Doppler Basics

7/31/2019 Doppler Basics

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ULTRASOUND AND

DOPPLER


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*Sound waves are longitudinal waves.

* The term longitudinal wave means that, the motion of particles in the

medium is parallel to the direction of wave propagation.


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Velocity of sound is independent of frequency & depends primarily on

Physical make up of the material through which sound is beingtransmitted.

Imp characteristics of transmitting mediumare 1.COMPRESSIBILITY2. DENSITY

GAS LIQUID SOLID


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Velocity of sound in some Biological MaterialsMaterial Velocity of Sound Impedance (Rayl x 10 -6)

Air 330 0.0004Fat 1450 1.38

Water 1480 1.48Average Human ST 1540 1.63

Brain 1540 NALiver 1550 1.65

Kidney 1560 1.62Blood 1570 1.61Muscle 1580 1.7

Lens of eye 1620 NASkull Bone 4080 7.8


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*Ultrasound by definition has a frequency of greater than20,000 cycles per sec.

* Audible sound has a frequency between 15 20,000 cycles/se

* The sonic beams that we use in diagnostic imaging have

frequencies from 10,00,000 to 20,00,000 cycles per sec.


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*Transducer is a device that can convert one form of energyinto another.

* Ultrasonic transducers are used to convert an electric signalinto ultrasonic energy that can be transmitted into tissues,& to convert ultrasonic energy reflected back from the tissuesinto an electric signal.


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PULSE ECHO


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Based upon thepulse-echo principle

occurring with ultrasound piezoelectriccrystals, ultrasound transducers convert:

Electricity into sound = pulse

Sound into electricity = echo


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* Piezoelectric effect Certain materials are such that ,theapplication of an electric field causes a change in their physicaldimensions, & vice versa.( first described in 1880)

The reverse of the piezoelectric effect converts the

energy back to its original form.


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* Piezoelectric crystals are made up of innumerable dipoles arrangedin a geometric pattern.

*When an electric field is applied, the dipoles realign themselves &in the process there is a mild change in the dimension of the crystal.

*Voltage between the plating electrodes produces theelectric

field, which in turn causes the crystal to change

shape.


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*Piezoelectric crystals behave as a series of vibratingpoints.

*Wave fronts are not uniform close to the crystal.

*The distance at which the waves become synchronousdepends on their wavelengths.


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*Between pulses, the transducer serves as a receiver.

*Commonly used rate is1000 pulses /sec.( range of between500 3000)

*At this rate the total time available for each pulse is 0.001

sec.

Approx one millionth of a sec is devoted to transmission,sothe transducer is a receiver almost thousand times

longer than it is a transmitter.


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*Intensity of ultrasound varies along the length of the beam.

*

Parallel component is near zone or Fresnel zone.* Diverging portion of the beam is far zone or fraunhoferzone.

Near zone increases in length with increasing frequency. Near zone increases in length with larger transducers.


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High frequency

Depth resolution is better,Fresnel zone is longer

Less penetration

Tissue absorption increases with increasing frequency

Low frequency

More penetration


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1.Reflection

2.Refraction

3. Absorption


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Specular reflection is responsible for bright appearance of boundariesbetween tissues. It occurs at tissue interfaces

Scatter gives rise to characteristic echo texture of image. Occurs atsmall boundaries that occur within tissues.


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Bending of waves as they pass from one medium to anotheris called refraction


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Absorption in ultrasound is a result of frictional forces,that oppose the motion of particles in the medium.


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Data is stored

/ converted

For display

Digital

AMPLIFIERPULSE GENERATOR

DISPLAY


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The frequency, f, is the number of cycles of displacements

passing through a point in the medium during 1 second (s)

The unit of frequency is the hertz (Hz), with 1 Hz being one

complete cycle per second


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PULSE =Set Of

Frequencies


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Acoustic Impedance

The acoustic impedance of a medium

is the impedance (similar to

resistance) the material offersagainst the passage of the sound

wave through it and depends on the

density and compressibility of the

medium


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Beam shape

The shape of the ultrasound beam

produced by a transducer will depend on

the shape of the element(s), on the

transmitted frequency and on whetherthe beam is focused.

The shape of the beam will affect the

region of tissue that will be insonatedand from which returning echoes will be

received.

INTERACTION OF


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INTERACTION OFULTRASOUND WITH

SURFACES When the ultrasound beam meets aboundary between two media,some of the ultrasound will be

transmitted

some will be reflected. A:

When the two media have similaracoustic impedances, the majorityof the ultrasound will be transmittedacross the boundary. B:

When the two media have differentacoustic impedances, most of theultrasound will be reflected.


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Specular reflections

Specular reflections occur at large smooth

interfaces (A), whereas ultrasound is

scattered by rough surfaces (B) and small

structures (C).


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Learning to use knobs effortlessly is an important part ofthe art of ultrasonic scanning.


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GAIN

Controls the degree of echo amplification or brightness of image

ZOOM


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TIME GAIN COMPENSATION ( TGC)

Attempts to compensate for acoustic loss by absorption,reflection & scatter & to show structures of same acousticstrength with the same brightness no matter what thedepth.

Operator controlled adjustment tocompensate for the attenuation ofsound as it travels into the tissue


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DYNAMIC RANGE

Refers to range of intensities from the largest to the

smallest echo that a system can display.

60 db 30 db


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CALIPERS

DEPTH


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*Artifacts related to instrumental problems

*Technique dependant artifacts

*Artifacts due to the way tissues affectsound.


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ARTEFACTS RELATED TO INSTRUMENTAL PROBLEMS

1. Artifactual noise

2.Calibration artifacts

2. Main Bang artifact


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3.Veiling Artifact

4.Side lobe artifact


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ARTEFACTS CAUSED BY TECHNIQUE

1. Noise

2.TGC problems3. Banding

4. Contact problem


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ARTEFACTS CAUSED BY SOUND TISSUE INTERACTIONS

1.Artefacts from strongly reflective structures

2. Enhancement artifact


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3 Mirror image artifact

4. Reverberation Artifact


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5. Comet tail artifact
http://bjr.birjournals.org/content/vol76/issue907/images/large/BJR23226-1.jpeg


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U/S CONTRAST AGENTS

microbubble contrast media - to improve

ultrasound signal backscatteris known

as contrast-enhanced ultrasound. This

technique is currently used inechocardiography, and may have future

applications in molecular imaging and

drug delivery
http://en.wikipedia.org/wiki/Backscatterhttp://en.wikipedia.org/wiki/Contrast-enhanced_ultrasoundhttp://en.wikipedia.org/wiki/Echocardiographyhttp://en.wikipedia.org/wiki/Echocardiographyhttp://en.wikipedia.org/wiki/Contrast-enhanced_ultrasoundhttp://en.wikipedia.org/wiki/Contrast-enhanced_ultrasoundhttp://en.wikipedia.org/wiki/Contrast-enhanced_ultrasoundhttp://en.wikipedia.org/wiki/Backscatter


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3D ULTRASOUND

3D ultrasound medical ultrasound technique pregnancy, providing three dimensional images of

the fetus.

Often these images are captured rapidly andanimated to produce a "4D ultrasound".

In 3D fetal scanning, however, instead of thesound waves being sent straight down andreflected back, they are sent at different angles.The returning echoes are processed by a highlysophisticated computer program resulting in a

reconstructed three dimensional volume image offetus's surface or internal organs; allowing one tosee width, height and depth of images in much thesame way as 3D movies but no movement isshown.
http://en.wikipedia.org/wiki/Medical_ultrasoundhttp://en.wikipedia.org/wiki/Pregnancyhttp://en.wikipedia.org/wiki/Three-dimensional_spacehttp://en.wikipedia.org/wiki/Fetushttp://en.wikipedia.org/wiki/Sound_waveshttp://en.wikipedia.org/wiki/3-D_filmhttp://en.wikipedia.org/wiki/3-D_filmhttp://en.wikipedia.org/wiki/Sound_waveshttp://en.wikipedia.org/wiki/Fetushttp://en.wikipedia.org/wiki/Three-dimensional_spacehttp://en.wikipedia.org/wiki/Pregnancyhttp://en.wikipedia.org/wiki/Medical_ultrasound


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3D Ultrasound was first developed by

Olaf von Ramm and Stephen Smith at

Duke University in 1987.[2]

4D baby scans are similar to 3D scansexcept that they show fetal movement.


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US - Advantages

Excellent soft tissue

contrast resolution

Dynamic

No radiation

Safe in pregnancy

Available, cheap


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OTHER USES

The ability to stimulate bone-growth

Potential to disrupt the blood-brain barrierfor drugdelivery.

Ultrasound is used in UAL (= ultrasound-assisted

lipectomy), orliposuction.

Doppler ultrasound is being tested for use inaiding tissue plasminogen activatortreatment instroke sufferers. This procedure is calledUltrasound-Enhanced Systemic Thrombolysis.

Low intensity pulsed ultrasound is used fortherapeutic tooth and bone regeneration.

A ti T t d D
http://en.wikipedia.org/wiki/Blood-brain_barrierhttp://en.wikipedia.org/wiki/UALhttp://en.wikipedia.org/wiki/Lipectomyhttp://en.wikipedia.org/wiki/Liposuctionhttp://en.wikipedia.org/wiki/Tissue_plasminogen_activatorhttp://en.wikipedia.org/wiki/Strokehttp://en.wikipedia.org/wiki/Ultrasound-Enhanced_Systemic_Thrombolysishttp://en.wikipedia.org/wiki/Low_intensity_pulsed_ultrasoundhttp://en.wikipedia.org/wiki/Low_intensity_pulsed_ultrasoundhttp://en.wikipedia.org/wiki/Ultrasound-Enhanced_Systemic_Thrombolysishttp://en.wikipedia.org/wiki/Ultrasound-Enhanced_Systemic_Thrombolysishttp://en.wikipedia.org/wiki/Ultrasound-Enhanced_Systemic_Thrombolysishttp://en.wikipedia.org/wiki/Strokehttp://en.wikipedia.org/wiki/Tissue_plasminogen_activatorhttp://en.wikipedia.org/wiki/Liposuctionhttp://en.wikipedia.org/wiki/Lipectomyhttp://en.wikipedia.org/wiki/UALhttp://en.wikipedia.org/wiki/Blood-brain_barrierhttp://en.wikipedia.org/wiki/Blood-brain_barrierhttp://en.wikipedia.org/wiki/Blood-brain_barrierhttp://en.wikipedia.org/wiki/Acoustic_Targeted_Drug_Deliveryhttp://en.wikipedia.org/wiki/Acoustic_Targeted_Drug_Delivery


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Acoustic Targeted DrugDelivery

High frequency ultrasound (from 1 MHz to

10 MHz)

intensities from 0-20 watts/cm2.

The acoustic energy is focused on thetissue of interest to agitate its matrix and

make it more permiable to therapeutic drugs

Enhanced drug uptake
http://en.wikipedia.org/wiki/Acoustic_Targeted_Drug_Deliveryhttp://en.wikipedia.org/wiki/Acoustic_Targeted_Drug_Deliveryhttp://en.wikipedia.org/wiki/Acoustic_Targeted_Drug_Deliveryhttp://en.wikipedia.org/wiki/Acoustic_Targeted_Drug_Delivery


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DISADVANTAGES Trouble penetrating bone.

Performs very poorly when there is a gasbetween the transducer and the organ ofinterest, due to the extreme differences inacoustic impedance.

The depth penetration of ultrasound is limited,making it difficult to image structures deep in thebody, especially in obese patients.

The method is operator-dependent.
http://en.wikipedia.org/wiki/Bonehttp://en.wikipedia.org/wiki/Impedancehttp://en.wikipedia.org/wiki/Impedancehttp://en.wikipedia.org/wiki/Bone


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SIDE EFFECTS Ultrasound energy produces a mechanical pressure

wave through soft tissue.

This pressure wave may cause microscopic bubbles inliving tissues, and distortion of the cell membrane,

influencing ion fluxes and intracellular activity.

When ultrasound enters the body, it causes molecularfriction and heats the tissues slightly. This effect is veryminor as normal tissue perfusion dissipates heat.

With high intensity, it can also cause small pockets ofgas in body fluids or tissues to expand andcontract/collapse in a phenomenon called cavitation


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Doppler ultrasound

Creation of a color flow image

Blood flow and its appearance on

color flow imaging

Spectrum


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Doppler effect

The Doppler effect is the change in the

observed frequency due to the relativemotion of the source and the observer

Austrian physicist named Christian Doppler in 1842.


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DOPPLER EFFECT


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A Doppler transducer placedon the skin and aimed at anangle, , towards a bloodvessel, which contains blood

flowing with a velocity of um/s, at any instant.

The transducer emitsultrasound waves offrequency, fo, and echoes

generated by movingreflectors in the blood, e.g. redblood cells, have a frequency,fr.

The difference between these

two frequencies, f, is relatedto the velocity of the flowingreflectors throught thefollowing equation:
http://upload.wikimedia.org/wikibooks/en/4/4b/DopplerEq1.gifhttp://upload.wikimedia.org/wikibooks/en/9/92/DopplerBloodFlowDiag.gif


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The detected Doppler shift frequency changes

as the angle of insonation changes.


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The Doppler shift frequency can then be

extracted from the received signal by a

process known as demodulation.

Here, the received signal is multiplied by

the transmitted signal and the product is

filtered to remove the high frequencies,thus providing the Doppler shift

frequency.


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The received signal has a different frequency

from the transmitted frequency, owing to the

Doppler effect, and a lower amplitude, owingto attenuation of the signal by overlying

tissue.

As mentioned earlier, once the Doppler shift

frequency has been extracted (by

demodulation) and amplified, it can simply be

output to a loudspeaker or investigated using

a spectrum analyzer


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Demodulation

This is used to

extract the

Doppler

frequency


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The velocity of the blood cells will vary

with time, owing to the pulsatile nature

of arterial blood flow.

This means that the Doppler shift signal

obtained from flowing blood will contain

a range of frequencies, due to the rangeof velocities present, and the frequency

content will vary with time.


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Color flow

Overall view of flow in a region

Limited flow information

Poor temporal resolution/flow dynamics

(frame rate can be low when scanningdeep)

color flow map (diferent color maps)

direction information velocity information (high velocity & low

velocity)

turbulent flows


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EFFECT OF PRF


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Spectral Doppler

Examines flow at one site

Detailed analysis of distribution of flow

Good temporal resolution can examine

flow waveform

Allows calculations of velocity and

indices


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Spectral analysis can be used to break downthe Doppler signal into its component

frequencies and to show how these componentfrequencies vary with time.

a spectrum is displayed, with time along the

horizontal axis and the Doppler shift frequencyalong the vertical axis.

The third axis, the brightness of the display,shows the back-scattered power of the signal ateach frequency (i.e., the proportion of the bloodcells moving at a particular velocity).


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Continuous wave (CW) Doppler


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Continuous wave (CW) Doppler

Continuous wave (CW) Doppler continuously emits a single

frequency while the receiving element continuously detectsany echoes from the sensitive region of the beam (i.e., wheretransmitted and received beams overlap)

This region usually covers a depth of a few centimeters, andany flow within this area will be detected.

unable to provide information about the depth from which theDoppler signal is returning.

CW Doppler is therefore said to have poor range resolution.

Veins often lie adjacent to arteries and so, in many cases, theCW Doppler will simultaneously detect arterial and venousflow.


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Pulsed Doppler


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Pulsed Doppler

The poor rangeresolution of CWDoppler can beovercome byusing a pulse ofultrasound energyand only acquiringthe returningsignal at a known

time after thepulse has beentransmitted.

pulse of ultrasound is transmitted and the

receiver then waits a given time before

acquiring the signal over a short period of

time.


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Power/energy/amplitude flow

Sensitive to low flows

No directionalinformation in some

modes

Very poor temporal

resolution Susceptible to noise


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APPLICATIONS


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Doppler Basics