Ky Thuat Va Thiet Bi Sieu Am (4 Slides p.p)

7/30/2019 Ky Thuat Va Thiet Bi Sieu Am (4 Slides p.p)

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K thut v thit b siu(Ultrasound diagnostics)

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K thut siu m (Ultrasounddiagnostics)

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OutlineI. Physical properties of ultrasound and acoustic parameters of mediumII. Ultrasonography (Chn on hnh nh bng siu m)

Impulse reflection method

A-mode one-dimensional B-mode two-dimensional M-mode Basic characteristics of US images Interventional sonography Echocontrast agents Harmonic imaging

Principle of 3D imagingIII. Doppler flow measurement

Principle of Doppler effect Principle of blood flow measurement CW Doppler system Systems with pulsed wave PW Doppler Duplex and Triplex methods

Power Doppler method Tissue Doppler Imaging (TDI)

Ultrasonic densitometryIV. Advantages Patient Safety: reducing Ultrasound Doses

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History Ultrasound has been used as a navigational and detection aid by the

bat for millions of years. It was not until the second world war, however,that man started extensive use of ultrasound for the same purpose.With the enormous potential of military research programs, ultrasoundtechnology rapidly developed.

Although ultrasound had already been used in the therapy (cha bnh)and was proposed by S.Y. Sokolov for diagnostic use (mc ch iutr) in 1937, no successful attempt to apply the ultrasound echo-sounder principle to medical diagnosis was made until the early 1950s.

Most of the equipment used at that time were industrial-type ultrasounddevices for detecting flows in metal, but soon ultrasonic devicesgenerally known as ultrasonoscopes specifically intended for medicaldiagnostics were developed. The major advantages of these devicesare the non-invasive and non-ionizing nature of the examination andtheir relatively low cost when compared to X-Ray, MagneticResonance (MR), CT and Isotopic Scanning techniques.

Over the last decade, the diagnostic usefulness of the equipment hasbeen vastly improved, as better instruments were developed and moreclinical experience gained, and in several diagnostic fields, ultrasoundtechnique has shown to be superior to other methods.


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Siemens P10 handheldultrasound machine


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Contrast media sensitivities in imagingonly nuclear imaging has the sensitivity

10-5MRS

10-5MRI

10-9 - 10-12PET

10-9 - 10-12Nuc Med

10-3CT

10-3Sono

Contrast media concentration(mol/kg BW)

Imaging method

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Ultrasound diagnostics

K thut chun on siu m (ultrasound diagnostics) c pht trin u

nhng nm 50 ca th k 20. K thut cho phpthu c cc nh ct lp

(cross-sectional images) ca c th con ngi vi ccthng tin v sinh l

(physiology) v bnh l (pathology).

Nguyn l chung: da trnhin tngphn x (reflection) ca sng siu

m ti c c b mt tip gipm hc (acoustical interfaces)

Mt s ni dung k thut chnh v siu m:

Ultrasonography (A, B and M mode, 3D and 4D imaging)

Doppler flow measurement, including Duplex and Triplex methods

(Duplex, Colour, Triplex, Power)

Tissue Doppler imaging

Ultrasound densitometry

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I. Physical properties (cc tnh cht Vt l) casng siu m

Questions: what is ultrasound and what are the main acousticalproperties of medium ?

Sound is a periodic disturbance (vibrations) that in fluids density,propagates as longitudinal waves (Mechanical vibration or wave).

Ultrasound is sound with a frequency over 20,000 Hz, which isabout the upper limit of human hearing.

Obeys the same physical laws as wave.

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Basic Principles of Sound

Sound is produced by a vibrating body and travels in the form

of a wave.

Sound waves travel through materials by vibrating the

particles that make up

the material.

The pitch ( cao/mc )of the sound is determined

by the frequency of the wave

(vibrations or cyclescompleted in a certain

period of time).


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Sound spectra

Diagnostic

Imaging

0 20 Hz 20 kHz 1 MHz 30 MHz

Infrared Audible NDTSound Sound Cleaning

Sng sium(US) l cc daong c hc (mechanical oscillations)vi tn s > 20 kHz lan truyn trong mt mi t rng n hi (elasticmedium). For medical diagnosis, typically ranging from 1 to 30 MHz.

Trong c ht lng v kh, US lan truyn nh mt sng dc

(longitudinal waves). Trong cht rn, US lan truyn nh mt sngngang (transversal waves).

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In medical US, images representing human organs (b phn c th) areformed by transmitting sound waves into the body and receiving back and

processing the resultant echoes from the tissues.

To accomplish this, medical ultrasound uses a process very similar to anocean-going vessels depth sounding equipment or oceanic surveyequipment. All of these systems make use of sound waves and their

reflections.

Sea

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Several wave modes ofvibration are used in

ultrasonic inspection.

The most common arelongitudinal, shear, and

Rayleigh (surface)

waves

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Ultrasound generation

High frequency ultrasound is generated with a transducer.

The transducer is capable of bothtransmitting and receiving soundenergy.

A piezoelectric element in thetransducer converts electricalenergy into mechanicalvibrations (sound), and viceversa.


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Tng tc ca US vi m (tissue) Phn x (reflection): xy r a vi c c mt phn cch ngnht, nhn, c

kch thc > b rng chm tia (smooth homogeneous interfaces of sizegreater than beam width), VD: cc phn t hu c (organ outlines)

Tn x Rayleigh (Rayleigh scatter): xy r a vi cc kchthc phn hi mnh (small reflector sizes), VD cc t bo mu trongmt mi t rng khngngnht.

Khc x (refraction): xy r a vi cc mi trng khng cht, c (lessdense to denser medium), khc vi nh sng ikhi to r a s bin dng(distortion)

Hp th (absorption) (sng m chuyn thnh nhit - sound to heat) Hin tng tngtheo tn s f, ngc vi t ia X Hp th mnh trong phi (lungs), gim dn trong xng (bone) v cc

m mm (soft tissue), ngc vi t ia X.

Giao thoa (interference): Cc vn hoa (speckles) trn nhUS l do sgiao thoa gia cc sng tn x Rayleigh.

Nhiu x (diffraction)

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Reflection

Medium 1 Medium 2

Transmitted waveIncident wave

Reflected wave

- One of the basic principles of medical ultrasound diagnosis.

- Occurs at areas of acoustic impedance mismatch.

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Refraction

Medium 1

Medium 2

i ncident wave Ref lected wave

Transmitted wave

When a propagating ultrasoundwave encounters a interface at anoblique angle, it is Refracted inthe same way that light is refractedthrough a lens. The portion of the

wave that is not reflectedcontinues into the second medium.It is dependent on the velocities ofthe two medium. If the velocitiesare equal, There would be no

refraction occurred and the beamgoes straight into the secondmedium. For the velocities ofthe different tissues in the human

body are quite close, refraction'scan be ignored.

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Diffraction

DiffractingObject

If an ultrasound beam passes

an obstacle within a distanceof 1 or 2 wavelengths, itsdirection of propagation is

deflected by diffraction asshown in the figure. Thecloser the beam is to the

diffracting object, the greaterthe deflection is.

1 or 2 wavelengths

Deflecting beam


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Scattering

Spherical Scatter-waveOccurs when small particlesabsorb part of the ultrasound

energy and re-radiate it in alldirections as a spherical field.This means that the transducer

can be positioned at any angleto the ultrasound beam and

still receive echoes back.Scattering allows reflectionsfrom objects even smaller than

the wavelength. Manybiological interfaces haveirregular surfaces, tending to

give scatter-like reflection,which is quite useful, as it will

give at least some echoes eventhough the beam is not directlyperpendicular to the reflectinginterface.

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Backscatter

Backscatter or Rayleigh scattering occurs

with structures smaller than the transmittedwavelength. Reflected energy is very low,but contributes to the texture of the image.

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CCCCcccc thngthngthngthng ssss ssssngngngng mmmm ((((acoustic parameters)))) ccccaaaamimimimi trtrtrtrngngngng

Tng tc ca USvi mi t rng phn x, tn xngc (back-scattering), khc

x, suy gim(attenuation: dotnx v hp th)

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Cc thng s sng m

Tc truyn sng (speed) of US cph thuc vo tnh n hi vmt (elasticity and density r) ca mi t rng. K- modulnn(modulus of compression).

- trong nc v m mm c= 1500 - 1600 m.s-1

- in bone c ~ 3600 m.s-1

Ph thuc c nhit (temperature) mi trng

Velocity (c) = Frequency (f) Wavelength ()

[ ]1. = smKc

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S suy gim (attenuation) of US th hin qua vic bin snggim dn trong qu trnh truyn (decrease of wave amplitudealong its trajectory). Suy gim ny ph thuc vo tn s:

Ix = Io e-2ax = .f2

Ix final intensity, Io initial intensity, 2x medium layer thickness

(reflected wave travels to and fro), - linear attenuation

coefficient (increases with frequency).

V: =log10(I0/IX)/2x

nn n v ca l dB/cm.

Ti f = 1 MHz = 1.2 vi c bp (muscle), 0.5 (gan - liver), 0.9 (no- brain), 2.5 (connective tissue - m ni), 8.0 (xng - bone)

Cc thng s sng m

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Attenuation

Attenuation of ultrasound wave occurs when it is propagating

through the medium. Loss of propagating energy will be in the

form of heat absorbed by the tissue, approximately 1 dB/cm/MHz,

or caused by wavefront dispersion (s phn tn) or wavescattering.


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Cc thng s sng m

Attenuation ofAttenuation ofAttenuation ofAttenuation of

ultrasoundultrasoundultrasoundultrasound

When expressing intensity of

ultrasound in decibels, we can

see the amplitudes of echoes(ting vng) to decreaselinearly.

xk

I

Ix

I

Ie

I

Ixxxx ,

00

2

0

log2ln ===

depth[cm]

I or P

[dB]attenuation

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Acoustic impedance (tr khng): product of US speedcand medium density

Z = . c (Pa.s/m)

Z.10-6: muscles 1.7, liver 1.65, brain 1.56, bone 6.1,

water 1.48

Acoustic parameters of medium

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We suppose perpendicular incidence of US on an interface betweentwo media with different Z - a portion of waves will pass through and aportion will be reflected (the larger the difference in Z, the higherreflection).

Acoustic parameters of medium: USreflection and transmission on interfaces

P1 Z 2 - Z 1R = ------- = ---------------

P Z2 + Z1

P2 2 Z 1D = ------- = ---------------

P Z2 + Z1

Z

P P2

1P

Z1 2

Coefficient of reflection R ratio of acoustic pressures (p sut mthanh) of reflected and incident waves

Coefficient of transmission D ratio of acoustic pressures oftransmitted and incident waves


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Near field (Fresnel area) this part of US beam is cylindrical thereare big pressure differences in beam axis

Far field (Fraunhofer area) US beam is divergent (phn k, lchng) pressure distribution is more homogeneous

Increase of frequency of US or smaller probe diameter causeshortening of near field - divergence of far field increases

Acoustic parameters of medium: Near field andfar field (trng gn v xa)

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II. Ultrasonography (Chn on hnh nh bng siu m)

Passive US low intensity waves which cannot cause substantial

changes of medium.

In US diagnostics (ultrasonography = sonography = echography) -

frequencies used are 2 - 40 MHz with (temporal average, spatial peak)

intensity of about 1 kW/m2

Impulse reflection method (phn x xung in): a probe with one

transducer (b chuyn i) which is source (b pht sng US) as well

as detector (u thunhn tnhi u) of US impulses. A portion of emitted

(pht ra) US energy is reflectedon the acoustic interfaces and the

same probe then receives reflected signal. After processing, the signal

is displayed on a screen.


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Transducer

The transducer is the component which, when connected to

the ultrasound equipment, transmits the ultrasound andreceives its reflections or echoes from tissues.

Transducer is one of the most important component of theultrasound system. For more detail information, please referto System Components.

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Transducer is one of the most important component of the ultrasound system.The transducer is the component which, when connected to the ultrasound

equipment, transmits the ultrasound and receives its reflections or echoesfrom tissues.

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Matching Layer

Transducer

Crystal

Tissue

Impedance Matching

TransducerCase

-To transmit as much power as possible from transducer to the tissue.


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Ultrasonography



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Ultrasonography


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Ultrasonography


Main parts of the US apparatus (thitb):

Common to diagnostics and therapy

probe with electroacoustic transducer

(transducers)generator of electric oscillations

(continuous, pulsed)

Special parts of diagnostic apparatus

electronic circuits for processing ofreflected signal

display unit

recording unit

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Ultrasonography

A-mode one-dimensional

Distances between reflecting interfaces and the probe are

shown (cho bit khong cch gia cc mt phn x tipgip)

Reflections from individual interfaces (boundaries of

media with different acoustic impedances) are representedby vertical deflections( lchng lch theophng tr c y) of base line, i.e. the echoes.

- Echo amplitude is proportional to the intensity of reflected

waves (amplitude modulation)

- Distance between echoes shown on the screen is

proportional to real distance between tissue interfaces.Today used mainly in ophthalmology (nhn khoa).

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Ultrasonography

A-mode one-dimensional

IH1


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Slide 48

IH1 olej se ji dvno nepouv, doporuuji opravit na gel filmIvo Hrazdira, 11/15/2008


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A tomogram (phng php chp ri) is depicted basedon the following principle:

- Brightness of points on the screen represents intensityof reflected US waves (brightness modulation).

- Static B-scan: a cross-section (ct ngang) image ofexamined area in the plane given by the beam axis anddirection of manualmovement (di chuyn bng tay) ofthe probe on body surface. The method was used in 50

and 60 of 20th century

Ultrasonography

B-mode two-dimensional

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Ultrasonography

B-mode two-dimensional - static

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Ultrasonography M-mode

One-dimensional static B-scan shows movement of reflecting

tissues. The second dimension is time in this method.

Static probe detects reflectionsfrom moving structures. The bright

pointsmove verticallyon the screen, horizontal shiftingof the record

is given by slow time-base.

Displayed curves represent movementof tissue structures

chest wallchest wallchest wallchest wall

((((llllngngngngngngngngcccc))))

LungsLungsLungsLungs((((phphphphiiii))))

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Ultrasonography

Comparison of A-, B- and M-mode principle


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Repetitive formation (s hnh thnh/sp xp lp li) ofB-mode images of examined area by fast deflection( lch nhanh) of US beam mechanically (in the past)or electronically in real time today.

Electronic probes consist of many piezoelectrictransducers which are gradually activated (s dngtng bc mt).

Ultrasonography

B-mode - dynamic

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Ultrasound probes for dynamic B-mode: electronic and

mechanical (history), sector (hnh qut) and linear (tuyn tnh).

Ultrasonography B-mode - dynamic

Abdominal cavity (khongt rng, l hngtrongbng) isoftenexamined byconvex probe (u d li) a combinationof a sector and linearprobe.

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Modern ultrasonography - digital processing of image

Analogue part detection system

Analogue-digital converters (ADC)

Digital processing of signal possibility ofprogramming (preprocessing, postprocesssing), image storage

(floppy discs, CD, flash cards etc.)


MEMORY

MEMORY

MEMORY

MEMORY

samplingsamplingsamplingsampling

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Degree of reflectivity (mc phn x) echogenity( vng). The images of cystic (nang/u nang, liquid-filled)and solid structures are different. According to the intensityof reflection we can distinguish structures:

- hyperechogenic, izoechogenic, hypoechogenic,anechogenic.

Solid structures acoustic shadow (vng ti m)(caused by absorption and reflection of US)

Air bubbles and other strongly reflecting interfacescause repeating reflections (reverberation s di m,comet tail ui sao chi).

Ultrasonography

Basic characteristics of US images

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Acoustic shadow caused byabsorption and reflection of US

by a kidney stone (si thn,see the arrow)

Hyperechogenic area below acyst (nang/u nang) (lowattenuation of US duringpassage through the cystcompared with the surroundingtissues see the arrow)

Ultrasonography

Si thn

Nang thn

IH3

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Limitation! absorption of US increases with frequency ofultrasound = smaller penetration depth

Ultrasonography

Spatial (v khng gian)resolution of US imagingsystem is determined bythe wavelength of theUS. When the objectdimension is smaller thanthis wavelength only

scattering occurs.Hence higher spatialresolution requires higherfrequencies

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Higher frequency ultrasound gives better resolution, but attenuationin the tissue also increases with increased frequency. Therefore, acompromise has to be made between resolution andpenetration depth.

Compromise frequency3-5 MHz penetration in depth of about20 cm

Frequency Low High

Resolution Better

Penetration Better


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Slide 58

IH3 doplnny ipkyIvo Hrazdira, 11/15/2008


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Axial (theo trc) spatial resolution - it is given by the shortestdistance of two distinguishable structures lying in the beam axis

it depends mainly on frequency (at 3.5 MHz about 0.5 mm)

Lateral (bn/ bn) spatial resolution - it is given by the shortestdistance of two distinguishable structures perpendicularly to the

beam axis depends on the beam width Elevation (mt/mt chiu) resolution ability to distinguish two

planes (sections) lying behind or in front of the depicted tomographic(chp ri) plane it depends on frequency and beam geometry

Ultrasonography Spatial Resolution

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The best resolving power (kh nng phn gii caonht) can be found in the narrowest part (phn hpnht) of the US beam profile.

Focusing US beam is converged (hi t) at theexamined structure by means of acoustic lenses (shapesof the layer covering the transducer) or electronically.

The probes can be universal or specially designedfor different purposes with different focuses.

The position of focus can be changed in most sectorprobes.

Ultrasonography Spatial Resolution

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Typical acoustic lens with logarithmic profile

By changing the lens shape, the beam diameter and length of focal zonecan be controlled.

A transducer with an axially symmetric logarithmic acoustic lens will form anarrow axially symmetric weakly diverging US beam. A transducer with acylindrical logarithmic lens will create a "knife-like" ultrasonic beam: narrow

and weakly diverging in one direction but wide and uniform in theperpendicular direction. If the logarithmic surface of the lens is attached to the piezoelement and the

front (radiating) surface is flat the probe can be used as both an immersionand a contact transducer.

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Ultrasonography

Interventional sonography (chn on c can thip)

Interventional sonography is used mainly for guidingpunctures (chch/chch l):

- diagnostic (chn on) thin needle (kim tim)punctures to take tissue samples for histology (tin s cam)

- therapeutic (cha bnh) for aspiration (ht) of a cyst(nang) or an abscess (p-xe) content or an exudate (chtdch), etc.

Puncture can be done by free hand the probe is next tothe puncture site or the puncture needle is guided by aspecial probe attachment.


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UltrasonographyUltrasonographyUltrasonographyUltrasonography

Echocontrast agents (Cc cht tng tng phn)

- increase echogenity ( vng) of streaming blood, gasmicrobubbles(mainly air or volatile

hydrocarbons)

- free- enclosed inbiopolymer

envelope

A SEM micrograph ofencapsulatedechocontrast agent

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UltrasonographyUltrasonographyUltrasonographyUltrasonography

Echocontrast agents - application

Enhanced demarcation (ranh gii) of heart ventricle (tmtht) after application of the echocontrast agent

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An impulse (xung in) with basic frequencyf0 is emitted into the tissue. The receiver,however, does not detect the reflected USwith this same frequency but with the secondharmonic frequency 2f0. Its source is tissue

itself (advantage in patients ). The method isalso used with echocontrast agents sourceof the second harmonic are oscillatingbubbles. Advantageous when displayingblood supply of some lesions (vt thng).

Conventional (left) and

harmonic (right) images of akidney with a stone.

Ultrasonography

Harmonic imaging (to nh iu ha)

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- The probe is linearly shifted, tilted or rotated.The data about reflected signals in individual planes are stored inmemory of a powerful PC which consequently performs mathematicalreconstruction of the image.

- Disadvantages of some 3D imaging systems: relatively long timeneeded for mathematical processing, price.

Ultrasonography

Principle of three-dimensional (3D) imaging


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Four-dimensional (4D) imageThe fourth dimension is time

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III. Doppler flow measurement(o dngchy Doppler)

Hiu ng Doppler:- The Doppler effect: frequency shift of waves formed or reflected ata moving object.

- Application: can be used for detection and measurement of bloodflow, as well as, for detection and measurement of movements ofsome acoustical interfaces inside the body (foetal heart tim thai,blood vessel walls vch mch mu)

Christian. A. Doppler (1803-1853), Austrian physicistand mathematician, formulated his theory in 1842during his stay in Prague.

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perceived (nhn/thu c)frequency corresponds with

source frequency in rest (trngthi ngh)

perceived frequency is higher

when approaching (vt chuynng li gn ngun thu)

perceived frequency is lowerwhen moving away (di chuynra xa)

Doppler flow measurement

Principle of Doppler effect

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When something moves toward you, radiation emited from it has anapparently shorter wavelength, and for away moving, longerwavelength:

= change in wavelength

= wavelength

v= relative velocity (speed)

c= speed of radiation

(in this case, light)




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Application of Doppler effect

in blood flow velocitymeasurement

Moving reflector (backscatterer) = erythrocytes

(erythrocyte)



(hng cu)

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US Doppler blood flow-meters (my o lu lng mu):are based on the difference between the frequency ofultrasound (US) waves emitted by the probe and those

reflected (back-scattered) by moving erythrocytes.

The frequency of reflected waves is (in comparison withthe emitted waves):

- higher in forward blood flow (towards the probe)

- lower in back blood flow (away from the probe)

The difference between the frequencies of emitted andreflected US waves is proportional to blood flow velocity.


Principle of blood flow measurement

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General principle of blood flow measurement

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1) Calculation of Doppler frequency change fd2) Calculation of reflector (erythrocytes) velocity v

1) 2)

fv- frequency of emitted US waves

- angle made by axis of emitted US beam and the velocityvector of the reflector

c US speed in the given medium (about 1540 m/s in blood)

c

vff

vd

cos2 = cos2

= v

d

f

cf

v



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Dependence of velocityoverestimation on theincidence angle (if thedevice is adjusted fora = 0, i.e. cosa = 1)

a - angle made by axis of emitted

US beam and the velocity vector of

the reflector


Angle alpha

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1) Systems with continuous wave CW. They are used for measurement onsuperficial (trn b mt) blood vessels. High velocities of flow can bemeasured, but without depth resolution. Used only occasionally.

2) Systems with pulsed wave - PW. It is possible to measure blood flow withaccurate depth localisation (v trchnh xc). Measurement of high velocitiesin depths is limited.


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The probe has only one transducer which acts alternately asemitter and receiver.

The measurement of velocity and direction of blood flow in the

vessel is evaluated in the so-called sampling volume withadjustable size and depth.

The pulse duration defines the size of the sampling volume

(this volume should involve the whole diameter of the examinedblood vessel).


Systems with pulsed wave - PW

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Aliasing (s mo tn hiu) at high repetition frequencyof pulses the upper part of the spectral curve can appearin negative velocity range

- at velocity above 4m/s aliasing cannot be removed

NyquistNyquistNyquistNyquist limitlimitlimitlimit

Doppler methods

Pulse wave (PW) systems


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DUPLEX method

is a combination

of dynamic B-mode imaging (the morphology ofexamined area with blood vessels is depicted)

and the PW Doppler system (measurement of velocityspectrum of blood flow).

It allows to examine blood flow inside heart or in deep blood

vessels (flow velocity, direction and character)

Doppler methods

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Doppler methods

Scheme: sector imagewith sampling volume

DUPLEX method

Image of carotid (ng mch) withspectral analysis of blood flow velocity

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Doppler methods DUPLEX method

Placement of sampling volume (left) and the record of blood flow

velocity spectrum in stenotic (hp) a. carotis communis(ngmch/tnh mch chung) (right)

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Doppler methods

Colour Doppler imaging (nh Doppler mu)

Principle:- The image consists of black-white and colour part.- The black-white part contains information about reflectivityand structure of tissues.

- The colour part informs about movements in the examinedsection. (The colour is derived from average velocity of flow.)

The apparatus depicts distribution and direction of flowing bloodas a two-dimensional image:

- BART rule blue away, red towards. The flow away from theprobe is coded by blue colour, the flow towards the probe iscoded by red colour. The brightness is proportional to the velocity,

turbulences (s hn lon) are depicted by green patterns.


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Doppler methods

Colour Doppler imaging

Carotid bifurcation(chia/r nhnh ngmch)

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Doppler methods TRIPLEX method

A combination of duplex method (B-mode imaging withPW Doppler) and color flow mapping

Normal finding of blood flow in a. carotis communis(left) andabout 90%-stenosis (hp) of a. carotis interna(right)

IH6

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Doppler methods TRIPLEX method

stenosis

of

a. carotis

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Doppler methodsPOWER DOPPLER method

- the whole energy of the Doppler signal is utilised

- mere (phn/on) detection of blood flow only l ittle dependson the so-called Doppler incidence angle- imaging of even very slow flows (blood perfusion of tissuesand organs)- flow direction is not shown


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Slide 86

IH6 opravena chybn formulaceIvo Hrazdira, 11/19/2008


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Tissue Doppler Imaging (TDI)

Colour coding of information about velocity and direction of

movements of tissues

Velocities 1-10 mm/s

are depicted.

TDI of a. carotis

communisduringsystole (tm thu)

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Ultrasonic densitometry (o ttrng bng siu m)It is based on both the measurement of speed of ultrasound in bone

and the estimation of ultrasound attenuation in bone. In contrast to X-raymethods, ultrasound densitometry also provides information on thestructure of bone and its elastic properties:

The speed of ultrasound depends on the density and elasticity of the

measured medium. The anterior ( pha trc) area of the tibia (xngchy/xng ng chn) and the posterior ( pha sau) area of the

calcaneus (xng gt) are frequently used as places of measurement.The speed of ultrasound is given by the quotient of measured distanceand the transmission time.

Ultrasound attenuation depends on the physical properties of the givenmedium and the frequency of the ultrasound applied. For the frequencyrange 0.1 - 1 MHz the frequency dependence is nearly linear.

Attenuation is currently expressed in dB/MHz/cm.

Clinical importance: diagnostics of osteoporosis (chng long xng)

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Ultrasonic densitometry

Ultrasound measurements usedto assessbone density at the calcaneus

US advantages/disadvantages


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Advantages

By comparison with X-ray, CT, MR and other diagnostic methods,Ultrasound diagnosis, especially for soft tissues and moving organ

like heart and blood flow, has shown great advantages as following:

* Real Time Imaging (except MR)

* Non-invasive (except MR)

* Non-ionizing Radiation (except MR)

* Relatively Low Cost

* Wide Applications

* Mobility

* Flexible Imaging

* Biopsy (sinh thit)

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Limitations

Surface must be accessible to transmit ultrasound.

Skill and training is more extensive than with some other methods.

Normally requires a coupling medium to promote transfer of sound

energy into test specimen.

Materials that are rough, irregular in shape, very small, exceptionally

thin or not homogeneous are difficult to inspect. Cast iron and other coarse grained materials are difficult to inspect

due to low sound transmission and high signal noise.

Linear defects oriented parallel to the sound beam may go

undetected.

Reference standards are required for both equipment calibration,and characterization of flaws.

Patient Safety: reducing UltrasoundDoses

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Prudent use of Ultrasound

US is non-ionising BUT since many bioeffects ofultrasound have not yet been studied fully, prudent

(cn trng) use is recommended

ALARA as low as reasonably achievable (exposure)

In practice prudent = justification (cn chnh) +optimisation (ti u)


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Biological Effects

Possible bioeffects: inactivation of enzymes, altered (bin i) cellmorphology, internal haemorrhage (chy mu/xut huyt trong), freeradical formation

Mechanisms of bioeffects:

Mechanical effects

Displacement and acceleration of biomolecules

Gas bubble cavitation (to l hng) (stable and transient vnhvin v tm thi)

Elevated tissue temperatures (absorption of ultrasound andtherefore increase in temperature high in lungs, less in bone, leastin soft tissue)

All bioeffects are deterministic with a threshold (cavitation) or withoutit (heating).

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Output Power from Transducer

varies from one machine to another

Increases as one moves from real-time imaging tocolour flow Doppler

M-mode output intensity is low but dose to tissue ishigh because beam is stationary

99

Risk Indicators

To avoid potentially dangerous exposures, two indices were

introduced: TI and MI. Their values (different for different organs)are often displayed on device screens and should not be

exceeded.

Thermal Index (TI): TI = possible tissue temperature rise if

transducer is kept stationary

TIS: soft tissue path

TIB: bone near focus of beam

TIC: Cranium (near surface bone)

Mechanical Index (MI): measure of possible mechanical bioeffects

100

More on the TI and MI

Thermal index device power divided by the power that wouldincreased the temperature by one degree under conditions ofminimum heat loss (without perfusion).

Mechanical index (for assessment of cavitation-conditioned

risk, increased danger when using echocontrast agents):


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Justification

No commercial demos on human subjects

No training on students

No see baby just for fun or excessive screening inobstetrics

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Optimisation of Dose 1

Minimise TI and MI and use appropriate index (TIS, TIB,TIC), care in cases when these underestimate

Check acoustic power outputs on manual

Use high receiver gain when possible as opposed to hightransmit power

Start scan with low transmit power and increasegradually

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Optimisation of Dose 2

Avoid repeat scans and reduce exposure time

Do not hold transducer stationary

Greater care when using contrast agents as these increasethe possibility of cavitation

Exceptional care must be taken in applying pulsed Doppler inobstetrics

Regular quality control of the ultrasound device

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Other applications

Some of the applications for which ultrasonic testing may be

employed include:

Flaw detection (cracks, inclusions, porosity, etc.)

Erosion & corrosion thickness gauging

Assessment of bond integrity in adhesively joined and brazedcomponents

Estimation of void content in composites and plastics

Measurement of case hardening depth in steels

Estimation of grain size in metals


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Thickness Gauging

Ultrasonic thickness gauging isroutinely utilized in thepetrochemical and utilityindustries to determine variousdegrees of corrosion/erosion.

Applications includepiping systems,storage andcontainment facilities,and pressure vessels.

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Flaw Detection De-laminations

Contact, pulse-echo inspection for delaminations on 36rolled beam.

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Flaw Detection in Welds

One of the most widely usedmethods of inspecting

weldments is ultrasonic

inspection.

Full penetration groove welds

lend themselves readily toangle beam shear wave

examination.

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Applications: Equipment

Equipment for ultrasonic testing is very diversified. Proper

selection is important to insure accurate inspection data as

desired for specific applications.

In general, there are three basic components that comprisean ultrasonic test system:

- Instrumentation

- Transducers

- Calibration Standards


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Transducers

Transducers are manufactured in a variety of forms,shapes and sizes for varying applications.

Transducers are categorized in a number of ways whichinclude:

- Contact or immersion

- Single or dual element

- Normal or angle beam

In selecting a transducerfor a given application, itis important to choose thedesired frequency,

bandwidth, size, and in some cases focusingwhich optimizes the inspection capabilities.

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Instrumentation

D-meters or digital thicknessgauge instruments provide theuser with a digital (numeric)readout.

They are designed primarily for

corrosion/erosion inspectionapplications.

Some instruments provide the user with both a digital readout anda display of the signal. A distinct advantage of these units is thatthey allow the user to evaluate the signal to ensure that the digitalmeasurements are of the desired features.

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Flaw detectors are instruments

designed primarily for theinspection of components for

defects.

However, the signal can be

evaluated to obtain other

information such as materialthickness values.

Both analog and digital display.

Offer the user options of gating

horizontal sweep andamplitude threshold.

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