Rouleaux flow, Sagittal. Rouleaux flow, transverse. Doppler Overview. Doppler Overview. Why we hate Doppler physics!!

Transcription

1 Doppler Technology & Optimization Doppler From A Z Donald T. Milburn, RDCS, RVT, FSDMS Prescott, Az. San Diego local ultrasound society May 12, 2009 Donald T. Milburn, RDCS, RVT, NACT, FSDMS Director, Radiology & Vascular Education Programs 2 I d like to begin with this quote; The best interest of the patient, is the only interest to be considered. William J. Mayo What we will do 2day Doppler Overview Doppler Equation PW Doppler Color Doppler Power Doppler 3 4 Who was Mr Doppler & What is Doppler Technology? A class of technologies named after Christian Doppler, a famous Austrian physicist from the late 1800 s. He proved that any movement between a light emitting source and a receiver, will produce a detectable frequency shift. In his case, he was observing the colored lights of double star systems. His theory is the basis for all ultrasoundbased Doppler blood detection technologies. 5 Mr. Doppler was talking about light frequencies White light refracted through a prism (or atmosphere) will separate into colors In our visual world, red = the fastest frequency shifts and violet = the slowest Visible light Incidence and refraction angles (i & r) bend white light into constituent colors (through n2). Violet rays result from the highest deviations (freq. Shifts) and red from the least. In CDI, Red = positive shift, blue = negative CDI display 6

2 Why we hate Doppler physics!! The received frequency response is directly proportional to the transmitted frequency, (fo), and the transmissionreceive angle. Therefore the unconverted frequency shift estimations, f, will degrade as a function of the cosine of the angle from which the transmitted beam is displaced from the line of sight of the insonated media being tracked, (cos 0). When understanding and therefore compensating for angle limitations and the overunderestimation of frequency shifts due to the intrinsic spectral broadening inherent in multi-element electronic array probes, Spectral Doppler s Fast Fourier Transform analysis can provide increased accuracy in frequency shift estimation due to the employment of non-averaging algorithms versus the commonly accepted autocorrelation techniques used in color Doppler which preclude velocity estimation and limit CDI to a display of non-angle corrected, average frequency shifted data. 7 It s just that simple,,,,,, not! The transition between laminar and turbulent flow occurs when a critical number is exceeded. What is this # is called? Reynolds Number Laminar Flow occurs at low Reynolds numbers, where viscous forces are dominant, and is characterized by smooth, constant fluid motion, while Turbulent Flow, on the other hand, occurs at high Reynolds numbers and is dominated by inertial forces, producing random eddies, vortices and other flow fluctuations. 8 Doppler Overview Doppler Overview (Parabolic Flow) 9 Turbulent Flow will revert to Laminar Flow distal to a 10 flow reducing lesion Rouleaux flow, transverse Rouleaux flow, Sagittal Red Blood Cells (RBCs) clump together in coin-like stacks (or rouleaux formation) wherever space allows them to. They do this, actually, to reduce the viscosity of the blood and for the protection that the red cells receive by traveling in these train-like processions. So RBCs in the main arteries and veins travel, naturally, in these "rouleaux" stacks

3 Normal flow to reduced flow to high speed flow, then losing energy and returning to laminer flow Rayleigh scattering enhanced with harmonics Rayleigh scattering enhanced with harmonics Doppler Overview Flow Patterns Laminar flow: Blood traveling in parallel layers in a vessel. Central layers travel at an increased velocity relative to outer layers. Plug flow: A type of laminar flow where multiple layers of blood travel at similar speeds. Disturbed flow: Flow in which the layers of flow have been disrupted. 15 Turbulent flow: Chaotic flow in multiple directions. Arterial Vascular Hemodynamics Owen CA, Roberts M Journal of Diagnostic Medical Sonography : 16 Doppler Overview Doppler Overview Concept 1 Concept 2 Concept

4 To Perform Doppler, We Need An Angle degrees 90 degrees 0 degrees We use the Doppler theory and apply it to sound waves to detect frequency shifts ( speed ) and direction f 2 fo V C 0 The big problem with Doppler is operator vs. interpreter variability Doppler Equation Doppler Equation Cowboy s Understood Doppler Doppler Equation!!! I love hearing the lonesome wail of the train whistle as the magnitude of the frequency of the wave changes due to the Doppler effect ya ll! Lynx Lake, Prescott, Arizona 23 24

5 We witness visually and hear Doppler audibly, the Doppler Equation effect in our to day lives. I think Mr. Doppler would be proud that his theory of detecting the motion of heavenly bodies, has made stars of those whose job it is to detect the motion of blood cells within our terrestrial bodies!!! D. Milburn 25 What are the Doppler-based Blood Detection Technologies? Pulsed Wave Doppler (PW) High Pulsed Repetition Freq. Doppler (HPRF) Continuous Wave Doppler (CW) Color Doppler Imaging (CDI) Power Doppler Imaging (PDI) 26 Color Doppler Pulsed Wave Doppler PW Types of Doppler Instrumentation! High PRF Doppler HPRF Continuous Wave Doppler X-mit Receive Power Doppler CW Continuous Wave Doppler (CW) CW Doppler uses two crystals or two subsets of crystals to transmit and receive. It does not use a discrete sample volume and is not restricted by the low PRFs employed in Pulsed Doppler. CW Doppler will not alias. CW Doppler is used to detect high velocities. = CW Sample area Transmit No Aliasing Detects high velocities Receive No range location Pulsed Wave (PW) PW Doppler uses a discrete sample volume (SV) to sample blood at precise locations. The Doppler frequency is pulsed thousands of times per second, (KHz). PRF determines the speed of blood that may be detected unambiguously. f 2 f0 V Sample volume High Pulsed Repetition Frequency (HPRF) HPRF employs multiple sample volumes placed at intervals along the Doppler cursor. These sites can be sampled at a faster rate than PW Doppler and thus higher PRFs can be used. Higher PRFs will detect and correctly display higher blood flows. SV 1 SV 2 f 2 f0 V SV 3 SV 1 SV 2 Crystal 2 Accurate range location Excellent spectral quality Aliasing 29 Multiple sample volumes Equidistant spacing Good range location Adequate spectral quality Increased Nyquist limits Multiple sample volumes 30

6 It s all based on the Doppler Equation The Doppler Equation is not 2 B feared! The Doppler equation? I have to know the Doppler equation??? f = 2 f0 v cosø c Important: Frequency shift size is linear and proportional to Doppler frequency Scattering (Rayleigh) is proportional to the 4 th power of the Doppler frequency What is Spectral Doppler? PW Doppler Controls The ultrasonic evaluation of blood flow. Pulsed Wave (PW), High Pulse Repetition Frequency (HPRF) and Continuous Wave Doppler (CW) are all Spectral Doppler technologies. The differences between these technologies must be understood PW Doppler Controls SV Length The sample volume must be adjusted for every patient and every clinical situation. Sample volume length is adjusted to optimize data acquisition. Small sample volumes are used to acquire data from discrete areas and for peak velocity detection. Large sample volumes are used to analyze complex vasculature (tumor flow) and to calculate flow volumes. Use caution on HPRF (multiple sample volumes) < SV = < signalnoise > SV = > signalnoise 35 36

7 X The Sample Volume (SV) Length must be adjusted for each vessel and pathology! Z Intrinsic Spectral Broadening What happened to my spectral window? The waveform is filled in,,, is that not signal? Y Think about spatial position 37 Facts: In the beginning, (1970 s80 s),most clinical vascular spectral Doppler was performed using single crystal mechanical transducer technology. Single crystal technology had the advantage of a large, round transmit and receive surface with excellent focus capabilities. Spectral broadening in the Doppler waveforms acquired with single crystal transducers primarily was related to hemodynamics and in normal blood flow conditions, was minimal as Doppler data was received across one large collection surface However,,,,,, slow flow detection could be compromised if the blood flow was slower than the mechanical crystal could be moved through it s field of travel Facts: Phased array, electronic linear and curved array transducer technologies have the advantage of no moving parts and were therefore inherently superior in detecting slow blood flow with Doppler. However,,, multiple elements in the transducer array transmit (delay lines) & receive the Doppler reflections at different points in time in relation to their position in the receive aperture, which is dynamic (changes size). Multiple receive angles result in many slightly different Doppler frequencies being processed and included in the waveforms This creates Intrinsic spectral broadening created by the transducer technology,,, not abnormal blood flow 39 Graphic courtesy of Rick Duncan 40 Transducer elements = Aperture = Doppler cursor = Receive elements Single crystal mechanical probe Vessel = receive to one collecting surface Spectral broadening 41 42

8 So; Spectral broadening should not be relied on as an absolute diagnostic criteria when generating Doppler results with electronic array probes. That being said; If I am not so concerned about spectral broadening I can now use larger sample volumes to increase sensitivity to slowlow flow! Scale (PRF - Pulsed Repetition Frequency) Spectral Doppler aliasing is controlled using Scale (PRF). High velocity flow requires a higher Scale (PRF) Low velocity flow requires a lower Scale (PRF) Aliasing occurs when the velocity of the blood exceeds ½ of the PRF (Nyquist Limit) What is the most common Doppler artifact that Scale (PRF) will reduceeliminate? Aliasing Which way is the wheel spinning? 45 Clockwise or Counter Clockwise? 46 Baseline This control changes the spectrum baseline to accommodate higher velocity blood flow. It can be used to reduce aliasing by displaying a greater range of forward flow with respect to reverse flow and vice versa. The baseline displays as a solid line running across the spectrum. The baseline is raised and lowered in equal increments, depending on the current Doppler scale factor. Notice the baseline color? What color is a CDI baseline? 47 48

9 Invert Inverts the directional display of detected flow without affecting the baseline position. Allows negative flows to be displayed in a positive direction and viceversa. Can be used to reverse Doppler waveform presentation rather than reversing probe orientation. Useful when examining vessels from different probe positions as it keeps the waveform direction uniform. Spectral Doppler Gain Doppler Gain amplifies the overall strength of the received Doppler signal. Gain must be used with caution as it can increase or decrease spectral broadening. Spectral broadening is used as a diagnostic criteria. Too much Gain = noise Too little Gain = not enough signal Spectral Doppler Gain Receive Gain only Turn it up!!! Too much Gain = noise Too little Gain = not enough signal M&A caution! Wall Filter These are used to filter out tissue signals, motion artifacts from breathing and other patient movement. Filters will be automatically adjusted with the Scale (PRF). The higher the Scale the higher the Wall Filter and vice versa. However, if there is still motion artifact the Wall Filter can be adjusted manually. Wall Filters are electronic eraser s You will see no information within the bounds of the green bar. Filters remove noise and low flow signals. Angle Correct Optimizes the accuracy of the flow velocity. This is especially useful in vascular applications where you need to measure velocity. (converts Hz. to velocity) For optimum velocity measurements, the angle of incidence should be 60 degrees or less for vascular applications. ICAVEL requires 60 degrees or less 53 54

10 Quick Angle Quickly adjusts the angle by 60 degrees. Toggles through 60, 0 and +60 degrees. Angle Steer You can Steer the Doppler Cursor left, right or center to get more information without moving the probe. This Steer function only applies to linear probes. Linear probes can be steered left (20 degrees), center, or right (20 degrees) Fine Steer Fine Steer steers the Doppler cursor leftright between 0 and 30 degrees in 2 degree increments. Provides a Doppler cursor angle suitable for linear probe orientation. Beneficial in Vascular applications. 60 degree angle from parallel vessel 20 degrees > 20 degrees (Fine Angle Steer) Newton s 3 rd law 57 Beam steer with heeltoe maneuver 58 Sweep Speed Changes the speed at which the timeline is swept. You can speed up or slow down the timeline to see more or fewer occurrences over time. The slower the sweep speed, the closer together the spectral waveforms. The faster the sweep speed the further apart the spectral waveforms. Colorize Enhances the eye s discrimination and ability to resolve changes in shades of gray. The eye can perceive slight changes in color easier than in black and white. Useful when the spectral trace is difficult to see adequately

11 Auto Calcs Allows the selection of the automatic calculations to be either Off, Live or Frozen. Once you have a Spectral waveform you are able to turn this function on and off. Simultaneous (Duplex vs. Triplex) Simultaneous This controls if the system will use Duplex or Triplex Duplex is usually recommended as you get a cleaner Doppler trace. This results from the machine only acquiring Doppler information instead of B-mode, Color and Doppler simultaneously. Caution: Triplex can limit Scale (PRF) and high speed flow detection Spectral Doppler Frequency Doppler frequency will affect the waveform size. This can be used to increase small signals (increase sensitivity) and reduce the size of large signals. If you are scanning superficial structures, use a higher frequency as this will provide better signal to noise ratio. If scanning deeper structures and you need more penetration use a lower frequency There are lot s of automated features on new Doppler systems To this I say Midway Review Doppler Concepts? Doppler Equation? Pulsed Wave Doppler? Spectral Doppler GE Controls E9 Doppler Optimization?

12 Let s talk about Color Doppler What is Color Doppler Imaging? Color is a Pulsed Doppler technology that provides data on: - Existence of flow - Direction of flow - Flow turbulence - Flow quality Why do we need Spectral Doppler? It is the only method available to obtain reproducible, noninvasive, quantitative parameters on blood flow Why do we use Color Doppler? Color Doppler is used as a road map for accurate spectral Doppler sample volume placement. A flow disturbance can be located and Qualified with CDI and then Quantified with spectral Doppler Color Doppler saves time over blind spectral mapping. Color Doppler Processing (Autocorrelation) Autocorrelation processing looks at Doppler data from multiple sample volumes. All frequency shifts from all sample volumes are analyzed for direction, strength and mean values. All data from all sample volumes are then averaged together into a display of mean frequency shifts

13 Color Doppler menu Color Doppler menu What are the standard CDI displays? BART: Blue Away, Red Towards RABT: Red Away, Blue Towards Size - [Region of Interest (ROI)] This determines the region of color displayed on the image The larger (lateral width) the Color Box Size, the slower the frame rate To increase frame rate decrease the width of the Color Box Size. The height (axial length) of the Color Box Size does not have as much affect on the frame rate. Don t forget you can also change the B-mode Sector Width to increase frame rates!!! It s best to keep the Color Box Sized to the area of interest Color Scan Area (ROI) Color Invert Large ROI Small ROI This control allows you to reverse the displayed color of the blood flow towards and away from the transducer This may be inverted in real time or freeze frame Usually red is used for flow towards the transducer and blue for flow away from the transducer 77 78

14 How We See Color: Color blindness affects 1 in 10 men and 1 in 100 women! Disorder is passed from mother to son on the X chromosome The most common is the inability to distinguish red from green This is why stoplights are always in the same order This is why men are not allowed color to pick out their own clothes Color Maps There are several color maps available. These are velocity maps showing blue away red towards, variance maps which add green as a measure of turbulence, additive maps which add more whites to the color, and finally the symmetric maps which show positive and negative flow using the same color (useful for 3D). There are no correct maps. Use whichever map presents the most useable frequency data to your eye. Select Maps and scroll to change the map Angle Steer Allows steering (slanting) of the color box region of interest (ROI) on linear transducers This is used to improve color sensitivity without moving the probe The color ROI may be steered left, right or straight depending on the angle of the vessel Which angle steer provides maximum sensitivity? Caution!!! M&AAngle Gain artifact!! Angle Steer Angle Steer Color Gain Color Gain is used to amplify the overall strength of echo s displayed in the color ROI. To correctly set the Color Gain turn the Gain up until there is bleeding of the color. Then reduce the Color Gain until the noise disappears. This control is on the Color Flow (CF) button. Spectral Doppler Gain works the same way

15 Color Wall Filters Color Wall Filters (Thump, Wall, High Pass filter whatever) These are used to filter out motion artifacts from breathing and other patient movement. This will be automatically adjusted with the Scale (PRF). The higher the scale the higher the Wall Filter and vice versa. However if there is still motion artifact the Wall Filter can be adjusted manually. There are separate Wall Filters for Doppler and Color Doppler. Wall filters are electronic erasers NOTE!! Wall Filters can increase or decrease automatically with vertical ROI movement High Pass Filter Facts amplitud e Tissue signals are db greater in amplitude than Blood signals Tissue = High AmplitudeLow Velocity signals Blood = High VelocityLow Amplitude signals High Pass Filters remove tissue signals and preserve blood signals < HPF # = > display of low freq. blood and > tissue noise > HPF # = < display of low freq. blood signals *** Settings too high will eliminate blood signals!!! HPF # = Tissue 87 velocity Blood 88 Why do we see color here? Color Threshold There are sample volumes in both area s Color Threshold Assigns the gray scale level at which color information stops The higher the Color Threshold, the more color that will be displayed and vice versa This is generally set at about 70-90%. If there is still bleeding then you can reduce this value. **Key point some vessels are so small that they cannot be visualized. If the scanner does not see the black from blood, it will not write color. Threshold makes the system write color to the detected Doppler shift, even if the vessel is not seen. And we see no color here? 89 90

16 D:\Documents and Settings\ \Desk top\clinical Images\Lori's images\rra CLR.jpg Color Threshold Color Sample Volume Allows you to improve the color sensitivity, axial resolution and penetration of color Doppler Sample Volume size may be set at zero (0) for the highest spatial resolution, and set at four (4) for maximum penetration. Sample volume size can affect axial bleeding! > PS < PS Color Doppler Frequency Color Doppler Frequency Color Frequency may be adjusted on some transducers. If you are scanning superficial structures use a higher Frequency as this will give you better signal to noise ratio. If scanning deeper structures and you need more penetration use a lower Frequency CDI Frequency will lower automatically with ROI depth increase Color ScalePRFVelocity PRFVelocityScale (Doppler) Spectral Doppler aliasing is controlled with the Scale (PRF) High speed flow requires a higher Scale (PRF) Low speed flow requires lower Scale (PRF) Aliasing occurs when the speed of the blood exceeds ½ of the PRF Scale & Baseline control Aliasing 95 96

17 Accumulation Accumulation Color & Spectral Doppler Baseline This control changes either the Spectral Baseline or the Color Bar to accommodate higher velocity blood. It can be used to reduce aliasing by displaying a greater range of forward flow with respect to reverse flow and vice versa. This control works the same in Spectral and Color Doppler Baseline is usually set at the midpoint between forward and reverse flow. Color Baseline CDI Baseline shift SDI Baseline shift DopplerColor Baseline This control changes either the spectrum baseline or the color bar to accommodate higher velocity blood flow. It can be used to reduce aliasing by displaying a greater range of forward flow with respect to reverse flow and vice versa. This control works the same in spectral and color Doppler Baseline is usually set at the midpoint between forward and reverse flow. CDI baseline can be used to eliminate aliasing wo eliminating so flow signals CDI Baseline shift Aliasingflow reversal??? SDI Baseline shift

18 Power Doppler Power Doppler Imaging Power Doppler Imaging (PDI) is a mapping technique that looks at the strength (amplitude) of the Doppler signal instead of the frequency shift. PDI appears to be more sensitive than Color Flow when switching between the two modes. The reason is that PDI parameters are set to detect lower velocities Why Use Power Doppler? Power Doppler is used whenever extreme sensitivity to low or slow flow is needed Power Doppler is also used to eliminate aliasing and angle of insonation artifacts Power Doppler is superior to color in outlining vessel walls and depicting residual lumens in high grade stenotic lesions Color Doppler vs. Power Doppler A 3 db increase in signal strength! Weak 90 degrees are avg d to 0 i.e.: black C D I P D I Weak 90 degrees are summed to a positive number Power Doppler Transmit Frequency As in color Doppler, changing the transmit Doppler frequency in PDI can have a dramatic effect on the received signal. Flash Suppression CAUTION! Use no flash suppression with Fremitus!!!

19 To Increase Color Sensitivity Adjust Size of ROI and Angle Steer Adjust Color Focal Position Increase Color Gain and Decrease the Scale (PRF) Decrease Wall FilterIncrease Threshold Adjust Color Sample VolumeAdd Accumulation DecreaseIncrease Color Frequency Change Transducers Mr. Sensitivity Follow the Doppler Recipe s 4 Success! To Reduce Color Bleeding Decrease Color Gain Increase Color Scale (PRF) Decrease the Color Threshold Decrease Sample Volume Size Increase Color Frequency To Reduce Flash Artifact Increase the Scale (PRF) which will in turn increase the Wall Filter Increase Flash Suppression (menu page 2) Be careful when using the Fremitus technique To Eliminate Aliasing Increase the Color Scale (PRF) Increase Spectral Doppler Scale (PRF) Decrease the Color Baseline Shift the Spectral Doppler Baseline To Increase Frame RateFlow Dynamics Decrease the Color Scan Area Size (ROI) Exit Color Flow and Decrease B-mode Sector Width Decrease Depth Increase Color Scale (PRF) Change Line Density Decrease Frame Averaging High PRF may not eliminate aliasing in a 0 degree vessel Holy Cow I ve been cloned!!

20 Remember, Enough GAS Solves All Problems!! OK, time for the test! G = GAIN A = ANGLE S = STEERING Beam Steer is important for image quality and Doppler acquisition! Both images have acceptable Doppler data. Which image is steered correctly and why? B-mode image steered for IQ. CDI ROI & PW Cursor angled correctly Angle corrected to vessel wall for velocity conversion Results? QUALITY DIAGNOSTIC DATA! The optimal B- mode image interface angle is 90 degrees Optimal Doppler interface angle is 0 degrees Describe the Alternating colors Alternating colors are caused by: Look at the angle of insonation and the target interface angles 1. Shadowing degree Doppler angle 3. Refraction 4. Diffraction 5. None of the above Note the color bar orientation

21 Sound Diffraction Alternating colors are caused by 1.Shadowing 2.90-degree Doppler angle 3.Refraction 4.Diffraction 5.None of the above Why the difference in low flow sensitivity Angle and aperture Color Optimization Which CDI control has been adjusted? You have presented these two images to the interpreting physician. Heshe is confused by the displayed colors. What control change could cause the difference in the below images?

22 D:\Documents and Settings\ \Desk top\clinical Images\Lori's images\rra CLR.jpg Color bar directionorientation combined with variation in cardiac cycle What are the standard CDI displays? BART: Blue Away, Red Towards RABT: Red Away, Blue Towards Color Doppler Controls: Would Gain or scale adjustment create the difference in these displays? Color Gain Color Gain is used to amplify the overall strength of echo s displayed in the color ROI. Gain does not increase the insonation power. To correctly set the Color Gain, increase the Gain until there is bleeding of the color. Next, reduce the Color Gain until the noise disappears. This control is on the Color Flow (CF) button. Spectral Doppler Gain works the same way Color ScalePRFVelocity Review 131 We reviewed basic hemodynamics We discussed all Doppler types We learned how the Doppler Equation works and how to apply it to our advantage We showed all relevant Doppler menu s and controls and provided clinical examples I gave you our Doppler Recipes for 132

23 A thousand thank-you s for your kind attention! The E a