Presentation on theme: "Doppler Ultrasound: Principles. Doppler Effect Shift in perceived frequency when either source or listener are moving relative to one another Familiar."— Presentation transcript:
Doppler Ultrasound: Principles
Doppler Effect Shift in perceived frequency when either source or listener are moving relative to one another Familiar occurrence in audible sounds Also occurs in medical ultrasound
Waves from stationary and moving sources StationaryMoving
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.
Doppler shift Doppler shift is the difference between the transmitted and received frequencies. Transmitted and received frequencies are in the MHz range Doppler shift frequencies often in audible range
Scattering from blood Source of signals for flow imaging: –Red blood cells –Much smaller than –Rayleigh scatterers Scattering increases with frequency Scattering increases with number of targets –Double the number of scatterers, scattered intensity doubles!
Scattering from blood increases with frequency Blood behaves as Rayleigh scatterers –Intensity varies with f 4 –Double the frequency, intensity increases by 2 x 2 x 2 x 2 = 16! High frequency enhances blood signals if the path is short Attenuation prohibits use of super high frequencies in most areas 40 MHz IVUS image of a coronary artery
Doppler equation Relationship between Doppler shift (or just Doppler) frequency, F D and reflector velocity, v: f o is the ultrasound frequency, or the transmitted beam frequency. v is the reflector velocity (m/s; cm/s) is the Doppler angle c is the speed of sound
If the beam direction is perpendicular to the direction of flow, the Doppler frequency is ZERO!
Cosine Function a h cos = a/h cos = a h a cos = 0.5 h
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 flow Doppler frequency varies with the cosine of the angle. –Cosine = 1 for 0 o –Cosine = ½ for 60 o –Cosine = 0 for 90 o –For angles between 0 and 10 o, cosine is close to 1. The larger the angle (up to 90 o ), the smaller the cosine
Angle Correct Cursor Angle correct is needed to convert the Doppler frequency to a reflector velocity Operator adjusts the cursor parallel to the flow direction Machine then computes the Doppler angle
Angle Correct Cursor Angle correct is needed to convert the Doppler frequency to a reflector velocity Operator adjusts the cursor parallel to the flow direction Machine then computes the Doppler angle Most displays indicate the reflector velocity rather than Doppler frequency
Angle Correct Error
Effect of 5 o error in setting the angle correct cursor Actual Angle 0 o 20 o 40 o 60 o 80 o Assumed Angle 5 o 25 o 45 o 65 o 85 o Estimated Velocity 50.2 cm/s 51.8 cm/s 54.2 cm/s 59.2 cm/s 99.6 cm/s Actual velocity is 50 cm/s
Flow Patterns Laminar flow –Highest in center –Zero at wall Turbulent flow –Larger distribution of velocities
Ways to “analyze” complex Doppler signals Listen to signals on the loudspeaker –Ear is sensitive to variations in loudness, pitch –Interpretation is subjective Display analog signal derived from velocity or flow rate –Example: zero crossing detector Display Doppler signal spectrum –Usually displayed as a velocity spectral waveform
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).
Spectral analysis done using a FFT device FFT = Fast Fourier Transform
Spectral Display (Frequency)
Calculate Velocity from Doppler Frequency note
Spectral Display (velocity)
Mirror Image Artifact (Spectral Doppler) Origin of artifact: - ~ perpendicular to flow; - over gaining
“Simplified” Doppler equation (When the Doppler angle is zero) Let f 0 be expressed as “F (MHz)”, such as 3, 5, etc; let v be V m/s; also, let c=1540m/s
74. With Doppler ultrasound, if the echo frequency is lower than the transmitted frequency, we can conclude that the reflector is moving _______ the transducer. A.toward B.away from C.perpendicular to D. at a 60 o angle to
75. If blood is flowing directly toward a 5 MHz transducer at 2 m/s, the Doppler frequency would be: A. 1.3 Hz B. 13 kHz C MHz D. 13 MHz
75. If blood is flowing directly toward a 5 MHz transducer at 2 m/s, the Doppler frequency would be: A. 1.3 Hz B. 13 kHz C MHz D. 13 MHz
76. The pressure amplitude of scattered echoes from red blood cells that produce Doppler signals is LEAST affected by: A. speed of sound B. attenuation by soft tissue C. transducer frequency D. hematocrit
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 kHz B.4 kHz C. 8 kHz D.16 kHz
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 kHz B. 12 kHz C. 24 kHz D. 10 MHz
79. An artery is interrogated with a 2 MHz beam with a Doppler angle of 45 o. A peak Doppler shift of 1.8 kHz is detected. If the Doppler angle is increased to 60 o, you would expect the peak Doppler shift to be: A. 1.3 kHz B. 1.8 kHz C. 2.1 kHz D. 2.3 kHz
80. A small error in the estimation of the Doppler angle will result in the largest velocity miscalculations when the Doppler angle is: A. 0 o B. 10 o C. 45 o D. 80 o
81. A Doppler device without a spectral analyzer can NOT display: A. peak velocity B. mean velocity C. velocity changes over time D. instantaneous distribution of velocities
82. On most instruments, the wall filter eliminates: ________ signals.: A. low amplitude B. high amplitude C. low frequency D. high frequency
Basic Hemodynamics Vascular system –Arrangements of pumps, conduits, valves –Aorta-arteries-arterioles- capillaries-venules-veins-vena cava Travels from high to low pressure –Pressure gradient exists over any region –Total area of vasculature system varies with distance from the heart
Pulse Pressure = systolic pressure - ?? = measure of amplitude of blood pressure wave
Laminar Flow Ideal model Highest velocity in center, lowest near walls Tortuousity, bends, branching all change the velocity profile r
Laminar Flow Energy associated with moving fluid –Potential Elastic expansion of vessels, like stretching a spring –Kinetic Energy due to motion and related to the velocity Viscous losses tend to dissipate the energy with increasing distance L P2P2 P1P1 r
Poiseuille (pwazou r z) 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 /8. In honor of his early work the equation for flow of liquids through a tube is called Poiseuille's Law.
Laminar Flow V = velocity (cm/s, m/s) Q = volume flow rate (cm 3 /s, l/s) Pressure Volume relationship P 2 and P 1 are pressures, r is the radius, L is the length, and is the coefficient of viscosity. Viscosity: measure of a fluid’s resistance to flow; describes the internal frictionof a moving fluid –Syrup has a high viscosity –Water has a low viscosity L P1P1 P2P2 r Poiseuille equation
Bernoulli Equation ( Expresses conservation of energy) The simplified version of the Bernoulli Equation is where P, , 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.)
Applying the Bernoulli Equation ( Expresses conservation of energy) The modified Bernoulli Equation is where P now is in mm Hg and V is in m/s. The density, , of blood is taken to be 1060 kg/m 3, and the term 760/1.01 x 10 5 converts pressure from n/m 2 to mm Hg. P1P1 P2P2
Applying the Bernoulli Equation ( Expresses conservation of energy) The modified Bernoulli Equation is where P now is in mm Hg and V is in m/s. The density, , of blood is taken to be 1060 kg/m 3, and the term 760/1.01 x 10 5 converts pressure from n/m 2 to mm Hg. The pressure drop on the left is about 4 x (5.8) 2 = 135 mm Hg.
Continuous Wave (CW) Doppler Ultrasound transmitted continuously rather than in pulses Some units have two-element transducers, 1 transmitting, the other receiving Arrays are sometimes used with CW; different groups transmit than recieve
~MHz Difference (audible)
Common user controls: -Volume -Gain (sometimes) -Wall filter (sometimes)
Directional Doppler Usually want to know whether flow is towards or away from the transducer Quadrature detection is used Tracing shows velocities above and below the baseline Speakers present velocities on left or right speaker, depending on direction
Range (depth) selection done by a procedure called “gating” Gating +beam dimensions, define “sample volume”
Pulsed Doppler Important User Controls Output Frequency (may be different than B-mode frequency) Gain Wall filter Gate position Gate size (SV length) Beam angle Angle Correct Velocity scale (prf) Spectral Display Sweep speed Gray scale maps ….
Sample Volume (gate)
Nyquist Sampling Limit Require PRF = 2 x Doppler Frequency Example, 3 kHz Doppler signal –Need 6 kHz PRF to sample Example, 6 kHz Doppler signal –Need 12 kHz PRF to sample
Nyquist Sampling Limit The Maximum Doppler frequency that can be sampled is ½ the PRF Example, if PRF = 8 kHz –Max Doppler frequency is 4 kHz Example, if PRF = 4 kHz –Max Doppler frequency is 2 kHz
If the Doppler frequency exceeds ½ the PRF, aliasing occurs Aliasing produces false frequencies, reversal, etc.
Manifestation of Aliasing
After increasing the Velocity Scale (automatically increases the PRF)
PRF Maximum PRF depends on depth of sample volume –When sample volume is shallow, PRF can be higher –When it is deep, PRF must be lower Thus, for a given ultrasound frequency, a higher velocity can be detected at shallower depths than at deeper depths.
To get rid of aliasing: Change the velocity scale Change the baseline Use a lower ultrasound frequency Get closer!
Advantages of Pulsed Doppler Precise depth at which flow is detected can be specified Flow information from a small portion of a vessel can be isolated and analyzed without interference from flow in adjacent areas
Advantages of CW Doppler Instruments can be made extremely simple, inexpensive Useful when you do not have good information (such as a B-mode image) to help pinpoint vessel of interest DOES NOT ALIAS
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.
Maximum Velocity Detectable:
Introduces range ambiguity. Allows higher velocities to be detected.
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.
Flow velocity is usually indicated by color brightness.
In Spectral Doppler, the entire range of velocities within a gated region is displayed. In color, only the mean velocity is displayed from each region.
Most manufacturers allow the operator to select from a variety of color maps.
Color Threshold (emphasis on B-mode) Only echoes exceeding the gray level indicated by the line through gray bar will be displayed in place of color.
Color Threshold (emphasis on color) Only echoes exceeding the gray level indicated by the line through gray bar will be displayed in place of color.
Energy Mode (Power mode; color power angiography mode) Does not display Doppler shift frequency Displays amplitude, intensity or energy in the Doppler signal
Direction, Energy vs. Velocity Energy mode does not display direction. (Energy also not as direction dependent.) Aliasing not displayed in energy mode.
Aliasing, Velocity vs. Energy Energy mode does not display aliasing.
Digital Decoder Digital Encoder For each line in the B-Flow image: 1) Transmit coded sound waves 2) Decoder enhances flow signal 3) Flow and tissue displayed as in B-mode Probe Body How B-Flow Images are Formed LOGIQ 9 Bmode Process Display Monitor (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.)
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.)
Solution: Use coded excitation to 1) Increase sensitivity to blood reflectors (codes can be made sensitive to motion) 2) Equalize tissue signal (not sure how this is done) Problem: Blood echoes are very weak Detecting Blood Reflectors Blood Tissue Noise (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.)
Decoder Tissue Echo + Blood Echo Increase sensitivity to flow Equalize tissue signal Blood Reflectors Seen in B-Mode Encoder Body B-Flow Processing (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.)
Conventional Color Doppler Imaging Overlay color on B overwrite lumen Overlay Separate B-mode and color firings frame rate hit Flash 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.)
(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: Aliasing Signal dropout at orthogonal detection angles Wall 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 resolution High frame rates Blood and tissue displayed together - no overlay Intuitive display No 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.)
B-Flow Image Simultaneous tissue and flow without overlay Intuitive B-mode-like display with full field of view No separate firings for flow higher frame rate B-Flow Process B-mode Image B-Flow provides visualization of blood reflectors with: High spatial resolution High 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.)
83. Gating determines the: A. transmitting frequency B. Doppler frequency C. sample volume length D. sample volume width 84. How often a Doppler signal is sampled is determined by the _______ frequency. A. transmitted B. received C. Doppler shifted D. pulse repetition
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 kHzC. 12 kHz B. 6 kHzD. 18 kHz 86. Aliasing occurs at lower frequencies when the sample volume is: A. moved toward the transducer B. moved away from the transducer C. increased in width D. operated in continuous wave mode
87. Methods of compensating for aliasing do NOT include: A. transmitting at a higher frequency B. adjusting the spectral baseline C. increasing the velocity scale range D. switching to continuous wave 88. Range discrimination is POOREST with: A. M-Mode B. continuous wave Doppler C. pulsed Doppler D. color Doppler
89. The highest velocities can be accurately evaluated by: A. real-time B-mode B. pulsed Doppler C. continuous wave Doppler D. color Doppler 90. Increasing the packet size results in: A. better velocity estimates, higher frame rates B. better velocity estimates, lower frame rates C. worse velocity estimates, higher frame rates D. worse velocity estimates, lower frame rates
91. The term “variance” refers to: A. peak velocity B. mean velocity C. velocity range D. velocity threshold 92. The most accurate display of the distribution of velocities at a particular depth occurs with: A. continuous wave Doppler B. pulsed Doppler C. color Doppler D. B-mode imaging