Unit 19: Doppler Physics and Instrumentation

Introduction

• Focus on Doppler physics and math in ultrasound.
• Unit 20 will focus on clinical applications.
• Topics covered: Doppler effect, Doppler shift, Doppler equation, continuous wave Doppler, pulse wave Doppler, color Doppler, and instrumentation.

Section 19.1: Doppler Effect

• Doppler Effect: Change in frequency and wavelength due to motion of sound source, receiver, or reflector.
• Example: Siren on a moving vehicle.
  • Stationary truck: Perceived frequency = 800 Hz
  • Moving towards: Perceived frequency = 820 Hz (shorter wavelengths, higher frequency)
  • Moving away: Perceived frequency = 780 Hz (longer wavelengths, lower frequency)
• Key Point: A sound source moving toward the receiver causes high frequency (higher pitch); moving away causes low frequency (lower pitch).

Section 19.2: Doppler Shift

• Doppler Shift: Change in frequency due to motion.
• Calculated as: Doppler shift = Frequency received - Frequency transmitted
• Examples:
  • No motion: Doppler shift = 0 Hz
  • Moving towards: Doppler shift = +20 Hz
  • Moving away: Doppler shift = -20 Hz
• Red blood cells towards transducer = Positive shift; away = Negative shift.
• Blood velocity calculated using ultrasound values:
  • E.g., 5 MHz transducer, positive shift = 3000 Hz.

Section 19.3: Doppler Equation

• Doppler Equation: Used to calculate Doppler shift and velocity.
• Doppler Shift = (2 * Operating frequency * Velocity * Cosine(theta)) / Propagation speed
• Frequency of Doppler shift is directly related to operating frequency, velocity, and cosine of theta.
• Key Concepts:
  • 0° or 180° angles give the most accurate readings (cosine = ±1).
  • 90° angle yields no Doppler shift.
  • Never exceed 60° for Doppler angle in practice.*

Section 19.4: Velocity of Blood

• Velocity Equation: Rearranged Doppler equation to solve for blood velocity.
• Velocity = (Propagation speed * Doppler shift) / (2 * Operating frequency * Cosine(theta))
• Key relationships:
  • Increase in Doppler shift increases velocity.
  • Decrease in operating frequency increases velocity.
  • Decrease in cosine value increases reported velocity.*

Section 19.5: Doppler Instrumentation

• Bidirectional Doppler: Detects flow towards and away using phase quadrature.
• Types: Continuous wave Doppler, pulse wave Doppler, color Doppler.

Section 19.6: Continuous Wave Doppler

• Uses two crystals: one transmits, one receives continuously.
• No anatomical imaging; used in cardiac applications.
• Advantages: Can detect very high velocities (no aliasing).
• Disadvantages: Range ambiguity, no TGC.

Section 19.7: Pulse Wave Doppler

• Uses one crystal; allows duplex and triplex imaging.
• Advantages: Range resolution, adjustable sample volume.
• Disadvantages: Limited by Nyquist limit; subject to aliasing.
• Uses Fast Fourier Transform for detailed velocity data.

Section 19.8: Color Doppler

• Pulse wave technique showing average velocities as 2D overlay.
• Velocity Mode Map: Shows direction (towards/away) and velocity.
• Variance Mode Map: Adds laminar vs. turbulent flow indication.
• Uses autocorrelation for faster processing.
• Power color Doppler detects any motion, sensitive but not angle dependent.

Summary

• Understand Doppler effect, shift, and equations.
• Recognize instrumentation types and their advantages/disadvantages.
• Review key terms like phase quadrature, autocorrelation, angle correct, etc.
• Prepare for clinical application in Unit 20.