Section 1
The Basics: What You need to know

DOPPLER PRINCIPLES AS THEY APPLY TO ACCURACY AND QUALITY OF THE EXAM

• Doppler shift
  • Johann Christian Doppler
    • Discovered that sound waves change frequency (frequency shift) when there is a change in position between the source of the sound waves and the receiver of the reflected sound waves (Figure 1.1 and Video 1.1)
  • What is a Doppler shift
    • We hear a Doppler shift when we hear a siren
      • As the pitch (frequency) increases we know the vehicle is coming toward us
      • As the pitch (frequency) decreases we know the vehicle is moving away from us
    • The Doppler shift in diagnostic ultrasound (Figure 1.2)
      • When Doppler sound is sent into the heart
        • Blood cells moving away from the transducer send back lower-frequency sound, creating a negative frequency shift (i.e., out with 5 MHz - back with 3.5 MHz)
          • The sound wave is trying to catch the cells - so it interacts with the cells less frequently
        • Blood cells moving toward the transducer send back higher-frequency sound, creating a positive frequency shift (i.e., out with 5 MHz - back with 7.0 MHz)
          • The sound wave is running into the cells - so it interacts with the cells more frequently

            Figure 1.1 As this ambulance drives toward the person on the right it encounters the wave front and wavelength frequency increases. As it moves away from the person on the left, it encounters the wave front less frequently so the wavelength frequency decreases.

            
            Video 1.1

            Figure 1.2 (a) As sound waves encounter blood cells moving away from the transducer the reflected sound is at a lower frequency, and the machine processes this negative frequency shift as blood cells moving away from the transducer. (b) As sound waves encounter blood cells moving toward the transducer the reflected sound is at a higher frequency, and the machine processes this positive frequency shift as blood cells moving toward the transducer.

• Types of Doppler
  • Pulsed wave (PW)
    • PW Doppler sends out pulses of sound to a specific location in the heart (Figures 1.3a and b)
    • Reflected sound from the red blood cells at that specific location needs to return to the transducer and be processed (frequency shift calculated) before the next pulse is transmitted

      Figure 1.3 (a) A Doppler cursor placed in the aorta on an apical 5 chamber plane records blood flow moving away from the transducer, and the spectral waveform is displayed below the baseline which represents zero velocity. The pulsed wave (PW) gate here (small equal sign) is placed at the aortic valve, and the spectral flow profile is very specific for that location. (b) A Doppler cursor placed at the mitral leaflets on an apical 4 chamber plane records blood flow moving toward the transducer, and the spectral waveform will be displayed above the baseline which represents zero velocity. The PW gate here (small equal sign) is placed at the tips of the mitral valve leaflets, and the flow is very specific for that location.

      Figure 1.4 The Doppler cursor is aligned with aortic flow leaving the left ventricle on this subcostal 5 chamber view. Using continuous wave (CW) Doppler, flow is recorded all along the Doppler cursor with no site specificity. Flow along this cursor varies dependent on location, and so the flow profile shows all of these velocities. The flow profile is filled in because of these various velocities, but the highest flow is what is seen at the peak of the flow profile.

  • Continuous wave (CW)
    • CW Doppler sends sound out continuously along a line through the valves or vessels in the heart. There is no site specificity with CW Doppler
    • Reflected sound is processed continuously (frequency shifts are constantly calculated as blood cells all along the line send reflected sound back to the transducer) (Figure 1.4)
  • Color flow
    • Color flow Doppler is a form of PW Doppler
    • The color box has multiple Doppler lines with multiple gates along each line
    • Each gate sends flow information back to the transducer, which is assigned a color depending on which direction the blood is flowing
    • Flow toward the transducer is typically coded in red (Figure 1.5a and Video 1.5a)
    • Flow away from the transducer is typically coded in blue (Figure 1.5b and Video 1.5b)
    • Green variance maps are available to highlight a lot of variance in the flow being investigated
      • Variance is seen in turbulent flow
      • Not all green represents turbulence, however

• Advantages and disadvantages of each type of Doppler
  • PW
    • Advantages
      • Site specific - velocity only recorded at the gate (Figure 1.3)
    • Disadvantages
      • Cannot record high-velocity flow
  • CW
    • Advantages
      • Able to record high-velocity flow
    • Disadvantages
      • No site specificity - records velocities all along the Doppler line (Figure 1.4)
  • Color flow
    • Advantages
      • Potential abnormal flow is readily identified
    • Disadvantages
      • There is minimal quantitative information associated with color flow Doppler (Figure 1.6 and Video 1.6)

• The spectral display
  • Baseline
    • The baseline is moved up or down as necessary to display either upward or downward flow or both (Figures 1.7a-c)
    • When both upward and downward flows are being investigated and CW is used, the baseline is positioned and scale adjusted so that each flow profile is seen in its entirety
    • When there is upward and downward flow but PW is used, only one or the other can be recorded accurately since the gate is very site specific
    • The baseline is positioned to the top or bottom of the display and scale is adjusted to ideally display the full PW flow profile
  • Velocity scale
    • Velocity scales are seen both above and below the baseline
    • This scale is adjusted lower or higher by a velocity, PRF, or scale knob
    • As the baseline moves up or down the scale changes on each side of the baseline

      Figure 1.5 The multiple lines of Doppler sound in a color flow sector are filled with PW gates. Each gate records a positive or negative frequency shift that is displayed as a specific color. (a) When reflected sound sends back a positive frequency shift from a PW gate, meaning blood cells are moving toward the transducer, the flow at that gate is color coded in red. Here blood flow is moving up through the mitral valve on this apical 4 chamber view, and the image shows red flow moving through the mitral valve into the left ventricle. (b) When reflected sound sends back a negative frequency shift from a PW gate, meaning blood cells are moving away from the transducer, the flow at that gate is color coded in blue. Here blood flow is moving down into the pulmonary artery on this right transverse view, and the image shows blue flow moving from the right ventricle into the pulmonary artery.

      
      Video 1.5

    • If the velocity scale is not adjusted while the baseline is moved, the total velocity remains the same; it is just redistributed above or below the baseline
    • Velocity and baseline are adjusted to show a complete spectral flow profile and to eliminate aliasing if possible (Figures 1.7b and c)

      Figure 1.6 While color flow Doppler locates possible areas of abnormal flow rapidly, it provides minimal quantitative information. Tricuspid regurgitation (TR) in this foreshortened apical 4 chamber view is considered mild based upon the size of the color jet, but no hemodynamic information is provided.

      
      Video 1.6

  • Information obtained from spectral display
    • Velocity (Figures 1.7b and c)
      • Velocity in m/sec or cm/sec is displayed above and below the baseline
    • Direction (Figures 1.7a-c)
      • Blood cells moving toward the transducer are displayed above the baseline
      • Blood cells moving away from the transducer are displayed below the baseline
    • Timing (Figures 1.7a-c)
      • The horizontal axis of a spectral display represents time
      • Blood flow during diastole and systole are displayed at their respective times on the horizontal axis
      • An ECG may make identification of event timing more accurate
    • Intensity (Figure 1.7b)
      • The density of a spectral display is related to the number of blood cells involved in the flow
      • A small volume of blood cells would have less density than a large volume
      • This is accurate when the spectral displays are created with the same Doppler settings and alignment is parallel with both flows

• The color display
  • Direction (Figure 1.5)
    • Blood...