Module 3

Course Overview:
  - Subject: Vascular Sonography 1
  - Focus: Doppler Principles
  - Institution: Southern Alberta Institute of Technology (SAIT), School of Health and Public Safety
  - Term: Winter 2023

Overview of Doppler Principles related to Venous Blood Flow:
  - Objectives:
    - 3.1 Review the Doppler principle.
    - 3.2 Describe the technical principles and optimization of colour Doppler.
    - 3.3 Describe the technical principles and optimization of spectral Doppler.
    - 3.4 Compare the features of pulsed wave (PW) and continuous wave (CW) Doppler.
    - 3.5 Identify colour and spectral Doppler artifacts.
    - 3.6 Differentiate normal from abnormal venous colour and spectral waveforms.
Recommended Readings:
- Kupinski, 3rd Ed., Chapter 2: Pages 11-26
- Daigle, 5th Ed., Chapter 1: Pages 1-31

Doppler Effect:
  - Definition: A phenomenon where there is a change in the frequency of sound due to the motion of the source, the observer, or both.
  - In Sonography, specifically relates to the motion of red blood cells (RBCs):
    - Antegrade Flow: RBCs moving toward the transducer yield a larger echo frequency than the transmitted frequency.
    - Retrograde Flow: RBCs moving away from the transducer yield a smaller echo frequency than the transmitted frequency.
Methods for Detecting and Analyzing Doppler Shifts:
  - Spectral waveforms
  - Colour flow imaging
  - Audible sounds
Doppler Shift Calculation:    $\Delta f = f_r - f_t$
  - Where:
    - fr = returned frequency
    - ft = transmitted frequency
Factors Influencing Doppler Shift:
  - Transmitted frequency (
     $f_o$ )
  - Velocity of moving blood ( $v$ )
  - Angle between moving blood and sound beam ( $\theta$ )

Revised Doppler Shift Equation:
  - $\Delta f=\frac{2f_{o}v{cos}(\theta)}{c}$
  - Where:
    - $c$ = speed of sound in soft tissue (1540 m/s)
    - The angle of insonation ( $\theta$ ) is critical in determining RBC velocity.
    - In venous flow, angle correction is less critical, except in specific cases like MPV (Mesenteric Phlebography).

Fast Fourier Transform (FFT):
  - Utilized to separate received Doppler shift frequencies into individual frequency components.
  - Displayed as a spectrum plotted on a graph:
    - Horizontal X-axis: Time
    - Vertical Y-axis: Velocity
    - Brightness (Power/Z-axis): Reflects pixel intensity.

Mechanism:
  - Grey scale for stationary reflectors and colour for moving reflectors within the region of interest.
  - Colour represents a mean frequency shift calculated from three pulses.
  - Utilizes autocorrelation to sample multiple sites for real-time imaging.
Information from Returning Echoes:
  - Direction
  - Mean velocity
  - Amplitude
  - Variance
Qualitative Nature:
- Provides mean velocities rather than absolute values.
Impact of Colour Doppler on B-mode Images:
- Activating colour reduces Pulse Repetition Frequency (PRF) leading to lower frame rates, affecting temporal resolution.
- Narrow colour boxes can enhance frame rates due to reduced scan lines.

Flow Representation:
  - A colour map indicates mean frequency and direction of RBCs:
    - Blue: Antegrade flow towards the probe.
    - Red: Retrograde flow.
  - Flow Patterns:
    - Example of laminar flow with highest velocities in the center, peak mean velocities around 20 cm/s.

Mechanism:
  - Measures signal intensity (density of RBCs) rather than frequency shifts.
  - Less dependent on angle; more sensitive to flow, good for small vessels and slow flow monitoring.
  - Issues: Very slow frame rate; not suitable for unstable tissue or patient movement.
  - Direction determination via standard technology with modern machines.

Importance of Optimization:
- Extracting accurate information requires critical thinking and optimization of controls.
Key Controls in Optimization:
  - Gain: Directly affects the amplitude of returned Doppler shifts.
    - Adjustment: Too low may hide signals; too high can introduce noise and artifacts.
    - Optimal gain determined by increasing until noise appears then reducing just below that threshold.
  - Scale & PRF:
    - Adjust to represent peak and minimum velocities; higher settings avoid aliasing.
  - Baseline:
    - Allows complete spectral display; important to leave space for antegrade and retrograde flows.
    - Misalignment can lead to aliasing.
  - Gate Size (for spectral Doppler):
    - Set to one-third of vessel diameter for optimal signal quality.
  - Colour Box Adjustments:
    - Must cover the vessel completely; width impacts frame rate.

Purpose:
- Eliminates low-frequency noise caused by vessel movements, bowel peristalsis, and patient breathing, improving signal clarity.
Typical Settings:
- Low Filter: 55 Hz
- High Filter: 200 Hz

Common Artifacts:
  - Noise or Blooming
  - Flash or Clutter
  - Aliasing
  - Mirror Image
Noise or Blooming:
- Caused by excessive gain; appears as colour outside vessels or falsely elevated velocities.
Flash or Clutter:
- Non-RBC motion introduces unwanted colour flashes; control via gain adjustment can reduce this.
Aliasing:
  - Occurs when Nyquist limit is surpassed; indicates potential stenosis.
  - Compensation Methods:
    - Adjust baseline
    - Increase PRF
    - Change angle or frequency
Mirror Image Artifact:
- Artificially reflects flow on both sides of the baseline, resulting from over-gaining or incorrect angles.

Pulsed Wave Doppler:
  - Produces sound pulses at intervals; allows depth-specific signal isolation.
  - Sampling limitations can result in aliasing and range ambiguity.
  - Supports duplex imaging (2D image with waveform).
Continuous Wave Doppler:
  - Simpler device; continuous transmission and signal reception.
  - No depth specificity; requires anatomical knowledge to accurately assess sites.
  - Best for high-velocity measurements; avoids aliasing due to absence of Nyquist limit.

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