Unit 19: Doppler Physics and Instrumentation

Introduction

• Doppler Shift: Experience in daily life (e.g., sirens on emergency vehicles).
• Units 19 & 20: Focus on math/physics (Unit 19) and clinical applications (Unit 20).

Key Topics Covered

• Doppler Effect
• Doppler Shift
• Doppler Equation
• Continuous Wave Doppler
• Pulse Wave Doppler
• Color Doppler
• Instrumentation

Section 19.1: Doppler Effect

• Concept: Change in frequency/wavelength due to motion of sound source, receiver, or reflector.
• Example: Firetruck siren at 800 Hz. As it moves towards you, perceived frequency increases.
  • Shorter wavelengths = Higher frequency (e.g., perceive 820 Hz when approaching).
  • Longer wavelengths = Lower frequency (e.g., perceive 780 Hz when moving away).

Section 19.2: Doppler Shift

• Definition: Change in frequency due to motion.
• Calculation: Doppler Shift = Received Frequency - Transmitted Frequency.
• Examples:
  • Stationary truck: No Doppler shift (0 Hz difference).
  • Moving truck towards: Positive Doppler shift (+20 Hz).
  • Moving truck away: Negative Doppler shift (-20 Hz).
• Application in Ultrasound: Red blood cells moving towards/away from transducer demonstrate Doppler shifts.

Section 19.3: Doppler Equation

• Equation: Doppler Shift = 2 * (Transmitted Frequency * Velocity * Cosine(Theta)) / Propagation Speed.
• Variables:
  • Frequency of Doppler Shift: Calculated from received/transmitted frequencies.
  • Velocity: Calculated by rearranging the Doppler equation.
  • Cosine Theta (θ): Angle between sound beam and direction of blood flow.
  • Propagation Speed: Constant in soft tissue (1540 m/s or 154,000 cm/s).
• Velocity Calculation: Rearrange to solve for blood velocity.*

Section 19.4: Velocity of Blood

• Importance: Doppler shift helps calculate velocity, which is of diagnostic value.
• Relationships:
  • Velocity directly related to Doppler shift.
  • Inversely related to operating frequency and cosine of theta.

Section 19.5: Doppler Instrumentation

• Bidirectional Doppler Detection: Recognizes flow direction relative to transducer.
• Phase Quadrature: Mathematical method to analyze Doppler signals.

Section 19.6: Continuous Wave Doppler

• Features:
  • Uses two crystals: one for transmitting, one for receiving.
  • Detects very high velocities without aliasing.
  • Range ambiguity: Cannot determine exact origin of received signals.
  • No TGC (Time Gain Compensation).

Section 19.7: Pulse Wave Doppler

• Features:
  • Single crystal used to produce Doppler spectral tracings.
  • Duplex (2D + Doppler) & Triplex (2D + Color + Doppler) imaging.
• Advantages: Range resolution allows precise sampling.
• Disadvantages: Limited by Nyquist limit, potential for aliasing.

Section 19.8: Color Doppler

• Pulse Wave Technique: Displays average velocities as a 2D overlay.
• Velocity Mode Map: Shows direction and speed.
• Variance Mode Map: Adds laminar vs. turbulent flow.
• Packet/Ensemble: Multiple pulses used to create color Doppler information.
• Power Doppler:
  • Sensitive to slow flows and deeper vessels.
  • Not angle dependent, but lower frame rate.

Important Concepts and Terms

• Doppler Effect & Shift: Understanding changes in frequency due to motion.
• Doppler Equation: Relationships between variables.
• Velocity Calculation: From Doppler shift.
• Instrumentation: Continuous vs. pulse wave Doppler.
• Color Doppler: Average velocities, color maps, and power Doppler.

Conclusion

• Understanding of Doppler effect, shift, and their equations.
• Distinction between continuous and pulse wave Doppler.
• Importance of color mapping and instrumentation terms for clinical application.