Doppler Ultrasound

Aka: Doppler Ultrasound, Cardiac Doppler Ultrasound, Pulsed Wave Doppler, Continuous Wave Doppler, Color Flow Imaging, Color Doppler Imaging, Color Doppler Ultrasound

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II. Precautions

  1. Doppler imaging requires good two dimensional non-doppler, black and white images
    1. Optimize the view and center structures of interest before applying doppler
  2. Cardiac Cycle timing
    1. Measurements (2D or M-Mode) are performed in relation to Cardiac Cycle (systole and diastole)
      1. Often this can be estimated by visualizing wall motion and valve position
    2. However, interpretation of doppler signals must be in the context of Cardiac Cycle timing
      1. Valvular defects have specific characteristics and timing in relation to the Cardiac Cycle
      2. Freeze the image, and use the cine feature to scroll back and forth through the images
      3. Attach ekg lead for rhythm strip to Ultrasound machine if doppler is being performed
        1. Not usually done in most Emergency Department Bedside Ultrasounds
        2. Systole will correlate with start of QRS Complex to start of T Wave
          1. Early systole will coincide with QRS Complex
          2. Mid systole will coincide with ST Segment
          3. Late systole will coincide with start of T Wave
        3. Diastole will correlate with the end of the T Wave to the start of the next QRS Complex
          1. Early diastole will coincide with the end of the T Wave (or U-Wave if present)
          2. Mid diastole will continue until the P Wave
          3. Late diastole will coincide with the P Wave and PR Interval
  3. Confirm abnormal findings in multiple views and with additional modalities (e.g. PW, CW, Color)
    1. Confirm the timing of the abnormality in terms of the Cardiac Cycle
    2. Check that the machine settings are appropriate (e.g. Color Scale)
    3. Recheck the abnormality in several views (just as a single EKG lead finding needs confirmation in other leads)
  4. Pressure waves may demonstrate physical findings
    1. See Jugular Venous Pulsations

III. Background: Pulsed and Continuous Wave Doppler signals

  1. Doppler signals are only interpretable when flow is parallel to probe!
    1. Mitral valve and Tricuspid valve are parallel in Apical 4 Chamber View
    2. Aortic Valve is parallel in Apical 5 Chamber View
    3. Deviations from parallel flow underestimate velocity
      1. Small mis-measurements in velocity result in large deviations in pressure (velocity is squared)
      2. Small deviations (<20 degrees) from parallel flow measurement result in <10% error
  2. Pressures are measured across valve openings
    1. Measured peak velocity and mean velocity are converted to pressure using Bernoulli Equation
    2. Heart maintains constant volume from one cardiac chamber or vessel to the next for forward flow
      1. Velocity (and pressure gradient) increases at valve narrowing to maintain constant volume
      2. Simplified Bernouli Equation: Pressure Gradient = 4 * V^2
        1. Velocity of 2 m/s translates to a pressure gradient of 4*2^2=16 mmHg
        2. Velocity of 3 m/s translates to a pressure gradient of 4*3^2=36 mmHg
        3. Velocity of 4 m/s translates to a pressure gradient of 4*4^2=64 mmHg
        4. Velocity of 5 m/s translates to a pressure gradient of 4*5^2=100 mmHg

IV. Technique: Pulsed Wave Doppler (PW)

  1. Cursor is moved to a specific heart location to listen to a specific signal over a brief, fixed window
    1. The cursor location represents the Sample Volume, and is the only source of signals
  2. Indications
    1. Aortic Stenosis
      1. Measure peak velocity with Sample Volume at LVOT in apical 5 chamber view)
    2. Diastolic function
      1. Sample Volume at mitral valve, early diastolic inflow in apical 4 chamber view
      2. Peak Mitral E-Wave Velocity (cm/sec)
      3. E-Wave deceleration time
      4. E-Wave/A-Wave Ratio
  3. Aliasing (signal noise) occurs when frequency is high and Nyquist limit is exceeded
    1. Occurs when target object frequency is more than half of pulse repetition frequency (PRF)
    2. Aliasing occurs when RBCs move faster than 1/2 of PRF (RBC velocity too high)
    3. Methods to reduce aliasing
      1. Switch to continuous wave (CW) doppler OR
      2. Place Sample Volume indicator at a shallower, decreased depth (closer to the probe) OR
      3. Increase the pulse wave (PW) doppler scale (display zooms out)
        1. Show more of the m/s axis (zero-baseline shifts up)
        2. Allows for higher velocity flow appearing on the graph

V. Technique: Continuous Wave Doppler (CW)

  1. Returns signal from all Red Blood Cells moving across the Ultrasound beam (line)
  2. Indications
    1. Evaluate valve stenosis and regurgitation (identifying peak and mean velocity and pressure)
      1. Mitral Stenosis (peak and mean velocity and pressure)
      2. Aortic Regurgitation (pressure half-time in msec) during diastole
    2. Estimate Right Ventricular Systolic Pressure or RVSP (and Pulmonary Artery Systolic Pressure)
      1. Uses even trace tricuspid regurgitation (present in most patients)
      2. RVSP = RA Pressure (estimated based on IVC) + 4*Vtr^2
    3. Left ventricular outflow tract velocity time integral (LVOT VTI)
      1. Aortic outflow velocity wave is traced to estimate peak and mean systolic pressures

VI. Technique: Color Flow Imaging (Color Doppler Imaging or CDI)

  1. Displays Blood Flow velocities (doppler shifts) in 2 dimensions, showing multiple locations of flow
    1. Pulsed wave values from a grid of locations within a color box (color sample volume)
  2. Provides a real-time spatial map of velocities (as represented by colors) in relation to 2D structures
    1. Color demonstrates relative velocities to one another
    2. Interpret color flow and direction of signal in relation to the Cardiac Cycle (e.g. Mitral Regurgitation in systole)
    3. Unlike Pulsed Wave Doppler and Continuous Wave Doppler, color flow does not result in specific velocity values
      1. Results are qualitative, not quantitative, and severity of findings is best evaluated with PW and CW
  3. Color flow doppler may be used in most views (even when not parallel to flow)
    1. BLUE is flow AWAY from probe and RED is flow TOWARDS probe (Mnemonic: BART)
      1. With apical 4 chamber view, normal mitral valve flow in diastole is toward the probe (red)
      2. With apical 5 chamber view, normal aortic valve flow in systole is away from the probe (blue)
  4. Aliasing (signal noise) also occurs (as with PW) when frequency is high and Nyquist limit is exceeded
    1. Appears a mix of multiple random colors in the color box
    2. Aliasing occurs with higher red cell velocities, and therefore may be associated with more severe pathology
    3. See Pulsed Wave Doppler (PW) above regarding mechanism of aliasing and associated fixes
    4. Color gradient scale must be adjusted for accurate interpretation (as with PW doppler scale adjustments)
      1. Typically keep color scale between 50-60 and adjust up or down for specific findings, then reset scale back
      2. Adjust the color scale up (zooming out) to evaluate higher velocities (e.g. ventricles)
      3. Adjust the color scale down (zooming in) to evaluate lower velocities (e.g. atria in ASD evaluation)
        1. However, setting scale too low will exaggerate low flow signals and may appear as pathology
        2. Incorrectly set scale if not caught, may result even in unnecessary Valve Replacements
        3. In the Phen-Fen lawsuits, decreased color scale was among fraudulent techniques used
            1. https://www.law.com/americanlawyer/almID/1109128224002/?slreturn=20200027182837
  5. Frame Rates
    1. Frame rates (and processing speed, quality) are decreased when color box (sample volume) is enlarged
      1. Trade-off of greater sample volume at the cost of reduced frame rates
      2. Keep frame rates (displayed on screen) above 20 hz
    2. Decrease the color box size to focus on specific regions of interest (ROI width)
      1. Not all information from a view needs to be captured on a single image
      2. Better to keep the ROI width (color box) narrow and capture different regions in different images
      3. Frame rate optimization becomes even more important when windows are poor (e.g. COPD, Obesity)
  6. Gain
    1. Adjust the color gain to help identify abnormalities (e.g. regurgitation jets)
    2. However, avoid adding too much gain that will create the appearance of abnormalities (as with decreasing scale)
  7. Indications
    1. Best at displaying valvular regurgitation (e.g. Mitral Regurgitation, Aortic Regurgitation)
      1. Demonstrates pathology even when not parallel to flow (unlike PW and CW)
    2. Used as a quick screening tool for pathology (e.g. PLAX View of Aorta outflow and mitral valve)
      1. Use PW or CW to quantify in parallel views (e.g. apical 5 chamber and apical 4 chamber)

VII. References

  1. Jordan (2019) Cardiac Ultrasound Protocol Manual, Gulfcoast Ultrasound, p 13-22
  2. Reynolds (2018) The Echocardiographer's Pocket Reference, Arizona Heart Association, p. 323-4
  3. Palma, Bourque and Jordan (2019) Introduction to Adult Echo Ultrasound Conference, GulfCoast Ultrasound, St. Petersburg

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