Doppler Effect (19) PDF

19 - Doppler The Doppler Effect The Doppler Effect is the change in frequencies of a wave secondary to motion in the wave’s source Wave’s Source = The thing that is making sound This includes sound bouncing off that thing. If we bounce sound off something, that thing is considered...

19 - Doppler The Doppler Effect The Doppler Effect is the change in frequencies of a wave secondary to motion in the wave’s source Wave’s Source = The thing that is making sound This includes sound bouncing off that thing. If we bounce sound off something, that thing is considered to be “making” the sound. The Doppler Effect When an object that is creating waves moves, the direction of that motion tends to change the shapes of the waves Waves traveling in the same direction as the moving object will have a higher frequency Waves travelling the opposite Lower direction as the moving object will Frequency Higher have a lower frequency Frequency  Stationary Moving Sound Source Sound Source Doppler Frequencies This is also how RADAR and SONAR work! The frequency changes in those waves being given off by a moving source RADAR comes out of the tower with a known frequency When those waves hit the airplane and bounce off, their frequency is These changed changed based on the Airplane’s frequencies are called movement direction and speed “Doppler Frequencies” Doppler Shift A “Sonic Boom” is what occurs if a sound source travels faster than the sound waves travelling The amount of difference between ahead of it! the Transducer Frequency (Fundamental Frequency) and the Reflected Frequency (from the Red Blood Cell) The faster an object With this knowledge, we moves, the greater the can tell how fast Doppler Effect, with something is moving by increasingly higher comparing the original frequencies ahead and ”RADAR Tower” lower frequencies Frequency to the behind Frequency bouncing off the reflector Fundamental Frequency The frequency of the Transducer / Operating frequency / Transmitted Frequency What the sound frequency begins as Fundamental Frequency from the Transmitter Reflected Doppler Frequency from the moving object Reflected Frequency The Doppler Effect is also used by Weather Radar to track the The frequency of the sound waves motion of storm systems being reflected off of the RBC The known frequency of the RADAR tower is compared to the frequency of the waves reflected off water particles in the air We’ll do the same thing, except with blood cells instead of rain! Doppler Shift Equation DS = RF – FF DS = Doppler Shift (Hz) RF = Reflected Frequency (Hz) FF = Fundamental Frequency (Hz) If the DS is a positive number, the RBC is moving toward the Tx If the DS is a negative number, the RBC is moving away from the Tx Doppler Shift Equation (Simplified) You are operating a 4MHz Transducer. You receive back a frequency of 2.7 MHz Was the RBC you reflected your waves from coming toward you or away from you? Shift = Reflected – Original 2.7 4 Shift = 2.7 – 4.0 4 MHz Shift = -1.3 Negative Shift = Going AWAY from the Transducer 4.0 The pulse MHz we sent This is the reason that Doppler has a (+) sign above baseline and a (-) sign below baseline The pulse 2.7 we got back MHz Doppler Notes Doppler is used to determine Blood Velocity Speed Velocity = Speed + Direction Speed = How fast Direction Direction = Which way Doppler determines how fast (by the degree of the shift) Doppler determines which way (by the positive or negative shift) Doppler Shift Equation (Expanded) DS = ( 2 x V x FF x Cos ) ÷ C DS = Doppler Shift (Hz) V = Blood Velocity (cm/s) FF = Fundamental Frequency (Hz) Cos = Cosine of the Doppler Angle C = Propagation Speed Doppler Shift Equation (Expanded) Doppler Shift and Velocity have a Direct Relationship Direct Relationship with Doppler Shift Doppler Shift and Transducer Frequency have a Direct Relationship Doppler Shift and Cosine have a Direct Relationship Doppler Shift and Prop. Speed have an Inverse Relationship Inverse Relationship with Doppler Shift Doppler Equation Notes Here the equation has been reorganized to solve for Blood Velocity Doppler Shift and Velocity have a Direct Relationship (important) DS = RF - FF This is how we determine velocity of the blood, when we measure it The machine solves for “Blood Velocity” using the Doppler Shift Equation Transducer Frequency, Cosine and Prop Speed are known values to the machine Sound Beam Direction vs Flow Direction Parallel = When two planes are oriented toward each other in such a way that they will not cross Parallel to Flow = When the angle of the doppler beam is as close to matching the angle of flow as possible (0 degree angle) Perpendicular = When two planes cross at a 90 degree angle Perpendicular to Flow = When the doppler beam’s angle is oriented 90 degrees from the angle of flow  Smallest Angles Strongest Signals Largest Angle Worst Signal Strength of the doppler signal is reliant on angle, not direction!  The Doppler Angle is travelling The doppler signal out from the Tx in a Fan-Shape gets weaker the closer identical to the Scan Lines of the the angle of incidence Curvilinear Probe gets to 90º Red flow is making (+) Doppler Shifts moving toward the Tx Blue flow is making (-) Doppler Shifts moving Notice the color of the blood flow away from the Tx through this abdominal Aorta (Blood is flowing from Left-to-Right) Cosine The cosine of an angle is a logarithmic function used in mathematics Here, we only need to know (3) Cosine values: Angle : Cosine 0 degree : 1.0 60 degree: 0.5 90 degree: 0.0 The Cosine of the Angle and the Doppler Shift are Directly Related The greater the cosine (closer to 0 degrees) the greater the Doppler Shift Cosine This Angle = 60º Measured Velocity = True Velocity x Cos If the true velocity through a vessel is… 100 cm/s Measured with a doppler angle of 60º MV = TV x Cos MV = 100 x 0.5 MV = 50 cm/s In Vascular Ultrasound, the Cosine of the Angle is Taken at 90º? maintained at 60º by the “Angle Correct” function. MV = TV x Cos MV = 100 x 0 So, the machine automatically doubles every value before putting it on the screen. This is why MV = 0 (no velocity measured at all) maintaining an angle of 60º is important. Sound Beam Direction vs Flow Direction Cosine and Doppler Shift have a Direct Relationship The lower the angle, the greater the cosine The lower the angle, the greater the doppler shift Types of Doppler (2) Main and (4) Subtypes: Spectral Doppler Pulse Wave Continuous Wave Overlay Doppler Velocity Mode (Color) Power Doppler The differences between these two types of doppler (Spectral vs Overlay) are in how the Doppler Shift information is used Overlay Doppler A Doppler “package” in which the doppler shift and velocity information is presented in a colored box, laid-over the grayscale image (overlay) The “Color Map” There are 2 Subtypes of Overlay Doppler: Color Box (Color Overlay) (Velocity Mode) Power Doppler The Doppler Shift and Velocity information is animated with colors corresponding to a “Color Map” similar to the Gray Map of B-Mode Different “Hues” (colors) represent different doppler shifts  Notice the flow is Doppler Scan fastest in the center of Line Angles the vessel and slowest against the walls Notice how those colors are represented Slower Faster Doppler Angle Flow Direction Spectral Doppler A Doppler “package” in which the doppler shift and velocity information is presented on a Velocity / Time graph There are (2) Subtypes of Spectral Doppler: Pulsed Wave Continuous Wave Continuous Wave Doppler Requires (2) crystals to function One crystal always transmitting One crystal always receiving (one microphone and one speaker) Advantage (strength): Can measure extremely high velocities High Velocities in the Does not Alias Heart are why Echo makes such extensive use of CW! Disadvantage (weakness): Cannot sample doppler shifts from specific depths Must sample the entire length of the doppler scan-line (the whole beam)  Target Flow Continuous Wave Scan Line Accessory Flow picked up by the other parts of the scan line Continuous Wave cannot discern depth, as depth information requires a measurement of time, which requires pulses Pulse Wave Doppler Requires only (1) crystal to function The doppler pulse (a specific pulse only used for detecting doppler information) is fired down the scan line. Its reflected frequency is compared to the original, and doppler shifts are measured. Then, depth information is added in. (Depth information for doppler is obtained the same way that depth information is obtained for B-Mode 2D Grayscale) (13 us rule) = (1 cm depth = 13 us round- trip time) Pulse Wave Doppler Advantage: Able to sample doppler shift information from specific depths Disadvantage: Not useful for High-Velocity situations Subject to Aliasing  Target Flow This selector is called the “Sample Volume” or the “Range Gate” Pulse Wave flow is often characterized by these “windows” making the waveforms appear hollow This is often a good thing, as turbulent flow does not create these uniform, clear “windows”  This is a sign that the flow is likely more turbulent, or that the sample volume is large, allowing many Notice the lack of clear different velocities to “windows” within these be present in the waveforms. recording Quantitative vs Qualitative Studies and Information taken with the Ultrasound machine can be “Quantitative” images or “Qualitative” images Quantitative = Things you can Measure (think Quantity) Qualitative = Things you can Observe (think Quality) Quantitative vs Qualitative Spectral Doppler is a Quantitative Observations Study Overlay Doppler is a Qualitative Study What is the value? Spectral = Specific, math-ready Measurable Numbers numbers Look at different points in time on 1 image Overlay = Big Picture flow profiles (is it turbulent or parabolic, etc.) Live display of flow (it’s a video, real time) Aliasing The most common error associated with doppler ultrasound An error that occurs when the doppler shift exceeds the Nyquist Limit The Nyquist Limit: NL = PRF / 2 Aliasing If the velocity of the blood is high enough to create a value greater than the Nyquist Limit, the velocity information is unusable and false. This is considered an “Artifact” Artifact = False information displayed on the US study (Something that isn’t there, or isn’t shaped correctly, etc.)  The Nyquist Limits of the Pulse Wave The velocities that cannot be properly displayed with the current Scale/PRF settings are “wrapped-around” to the other end of the spectrum  As the doppler shifts exceed the Nyquist limit, they are wrapped-around to the other The Nyquist Limits of side of the spectrum the Color Overlay Notice that this creates areas where the maximum positive color borders the maximum negative color In this case Light-Blue touching Light-Orange  The flow velocity exceeds the Nyquist Limit of the Color Scale but not of the Spectral Scale This is why we see Aliasing in the Color Box but not on the PW Waveform!