www.psych.nmsu.edu/~jkroger/lab/EEG_Introduction.html]
© 2015. Dr. Felipe Orihuela-Espina
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EEG
Figure: [Jurcak2007]
Figure: [Self elaboration from LACCIR project]
© 2015. Dr. Felipe Orihuela-Espina
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EEG
Figure: [Self elaboration from LACCIR project]
© 2015. Dr. Felipe Orihuela-Espina
37
EEG: Information processing
Perceptual analysis
Deployment
of covert
attention
Sensory
process
Object
recognition
and
categorization
Stimulus evaluation and context
updating (working memory)
Error processing
and reinforcement
learning
Not
understood
Brain information
transformation:
From raw sensory input to
behavioural response
(in a visual search task)
Figure modified from [WoodmanGF2010]
16/07/2015
INAOE
38
MEG
 Magnetoencephalogra
phy (MEG) records
magnetic fields
produced by electrical
currents occurring in
the brain.
 The MEG (and EEG)
signals derive from the
net effect of ionic
currents flowing in the
dendrites of neurons
during synaptic
transmission.
Figure: [Wikipedia:Magnetoencefalografía]
© 2015. Dr. Felipe Orihuela-Espina
39
MEG
Figure from: [http://www.slideshare.net/Anuj0909/magnetoencephalography-by-anuj-malik]
© 2015. Dr. Felipe Orihuela-Espina
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MEG
Figure from: [Kim et al (2013)
Korean J Pediatr
2013;56(10):431-438]
© 2015. Dr. Felipe Orihuela-Espina
41
MEG
Figure from: [Foley et al. (2014) Epilepsy & Behavior 30:38-42]
© 2015. Dr. Felipe Orihuela-Espina
42
ECG or EKG
 Electrocardiogram (e-lek-tro-KAR-de-ogram), ECG or EKG records the heart's
electrical activity.
 An ECG shows:
 How fast your heart is beating
 Whether the rhythm of your heartbeat is steady or
irregular
 The strength and timing of electrical signals as
they pass through each part of your heart
 Source: [http://www.nhlbi.nih.gov/health/healthtopics/topics/ekg]
© 2015. Dr. Felipe Orihuela-Espina
43
ECG
With each heartbeat, an electrical signal spreads from the top of the
heart to the bottom. As it travels, the signal causes the heart to contract
and pump blood.
Figure from: No longer available
[http://ecgguru.com/ecg/ecg-waveformillustration]
Figure from:
[http://drmedicineguide.blogspot.mx/2013/03
/ecg-electrode-placement.html]
© 2015. Dr. Felipe Orihuela-Espina
44
ECG
Figure from:
[http://www.swharden.com/blog
/2009-08-14-diy-ecg-machineon-the-cheap/]
Figure from:
[http://www.nhlbi.nih.gov/health/healthtopics/topics/hb/understanding/]
© 2015. Dr. Felipe Orihuela-Espina
45
ECG
Electrode positioning in ECG obeys to sensitivity in detecting myocardial
infarction. A classical ECG (small picture) has 10 electrodes (12 leads systems) but
other montages (large picture) are possible
Small figure from: [nuclearcardiologyseminars.com]
Large figure from: [http://drmedicineguide.blogspot.mx/2013/03/ecg-electrode© 2015. Dr. Felipe Orihuela-Espina
46
placement.html]
EMG
 Electromyography (EMG) recording the
electrical activity produced by skeletal
muscles (those voluntarily controlled)
 When muscles contract, summation of motor
neurons action potentials give off an electrical
burst strong enough to be detected by an
electrode placed on the skin.
 It uses needle electrodes (better SNR) or
pads (less invasive)
© 2015. Dr. Felipe Orihuela-Espina
47
EMG
Figure from: [www.protokinetics.com]
Figure from:
[http://www.lookfordiagnosis.com/mesh_info.php?te
rm=Electromyography&lang=1]
© 2015. Dr. Felipe Orihuela-Espina
48
EMG
Figure from: [http://www.a-tech.ca/subcat.php?id=106]
© 2015. Dr. Felipe Orihuela-Espina
49
EMG
EMGs of the human soleus, bipolar
surface electrodes.
(a) Very slight contraction, separate
motor unit potentials are visible.
(b) Strong contraction, showing an
‘interference pattern’. The individual
motorunit potentials are
no longer discernible. Note the
different voltage scales.
Figure from: [Hof AL (1984) Human
Movement Science 3:119-153]
© 2015. Dr. Felipe Orihuela-Espina
50
EMG
Figure from: [De Luca CJ (1997) Journal of Applied Biomechanics, 13 (2): 135-163]
© 2015. Dr. Felipe Orihuela-Espina
51
ULTRASOUND
© 2015. Dr. Felipe Orihuela-Espina
52
Disclaimer
 ☞ This section is roughly based on
University of Washington at Seattle’s
course on “Bioengineering 508 Physical
Aspects of Medical Imaging ” by Prof. Paul
Kinahan and colleagues.
 You can find the original presentation here:
 http://courses.washington.edu/bioen508/Lectu
re6-US.pdf
© 2015. Dr. Felipe Orihuela-Espina
53
Ultrasound
 Remember:
 Ultrasound waves:
 …are mechanical pressure
waves (need an elastic
medium to propagate)
 …propagate longitudinally Figure from [http://www.olympus-ims.com/en/ndttutorials/flaw-detection/ultrasound/]
 An elastic medium is one
that deforms under force.
 region of increased
pressure on a sound wave
is called a compression (or
condensation). A region of
decreased pressure on a
sound wave is called a
rarefaction (or dilation)
Figure from
[http://physics.stackexchange.com/questions/123471/compressionvs-rarefaction-in-sound-waves]
© 2015. Dr. Felipe Orihuela-Espina
54
Ultrasound
 Remember that as a
wave; it still complies
with
 the relation: c=fλ
Figure from [University of Washington Course on “Bioengineering 508 Physical Aspects of
Medical Imaging ”]
© 2015. Dr. Felipe Orihuela-Espina
55
Ultrasound
 Remember that as a
wave; it still complies
with
 Snell’s law
Figures from [University of Washington
Course on “Bioengineering 508 Physical Aspects of56
© 2015. Dr. Felipe Orihuela-Espina
Medical Imaging ”]
Ultrasound
 Remember that as a
wave; it still complies
with
 Attenuation as per
Beer-Lambert law
 Becuase of
attenuation the
intensity of echoes
decrease.
Attenuation measurements can be made by
examining the exponential decay of
multiple back surface reflections.
Figure from: [https://www.ndeed.org/EducationResources/CommunityColl
ege/Ultrasonics/MeasurementTech/attenua
tionmeasure.htm]
© 2015. Dr. Felipe Orihuela-Espina
57
Ultrasound
Figure from:
[http://galleryhip.com/ultrasoundmachine.html]
Figure from: [medstandard.org]
© 2015. Dr. Felipe Orihuela-Espina
58
Ultrasound
 Basic idea:
1. Irradiate sound waves
2.
3.
4.
into the sample.
Sound waves travel at
different speed
depending on the
medium through which
it propagates.
Waves are reflected at
the interfaces between
the different tissues
The return time of the
waves tells us of the
depth of the reflecting
surface
Figure from [University of Washington Course on
“Bioengineering 508 Physical Aspects of Medical
Imaging ”]
© 2015. Dr. Felipe Orihuela-Espina
59
Ultrasound
Figure from [Coletta, Chapter 16 “Mechanical Waves: Sound”]
© 2015. Dr. Felipe Orihuela-Espina
60
Ultrasound
 Propagation of sound waves in tissues:
 An echo is a wave sound that reflects as the medium changes.
 Echoes can be:
 Specular echoes*:
 Originate at smooth boundaries from relatively large media
 Intense and angle dependent
 Reflection from large surfaces
Figure from [University of Washington Course on “Bioengineering 508 Physical Aspects
of Medical Imaging ”]
* The name of specular echoes is ambiguous; as they refer to all reflection
echoes whether specular or not
© 2015. Dr. Felipe Orihuela-Espina
61
Ultrasound
 Propagation of sound waves in tissues:
 An echo is a wave sound that reflects as the medium changes.
 Echoes can be:
 Scattered echoes:
 Originate from objects that are size of wavelength of smaller
 Weak and less angle dependent
 Reflection from small surfaces
Figure from [University of Washington Course on “Bioengineering 508 Physical Aspects of
Medical Imaging ”]
© 2015. Dr. Felipe Orihuela-Espina
62
Ultrasound
 The bulk modulus  of a substance is its
resistance to uniform compression.
 It is defined as the ratio of the infinitesimal
pressure increase to the resulting relative
decrease of the volume.
 Its SI unit is the Pascal.
Figure from: [http://www.engineeringtoolbox.com/bulk-modulus-elasticity-d_585.html]
© 2015. Dr. Felipe Orihuela-Espina
63
Ultrasound
 The bulk modulus  of
a substance:
 where V is the initial

volume.
And since density ρ is
the mass per unit of
volume;
Figure from: [http://www.engineeringtoolbox.com/bulk-modulus-elasticity-d_585.html]
© 2015. Dr. Felipe Orihuela-Espina
64
Ultrasound
 Compressibility is a measure of the
relative volume change of a fluid or solid
as a response to a pressure.
 Compressibility is the inverse of the bulk
modulus.
 ☞ Watch out! Sometimes compressibility is
denoted as  which can be confusing.
© 2015. Dr. Felipe Orihuela-Espina
65
Ultrasound
 The speed of sound c in a certain
medium depends on the medium density ρ
and elasticity or stiffness represented by
the bulk modulus  and can be
approximated by:
© 2015. Dr. Felipe Orihuela-Espina
66
Ultrasound
 ☞Beware! It’s a bit more
complicated…
 …since it depends on
compressibility (a relation of
pressure and volume), it also
depends on temperature but
you are unlikely to see this
level of detail in an introductory
text.
 You may know the classical
gas state equation: PV=nRT
 For solids, there is also a state
equation relating P, V and T,
but it is rather more
complicated.
Figure from: [Nasoni et al (1980)
Ultrasonics symposium, 10771082]
© 2015. Dr. Felipe Orihuela-Espina
67
Ultrasound generation
 Piezoelectric effect
 Piezoelectricity means electricity

resulting from pressure*.
The piezoelectric effect is the capacity of
certain solid materials to accumulate
electric charge in response to applied
mechanical stress or viceversa.
 Externally apply mechanical stress and

generate internal electricity
Externally apply electricity and generate
internal mechanical strain
 First demonstration of the piezoelectric
effect was in 1880 by siblings Pierre
Curie (Nobel Prize Winner) and Jacques
Curie
Figure from: [Wikipedia: Piezoelectricity]
 ☞ For further details,check back up
slides.
*In greek, piezo means squeeze.
© 2015. Dr. Felipe Orihuela-Espina
68
Ultrasound generation
 Ultrasound
generators are
transducers based on
the piezoelectric
effect.
 Apply electricity and
get a mechanical
strain (sound)
Simple ultrasound generator. The SPKR is a
piezoelectric material.
Figure from:
[http://www.discovercircuits.com/U/ultrasonic.
htm]
© 2015. Dr. Felipe Orihuela-Espina
69
Ultrasound generation
 Typically, the active element of the
transducer is a thin piezoelectric
ceramic or piezocomposite
element.
 The active element, often referred
to as the crystal, is:
 protected from damage by a

wearplate or acoustic lens, and
backed by a block of damping
material that quiets the transducer
after the sound pulse has been
generated.
 Dual transducers differ in that they
have separate transmitting and
receiving elements separated by a
sound barrier.
 Source: [http://www.olympus-
ims.com/en/ndt-tutorials/flawdetection/generating-ultrasound/]
Figure from: [http://www.olympusims.com/en/ndt-tutorials/flawdetection/generating-ultrasound/]
© 2015. Dr. Felipe Orihuela-Espina
70
Ultrasound generation
Figure from:
[University of Washington Course
onDr.
“Bioengineering
508 Physical Aspects of Medical Imaging
© 2015.
Felipe Orihuela-Espina
71 ”]
Ultrasound image formation
1.
2.
The ultrasound machine irradiates high
frequence sound pulses to tissue.
The sound waves interact with matter.
During this interaction it hits
boundaries between tissues
 Some sound waves will be reflected
others refracted to continue travelling
deeper in the matter
3.
Reflected waves who return to tissue
surface are detected by the probe and
passed to a CPU.
 The probe again capitalises on the
piezoelectric effect to convert back sound
to electricity.
4.

The CPU estimates the distance from
the probe to the tissue boundaries
using echoes times.
In the most basic ultrasound image pixel
intensities are proportional to echoes
strength. Distance is represented along
the Y axis.
Figure from:
[University of Washington Course on “Bioengineering 508 Physical Aspects of Medical Imaging ”]
© 2015. Dr. Felipe Orihuela-Espina
72
Ultrasound image formation
 Echo Display Modes:
 A-mode (amplitude):
display of processed
information from the
receiver versus time
 Speed of sound equates
to depth
 B-mode (brightness):

Conversion of A-mode
information into
brightness-modulated
dots
M-mode (motion): uses
B-mode information to
display the echoes from
a moving organ
Source and Figure from:
[University of Washington Course
onDr.
“Bioengineering
508 Physical Aspects of Medical Imaging
© 2015.
Felipe Orihuela-Espina
73 ”]
Ultrasound image formation
Source and Figure from:
[University of Washington Course on “Bioengineering 508 Physical Aspects of Medical Imaging ”]
© 2015. Dr. Felipe Orihuela-Espina
74
Ultrasound
 Mechanical absorption coefficient of a material is
generally dependent on frequency.
 A common model for this dependency is:
 Where α is the attenuation coefficient and a=8.7.
Often; b~1
 With this model, we can estimate the depth of
penetration* of the sound wave as a function of
frequency and its attenuation in different tissues.
* Depth of signal is related to reflection time, so as time progresses, the signal will be increasingly
attenuated. Time-dependent attenuation causes severe signal loss if not compensated. Thus, all systems
nowadays are equipped with circuitry that performs time-gain compensation (TGC), a time-varying
amplification. Source: [University of Washington Course on “Bioengineering 508 Physical Aspects of Medical
Imaging ”]
© 2015. Dr. Felipe Orihuela-Espina
75
Ultrasound image formation
 In ultrasound, spatial
resolution is proportional to
sound frequency.
 Higher frequencies yield
better image resolution.
 …but the price to pay is
lower penetration.
 Lower frequencies penetrate
deeper in the tissues.
 …but don’t be fooled! There
is no need to compromise.
Use broadband irradiation
and get the best of both
Figure from: [http://www.usra.ca/transducer.php]
worlds!
© 2015. Dr. Felipe Orihuela-Espina
76
Ultrasound
Ultrasound image of a fetus in the womb, viewed at 12 weeks of
pregnancy. Figure from: [Wikipedia:Ultrasound]
© 2015. Dr. Felipe Orihuela-Espina
77
3D/4D Ultrasound
Figure from: [imgarcade.com]
© 2015. Dr. Felipe Orihuela-Espina
78
HD Ultrasound
Figure from: [https://www.youtube.com/watch?v=BD7quHKgEuk]
© 2015. Dr. Felipe Orihuela-Espina
79
Ultrasound: Echocardiography
Figure from:
[https://www.youtube.com/watch?v=4v
BJoWP-zBM]
For the analysis of valves and blood flow,
ultrasound Doppler is used
Figure from:
[http://www.ntnu.edu/isb/echocardiogra
phy]
© 2015. Dr. Felipe Orihuela-Espina
80
Ultrasound
Thyroid
Liver
Spine
All figures from: [http://www.oit.edu/academics/degrees/diagnostic-medical-sonography]
© 2015. Dr. Felipe Orihuela-Espina
81
Ultrasound
 A excellent introductory slideshow:
 http://www.slideshare.net/KhalisKarim/physicultrasound
© 2015. Dr. Felipe Orihuela-Espina
82
THANKS, QUESTIONS?
© 2015. Dr. Felipe Orihuela-Espina
83
BACK UP
© 2015. Dr. Felipe Orihuela-Espina
84
Piezoelectric effect
 The piezoelectric
effect results from the
change of polarization
when applying a
mechanical stress.
 The change in
polarization appears
as a variation of
surface charge density
upon the object faces
Figure from: [http://www.cosmic-energy.org/?page_id=771]
© 2015. Dr. Felipe Orihuela-Espina
85
Piezoelectric effect
Figure from: [http://global.kyocera.com/fcworld/charact/elect/piezo.html]
© 2015. Dr. Felipe Orihuela-Espina
86
Piezoelectric effect
Figures from [University of Washington Course on “Bioengineering 508 Physical Aspects of
Medical Imaging ”]
© 2015. Dr. Felipe Orihuela-Espina
87
Ultrasound
 Impedance is the product of the density of
a material ρ and the speed of sound c in
that material
© 2015. Dr. Felipe Orihuela-Espina
88