5 Please note: the ‘squishing’ of the wave-fronts in the middle diagram and the stretching in the lower diagram are exaggerated. Realistically the Doppler shifts are so small in ultrasound you would hardly see any difference in the wave-fronts compared to the unshifted one on top. These are artists’ diagrams.
6 Doppler shiftDoppler shift is the difference between the transmitted and received frequencies.Transmitted and received frequencies are in the MHz rangeDoppler shift frequencies often in audible range
7 Scattering from blood Source of signals for flow imaging: Red blood cellsMuch smaller than lRayleigh scatterersScattering increases with frequencyScattering increases with number of targetsDouble the number of scatterers, scattered intensity doubles!
8 Scattering from blood increases with frequency Blood behaves as Rayleigh scatterersIntensity varies with f4Double the frequency, intensity increases by 2 x 2 x 2 x 2 = 16!High frequency enhances blood signals if the path is shortAttenuation prohibits use of super high frequencies in most areas40 MHz IVUS image of a coronary artery
9 Doppler equationRelationship between Doppler shift (or just Doppler) frequency, FD and reflector velocity, v:fo is the ultrasound frequency, or the transmitted beam frequency.v is the reflector velocity (m/s; cm/s)q is the Doppler anglec is the speed of sound
14 The frequency of the Doppler shift is proportional to the cosine of the Doppler angle Angle formed by the ultrasound beam and the direction of flowDoppler frequency varies with the cosine of the angle.Cosine = 1 for 0oCosine = ½ for 60oCosine = 0 for 90oFor angles between 0 and 10o, cosine is close to 1.The larger the angle (up to 90o), the smaller the cosine
15 Angle Correct CursorAngle correct is needed to convert the Doppler frequency to a reflector velocityOperator adjusts the cursor parallel to the flow directionMachine then computes the Doppler angle
16 Angle Correct CursorAngle correct is needed to convert the Doppler frequency to a reflector velocityOperator adjusts the cursor parallel to the flow directionMachine then computes the Doppler angleMost displays indicate the reflector velocity rather than Doppler frequency
18 Effect of 5o error in setting the angle correct cursor Actual velocity is 50 cm/sActual Angle0o20o40o60o80oAssumed Angle5o25o45o65o85oEstimated Velocity50.2 cm/s51.8 cm/s54.2 cm/s59.2 cm/s99.6 cm/s
19 Flow Patterns Laminar flow Turbulent flow Highest in center Zero at wallTurbulent flowLarger distribution of velocities
20 Ways to “analyze” complex Doppler signals Listen to signals on the loudspeakerEar is sensitive to variations in loudness, pitchInterpretation is subjectiveDisplay analog signal derived from velocity or flow rateExample: zero crossing detectorDisplay Doppler signal spectrumUsually displayed as a velocity spectral waveform
23 Doppler Instrument with built-in Zero-Crossing Detector Provides an estimate of Doppler signal frequency vs. time.Output may be to a chart recorder.Output may also be superimposed on a spectral display (rare).
24 Spectral analysis done using a FFT device FFT = Fast Fourier Transform
33 Mirror Image Artifact (Spectral Doppler) Origin of artifact:- ~ perpendicular to flow;- over gaining
34 “Simplified” Doppler equation (When the Doppler angle is zero) Let f0 be expressed as “F (MHz)”, such as 3, 5, etc; let v be V m/s; also, let c=1540m/s
35 74. With Doppler ultrasound, if the echo frequency is lower than the transmitted frequency, we can conclude that the reflector is moving _______ the transducer.towardaway fromperpendicular toD. at a 60o angle to
36 75. If blood is flowing directly toward a 5 MHz transducer at 2 m/s, the Doppler frequency would be: A. 1.3 HzB. 13 kHzC MHzD. 13 MHz
37 75. If blood is flowing directly toward a 5 MHz transducer at 2 m/s, the Doppler frequency would be: A. 1.3 HzB. 13 kHzC MHzD. 13 MHz
38 76. The pressure amplitude of scattered echoes from red blood cells that produce Doppler signals is LEAST affected by:A. speed of soundB. attenuation by soft tissueC. transducer frequencyD. hematocrit
39 77. A peak Doppler shift of 4 kHz is detected in a large artery 77. A peak Doppler shift of 4 kHz is detected in a large artery. If the peak velocity in the vessel were to double, the detected peak Doppler shift would be:A. 2 kHz4 kHz8 kHz16 kHz
40 78. A 5 MHz transducer detects a peak Doppler shift of 12 kHz in an artery. With a 2.5 MHz transducer, the same vessel would be expected to produce a peak Doppler shift of:A. 6 kHzB. 12 kHzC. 24 kHzD. 10 MHz
41 79. An artery is interrogated with a 2 MHz beam with a Doppler angle of 45o. A peak Doppler shift of 1.8 kHz is detected. If the Doppler angle is increased to 60o, you would expect the peak Doppler shift to be:A kHzB. 1.8 kHzC. 2.1 kHzD. 2.3 kHz
42 80. A small error in the estimation of the Doppler angle will result in the largest velocity miscalculations when the Doppler angle is:A. 0oB. 10oC. 45oD. 80o
43 81. A Doppler device without a spectral analyzer can NOT display: A. peak velocityB. mean velocityC. velocity changes over timeD. instantaneous distribution of velocities
44 82. On most instruments, the wall filter eliminates: ________ signals A. low amplitudeB. high amplitudeC. low frequencyD. high frequency
45 Basic Hemodynamics Vascular system Travels from high to low pressure Arrangements of pumps, conduits, valvesAorta-arteries-arterioles-capillaries-venules-veins-vena cavaTravels from high to low pressurePressure gradient exists over any regionTotal area of vasculature system varies with distance from the heart
46 Pulse Pressure = systolic pressure - ?? = measure of amplitude of blood pressure wave
47 Laminar Flow Ideal model Highest velocity in center, lowest near walls Tortuousity, bends, branching all change the velocity profile
48 Laminar Flow Energy associated with moving fluid P2P1LEnergy associated with moving fluidPotentialElastic expansion of vessels, like stretching a springKineticEnergy due to motion and related to the velocityViscous losses tend to dissipate the energy with increasing distance
49 Poiseuille (pwazourz) Schooled in physics and mathematics Poiseuille developed an improved method for measuring blood pressure.Poiseuille’s interest in the forces that affected the blood flow in small blood vessels caused him to perform meticulous tests on the resistance of flow of liquids through capillary tubes. In 1846, he published a paper on his experimental research. Using compressed air, Poiseuille forced water (instead of blood due to the lack of anti-coagulants) through capillary tubes. Because he controlled the applied pressure and the diameter of the tubes, Poiseuille’s measurement of the amount of fluid flowing showed there was a relationship. He discovered that the rate of flow through a tube increases proportionately to the pressure applied and to the fourth power of the tube diameter. Failing to find the constant of proportionality, that work was left to two other scientists, who later found it to be p/8. In honor of his early work the equation for flow of liquids through a tube is called Poiseuille's Law.
50 Laminar Flow Poiseuille equation r P1 P2 L V = velocity (cm/s, m/s) Q = volume flow rate (cm3/s, l/s)Pressure Volume relationshipP2 and P1 are pressures, r is the radius, L is the length, and h is the coefficient of viscosity.Viscosity: measure of a fluid’s resistance to flow; describes the internal frictionof a moving fluidSyrup has a high viscosityWater has a low viscosity
51 Bernoulli Equation (Expresses conservation of energy) The simplified version of the Bernoulli Equation iswhere P, r, V, and z are pressure, density, velocity and height, respectively, and g is the acceleration due to gravity.The fact that the terms on the left sum up to a constant means that if V increases, P must decrease to keep the quantity a constant.The Bernoulli principle helps to explain how an airplane wing works. The greater velocities just above the wing result in a lower pressure on the top-front of the wing than on the undersurface, hence an upward force. (Lift also can be associated with a downward component of the air to the rear; Newton’s third law says there must be an upward lift.)
52 Applying the Bernoulli Equation (Expresses conservation of energy) The modified Bernoulli Equation isP1P2where DP now is in mm Hg and V is in m/s. The density, r, of blood is taken to be 1060 kg/m3, and the term 760/1.01 x 105 converts pressure from n/m2 to mm Hg.
53 Applying the Bernoulli Equation (Expresses conservation of energy) The modified Bernoulli Equation iswhere DP now is in mm Hg and V is in m/s. The density, r, of blood is taken to be 1060 kg/m3, and the term 760/1.01 x 105 converts pressure from n/m2 to mm Hg.The pressure drop on the left is about 4 x (5.8)2 = 135 mm Hg.
55 Continuous Wave (CW) Doppler Ultrasound transmitted continuously rather than in pulsesSome units have two-element transducers, 1 transmitting, the other receivingArrays are sometimes used with CW; different groups transmit than recieve
57 Common user controls:VolumeGain (sometimes)Wall filter (sometimes)
58 Directional DopplerUsually want to know whether flow is towards or away from the transducerQuadrature detection is usedTracing shows velocities above and below the baselineSpeakers present velocities on left or right speaker, depending on direction
59 Range (depth) selection done by a procedure called “gating” Gating +beam dimensions, define “sample volume”
69 After increasing the Velocity Scale (automatically increases the PRF)
70 PRF Maximum PRF depends on depth of sample volume When sample volume is shallow, PRF can be higherWhen it is deep, PRF must be lowerThus, for a given ultrasound frequency, a higher velocity can be detected at shallower depths than at deeper depths.
71 To get rid of aliasing: Change the velocity scale Change the baseline Use a lower ultrasound frequencyGet closer!
72 Advantages of Pulsed Doppler Precise depth at which flow is detected can be specifiedFlow information from a small portion of a vessel can be isolated and analyzed without interference from flow in adjacent areas
73 Advantages of CW Doppler Instruments can be made extremely simple, inexpensiveUseful when you do not have good information (such as a B-mode image) to help pinpoint vessel of interestDOES NOT ALIAS
74 Maximum Detectable Velocity Minimum PRF to avoid aliasing = 2 times the Doppler frequency.Maximum PRF set by gate depth (must wait between successive transmit pulses).Establishes a maximum detectable velocity, that depends on gate depth AND ultrasound frequency.
84 Frame Rate vs Color Box Width Wider color box requires more individual color beam lines.Each color beam line requires several pulse-echo sequences (pulse-packet size).More time is needed to acquire echo data for the wide color box than the narrow color box.Lower frame rate for the wide color box.
100 Energy Mode (Power mode; color power angiography mode) Does not display Doppler shift frequencyDisplays amplitude, intensity or energy in the Doppler signal
101 Direction, Energy vs. Velocity Energy mode does not display direction.(Energy also not as direction dependent.)Aliasing not displayed in energy mode.
102 Aliasing, Velocity vs. Energy Energy mode does not display aliasing.
103 How B-Flow Images are Formed DigitalEncoderDigitalDecoderLOGIQ 9Bmode ProcessDisplayMonitorProbeFor each line in the B-Flow image:1) Transmit coded sound waves2) Decoder enhances flow signal3) Flow and tissue displayed as in B-modeBody(Slides are based on a set given to Zagzebski by GE Medical. They are presented to attempt to understand B-flow, a complimentary technology to color flow imaging.)
104 B-flow processing (Previous) How B-Flow images are formed? Digital encoded ultrasound is used as an enabling technology. Coded sound waves are transmitted into the body and vasculature and the returning signals are then decoded and displayed as in B-mode.(Next)Enhancement of the returned signal from blood reflections is necessary due to the relative weakness of blood reflectors compared to tissue. (Tissue is typically db > reflective than blood.) Enhancement however increases the tissue signal as well so the tissue must be equalized to show enhancement of the blood reflectors only.(Slides are based on a set given to Zagzebski by GE Medical. They are presented to attempt to understand B-flow, a complimentary technology to color flow imaging.)
105 Detecting Blood Reflectors Problem:Blood echoes are very weakSolution:Use coded excitation to1) Increase sensitivity toblood reflectors (codes canbe made sensitive to motion)BloodTissueNoise2) Equalize tissue signal(not sure how this is done)(Slides are based on a set given to Zagzebski by GE Medical. They are presented to attempt to understand B-flow, a complimentary technology to color flow imaging.)
106 Blood Reflectors Seen in B-Mode B-Flow ProcessingBlood Reflectors Seen in B-ModeBodyEncoderIncrease sensitivity to flowEqualize tissue signalBlood Echo+Tissue EchoDecoder(Slides are based on a set given to Zagzebski by GE Medical. They are presented to attempt to understand B-flow, a complimentary technology to color flow imaging.)
107 Conventional Color Doppler Imaging OverlayOverlay color on B overwrite lumenSeparate B-mode and color firings frame rate hitFlash artifact obscures anatomy(Slides are based on a set given to Zagzebski by GE Medical. They are presented to attempt to understand B-flow, a complimentary technology to color flow imaging.)
108 Signal dropout at orthogonal detection angles (Previous)This is where the strength of digital encoded ultrasound lies in that the coded signal may be decoded into separate tissue and blood signals. It is now relatively simple to enhance the blood signal while preserving diagnostic gray scale.Detecting blood flow with Doppler methods provides valuable diagnostic information. However, Doppler technology constraints limit our ability to detect flow. Limitations such as:AliasingSignal dropout at orthogonal detection anglesWall filter limitations (Next)All affect our ability to detect all types of blood flow. As a B-mode imaging technology, B-Flow provides direct visualization of blood reflectors with:High spatial resolutionHigh frame ratesBlood and tissue displayed together - no overlayIntuitive displayNo complex parameters to optimize(Slides are based on a set given to Zagzebski by GE Medical. They are presented to attempt to understand B-flow, a complimentary technology to color flow imaging.)
109 B-Flow Image B-Flow Process B-mode Image B-Flow Image Simultaneous tissue and flow without overlayIntuitive B-mode-like display with full field of viewNo separate firings for flow higher frame rateB-Flow provides visualization of blood reflectors with:High spatial resolutionHigh frame rates(Slides are based on a set given to Zagzebski by GE Medical. They are presented to attempt to understand B-flow, a complimentary technology to color flow imaging.)
110 83. Gating determines the: A. transmitting frequencyB. Doppler frequencyC. sample volume lengthD. sample volume width84. How often a Doppler signal is sampled is determined by the _______ frequency.A. transmittedB. receivedC. Doppler shiftedD. pulse repetition
111 86. Aliasing occurs at lower frequencies when the sample volume is: 85. If a pulsed Doppler device is operating at a PRF of 6000/s, what is the maximum Doppler frequency that can be accurately detected?A. 3 kHz C. 12 kHzB. 6 kHz D. 18 kHz86. Aliasing occurs at lower frequencies when the sample volume is:A. moved toward the transducerB. moved away from the transducerC. increased in widthD. operated in continuous wave mode
112 87. Methods of compensating for aliasing do NOT include: A. transmitting at a higher frequencyB. adjusting the spectral baselineC. increasing the velocity scale rangeD. switching to continuous wave88. Range discrimination is POOREST with:A. M-ModeB. continuous wave DopplerC. pulsed DopplerD. color Doppler
113 89. The highest velocities can be accurately evaluated by: A. real-time B-modeB. pulsed DopplerC. continuous wave DopplerD. color Doppler90. Increasing the packet size results in:A. better velocity estimates, higher frame ratesB. better velocity estimates, lower frame ratesC. worse velocity estimates, higher frame ratesD. worse velocity estimates, lower frame rates
114 91. The term “variance” refers to: A. peak velocityB. mean velocityC. velocity rangeD. velocity threshold92. The most accurate display of the distribution of velocities at a particular depth occurs with:A. continuous wave DopplerB. pulsed DopplerC. color DopplerD. B-mode imaging