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Doppler physics by Dr Nani
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Doppler physics and artifacts
Student at Aligarh Muslim University
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Doppler physics by Dr Nani
- DOPPLER PHYSICS By Dr Nani Lampung Junior Resident Dept of Radiodiagnosis JNMCH AMU ALIGARH
- PRINCIPLE OF DOPPLER ULTRASOUND • BASIS – ‘ DOPPLER EFFECT’ • DISCOVERY – Physicist Christian Johann Doppler in 1842 • ‘When a sound source and the reflector are moving toward each other, the sound waves are spaced closer together and reach the reciever at a higher frequency than they were originally emitted ’.
- • Doppler shift isthedifference between thetransmitted and received frequencies • Transmitted and received Frequenciesarein theMHz range Doppler shift frequenciesoften in audiblerange DOPPLER SHIFT
- WHENRECEIVED FREQUENCY= TRANSMITTED FREQUENCY, NO DOPPLERSHIFT POSITIVE NEGATIVE
- • Relationship between Doppler shift (or just Doppler) frequency, FD and reflector velocity, v: fo istheultrasound frequency, or thetransmitted beam frequency. v isthereflector velocity (m/s; cm/s) q istheDoppler angle c isthespeed of sound DOPPLEREQUATION c cosθv2f F o D =
- Effect of Doppler angle on frequency shift c cosθv2f F o D =
- DOPPLER ULTRASOUND MODES/INSTRUMENTATIONS ContinuouswaveDoppler Pulsed waveDoppler Spectral doppler Colour doppler Power doppler
- CONTINUOUSWAVE DOPPLER Simplest form Usesseparatetransmit & receivecrystalsthat continuously transmit and receiveultrasound. Ableto detect thepresenceand direction of flow, - unableto distinguish signalsarising from vesselsat different depths. ( THEREFORE IT LACKSRANGE RESOLUTION.) Portableand inexpensive. Uses- CARDIAC SCANNERS( HIGH VELOCITIESIN AORTA ), bedsideto ascertain flow in superficial vessels.
- CONTINUOUSWAVE DOPPLER
- • Usesbrief pulsesof ultrasound energy using only one crystal. • Theecho delay time( Te) can beconverted into distance and theDEPTH of echo sourcecan bedetermined • Thesensitivevolumefrom which flow dataaresampled can becontrolled in termsof shape, depth, and position. PULSED-WAVE (PW) DOPPLER
- DUPLEX ULTRASOUND SCANNING Duplex ultrasound instrumentsarereal-timeB-modescanners with built-in Doppler capabilities. ● B-mode Imager( Outlineanatomic structures) ● Pulsed-Doppler ( Flow and movement patterns)
- PULSED WAVE DOPPLER PULSEREPETITION FREQUENCY ( PRF)- No. of pulses transmitted per second • With increasing scanning depth PRF decreasesasmoretimeis needed for theechoesto return • At aminimum, thePRF must beat least twice the frequency of theDoppler signal to construct thesignal successfully. • Thissetsupper limit to theflow velocitiesto beaccurately recorded • Usehigher PRF setting for high flow velocities, low PRF for slow venousflow
- A (PRF = 700 Hz) B (PRF = 4500 Hz)
- Continuous wave (CW) Doppler Pulsed wave (PW) Doppler
- SPECTRAL DOPPLER • Ultrasound isemitted in pulsessimilar to B-mode • Gate is used to determine the interval after emission when returning signalsarereceived and thereforethedepth from which thesampleistaken. • The Doppler shift data are displayed in graphic form as a time-varying plot of the frequency spectrum of the returning signal. • A fast Fourier transformation is used to perform the frequency analysis.
- • SPECTRAL ANALYSIS shows the Doppler shift spectrum and direction ,usually displayed as the maximum velocity rather than frequency • ProvidesQUANTITATIVE INFORMATION
- Theresulting Doppler frequency spectrum displaysthefollowing : Variation with time of the Doppler frequencies present in thevolumesampled. The envelope of the spectrum, representing the maximum frequencies present at any given point in time. Thewidth of the spectrum at any point, indicating therangeof frequenciespresent.
- SPECTRAL DOPPLER
- SPECTRAL DOPPLER
- DOPPLER SPECTRUM ASSESSMENT Assess the following 1. Presence of flow 2. Direction of flow 3. Amplitude 4. Window 5. Pulsatility
- DOPPLER SPECTRUM ASSESSMENT
- DOPPLER SPECTRUM ASSESSMENT • Increasepower or gain • Decreasethevelocity scale • Decreasing thereject or filter • Slowly increasing theSV size SENSITIVITY IMPROVEMENT
- DOPPLER SPECTRUM ASSESSMENT DIRECTION OF FLOW • Pulsed doppler usequadraturephasedetection to providebidirectional doppler information FLOWCAN EITHERBE • Mono-phasic • Bi-phasic • Tri-phasic • Bidirectional
- DOPPLER SPECTRUM ASSESSMENT
- DOPPLER SPECTRUM ASSESSMENT
- DOPPLER SPECTRUM ASSESSMENT
- DOPPLER SPECTRUM ASSESSMENT
- AMPLITUDE • Thespectrum displaysecho amplitude by varying the brightnessof thedisplay Theamplitudeof theechoesaredetermined by • Echo intensity • Power • Gain • Dynamic range DOPPLER SPECTRUM ASSESSMENT
- WINDOW Recieved doppler shift consistsof arangeof frequencies Narrow rangeof frequencieswill result in anarrow display line. Theclear underneath thespectrum iscalled thewindow DOPPLER SPECTRUM ASSESSMENT
- SPECTRUM BROADENING - Loss of spectral window is called SPECTRAL BROADENING OCCURS- • As the blood decelerates in diastole • SV placed close to the vessel wall • In small vessels ( parabolic velocity profile )
- SPECTRAL BROADEDING • Tortousvessels • Low flow states • Excessivegain/power/dynamic range ITISHALLMARKOFDISTURBEDAND/OR TURBULENTFLOW
- PULSATILITY • Measuresthedifferencebetween themaximum and minimum velocitieswithin thecardiac cycle • Indicesareunit less • All increasein valueasflow pulsatility increases • Can bemeasured without knowledgeof thedoppler angle DOPPLER SPECTRUM ASSESSMENT
- DOPPLERINDICES • PI = S-D/Vm ( GOSLING ) • RI = S-D/S( POURCELOT ) • S/D Ratio • Acceleration time( AT) & Acceleration Index ( AI) • SPECTRAL BROADENING
- COLOUR DOPPLER • Based on pulsed doppler technique. • Doppler shifts- converted to colour and the moving blood isdisplayed in colorsthat correspond to itsvelocity and direction.
- COLOUR DOPPLER • PositiveDoppler shiftsareencoded asred and negative shiftsareencoded asblue. • Velocity of theflow isrepresented in shadesof color - faster - brighter
- COLORDOPPLER. B A
- COLOR BOX • Operator adjustableareawithin the USimage • AffectsImageresolution & quality ( changein box size)- Framerate decreaseswith increasing box size • Assmall & superficial aspossible • Deep color box – slower PRF resultsaliasing COLORDOPPLER.
- COLOR BOX
- COLORDOPPLER.
- COLOUR DOPPLER Advantages It providesan overall view of flow in organ or structure Providesdirectional information about flow, Providesvelocity information about flow and shows turbulent typeflow Limitations • SEMI QUANTITATIVE • Angledependence • Aliasing • Artifactscaused by the noise • Poor temporal resolution
- Dopplerfrequency spectrum display A, Color Doppler maging B, Doppler frequency spectrum waveform B A
- SPECTRAL DOPPLER • DEPICTION OF DOPPLER SHIFT INFORMATION IN WAVEFORM COLOUR DOPPLER • UTILIZE DOPPLER SHIFT INFORMATION TO SHOW BLOOD FLOW IN COLOR
- SPECTRALDOPPLER ADVANTAGES • depictsquantitaiveflow at onesite • allowscalculationsof velocity and indices • good temporal resolution COLORDOPPLER ADVANTAGES • Overall view of flow • Directional information about flow • Averaged velocity information about flow
- POWER DOPPLER • Also known asEnergy Doppler / AmplitudeDoppler. • In thismode, colour isassigned to thepower /strength/ energy of theDoppler signal rather than theDoppler frequency shift. • Flow isusually displayed with onecolour USES • To detect very slow flow, flow in small vessels, or where transducer angling isawkward. • Power Doppler can beused in conjunction with contrast agentsof varying brightness
- POWER DOPPLER
- POWER DOPPLER Advantages Angleindependent No aliasing Improved signal to noiseratio Moresensitiveto detect low flow Better ableto define boundaries Limitations No directional information Poor temporal resolution (dueto relatively low PRF ). Dueto thelow PRF, power Doppler isparticularly sensitiveto flash artefact
- OPTIMISATION OFCOLORFLOW DOPPLEREXAMINATION 1. TransducerFrequency. For superficial structures 7-10MHZ isused. For deep abdominal structures– 3MHZ – 5MHZ isoptimal. Choiceof transducer frequency isparamount becausethe intensity of thescattered sound variesin proportion to the 4th power of theDoppler frequency.
- OPTIMISATION OFCOLORFLOW DOPPLEREXAMINATION 2. DopplerAngle. • Strongest signalsof Doppler resultswhen themotion isparallel to beam. • A Doppler angleof 900 doesnot display flow becauseno doppler shift detected.
- OPTIMISATION OFCOLORFLOW DOPPLEREXAMINATION 3. Sample Volume. • Ideal samplevolume– 2/3rd of thevessel width positioned in thecenter of thevessel. • If samplevolumeismore: - Spectral broadening • If samplevolumeisless: Measured velocity is too low.
- OPTIMISATION OFCOLORFLOW DOPPLEREXAMINATION 4. Wall Filters. • Cut off of thelow frequency noises, a cleaner high velocity blood flow signal is displayed. • If set too high theblood flow is discarded, if set low noisewill bemore. 5. DopplerGain. • Controlstheamplitudeof thecolour display in colour or power Doppler mode & thespectral display in pulseDoppler mode. Excess gain Proper gain Insufficent gain PSV = 75 cm/sec 60 cm/sec 50 cm/sec
- OPTIMISATION OFCOLORFLOW DOPPLEREXAMINATION 6. Velocity scale. • Controlstherangeof frequenciesdisplayed • Too high scale- dynamic rangeistoo largeand low velocity signalsaremissed simulating an areaof thrombosis • If scaleistoo low thedynamic rangeistoo small to display thehigh velocity signalsaccurately resulting in aliasing.
- DOPPLER ARTIFACTS
- • ALIASING An inaccuratedisplay of colour or spectral Doppler velocity when thevelocity rangeexceedsthescale availableto display it. Nyquist sampling rate- PRF = 2 x fD DOPPLER ARTIFACTS
- How to reduce ALIASING ? • Drop thebaseline • Increasetheavailablevelocity range. • DecreasetheDoppler frequency shift by using alower insolating frequency or • Increasing theDoppler angle.
- • PowerDopplerhas no aliasing becauseit hasno directional or velocity component. Advantage: - • 1. useful for localizing thehighest velocity region. • 2. It isused in identifying theabnormal areain TIPSS & in localizing AV fistulae. Disadvantage:- high velocity may not beaccurately measured.
- Blooming Artifact ( Colour Bleed ). • Caused by abnormally high gain settings. • Causes the obscuration of thrombus or plaques in the vessel. - Also seen with ultrasound contrast agents DOPPLER ARTIFACTS
- DOPPLER ARTIFACTS Flash Artifact • Manifestsasacolor signal dueto transducer or patient motion • It isseen in left lobeof liver dueto cardiac pulsation and in hypoechoic areassuch as cystsand fluid collections.
- DOPPLER ARTIFACTS MirrorImage artifact : • displays objects on both sides of a strong reflector, • The reflectors (diaphragm, pleural surface and aortic wall) directs some of the echoes to a second reflector before it returns them to the transducer resulting multipath reflection. • Eg Duplication of sub clavian artery (pleura reflector)
- DOPPLER ARTIFACTS Twinkling artifact - Appearsasdescretefocusof alternating colorsbehind echogenic object e.g: Renal Calculi, bladder calcification and cholesterol crystalsin thegall bladder • FB likeiron fillings, aneurysm coil producethisartifact.
- GUIDELINESFOR AN OPTIMAL COLOUR FLOW DOPPLER EXAMINATION. • Thecolour flow box should bekept assmall & superficial as possible • Adjust thegain and filter settings • Adjust thevelocity scale(PRF) and baselineaccording to the flow conditions.
- • Obtain an optimal Doppler angleby adjusting thebeam steering and probeposition. ( 600 or less) • Adjust thepulsed Doppler samplevolumesize(gate) appropriately (2/3rd of thevelocity diameter) • AVOID TRANSDUCER MOTION.
- RECENT INNOVATIONS IN DOPPLER US TECHNIQUES • Doppler USisexperiencing technical innovationsthat also contributeto improved B-modeimaging. 1 Extended field of view imaging 2 (a) wideband, widedynamic rangesystems- improvethe sensitivity of Doppler US; (b) Viaincorporation of custom-designed integrated circuitry.- Doppler USprocessing on even handheld US scanners. • 3 Contrast agentshavebecomemuch morethan simple“echo enhancers” for Doppler instrumentation.
- REFERENCES 1. Diagnostic Ultrasound, Carol M Rumack and others, 4th ed. 2. Grainger & Allison’sDiagnostic Radiology- A Textbook of Medical Imaging, 6th ed. 3. Teaching Manual of Color Duplex Sonography,2nd ed. 4. Frederick W. Kremkau, Diagnostic Ultrasound - Principlesand Instruments, 7th Edition, St. Louis,-Missouri, SaundersElsevier, 2006 5. Rubin JM ,BudeRO CarsonPL et al ;Power Doppler US; A potentiallyuseful alternativeto to mean frequency based color Doppler US.
- THANK YOU….
Notas del editor
- The basis for determining the velocity and direction of blood flow
- Doppler effect responsible for the apparent rising pitch of an approaching ambulance siren and the falling pitch as the ambulance moves away
- This magnitude of Doppler shift is not only proportional to the frequency of the original signal but is also proportional to. Blood flow velocity. Speed of sound in human tissue. Angle of the ultrasound beam relative to the long axis of the vessel.
- Effect of Doppler angle on frequency shift. At an angle of 60 degrees, the detected frequency shift detected by the transducer is only 50% of the shift detected at an angle of 0 degrees. At 90 degrees, there is no relative movement of the target toward or away from the transducer, and no frequency shift is detected. The detected Doppler frequency shift is reduced in proportion to the cosine of the Doppler angle. Because the cosine of the angle changes rapidly at angles above 60 degrees, the use of Doppler angles of less than 60 degrees is recommended in making velocity estimates.
- USES CONTINOUS TRNSMISSION AND RECEPTON OF ULTRASOUND DOPPLER SIGNALS ARE OBTAINED FROM ALL THE VESSELS IN THE PATH OF US BEAM A pencil probe contains two piezo-crystals, one to send and the other to receive signals (Fig. 1.9). The probe usually has air backing for greater sensitivity. It operates at only one continuous frequency, the optimum being around 8 MHz.
- The probe for continuous-wave Doppler The dark-shaded segment represents the area of maximum sensitivity to receive the signal and covers multiple vessels in the field.
- TRANSDUER BOTH SENDS AND RECIEVES THE SIGNAL. RETURNED SIGNAL IS GATED SO THAT ONLY INFORMATION ABOUT THE DESIRED DEPTH IS COMPUTED. Using pulses of sound permits use of the time interval between the transmission of a pulse and the return of the echo as a means of determining the depth from which the Doppler shift arises.
- Advantage - the position of the Doppler sample can be precisely controlled and monitored. provide complementary information because the scanner can best outline anatomic structures whereas a Doppler instrument yields information regarding flow and movement patterns.
- From the deeply positioned sample volume
- Pulse repetition frequency (PRF). Depending on the color map selected, velocity of the target, Doppler angle, and PRF, a given velocity may appear as any color with color Doppler. A and B are sonograms of identical vessels. A, PRF is 700 Hz, which results in aliasing of the higher Doppler frequency shifts in the carotid artery, but permits the identification of relatively slow flow in the jugular vein. B, PRF is 4500 Hz, eliminating aliasing in the artery but also suppressing the display of the low Doppler frequencies in the internal jugular vein.
- A, Continuous wave (CW) Doppler uses separate - Although able to detect the presence and direction of flow, CW devices are unable to distinguish signals arising from vessels at different depths (green-shaded area). B, Using the principle of ultrasound ranging (see Fig. 1-4), The operator is able to control the position of the sample volume and, in duplex systems, to view the location from which the Doppler data are obtained
- The amplitude of the Doppler signal is related to the number of targets moving at a given velocity. In many instruments the amplitude of each frequency component is displayed in gray scale as part of the spectrum. The presence of a large number of different frequencies at a given point in the cardiac cycle results in spectral broadening.
- The path of the Doppler ultrasound beam, sample volume size and position are displayed on the image
- The use of an offset, or baseline, allows both forward and reverse flows to be displayed on the same spectrum (A), which can be inverted if required (B).
- These indices can thus serve as a semiquantiative parameter for the evaluation of stenoses
- most common form of Doppler ultrasound to be used for radiology applications
- Each color pixel in a color Doppler image represents the Doppler frequency shift at that point, and it cannot be used to estimate velocity. Even though the points A and B have similar color values and therefore similar Doppler frequencies, the velocity at A is much higher than at B because of the large Doppler angle at A compared to B. THE VELOCITY REPRESENTED BY A GIVEN DOPPLER FREQUENCY INCREASES IN PROPORTION TO THE DOPPLER ANGLE.
- in which all the color doppler information is displayed ( STILL PROVIDING THE NECCESARY INFORMATION )
- Color scale display – beam steering and change in angle of color box Image of a bend in a carotid artery showing flow toward and away from the transducer in different colors. The path of the flow is shown by the arrows. No flow is displayed in the center of the image, where the flow is at right angles to the beam.
- A, Doppler frequency spectrum waveform shows changes in flow velocity and direction by vertical deflections of the waveform above and below the baseline. The width of the spectral waveform (spectral broadening) is determined by the range of frequencies present at any instant in time (arrow). A brightness (gray) scale is used to indicate the amplitude of each frequency component. B, Color Doppler imaging. the degree of the saturation of the color is used to indicate the frequency shift from moving red cells.
- The strength/power/energy of the flow information is displayed
- Power Doppler image to show the renal hilar, interlobar and cortical arteries
- The signal is not dependent on the angle of insonation, although there must be a Doppler shift. ● Aliasing does not occur with power Doppler since the power of the signal is independent of its apparent direction. ● The power of electronic noise is low so that the signal-to-noise ratio is improved, and a low PRF is used so that power Doppler is three to five times more sensitive to detecting low flow than colour Doppler, and is better able to define boundaries. Limitations ~ is provided so that adjacent vessels with flow in opposite directions appear the same, making it difficult to distinguish veins from arteries. There is a so that there is
- Unlike in gray – scale ultrasound imaging where by the best image is obtained perpendicular to the US beam, in Doppler Ultrasound, the strongest signals (and best spectra) result when the motion is parallel to the beam. The larger the angle greater the correction is needed and more prone to errors. The Doppler shift obtained depends upon the cosine of this angle When the angle is 0 or 180 then the true velocity is measured as the cosine of 0,and 180 is 1 This situation is akin to putting the transducer inside a vessel When the angle is 90 i.e. the beam is perpendicular to the flow direction, no shift is obtained (cosine of 90 is 0) If the angle is 45 (cosine is 0.7) then 70 % of flow velocity measured underestimating it by 30 %
- Definition:-The sample volume is the three dimensional space from which the Doppler frequency shifts are measured. happens (that may be incorrectly interpreted as post stenotic turbulence).
- WALL FILTERS- ( FUNCTION ELIMIMNATE ARTIFACTS DUE TO LOW FREQUENCIES FROM THE VESSEL WALLs ) Excessive gain will cause an overestimation of peak velocity (A), and insufficient gain will result in underestimation of velocity (C)
- The above mentioned parameters are very important for optimal Doppler study. If these are INACCURATE ARTIFACTS RESULTS in Doppler study
- These are grouped in three broad categories.
- Aliasing is a production of artifactual, lower-frequency signals when the sampling rate (the PRF) is less than twice the Doppler signal frequency it is the minimum sampling rate that can be used for a signal of a given frequency. If the sampling rate < Nyquist rate, aliasing occurs.
- Aliasing. Pulse repetition frequency (PRF) determines the sampling rate of a given Doppler frequency. A, If PRF (arrows) is sufficient, the sampled waveform (orange curve) will accurately estimate the frequency being sampled (yellow curve). B, If PRF is less than half the frequency being measured, undersampling will result in a lower frequency shift being displayed (orange curve). C, In a clinical setting, aliasing appears in the spectral display as a “wraparound” of the higher frequencies to display below the baseline. D, In color Doppler display, aliasing results in a wraparound of the frequency color map from one flow direction to the opposite direction, passing through a transition of unsaturated color.
- IRST, If the scale is still inadequate
- in transjugular intrahepatic portosystemic shunt
- In this artifact the colour spreads out from within the vessel and bleeds beyond the wall in to adjacent areas.
- 1.The artifact is due to a misregistration in the location of the Doppler signal secondary to reflection of the sound beam at the high acoustic interface with the lung. 2 . though they are located only on one side of it.
- It is believed to originate from a clock error when processing the strong returning echoes from the calcified plaques.
- to allow better frame rate for better resolution and sensitivity. to obtain an optimal colour signal and minimal colour noise. A low scale is used for low flows and velocities; however it may produce aliasing. A high scale reduces aliasing but is less sensitive for slow flows.
- to obtain accurate velocities.
- mode of imaging allows the operator to sweep the transducer across the anatomy of interest, creating a panoramic view that extends beyond the width of the field of view in the realtime image. An extended color flow image is superimposed on the same space as the extended B-mode image. Another innovation is contrast agents. The original motivation for the development of contrast agents was to enhance the Doppler signals and thereby make flow detection easier.
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